Electronics Manufacturing with Lead-Free Halogen-Free and Conductive-Adhesive

ELECTRONICSMANUFACTURING

WITH LEAD-FREE, HALOGEN-FREE,

AND CONDUCTIVE-ADHESIVE MATERIALS

http://dx.doi.org/10.1036/0071386246

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ELECTRONICSMANUFACTURING

WITH LEAD-FREE,

HALOGEN-FREE,

AND CONDUCTIVE-ADHESIVE

MATERIALS

John H. LauAgilent Technologies, Inc.

C. P. WongGeorgia Institute of Technology

Ning-Cheng LeeIndium Corporation of America

S. W. Ricky LeeHong Kong University of Science and Technology

http://dx.doi.org/10.1036/0071386246

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CONTENTS

Chapter 1. Introduction to Environmentally Benign Electronics Manufacturing 1.1

1.1 Trends in Industry 1.11.1.1 Automobile Industry 1.11.1.2 Electronics Industry 1.2

1.2 Trends in Worldwide Environmentally Benign Manufacturing 1.31.2.1 Government Activity 1.41.2.2 Industry Activity 1.41.2.3 R&D Activity 1.51.2.4 Education Activity 1.51.2.5 Worldwide Efforts on Environmentally Benign

Electronics Manufacturing 1.61.3 Trends in Environmentally Benign Electronics Manufacturing 1.6

1.3.1 IC Fabrication 1.91.3.2 IC Packaging 1.91.3.3 PCBs 1.91.3.4 Lead-Free Solders 1.101.3.5 Halogen-Free Flame Retardants 1.111.3.6 Conductive Adhesives 1.121.3.7 End-Of-Life Management 1.13

Acknowledgments 1.14References 1.14

Chapter 2. Chip (Wafer)-Level Interconnects with Lead-Free Solder Bumps 2.1

2.1 Introduction 2.12.2 UBM 2.1

2.2.1 Electroless Ni-P-Immersion Au UBM 2.12.2.2 Al-NiV-Cu UBM 2.6

2.3 Microball Wafer Bumping with Lead-Free Solders 2.62.3.1 Overview of Microball Wafer Bumping 2.62.3.2 Microball Preparation 2.62.3.3 Microball Management 2.92.3.4 Microball Wafer Bumping 2.12

2.4 Sn-Ag-Cu Solder Ball Mounting on Wafers 2.122.4.1 WLCSP 2.122.4.2 WLCSP with Stress-Relaxation Layer 2.15

2.5 Stencil Printing on Sn-Ag Solder on Wafers with Ni-Au UBM 2.202.5.1 The Interface Between Electroless Ni and Solders 2.202.5.2 Growth of the IMC and P-Rich Ni Layer 2.222.5.3 Bump Shear Fracture Surface 2.24

v

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2.6 Stencil Printing of Sn-Cu, Sn-Ag-Bi, and Sn-Ag-Cu Solders on Wafers with Ni-Au UBM 2.27

2.6.1 Interface of Reflowed Solder Bumps 2.272.6.2 Interface of Annealed Solder Bumps 2.292.6.3 Shear Strength of Solder Bumps 2.30

2.7 Stencil Printing of Sn-Cu, Sn-Ag-Bi, and Sn-Ag-Cu Solders on Wafers with Ti-Cu UBM 2.31

2.7.1 Interface of Reflowed Solder Bumps 2.312.7.2 Interface of Annealed Solder Bumps 2.31

2.8 Paste Printing of Solders on Wafers with Al-NiV-Cu UBM 2.34Acknowledgments 2.34References 2.35

Chapter 3. WLCSP with Lead-Free Solder Bumps on PCB/Substrate 3.1

3.1 Introduction 3.13.2 Solder Joint Reliability of SnAgCu WLCSP with a

Stress-Relaxation Layer 3.13.2.1 Finite Element Results 3.13.2.2 Thermal Cycling Results 3.23.2.3 Effects of the Stress-Relaxation Layer on Capacitance 3.4

3.3 Solder Joint Reliability of SnAg and SnAgCu WLCSPs with TiCu and NiAu UBMs 3.5

3.3.1 Isothermal Fatigue Test Results 3.53.3.2 Thermal Cycling Fatigue Test Results 3.8

3.4 Solder Joint Reliability of SnAg, SnAgCu, SnAgCuSb, and SnAgInCu WLCSPs with AlNiVCu UBM 3.15

3.4.1 Thermal Fatigue of SnAg, SnAgCu, SnAgCuSb and SnAgInCu WLCSPs on Ceramic Substrate 3.15

3.4.2 Thermal Fatigue of SnAgCu WLCSP on PCB 3.153.4.3 High-Temperature Storage of SnAgCu

WLCSP on PCB 3.153.4.4 Shear Strength of SnAgCu WLCSP on PCB 3.17


Chapter 4. Chip (Wafer)-Level Interconnects with Solderless Bumps 4.1

4.1 Introduction 4.14.2 Wafers for Electroless Ni-Au, Electroplated Au,

and Electroplated Cu Bumps 4.14.3 Electroless Ni-P-Immersion Au Bumps 4.1

4.3.1 Materials and Process 4.24.3.2 Passivation Cracking 4.2

4.4 Electroplated Au Bumps 4.64.4.1 Materials and Process 4.64.4.2 Bump Specifications and Measurement Methods 4.6

4.5 Electroplated Cu Bumps 4.84.5.1 Materials and Process 4.84.5.2 Special Considerations 4.8

vi CONTENTS

4.6 Electroplated Copper Wires 4.94.6.1 Structure 4.94.6.2 Fabrication Materials and Process 4.10

4.7 Wire-Bonding Microsprings 4.104.7.1 Materials and Process 4.114.7.2 Special Considerations 4.11

4.8 Wire-Bonding Au Stud Bumps 4.124.8.1 Materials and Process 4.124.8.2 Equipment 4.14

4.9 Wire-Bonding Cu Stud Bumps 4.174.9.1 Materials and Process 4.214.9.2 Shear Strength 4.23


Chapter 5. WLCSP with Solderless Bumps on PCB/Substrate 5.1

5.1 Introduction 5.15.2 Design, Materials, Process, and Reliability of WLCSPs

with Au Bumps, Cu Bumps, and Ni-Au Bumps on PCB with ACF 5.15.2.1 PCB 5.15.2.2 ACF 5.15.2.3 FCOB Assemblies with ACF 5.45.2.4 Thermal Cycling Test of FCOB Assemblies with ACF 5.95.2.5 SIR Test Results of ACF FCOB Assemblies 5.105.2.6 Summary 5.10

5.3 Copper Wired WLCSP with Solders or Adhesives on Substrates 5.115.4 Microspring WLCSP with Solders or Adhesives on PCB/Substrate 5.125.5 Au-Stud-Bumped WLCSP with ICA on PCB 5.12

5.5.1 Materials and Process Flow 5.135.5.2 Equipment for SBB Technology 5.14

5.6 Au-Stud-Bumped WLCSP with ICA on Flex 5.145.6.1 Materials and Process 5.165.6.2 Qualification Tests and Results 5.19

5.7 Au-Stud-Bumped WLCSP with ACP/ACF on PCB 5.225.7.1 ACF/ACP with Nonconductive Fillers 5.225.7.2 DSC Measurement Results 5.235.7.3 DMA Measurement Results 5.235.7.4 TMA Measurement Results 5.235.7.5 TGA Measurement Results 5.265.7.6 85°C/85% RH Test and Results 5.265.7.7 Thermal Cycling Test and Results 5.27

5.8 Au-Stud-Bumped WLCSP Diffused on Au-Plated PCB with NCA 5.295.8.1 Materials and Process 5.325.8.2 Reliability 5.35

5.9 Au-Stud-Bumped WLCSP Diffused on Au-Plated Flex with NCA 5.375.9.1 Materials and Process 5.375.9.2 Reliability 5.38

5.10 Cur-Stud-Bumped WLCSP with Lead-Free Solders on PCB 5.425.10.1 Materials and Process 5.425.10.2 Reliability 5.43


CONTENTS vii

Chapter 6. Environmentally Benign Molding Compounds for IC Packages 6.1

6.1 Introduction 6.16.2 Environmentally Benign Molding Compounds for PQFP Packages 6.1

6.2.1 Flame Resistance Systems: Addition-Type Retardants 6.26.2.2 Flame Resistance Systems: Novel Resin Systems 6.46.2.3 Effects of Raised Reflow Temperature

on Molding Compounds 6.46.2.4 Halogen-Free Molding Compounds for Lead-Free Soldering 6.8

6.3 Environmentally Benign Molding Compounds for PBGA Packages 6.10

6.3.1 Halogen-Free Flame-Retardant Agents 6.126.3.2 PBGA Package Warpage Controlled by Tg Dispersion 6.166.3.3 PBGA Package Warpage Controlled by

Stress-Absorbing Agents 6.196.4 Environmentally Benign Molding Compounds for

MAP-PBGA Packages 6.226.4.1 Halogen-Free Flame-Retardant Resins 6.226.4.2 Sample Preparation 6.226.4.3 Effects of Tg , Shrinkage, and Viscosity on Package

Coplanarity 6.256.4.4 Moisture Sensitivity Tests 6.27


Chapter 7. Environmentally Benign Die Attach Films for IC Packaging 7.1

7.1 Introduction 7.17.2 Environmentally Benign Die Attach Films 7.1

7.2.1 Silver-Filled Film DF-335-7 for Leadframe PQFP Packages 7.17.2.2 Insulating Film DF-400 for BT-Substrate PBGA Packages 7.6

7.3 Environmentally Benign In-Sn Die Attach Bonding Technique 7.107.3.1 In-Sn Phase Diagram 7.117.3.2 Design and Process of In-Sn Solder Joints 7.127.3.3 Characterization of In-Sn Solder Joints 7.14


Chapter 8. Environmental Issues for Conventional PCBs 8.1

8.1 Introduction 8.18.2 Influence of Electronic Products 8.2

8.2.1 Major Environmental Concerns 8.28.2.2 Energy Issues 8.58.2.3 Chemical Issues 8.78.2.4 Disposal and Recycling 8.148.2.5 Design for Environment 8.16

8.3 Environmental Research for the PCB Industry 8.188.3.1 Energy and Solvent Reduction 8.198.3.2 Renewable Resins for PCB 8.218.3.3 Reworkable Encapsulants for Disassembly 8.22

viii CONTENTS

8.4 International Driving Forces for Halogen-free Alternatives 8.238.4.1 Background and Challenge 8.238.4.2 Driving Forces 8.248.4.3 Material Availability 8.258.4.4 Design Measures and Performance 8.25

References 8.25

Chapter 9. Halogenated and Halogen-Free Materials for Flame Retardation 9.1

9.1 Introduction 9.19.2 Brominated Flame Retardants 9.2

9.2.1 Production Aspects 9.29.2.2 Classification 9.49.2.3 Risk Assessment 9.5

9.3 Toxicological Aspects of Halogen-Free Flame Retardants 9.79.3.1 Fundamentals 9.79.3.2 Denitrification 9.99.3.3 Bioassay Procedures 9.10

9.4 Environmentally Conscious Flame-Retarding Plastics 9.119.4.1 Flame-Retardant Polycarbonate Resin 9.12

References 9.19

Chapter 10. Fabrication of Environmentally Friendly PCB 10.1

10.1 Introduction 10.110.2 PCB DfE 10.1

10.2.1 Process Modeling 10.110.2.2 Health Hazard Assessment 10.210.2.3 Board Optimization 10.710.2.4 Life Cycle Analysis (LCA) 10.11

10.3 Implementing Green PCB Manufacturing 10.1110.3.1 Basic Processes 10.1210.3.2 Process Modifications 10.1310.3.3 Environmental Impact 10.16

10.4 Conformal Coating with Environmental Safety 10.1710.4.1 Fundamentals 10.1710.4.2 Coating Selection 10.1710.4.3 Curing Methods 10.1810.4.4 Dispensing Methods 10.1910.4.5 Process Issues 10.20

References 10.22

Chapter 11. Global Status of Lead-Free Soldering 11.1

11.1 Introduction 11.111.2 Initial Activities 11.111.3 Recent Activities 11.211.4 Impact of Japanese Activities 11.511.5 U.S. Reaction 11.511.6 What Are Lead-Free Interconnects? 11.7

CONTENTS ix

11.7 Criteria for Lead-Free Solder 11.811.8 Viable Lead-Free Alloys 11.8

11.8.1 Sn96.5/Ag3.5 11.811.8.2 Sn99.3/Cu0.7 11.811.8.3 SnAgCu 11.911.8.4 SnAgCuX 11.911.8.5 SnAgBiX 11.911.8.6 SnSb 11.1011.8.7 SnZnX 11.1011.8.8 SnBi 11.11

11.9 Cost 11.1111.10 PCB Finishes 11.1111.11 Components 11.1211.12 Thermal Damage 11.1211.13 Other Concerns 11.1311.14 Consortium Activity 11.1311.15 Opinions of Consortia 11.1311.16 What Are the Selections of Pioneers? 11.1411.17 Possible Path 11.1411.18 Is Pb-Free Safe? 11.1511.19 Summary 11.1511.20 Information Resources 11.16

11.20.1 Legislation 11.1611.20.2 Initiatives from Independent Corporations

and Electronics Industry Organizations 11.1611.20.3 Viable Alloys under Consideration 11.17

References 11.17

Chapter 12. Development of Lead-Free Solder Alloys 12.1

12.1 Criteria 12.112.2 Toxicity 12.112.3 Cost and Availability 12.412.4 Development of Lead-Free Alloys 12.4

12.4.1 Existing Alloys 12.412.4.2 Modification 12.5

12.5 Lead-Free Alloys Investigated 12.1312.6 Favorite Pb-Free Alloys 12.13

12.6.1 Japan 12.1312.6.2 Europe 12.1312.6.3 North America 12.3312.6.4 Comparison of Regional Preferences 12.33

12.7 Patent Issues 12.3612.8 Conclusion 12.37

References 12.37

Chapter 13. Prevailing Lead-Free Alloys 13.1

13.1 Eutectic Sn-Ag 13.113.1.1 Physical Properties 13.113.1.2 Mechanical Properties 13.113.1.3 Wetting Properties 13.613.1.4 Reliability 13.10

x CONTENTS

13.2 Eutectic Sn-Cu 13.1413.2.1 Physical Properties 13.1413.2.2 Mechanical Properties 13.1413.2.3 Wetting Properties 13.1413.2.4 Reliability 13.17

13.3 Sn-Ag-Bi and Sn-Ag-Bi-In 13.2313.3.1 Physical and Mechanical Properties 13.2313.3.2 Wetting Properties 13.2413.3.3 Reliability 13.26

13.4 Sn-Ag-Cu and Sn-Ag-Cu-X 13.3113.4.1 Physical Properties 13.3113.4.2 Mechanical Properties 13.3413.4.3 Wetting Properties 13.4213.4.4 Reliability 13.45

13.5 Sn-Zn and Sn-Zn-Bi 13.5413.5.1 Physical Properties 13.5413.5.2 Mechanical Properties 13.5513.5.3 Wetting Properties 13.5513.5.4 Reliability 13.56

13.6 Summary 13.59References 13.59

Chapter 14. Lead-Free Surface Finishes 14.1

14.1 Introduction 14.114.2 Options for PCB Lead-Free Surface Finishes 14.114.3 OSP 14.1

14.3.1 Benzotriazole 14.214.3.2 Imidazoles 14.714.3.3 Benzimidazoles 14.814.3.4 Preflux 14.14

14.4 NiAu 14.1414.4.1 Electrolytic Ni-Au 14.1514.4.2 Electroless Ni/Immersion Au 14.1814.4.3 Electroless Ni/Electroless (Autocatalytic) Au 14.25

14.5 Immersion Ag 14.2614.6 Immersion Bi 14.3614.7 Pd 14.38

14.7.1 Electrolytic Pd with or Without Immersion Au 14.3814.7.2 Electroless (Autocatalytic) Pd with or Without

Immersion Au 14.4214.8 Electroless NiPd(Au Flash) 14.4314.9 NiPd(X) 14.45

14.9.1 Electrolytic NiPdCoAu Flash 14.4514.9.2 Electroless NiPdNiAu Flash 14.45

14.10 Sn 14.4614.10.1 Electrolytic Sn 14.4714.10.2 Immersion Sn 14.50

14.11 Electrolytic NiSn 14.5514.12 Sn-Bi 14.59

14.12.1 Immersion Sn-Bi Alloy 14.5914.12.2 Electrolytic Sn-Bi Alloy 14.59

14.13 Sn-Cu (HASL) 14.60

CONTENTS xi

14.14 Electrolytic SnNi 14.6114.15 Solid Solder Deposition (SSD) 14.62

14.15.1 HASL 14.6214.15.2 Optipad 14.6414.15.3 Sipad 14.6514.15.4 PPT 14.6614.15.5 Solder Cladding 14.6714.15.6 Solder Jetting 14.6714.15.7 Super Solder 14.67

14.16 Summary for PCB Surface Finishes 14.6814.17 Options of Component Surface Finishes 14.7014.18 NiAu (ENIG) 14.7014.19 Electrolytic Pd 14.7114.20 Electroless NiPd 14.7114.21 Electrolytic PdNi 14.7214.22 Sn 14.7214.23 Electrolytic Sn-Ag 14.7214.24 Electrolytic Sn-Bi 14.7414.25 Sn-Cu 14.7514.26 Summary of Component Surface Finishes 14.76

References 14.76

Chapter 15. Implementation of Lead-Free Soldering 15.1

15.1 Compatibility of Lead-Free Solders with SMT Reflow Process 15.115.1.1 Experimental Design for Compatibility Evaluation 15.115.1.2 Results of Compatibility Study 15.715.1.3 Additional Factors to Be Considered 15.1615.1.4 Compatibility Assessment 15.19

15.2 Implementing Lead-Free Wave Soldering 15.1915.3 Effect of Reflow Profile on Lead-Free Soldering 15.2115.4 Flux Desired For Lead-Free Paste Soldering 15.2615.5 Flux Desired For Lead-Free Paste Handling 15.2915.6 Cleaning Performance of Lead-Free Solder Paste 15.2915.7 Flux Desired For Lead-Free Residue Cleaning 15.3115.8 Cleaning Chemistry/Process Desired for Lead-Free

Residue Cleaning 15.3115.9 Selection of Lead-Free Solder Paste 15.36

References 15.36

Chapter 16. Challenges for Lead-Free Soldering 16.1

16.1 Challenges for Surface Finishes 16.116.1.1 Black Pad 16.116.1.2 Extraneous/Skip Plating 16.416.1.3 Tin Whisker 16.516.1.4 Surface Finish Cleaning Resistance 16.10

16.2 Challenges for Soldering 16.1016.2.1 Intermetallic Compounds 16.1016.2.2 Dross 16.1116.2.3 Wave Solder Composition 16.1316.2.4 Lead Contamination 16.14

xii CONTENTS

16.2.5 Fillet Lifting 16.2016.2.6 Poor Wetting 16.2516.2.7 Voiding 16.2616.2.8 Rough Joint Appearance 16.28

16.3 Challenges for Reliability 16.2916.3.1 Tin Pest 16.2916.3.2 Intermetallic Compound Platelet 16.3016.3.3 Stiff Joint 16.3116.3.4 Thermal Damage 16.3316.3.5 Flux Residue Cleaning 16.3416.3.6 Conductive Anodic Filament 16.35

16.4 Unanswered Challenges 16.36References 16.39

Chapter 17. Introduction to Conductive Adhesives 17.1

17.1 Electronics Packaging: A Brief Overview 17.117.2 Overview of Conductive Adhesive Technology 17.4

17.2.1 ACAs 17.417.2.2 ICAs 17.8

17.3 Proposed Approaches for Fundamental Understanding of Conductive Adhesive Technology and Developing Conductive Adhesives for Solder Replacement 17.15

17.4 Research Objectives/Goals 17.1617.4.1 Fundamental Study of the Chemical Nature and

Behavior of Organic Lubricants on Silver Flakes 17.1617.4.2 Investigation of the Conductivity Mechanism of

Conductive Adhesives 17.1717.4.3 Identification of the Main Mechanisms Underlying

the Unstable Contact Resistance of ECAs on Non-Noble Metals and Approaches to Stabilization of Contact Resistance 17.17

17.4.4 Development of Conductive Adhesives with Satisfactory Conductivity, Stable Contact Resistance and Desirable Impact Strength 17.18

17.5 Outline of Research 17.19Acknowledgments 17.19References 17.19

Chapter 18. Conductivity Establishment of Conductive Adhesives 18.1

18.1 Introduction 18.118.2 Experiments 18.2

18.2.1 Materials 18.218.2.2 Transmission Electron Microscopy (TEM) Study of ECAs 18.218.2.3 Conductivity Establishment During Cure 18.218.2.4 Measurements of Cure Shrinkage 18.218.2.5 Conductivity Development of Ag Particles

and ECA Pastes with External Pressures 18.318.2.6 Conductivity Establishment of a Conductive Adhesive

and Lubricant Behavior of the Ag Flake 18.418.2.7 Measurements of Modulus Change During Cure 18.4

CONTENTS xiii

18.2.8 Cure Study of Conductive Adhesives 18.418.2.9 Measurements of Cross-Linking Density 18.4

18.3 Results and Discussion 18.518.3.1 Observation of Interparticle Contact Between

Silver Flakes 18.518.3.2 Conductivity Establishment of Conductive Adhesives

During Cure 18.518.3.3 Study of the Relationship Between Silver Flake

Lubricant Layer and Conductivity in ECAs 18.818.3.4 Study of the Relationship Between Cure Shrinkage

and Conductivity Establishment 18.1118.4 Conclusions 18.18

References 18.18

Chapter 19. Mechanisms Underlying the Unstable Contact Resistance of ECAs 19.1

19.1 Introduction 19.119.2 Experiments 19.3

19.2.1 Materials 19.319.2.2 Study of Bulk Resistance Shifts 19.319.2.3 Study of Contact Resistance Shifts 19.419.2.4 Study of Oxide Formation 19.5

19.3 Results and Discussion 19.519.3.1 Contact Resistance Shift Phenomenon 19.519.3.2 Investigation of Mechanisms Underlying

the Unstable Contact Resistance Phenomenon 19.719.3.3 Observation of Metal Oxide Formation 19.12

19.4 Conclusions 19.13References 19.15

Chapter 20. Stabilization of Contact Resistance of Conductive Adhesives 20.1

20.1 Introduction 20.120.1.1 Factors Affecting Galvanic Corrosion 20.120.1.2 Additives to Prevent Galvanic Corrosion 20.1

20.2 Experiments 20.320.2.1 Materials 20.320.2.2 Contact Resistance Test Devices 20.320.2.3 Study of Curing Behaviors of ECAs 20.320.2.4 Study of Dynamic Properties of ECAs 20.420.2.5 Measurement of Moisture Absorption 20.420.2.6 Measurement of Adhesion Strength 20.4

20.3 Results and Discussion 20.520.3.1 Effects of Electrolytes on Contact Resistance Shifts 20.520.3.2 Effects of Moisture Absorption on Contact Resistance Shifts 20.520.3.3 Stabilization of Contact Resistance Using Additives 20.7

20.4 Conclusions 20.1320.5 Summary 20.13

References 20.14

Index I.1About the Author A.1

xiv CONTENTS

PREFACE

Why did we want to write this book? Was it because of fear: fear of (potential) legis-lation, fear of trade barriers, fear of competition? Absolutely not! We wrote thisbook for ourselves, our children, and their children, so that we will all have a greenerenvironment to live in!

Books of this type can be huge and contain many different viewpoints, e.g., polit-ical, economic, cultural, and infrastructural. However, emphasis in this book isplaced on fundamental principles, engineering data, and manufacturing technolo-gies. There are four major subjects in this book: integrated circuit (IC) packaging(Chaps. 1 through 7), printed circuit board (PCB)/substrates (Chaps. 8 through 10),PCB/substrate assembly of IC packages (Chaps. 11 through 16), and novel conduc-tive adhesive materials (Chaps. 17 through 20).

Chapter 1 briefly discusses the trends in worldwide environmentally benign man-ufacturing, and especially electronics manufacturing. Chapter 2 presents chip(wafer)-level interconnects with lead-free solder bumps. Emphasis is placed on theunder-bump metallurgies (UBMs) and wafer bumping with microball mounting andpaste-printing methods. Chapter 3 examines the lead-free solder joint reliability ofwafer-level chip-scale packages (WLCSPs) on organic and ceramic substrates.

Chapter 4 discusses chip (wafer)-level interconnects with solderless bumps con-structed from Ni-Au, Au, and Cu, copper wires, gold wires, gold studs, and copperstuds.The design, materials, process, and reliability of WLCSPs with these solderlessinterconnects on PCB/substrate are presented in Chap. 5.

Halogen-free molding compounds for plastic quad flat pack (PQFP), plastic ballgrid array (PBGA), and mold array (MAP-PBGA) packages are briefly discussedin Chap. 6. Environmentally benign die attach films for PQFP and PBGA packagesand lead-free die attach bonding techniques for IC packaging are examined inChap. 7.

The environmental issues regarding conventional PCBs/substrates are discussedin Chap. 8. The influence of electronic products and the relevant environmentalresearch are reviewed and the international driving forces for alternative materialsare highlighted. In Chap. 9, halogenated and halogen-free materials are assessed indetail. Some environmentally conscious flame retardants are introduced. Theemerging technologies for fabricating environmentally friendly PCBs are describedin Chap. 10. The emphasis is placed on design for environment, green PCB manu-facturing, and environmental safety.

Chapter 11 reviews the global status of lead-free soldering activity, including leg-islation, consortia programs, and regional preference on lead-free solder alterna-tives. Chapter 12 discusses the criteria for lead-free solder, the approaches taken fordevelopment of lead-free solders, and the varieties of alloys and properties devel-oped by the industry.

Chapter 13 compares in detail the physical, mechanical, and soldering propertiesand the reliability of the prevailing lead-free solder options. Chapter 14 goes overthe lead-free surface finishes for both PCBs and component applications. Both man-ufacturing process and performance are discussed for each type of surface finish.

Implementation of lead-free soldering is analyzed in Chap. 15, with more empha-

xv

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sis on the requirement for reflow process.While lead-free soldering is inevitable, thechallenges of executing it definitely have to be addressed first. These are discussedin detail in Chap. 16.

Chapter 17 presents an overview of conductive adhesive technology and pro-poses approaches for a fundamental understanding. Chapter 18 examines the con-ductivity mechanisms of isotropic conductive adhesives. Emphasis is placed on therelationship between lubricant removal and conductivity in electrically conductiveadhesives (ECAs) and the relationship between cure shrinkage and conductivityestablishment.

Chapter 19 discusses the mechanisms underlying the contact resistance shifts ofECAs. Contact resistance stabilization of ECAs is presented in Chap. 20, withemphasis on determination of the effects of electrolytes and moisture absorption oncontact resistance shifts and on the stabilization of contact resistance using variousadditives.

For whom is this book intended? Undoubtedly it will be of interest to threegroups of specialists: (1) those who are active or intend to become active in researchand development in electronics and photonics manufacturing with lead-free, halo-gen-free, and conductive adhesive materials; (2) those who have encountered prac-tical lead-free, halogen-free, and conductive adhesive problems and wish tounderstand and learn more methods for solving such problems; and (3) those whohave to choose a reliable, creative, high-performance, robust, and cost-effectivepackaging technique for their green products.This book also can be used as a text forcollege and graduate students who have the potential to become our future leaders,scientists, and engineers in the electronics and photonics industry.

We hope this book will serve as a valuable reference to all those faced with thechallenging problems created by the ever increasing interest in lead-free, halogen-free, and conductive adhesive materials. We also hope it will aid in stimulating fur-ther research and development on environmental, electrical, and thermal designs;materials; processes; manufacturing; electrical, thermal, and end-of-life manage-ment; testing; reliability; and more sound applications of lead-free, halogen-free, andconductive adhesive technologies in electronic and photonic products.

Organizations that learn how to design lead-free, halogen-free, and conductiveadhesive technologies in their interconnect systems have the potential to makemajor advances in the electronics and photonics industry and to gain great benefitsin cost, performance, density, quality, size, weight, and market share. It is our hopethat the information presented in this book may assist in removing roadblocks;avoiding unnecessary false starts; and accelerating design, materials, and processdevelopment of lead-free, halogen-free, and conductive adhesive technologies. It isan exciting time for these technologies!

John H. Lau, PhD, PE, ASME Fellow, IEEE FellowPalo Alto, CA

C. P. Wong, PhD, NAE, IEEE Fellow, AIC FellowDuluth, Georgia

Ning-Cheng Lee, PhDNew Hartford, NY

S.-W. Ricky LeeKowloon, Hong Kong

xvi PREFACE

ACKNOWLEDGMENTS

Development and preparation of this book was facilitated by the efforts of a num-ber of dedicated people at McGraw-Hill and North Market Street Graphics. Wewould like to thank them all, with special mention to Stephanie Landis of NorthMarket Street Graphics, and Thomas Kowalczyk and Jessica Hornick of McGraw-Hill for their unswerving support and advocacy. Special thanks to Steve Chapman,executive editor of electronics and optical engineering, who made our dream of thisbook come true by effectively sponsoring the project and solving many problemsthat arose during the book’s preparation. It has been a great pleasure and fruitfulexperience to work with these people in transforming our messy manuscripts into avery attractive printed book.

The material in this book has clearly been derived from many sources, includingindividuals, companies, and organizations, and we have attempted to acknowledge inthe appropriate parts of the book the assistance that we have been given. It would bequite impossible for us to express our thanks to everyone concerned for their coop-eration in producing this book, but we would like to extend due gratitude.

Also, we want to thank several professional societies and publishers for permit-ting us to reproduce some of their illustrations and information in this book. Forexample, the American Society of Mechanical Engineers (ASME) conference pro-ceedings (e.g., International Intersociety Electronic Packaging Conference) andtransactions (e.g., Journal of Electronic Packaging), the Institute of Electrical andElectronic Engineers (IEEE) conference proceedings (e.g., Electronic Componentsand Technology Conference) and transactions (e.g., Advanced Packaging and Manu-facturing Technology), the International Microelectronics and Packaging Society(IMAPS) conference proceedings (e.g., International Symposium on Microelectron-ics) and transactions (e.g., International Journal of Microcircuits and ElectronicPackaging), the Surface Mount Technology Association (SMTA) conference pro-ceedings (e.g., Surface Mount International Conference and Exposition) and journals(e.g., Journal of Surface Mount Technology), the IBM Journal of Research and Devel-opment, Electronic Packaging and Production,Advanced Packaging, Circuits Assem-bly, Surface Mount Technology, Connection Technology, Solid State Technology,Circuit World, Microelectronics International, and Soldering and Surface MountTechnology.

John Lau would like to thank his former employers, Hewlett-Packard Companyand Express Packaging Systems, for providing him excellent working environmentsthat have nurtured him as a human being, provided job satisfaction, and enhancedhis professional reputation. He also would like to thank Steve Erasmus and Ted Lan-caster for their trust, respect, and support of his work at Agilent Technologies. Fur-thermore, he would like to thank his eminent colleagues at Hewlett-PackardCompany, Express Packaging Systems, Agilent Technologies, and throughout theelectronics and optoelectronics industry for their useful help, strong support, andstimulating discussions.Working and socializing with them have been a privilege andan adventure. He learned a lot about life and packaging and interconnection tech-nologies from them.

xvii


John Lau also thanks his daughter, Judy, and his wife, Teresa, for their love, con-sideration, and patience in allowing him to work many weekends on this book.Theirbelief that he is making a small contribution to the electronics and optoelectronicsindustry was a strong motivation for him. Knowing that Judy is going to Princetonfor her graduate studies in physics this fall, and that Teresa and he are in good health,he wants to thank God for His generous blessings.

C. P.Wong wants to thank his former colleagues at AT&T Bell Labs and GeorgiaTech—in particular, his current colleagues and good friend Rao Tummala at thePackaging Research Center at the Georgia Institute of Technology (GIT)—for alltheir support. Special thanks to his former PhD student, D. Lu, for his outstandingwork on electrical conductive adhesives.

C.P. also thanks his wife, Lorraine, and his children, Michelle and David, for theirsupport, love, and understanding all these years at Bell and GIT.

Ning-Cheng Lee would like to express gratitude to Indium Corporation of Amer-ica for providing a highly inspiring work environment. He also thanks his colleaguesat Indium for their encouragement and support of his pursuit of solutions for thenever ending challenges of this rapidly evolving world.

Ning-Cheng Lee wants to thank his mother, Shu-shuen Chang, for her encour-agement and blessing, and his wife, Shen-chwen, for her full support and patience—particularly for her tolerance toward his irregular work hours. He would also like tothank his sister Yu-Hsuan for her selfless and dedicated effort in taking care of theiraged mother so that he can focus on outside challenges.

Ricky Lee wishes to express his gratitude to his colleagues at Hong Kong Uni-versity of Science and Technology and his industrial partners in the Asia-Pacificregion.Without their efforts to establish a pro-electronic packaging environment, heprobably would not have begun his endeavor in this discipline. Special thanks arealso due to the Industry Department and Research Grant Council (RGC) of HongKong for their financial support to part of his research activities in electronic pack-aging.

Ricky Lee is also indebted to his family. During a certain period while working onthis book, he averaged only four hours of sleep a night.Without the spiritual supportfrom the family, he could never have struggled through that exhausting time!

xviii ACKNOWLEDGMENTS

CHAPTER 1INTRODUCTION TO

ENVIRONMENTALLY BENIGNELECTRONICS

MANUFACTURING

1.1 TRENDS IN INDUSTRY

Many thousands of industries have arisen during the four great Western transforma-tions—the Renaissance, the Reformation, the Industrial Revolution, and the Com-puter Age. The automobile and electronics industries are the largest and mostimportant of the current industries.They will be briefly discussed in the following text.

1.1.1 AUTOMOBILE INDUSTRY

Until 1996, the automobile industry was the largest industry in the world. Its earlyshift from workshop manufacturing to mass production made cars affordable for bil-lions of people. However, emissions of unburned hydrocarbons, nitrogen oxides, andcarbon monoxide spread over urban areas and into the countryside, which wasincreasingly buried under asphalt and concrete roads. And the synthesis of plastics(used in automobiles) grew into a large, highly energy-intensive industry generatinga variety of toxic pollutants previously never present in the biosphere and introduc-ing huge numbers of nondecaying wastes into the environment.

Post-1945 developments amplified these trends. New environmental risks wereintroduced as the developed world entered the period of its most impressive eco-nomic growth, terminated only by the 1973 to 1974 quintupling of oil prices. In just25 years, the consumption of primary commercial energy nearly tripled, electricitygeneration grew about 8-fold, car ownership increased 6-fold, and production ofmost kinds of synthetic materials grew more than 10-fold.

In the summer of 1970, the Massachusetts Institute of Technology first attempteda systematic evaluation of global environmental issues. Their summary of the Studyof Critical Environmental Problems indicated the following relative importance asperceived at that time: (1) emissions of carbon dioxide from fossil fuel combustion;(2) particulate matter in the atmosphere, cirrus clouds from jet aircraft, the effects ofsupersonic planes on stratospheric chemistry, the thermal pollution of waters, andthe impact of pesticides; and (3) mercury and other toxic heavy metals, oil on theocean, and the nutrient enrichment of coastal waters.

Just a month later, U.S. president Richard Nixon sent Congress the first report ofthe President’s Council on Environmental Quality. Soon afterward, the Environ-mental Protection Agency (EPA) was born and the environment entered big-timepolitics. It should be emphasized that this was the first time in history that a nationhad taken comprehensive stock of the quality of its surroundings. One of the EPA’s

1.1


greatest achievements is banning lead additives in gasoline, thus reducing the con-centration of lead in the air by 94 percent from 1980 to 1999.

1.1.2 ELECTRONICS INDUSTRY

Since 1996, the electronics industry has been the largest industry in the world (morethan $1 trillion).1–116 It is the most dynamic, fascinating, and important area of man-ufacturing.There are many categories of electronic products, such as consumer, com-puter, and communication items. Today, however, computers and their peripheralproducts account for the greatest percentage of the total revenue for electronicproducts. In 1992, 11.5 million personal computers (PCs) were shipped in the U.S.According to the data from the National Safety Council (1999), the number is pro-jected to be 55.8 million in 2005, as shown in Fig. 1.1.

In the past few years the electronics industry has been facing an impendingchange in light of upcoming halogen-free and lead-free technology legislations. Thisis because the electronics industry has relied on halogenated flame retardants andtin-lead solders for its products.

In 1998, the European Commission introduced two draft proposals called theWaste Electrical and Electronic Equipment (WEEE) and Reduction of HazardousSubstances (ROHS) directives. The primary objective of these complementary pro-posals is to minimize the risks and impacts that the production, use, treatment, anddisposal of waste electrical and electronic equipment have on human health and theenvironment. Additionally, the directives are intended to prevent uncontrolled dis-posal of electrical and electronic equipment and to foster the development of reuseand recycling methods in order to reduce the amount of waste for disposal. In short,they aim for “green” products!

1.2 CHAPTER ONE

FIGURE 1.1 In 1992, the number of PCs shipped in the U.S. was 11.5 million. Accordingto the National Safety Council, the projected number for 2005 is 55.8 million.

It is interesting to point out that green products sell! For example, Matsushita’smarket share of its lead-free MiniDisc player jumped from 4.6 to 15 percent in 6months (1999) in Japan. Toshiba’s bromine-free printed circuit boards (PCBs) helpthe company to sell its Libretto and Dynabook notebook computers in Europe andhas earned Toshiba some romantic names such as Blue Angel (in Germany), WhiteSwan (in Finland), and TCOGY (in Sweden).

The European Commission, which revised the original draft in 2000, includedJanuary 2, 2008 as the implementation date for the lead ban. In further revisions, theEuropean Parliament and the Council of Ministers proposed that the ban on leadtake effect on January 1, 2006 and January 1, 2007, respectively. Also, WEEE andROHS addressed several concerns about the use of halogened flame retardants, pri-marily (1) formation of dioxins and furans during incineration or recycling, and (2)persistence and bioaccumulation.

Prior to the proposed ban on halogen, lead, and other toxic materials, there areother projected milestones of the WEEE and ROHS directives; for instance, pro-ducers are expected to establish systems for recovering electronic waste by the endof 2003. Japan has begun its version of take-back legislation effective in 2001 for avariety of its domestic products. The Electric Household Appliance Recycling Lawpassed the obligation for collection and recycling of waste appliances to the produc-ers of those appliances.

It should be emphasized that worldwide interest in halogen-free flame retardantsand lead-free solders continues to grow, if not for environmental or regulatory rea-sons, then because of market differentiation. Many Japanese manufacturers areahead of the proposed regulated ban on halogen and lead. Also, most Japanese sys-tem manufacturers want their products to be labeled green for market share oppor-tunities, so they drive subassembly, component, and PCB manufacturers to make thechange to halogen-free and lead-free materials prior to impending regulationsbecoming effective.

1.2 TRENDS IN WORLDWIDE ENVIRONMENTALLY BENIGN

MANUFACTURING

Worldwide trends in environmentally benign manufacturing (EBM), especiallyin Europe, Japan, and the U.S., have been studied by Murphy62 in four categories:government, industry, research and development (R&D), and education. Thegovernment activities include take-back legislation, landfill bans, material bans,life cycle assessment (LCA) tool and database development, recycling infrastruc-ture, economic incentives, regulation by medium, cooperative/joint efforts withindustry, and financial and legal liability. The industrial activities include Inter-national Standards Organization (ISO) 14000 certification, water conservation,energy conservation/CO2 emissions, decreased releases to air and water, decreasedsolid waste/postindustrial recycling, postconsumer recycling, material and energyinventories, alternative material development, supply chain involvement, EBM as abusiness strategy, and life cycle activities. The R&D activities include relevant basicresearch (>5 years out) and applied R&D (<5 years out) such as polymers, elec-tronics, metals, automotive/transportation, and systems. The educational activitiesinclude courses, programs, focused degree programs, industry sponsorship, and gov-ernment sponsorship.

ENVIRONMENTALLY BENIGN ELECTRONICS MANUFACTURING 1.3

1.2.1 GOVERNMENT ACTIVITY

Governments in both Japan and Europe appear (at least outwardly) to operate at agreater level of interaction and cooperation with the private sector than does gov-ernment in the U.S. As a result, Japan and Europe tend to have a more proactiveapproach to problem solving. The U.S., largely in response to financial and legal lia-bilities associated with a significant amount of regulatory action, tends to solve prob-lems in a more reactive manner (Table 1.1).

1.2.2 INDUSTRY ACTIVITY

U.S. industry is focused on the reduction of liability, decreased consumption ofresources (especially water), and pollution prevention. Corporations in Japan areconcerned with energy conservation (reduced CO2 emissions), decreased solidwaste, and incorporation of environmental issues into business strategies. EuropeanUnion (EU) industries are very involved in end-of-life issues (Table 1.2).

1.4 CHAPTER ONE

TABLE 1.1 Government Activities in EBM

Activity Japan U.S. Europe

Take-back legislation ** — ****

Landfill bans ** * ***

Material bans * * **

LCA tool and database development *** ** ****

Recycling infrastructure ** * ***

Economic incentives ** * ***

Regulation by medium * ** *

Cooperative/joint efforts with industry ** * ****

Financial and legal liability * **** *

More asterisks indicate higher scores.

TABLE 1.2 Industrial Activities in EBM


ISO 14000 certification **** * ***

Water conservation ** *** *

Energy conservation/CO2 emissions **** ** **

Decreased releases to air and water * *** **

Decreased solid waste/postindustrial recycling **** ** ***

Postconsumer recycling ** * ****

Material and energy inventories *** * **

Alternative material development ** * ***

Supply chain involvement ** * **

EBM as a business strategy **** ** ***

Life cycle activities ** ** **


1.2.3 R&D ACTIVITY

R&D in the U.S. is heavily focused on materials and process technologies. Japan’sefforts are more closely aligned with applications and manufacturing systems. Euro-pean research is heavily weighted toward systems engineering, particularly in theareas of design for the environment and LCA (Table 1.3).

1.2.4 EDUCATION ACTIVITY

Higher education has begun to address EBM to a much greater degree in the Euro-pean countries than in either the U.S. or Japan. However, the U.S. places moreemphasis on the environmental consciousness of elementary students than Europeor Japan (Table 1.4). Overall, it appears that EBM in the U.S and Japan. is somewhatbehind that in the EU.


TABLE 1.3 Research and Development Activities in EBM


Relevant basic research (>5 years out)

Polymers ** *** **

Electronics ** *** *

Metals *** * **

Automotive/transportation ** * ***

Systems ** * ***

Applied R&D (<5 years out)

Polymers * *** **

Electronics *** ** **

Metals *** * **

Automotive/transportation *** * ***

Systems ** * ***


TABLE 1.4 Educational Activities in EBM


Courses ** ** ***

Programs * * **

Focused degree programs — — *

Industry sponsorship * ** ***

Government sponsorship * * **


1.2.5 WORLDWIDE EFFORTS ON ENVIRONMENTALLY BENIGN

ELECTRONICS MANUFACTURING

As mentioned in Sec. 1.1.2, alternative material systems for halogen-free flame retar-dants and lead-free solders are currently of intense interest in the electronics indus-try.This was initiated in part by the WEEE/ROHS directives and subsequently drivenby Japanese electronics system retailers and manufacturers in an attempt to increaseEuropean market share in advance of final legislation. European original equipmentmanufacturers have adopted some of these materials, albeit somewhat reluctantly.U.S. original equipment manufacturers are investigating the materials and are sup-plying components and PCBs that are halogen-free and lead-free in response to cus-tomer demand. At the same time, they are hesitant to adopt technologies that arelargely believed to be less reliable and not clearly of environmental benefit.62

1.3 TRENDS IN ENVIRONMENTALLY BENIGN ELECTRONICS

MANUFACTURING

The semiconductor is the heart of the electronics industry. The total semiconductormarket is expected to reach $224 billion in 2002, as shown in Fig. 1.2. It can be seen that30 percent of the market is for metal-oxide semiconductor (MOS) microcomponents,which includes microprocessors, microcontrollers, and microperipherals; 21 percent isfor MOS memory, which includes dynamic random access memory (DRAM),flash/electronically erasable programmable read-only memory (EEPROM), staticrandom access memory (SRAM), read-only memory (ROM), and erasable pro-grammable read-only memory (EPROM); 20 percent is for MOS logic, whichincludes application-specific integrated circuits (ASICs), SP logic, flat panel liquid

1.6 CHAPTER ONE

FIGURE 1.2 Total semiconductor market in 2002.

display (FPLD), display driver, and GP logic; 16 percent is for analog devices; 8.4percent is for discrete devices; 5 percent is for optoelectronics; and 0.36 percent is forbipolar digital devices.

The trends in semiconductor technology are shown in Table 1.5 for 8-in wafers. Itcan be seen that: (1) in 2000, most of the wafers were made using the 0.35-µm tech-nology, with only 15,000 made using 0.13-µm technology; (2) in 2001, most of thewafers were made using 0.25-µm technology; and (3) in 2003, most of the waferswere projected to be made using 0.18-µm technology and more than 1.6 million willbe made using 0.13-µm technology.

Semiconductor equipment bookings enjoyed the biggest growth between thesummer of 1998 and the summer of 2000, as shown in Fig. 1.3. However, the figuredrops like a rock after August 2000. The MOS wafer fabrication capacity utilizationalso reached new lows after the third quarter of 2000, as shown in Fig. 1.4. In Fig. 1.5,which shows the MOS wafer fabrication capacity utilization turning point forecast, itcan be seen that things go nowhere but down! As a matter of fact, foundry utilization


TABLE 1.5 Semiconductor Technology Trends and Forecast

Technology 1999 2000 2001 2002 2003 2004 2005

≥1.0 µm 595 676 472 555 538 571 574

0.8 µm 1,016 1,077 793 992 1,067 1,209 1,083

0.5 µm 2,226 2,318 1,762 1,907 1,798 2,202 2,015

0.35 µm 2,893 4,101 2,348 2,651 2,649 3,328 3,018

0.25 µm 2,253 3,567 2,621 3,543 3,708 4,303 4,045

0.18 µm 80 998 1,306 2,716 4,575 6,463 6,095

0.13 µm 0 15 172 630 1,622 3,047 4,049

<0.13 µm 0 0 0 0 19 344 1,235

Total 9,108 12,752 9,475 12,993 15,976 21,467 22,112

Values are in thousands of 8″ wafers.

FIGURE 1.3 Total semiconductor equipment bookings.

1.8

1.8 CHAPTER ONE

FIGURE 1.4 Wafer fabrication utilization reaching new lows.

FIGURE 1.5 Wafer fabrication capacity utilization forecast.

rates have reached the lowest level ever. Since semiconductors are the best index ofthe future of the electronics industry, there is no sign of recovery yet. System on a chip(SOC) could be their savior; however, there are many issues to be resolved.1–4

In general, an electronic product consists of semiconductor integrated circuits(IC) devices, IC packages, PCBs, and other components as well as materials such assolder and polymer. In the next section, EBM of these key elements will be brieflydiscussed.

1.3.1 IC FABRICATION

Most ICs are fabricated in a wafer with a diameter that can be as large as 300 mm.Usually IC fabrication facilities are very clean and have few environmental con-cerns. However, they use too much water, energy, and natural gas as well as producesignificant amounts of solid waste, which in turn affect the environment. This is dueto IC manufacturing processes such as (1) deposition of very thin layers and etchingof patterns at scales down to 0.13 µm (energy-intensive and causes perfluoro com-pound emission); (2) wafer surface (ultra) cleaning (uses large amounts of water);and (3) chemical mechanical polishing (uses significant amounts of water and cre-ates solid waste). Thus, IC fabrication companies should work very closely withequipment and material companies to qualify new materials and processes as well asto reduce usage of energy and water and emissions of solid waste and perfluorocompounds.

1.3.2 IC PACKAGING

After the wafer is made, it is ready for chip-level interconnects. Solder bumps on thewafer are one form of these interconnects. More than 12 different methods of mak-ing the tiny tin-lead solder bumps on wafers have been reported,5 and they will notbe repeated here. However, solder bumps with lead-free solders will be discussed inChaps. 2 and 3. Some companies that provide various types of wafer bumping areAdvanced Interconnect Solutions, Amkor Technology Inc., APack TechnologiesInc., Aptos Corp., ASE Inc., Carsem, Chipbond Technology Corp., Fujitsu Ltd., ICInterconnect, Kulicke & Soffa Flip Chip Division, Megic Corp., Pac Tech GmbH,SPIL Group, ST Assembly Test Services Ltd., STECO, and Unitive Advanced Semi-conductor Packaging.

For direct chip attach applications, the solder-bumped chip is placed directly onthe PCB.1–11 However, for solder-bumped flip chip in a package (FCIP) applications,the solder-bumped flip chip is surface mounted on the substrate of the FCIP first,and then the FCIP is surface mounted on the PCB with solder. Usually, the meltingpoint of the solder bumps on FCIP is higher than that of the solder joints on the PCBdue to packaging hierarchies. However, this is not necessary if the underfill encap-sulants are properly made.4–8

It has been proposed that pure tin (with a melting temperature of 232°C) shouldbe used for solder bumps in FCIP. However, for very-fine-pitch pads on the chip, tinwhisker growth could short the circuits.117–160

For most FCIPs, the housing (package) is usually made of flame-retardant mold-ing compounds that could cause environmental concerns.This topic will be discussedin Chaps. 6 and 7.

It should be noted that there are many different solderless chip-level intercon-nects4–9 such as Au stud bumps,Au bumps, Cu bumps, Ni-Au bumps, Cu wires, and Aumicrospring wires. These could be low-cost alternatives to lead-free technology andwill be discussed in Chaps. 4 and 5.

1.3.3 PCB

PCBs are used to support and link the components of an electronic product. Unlikethe bismaleimide triazine (BT) substrates (with a glass transition temperature>180°C) used in organic FCIP,10 most of the PCB materials have a glass transition


temperature ∼125°C. This may not be compatible with the lead-free surface-mounttechnology assembly process, which requires higher process temperatures as will bediscussed in Sec. 1.3.4. Thus, PCB manufacturers should investigate new materialsand processes for their PCBs for lead-free applications.

Another big challenge for PCB manufacturers is the use of brominated flameretardants (BFRs), because, as mentioned in Sec. 1.1.2, the WEEE/ROHS directivesban the use of many substances, including some BFRs. This topic will be discussedbriefly in Sec. 1.3.5 and in more details in Chaps. 8 through 10.

Today, most PCB shops use too much water, energy, and chemicals as well asproduce significant amounts of solid and trim wastes, which in turn affects the en-vironment. These are due to PCB manufacturing processes such as: (1) drilling(energy-intensive and creates solid waste); (2) plating (uses large amounts of waterand complex chemistries including organic and inorganic compounds); and (3) etch-ing (water-, energy-, and chemical-intensive). PCBs with build-up layers connectingthrough microvias4 could be inherently more environmentally friendly because theyhave smaller holes, less real estate, and better vision alignment (with higher-precision machines). All of these lead to less solid and trim waste as well as reducedresource consumption for the same functions.

1.3.4 LEAD-FREE SOLDERS

Low-cost tin-lead solders1–16 have been used as joining materials in the electronicsindustry for many years. The unique physical (well-defined eutectic with a relativelylow melting point) and mechanical (reasonably good thermal fatigue reliability)properties of the tin-lead solders have facilitated PCB assembly choices that havefueled creative advanced packaging developments, such as solder-bumped flipchips,1–9 ball grid array packages,1–10 and wafer-level chip-scale packages.1–7 For thesepackaging technologies, the tin-lead solders are the electrical and mechanical “glue”of the PCB assemblies. If the manufacturing process is well controlled and productsare recycled, there is little environmental risk in using tin-lead solder, since it is eas-ily recovered.

Since 1992, different bills have been introduced at the U.S. Congress to ban leadfrom a wide variety of applications, including solders.The reasons are, among others,that lead and its compounds are ranked as one of the top 10 hazardous materials andthat lead is the number one environmental threat to children. Also, according to theAmerican Association of Pediatrics, lead can damage the brain and nervous system,and even a low level of lead exposure can cause learning disabilities; hearing loss;speech, language, and behavior problems; and other serious health effects.

Of the 5 million tons of lead produced annually, only 0.5 percent is used in elec-tronic solder applications. However, this small amount of lead in solder poses moreconcern for human health than the lead in storage batteries (almost 100 percentrecycled). This is due to the fact that, although solder is only a small percentage byweight of electronic products such as televisions, refrigerators, computers, andphones, these products often end up in landfills after being disposed of, and the leadcould be ultimately leached out into the waste stream and the water supply.

Most of the lead-free solders,62–101 especially the most promising family (Sn-Ag-Cu-X), have a melting point (∼213°C) higher than that of tin-lead solder (183°C).These higher melting temperatures require higher process temperature profiles. Fortin-lead solder the maximum reflow temperature is ∼220°C, and for Sn-Ag-Cu sol-

1.10 CHAPTER ONE

der it is ∼260°C. Thus, in order to use the lead-free solders, all the materials of thePCB and the components on the PCB must be able to withstand the increased ther-mal exposure. This leads not only to requiring support of the infrastructure (materi-als, equipment, components, PCB, etc.) but also to increased cost and lowerreliability. Furthermore, significantly more energy is needed, which causes environ-mental concerns.

There are many reliability test and modeling data on tin-lead solders.1–16 How-ever, since lead-free solders are so new compared to tin-lead solders, there are notmany creditable, reliable data.Thus, there is a big risk in shipping products with lead-free solders, especially those that require very high levels of reliability.

Unlike tin-lead solder (with two components and a composition that is very easyto control), most of the potential lead-free solders have at least three components.(For a list of more than 100 lead-free solders, please read Ref. 5.) This could createnonuniformity in compositions, thus making manufacturing process control muchmore critical and increasing the likelihood of yield loss.

Due to the higher melting points of most lead-free solders, component rework ismore difficult, if not impossible, which leads to more manufacturing yield loss. Also,disassembly is more difficult, which limits component recovery at the end of life ofthe product.

How should lead-free solders be selected? What are the potential lead-free sol-ders? What are the correct PCB and component surface finishes for lead-free sol-dering? What are the optimal PCB assembly processes with lead-free soldering?How should low-alpha solders for flip chip applications be selected and used? Theanswers to these questions can be found in Chaps. 11 through 16.

1.3.5 HALOGEN-FREE FLAME RETARDANTS

Polymers are very important materials for most electronic products.19 They can beused for the housing (molding compounds) and the substrate (epoxy resins) of ICpackages. Since most polymers are highly flammable and their presence in elec-tronic products provides a ready source of heat, there is a need for flame retardantsof some type to be incorporated into the IC package and PCB (thermosets) used inelectronic products. Also, polymers have a poor environmental image, in large partdue to their contribution to litter and landfills.

Historically, electronic products have used halogenated flame retardants.102–116 Ahalogen is a chemical compound and is defined as any of a group of five chemicallyrelated nonmetallic elements including fluorine, chlorine, bromine, iodine, and asta-tine. Until recently, halogens have been widely used as fire-extinguishing agents.They may be incorporated into a system, such as the coating on a PCB. Althougheffective as flame retardants, this group of chemicals can have a negative impact onhuman and environmental health.

Bromine (Br) is a heavy, volatile, corrosive, reddish-brown, nonmetallic liquidelement that has a highly irritating vapor. It is used in producing gasoline antiknockmixtures, fumigants, dyes, and photographic chemicals.

Chlorine (Cl) is a highly irritating, greenish-yellow gaseous element that is capa-ble of combining with nearly all other elements. It is produced principally by elec-trolysis of sodium chloride and is used widely to purify water, as a disinfectant andbleaching agent, and in the manufacture of many important compounds includingchloroform and carbon tetrachloride.


Fluorine (F) is a pale yellow, highly corrosive, poisonous, gaseous element, themost electronegative and most reactive of all the elements. It is used in a wide vari-ety of industrially important compounds.

Astatine (At) is a highly unstable radioactive element, the heaviest of the halo-gen series, which resembles iodine in solution. Its longest-lived isotope has a massnumber of 210 and a half-life of 8.3 h.

Antimony (Sb) is a metallic element having four allotropic forms, the most com-mon of which is a hard, extremely brittle, lustrous, silver-white, crystalline material.It is used in a wide variety of alloys, especially with lead in battery plates, and in themanufacture of flameproofing compounds, paint, semiconductor devices, andceramic products.

Iodine (I) is a lustrous, grayish-black, corrosive, poisonous element havingradioactive isotopes, especially 131I. It is used as a medical tracer and in thyroid dis-ease diagnosis and therapy. Iodine compounds are used as germicides, antiseptics,and dyes.

While all these halogenated frame retardants are well know to have detrimentaleffects on both health and the environment, the BFRs are considered the safest.Currently, the phenolics (one family of BFRs), which include tetrabronmo-bisphenol A (referred to as TBBPA or TBBA), are used primarily for PCBs. How-ever, due to the WEEE/ROHS directives, there is a significant effort within theelectronics industry to find alternatives to BFRs, especially in Japan and Europe. Forexample, Toshiba, Sony, and Nortel Networks are using halogen-free PCBs (Chaps.8 through 10). Sumitomo, Nitto Denko, and Kumgang Korea Chemical are makinghalogen-free molding compounds for IC packages (Chap. 6).

1.3.6 CONDUCTIVE ADHESIVES

Recently, for the sake of a green environment, solderless materials such as adhe-sives161–180 have been evaluated for assembling flip chips on PCBs and substrates.There are many different kinds of adhesives, including isotropic conductive adhe-sives (ICAs), anisotropic conductive adhesives (ACAs), and nonconductive adhe-sives (NCAs).

ICAs electrically conduct in all directions. Usually, ICAs are made of epoxy withAg-Pd filler particles.They can be applied on the pads of PCBs or substrates by sten-cil printing or screening. But the most commonly used method is dipping the gold,copper, or Ni-Au bumps into the ICA, which will be discussed in Chaps. 4 and 5.

Because ACAs electrically conduct only in the vertical direction, they are alsocalled z-axis conductive materials. There are two groups of anisotropic conductivematerials, namely, anisotropic conductive films (ACFs) and anisotropic conductivepastes (ACPs). An ACF consists of thermosetting adhesive, conductive particles(solids or plated plastic spheres), and release film, and looks like paper. An ACPconsists of thermosetting adhesive and conductive particles and looks like a paste.For both adhesives, the solids often used are Au, Ni, and solder.

NCAs are conventional underfills and are not electrically conductive. Their con-tents are thermosetting adhesive and nonconductive fillers. In general, they are usedfor diffusion bonding and solder joint reliability in flip chip assemblies. When anNCA cures, it shrinks and brings the bumps and pads into a state of compression,which ensures long-term reliability. The applications of ICAs, ACFs, ACPs, andNCAs will be discussed in Chaps. 4 and 5. Novel developments in ICAs,ACFs,ACPs,and NCAs will be reported in Chaps. 17 through 20.

1.12 CHAPTER ONE

1.3.7 END-OF-LIFE MANAGEMENT

Historically, the precious and base metals such as the gold, palladium, copper, andlead on IC devices and PCBs, the steels of the product housings, and many expensivecomponents on PCBs have been recycled for profitability. However, because of theadvance of technologies (much less use of precious and base metals and much moreuse of plastic instead of steel for the housing) and shorter product lifetimes (cheapercomponents), recycling is becoming less economically attractive.


FIGURE 1.6 Average lifetime of PCs.

FIGURE 1.7 Number of PCs obsoleted in the U.S. was 17.5 million(1997). According to the National Safety Council, the number is projectedto reach 61.3 million by 2007.

On the other hand, the volume of electronic products being manufactured isgrowing at an unprecedented rate (see Fig. 1.1 for an example) and their lifetimesare being shortened (see Fig. 1.6), thus accelerating rates of obsolescence (see Fig.1.7). Consequently, there is more pressure to recycle in Europe and Japan, as dis-cussed in Sec. 1.1.2. The system manufacturers should design their electronic prod-ucts for recycling and for the environment!

ACKNOWLEDGMENTS

The authors would like to thank C. Murphy of the University of Texas, G. Pitts ofEcolibrium, the National Safety Council, and Semiconductor Equipment and Mate-rials International (SEMI) for sharing their useful and important information withthe industry.

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1.20 CHAPTER ONE

152. Kehrer, H. P., and H. G. Kadereit, “Tracer Experiments on the Growth of Tin Whiskers,”Applied Physics Letters, 16(11):411–412, June 1, 1970.

153. Key, P. L., “Surface Morphology of Whisker Crystals of Tin, Zinc and Cadmium,” IEEEElectronic Components Conference, pp. 155–157, May, 1970.

154. Furuta, N., and K. Hamamura, “Growth Mechanism of Proper Tin-Whisker,” Journal ofApplied Physics, 8(12):1404–1410, December 1, 1969.

155. Walker, R., “Internal Stress in Electrodeposited Metallic Coatings,” Metal FinishingMonograph, 32, 1968.

156. Arnold, S. M., “Repressing the Growth of Tin Whiskers,” Plating, 53:96–99, 1966.

157. Besancon, R. M., “Electrical Discharges in Gases,” in The Encyclopedia of Physics, Rein-hold, pp. 189–193, 1966.

158. Glazunova, V. K., and N. T. Kudryavtsev, “An Investigation of the Conditions of Sponta-neous Growth of Filiform Crystals on Electrolytic Coatings,” Translated form ZhurnalPrikladnoi Khimii, 36(3):543–550, March 1963.

159. Arnold, S. M., “Growth of Metal Whiskers on Electrical Components,” Proceedings ofElectrical Components Conference, pp. 75–82, 1959.

160. Frank, F. C., “On Tin Whiskers,” Philosophical Magazine, 44:854, 1953.

161. Lau, J. H., “Flip Chip on PCBs with Anisotropic Conductive Film,” Advanced Packaging,44–48, July/August 1998.

162. Miebner, R., R. Aschenbrenner, and H. Reichl, “Reliability Study of Flip Chip on FR4Interconnections with ACA,” Proceedings of IEEE Electronic Components and Technol-ogy Conference, pp. 595–601, June 1999.

163. Gustafsson, K., S. Mannan, J. Liu, Z. Lai, D. Whalley, and D. Williams, “The Effect of Tem-perature Ramp Rate on Flip-Chip Joint Quality and Reliability Using AnisotropicallyConductive Adhesive on FR-4 Substrate,” IEEE/ECTC Proceedings, pp. 561–566, May1997.

164. Watanabe, I., K. Takemura, N. Shiozawa, O. Watanabe, K. Kojima, A. Nagai, and T. Tanaka, “Anisotropic Conductive Adhesive Films for Flip-Chip Interconnection,”Proceedings of the 9th International Microelectronics Conference, pp. 328–332, Omiya,Japan, 1996.

165. Watanabe, I., N. Shiozawa, K. Takemura, and T. Ohta, “Flip Chip Interconnection Tech-nology Using Anisotropic Conductive Adhesive Films,” in Flip Chip Technologies, Lau,J. H., ed., McGraw-Hill, New York, pp. 301–315, 1996.

166. Aschenbrenner, R., R. Miebner, and H. Reichl, “Adhesive Flip Chip Bonding on FlexibleSubstrates,” Proceedings of IEEE Polymeric Electronics Packaging, pp. 86–94, October1997.

167. Wong, C. P., D. Lu, L. Meyers, S. Vona, and Q. Tong, “Fundamental Study of ElectricallyConductive Adhesives (ECAs),” Proceedings of IEEE Polymeric Electronics Packaging,pp. 80–85, October 1997.

168. Lu, D., C. P. Wong, and Q. Tong, “Mechanisms Underlying the Unstable Contact Resis-tance of Conductive Adhesives,” Proceedings of IEEE Electronic Components and Tech-nology Conference, pp. 342–346, June 1999.

169. Nguyen, G., J. Williams, F. Gibson, and T. Winster, “Electrical Reliability of ConductiveAdhesives for Surface Mount Applications,” Proceedings of International ElectronicPackaging Conference, pp. 479–486, 1993.

170. Nguyen, G., J. Williams, and F. Gibson, “Conductive Adhesives: Reliable and EconomicalAlternatives to Solder Paste for Electrical Applications,” Proceedings of ISHM Sympo-sium, pp. 510–517, 1992.

171. Li, L., and J. Morris, “Reliability and Failure Mechanism of Isotropically ConductiveAdhesive Joints,” Proceedings of IEEE Electronic Components and Technology Confer-ence, pp. 114–120, May 1995.


172. Chung, K., T. Devereaus, C. Monti, and M. Yan, “Z-Axis Conductive Adhesives as SolderReplacement,” Proceedings of International SAMPE Electronic Conference, vol. 7,pp. 473–481, 1994.

173. Yamaguchi, M., F. Asai, F. Eriguchi, and Y. Hotta, “Development of Novel AnitotropicConductive Film (ACF),” Proceedings of IEEE Electronic Components and TechnologyConference, pp. 360–364, June 1999.

174. Hotta, Y., “Development of 0.025 mm Pitch Anisotropic Conductive Film,” Proceedingsof IEEE Electronic Components and Technology Conference, pp. 1042–1046, June 1998.

175. Dernevik, M., R. Sihlbom, K.Axelsson, Z. Lai, J. Liu, and P. Starski, “Electrically Conduc-tive Adhesives at Microwave Frequencies,” Proceedings of IEEE Electronic Componentsand Technology Conference, pp. 1026–1030, June 1998.

176. Kang, S. K., and S. Purushothaman, “Development of Low Cost, Low Temperature Con-ductive Adhesives,” Proceedings of IEEE Electronic Components and Technology Con-ference, pp. 1031–1035, June 1998.

177. Yim, M., K. Paik, T. Kim, and Y. Kim, “Anisotropic Conductive Film (ACF) Interconnec-tion for Display Packaging Applications,” Proceedings of IEEE Electronic Componentsand Technology Conference, pp. 1036–1041, June 1998.

178. Wei, Y., and E. Sancaktar, “A Pressure Dependent Conduction Model for ElectricallyConductive Adhesives,” Proceedings of International Symposium on Microelectronics,pp. 231–236, 1955.

179. Liu, J., and R. Rorgren, “Joining of Displays Using Thermosetting Anisotropically Con-ductive Adhesives,” Journal of Electronics Manufacturing, 3:205–214, 1993.

180. Ito, S., M. Mizutani, H. Noro, M. Kuwamura, and A. Prabhu, “A Novel Flip Chip Technol-ogy Using Non-Conductive Resin Sheet,” Proceedings of IEEE Electronic Componentsand Technology Conference, pp. 1047–1051, June 1998.

1.22 CHAPTER ONE

CHAPTER 2CHIP- (WAFER)- LEVEL

INTERCONNECTS WITH LEAD-FREE SOLDER BUMPS

2.1 INTRODUCTION

Alternative material systems for lead-free solders1–71 are currently of intense inter-est in the electronics industry. This is initiated in part by the Waste Electrical andElectronic Equipment/Reduction of Hazardous Substances (WEEE/ROHS) direc-tives and subsequently driven by Japanese electronics retailers and manufacturers inan attempt to increase European market share in advance of final legislation. In thischapter, chip- (wafer-) level interconnects with lead-free solders are discussed.

Wafer bumping is the most important step in using solder-bumped flip chiptechnologies. The bump sizes60 may vary from 50 µm (for the conventional solder-bumped flip chip technology) to as large as 500 µm [for the wafer-level redistribu-tion solder-bumped chip-scale package (CSP)]. There are many different ways toput solder bumps on the wafer/die as reported in Ref. 60. If one accounts for cost,surface-mount technology (SMT) experience, material availability, and processingflexibility, microball mounting and paste printing are the two best methods tobump lead-free solders on the wafer/die. The only limitation is the pad pitch of thechip, which is 125 µm in mass production today.

Under-bump metallurgy (UBM) is the heart of solder wafer bumping.60 If it is notproperly made, then (1) during or right after reflowing the solder-bumped chip on theprinted circuit board (PCB)/substrate, the chip may fall off, and (2) the solder jointquality may not be adequate after multiple reflows. In this chapter, the electroless Ni-P (phosphorus)-immersion Au, Al-NiV-Cu, and the Ti-Cu or TiW-Cu UBMs willbe considered.

As mentioned earlier, the most likely alternatives to tin-lead solder are the Sn-Ag family of alloys. In this chapter, Sn-Ag, Sn-Ag-Cu, Sn-Cu, Sn-Ag-Bi, and Sn-Pb chip- (wafer-) level interconnects will be considered.

2.2 UBM

There are many different UBMs, as shown in Table 2.1. However, for microball andpaste-printing wafer-bumping methods, the Ti-Cu,TiW-Cu, electrolytic Ni, electrolessNi-P-immersion Au, and Al-NiV-Cu are the most commonly used UBMs. The mostcost-effective UBMs are the Ni-P-immersion Au (or, in short, Ni-Au) and Al-NiV-Cu.

2.2.1 ELECTROLESS Ni-P-IMMERSION Au UBM

Figures 2.1 and 2.2 show, respectively, the 5- and 12-µm electroless Ni-Au UBMsgrown on an Al pad. They are properly done and are ready for solder bumping. In

2.1


2.2 CHAPTER TWO

TABLE 2.1 Some Well-Known UBMs* for Gold, Copper, Aluminum,Solder, and Nickel Bumps

Bump UBM Bumping process

Gold Cr-Cu/Au Electroplating

Ti-Pd/Au Electroplating

Ti-W/Au Electroplating

Ti-Pt/Au Electroplating

Copper Cr-Cu Electroplating

Al-Ni-Cu Electroplating

Aluminum Ti Evaporating

Cr Evaporating

Solder Cr-Cu-Au Evaporating/printing/mounting

Ni-Cu Electroplating/printing/mounting

Ti-Cu Electroplating/printing/mounting

TiW-Cu Electroplating/printing/mounting

Ni-Au Electroless + printing/mounting

Au-Ni-Cu-Ti Electroplating/printing/mounting

Al-NiV-Cu Sputtering + printing/mounting

Nickel Ni Electroless Ni/Au

* UBM, under-bump metallurgy.

FIGURE 2.1 SEM photo of electroless Ni-P-immersion Au UBM (∼5 µm thick). (Source: Techni-cal University of Berlin/Fraunhofer Institute IZM.)

this section, some useful insights on Ni-Au UBM obtained by the Korea AdvancedInstitute of Science and Technology (KAIST) are reported.55

Unlike Ni layers prepared by vacuum deposition, the electroless deposited Ni-Pis plated by hypophosphite (H2PO2). It is well known that P in the electroless Nigreatly influences the interfacial reactions with SnX solders during reflow. Asexpected, the intermetallic compound (IMC) formed at the interface is mainlyNi3Sn4. However, a P-rich Ni layer is also formed as a by-product of a Ni-Sn reactionbetween the Ni-Sn IMC and the electronless Ni layer (i.e., the P that is accumulatedat the interface between the electroless Ni and IMC layer). The IMC between theelectroless Ni and the 96.5Sn-3.5Ag solder, the shear strength of the lead-free solderbumps, as well as the composition and growth of the P-rich Ni layer, have been inves-tigated in detail.55

Intermetallic compound formation is desired for cross-linking of the solder bumpwith the UBM metallization, and the process is characterized as self-limiting. How-ever, excessive IMC buildup will eventually result in a loss of contact between thesolder bump and the chip metal pad due to UBM consumption into the bulk solder.

The process flow of the electroless Ni-P plating developed by KAIST is shown inTable 2.2. It can be seen that before plating, the wafer is cleaned and the double-zincate process is performed to remove the Al2O3 layer and to activate the Al sur-face on the wafer.The pad sizes are 100 × 100 µm and its pad pitch is 400 µm.The Alpad is 1 µm thick.

CHIP (WAFER)-LEVEL INTERCONNECTS WITH LEAD-FREE SOLDER BUMPS 2.3

FIGURE 2.2 Electroless Ni-P-immersion Au round UBM (∼12 µm thick). (Source: Motorola.)

KAIST’s zincate solution is made of NaOH and ZnO, whose ratio has been opti-mized to create fine and numerous Zn particles on the Al pads. After the double-zincate process, a 5-µm electroless Ni-P UBM is formed in about 20 min. Thetemperature of the plating bath is controlled at 90 ± 0.5°C. Then, 0.2 µm of Au isimmersed on the Ni-P UBM to prevent Ni oxidation and to enhance initial solder-ability.

During the electroless Ni-P fabrication, the two most important parameters arethe P content and plating speed. On the one hand, the low-P (≤4 wt %) electrolessNi is a crystalline structure and has many defects (e.g., high stress and hardness, aswell as magnetic moment). On the other hand, the high-P (≥10 wt %) electroless Niacts like an impurity and generates various unexpected effects on the electroless Ni.Thus, an electroless Ni with medium P content (between 4 and 10 wt %) is consid-ered by KAIST. The plating speed affects the surface roughness of the electrolessNi. KAIST has suggested a reasonable plating rate of 0.25 µm/min.

The two variables affecting the P content and plating speed are (1) the electrolessNi plating solution and (2) the pH of the plating solution. As shown in Table 2.2,NiSO4 and NaH2PO2 are, respectively, used as a Ni source and a reducing agent.Also, two complexing agents, sodium acetate (CH3COONa) and lactic acid[CH3CH(OH)COOH], are used to affect the properties of the electroless Ni. Fur-thermore, thiourea (H2NCSNH2) is used to stabilize the electroless Ni process.

For example, Fig. 2.3 shows the effects of a complexing agent on P content andplating speed. It can be seen that when the concentration of the lactic acidincreases, the P content increases and the plating rate decreases. This is due to thecomplexing agent, which tends to reduce the concentration of free Ni ions in plat-ing solution.55

Figure 2.4 shows the effects of pH on P content and plating speed. It can be seenthat when the pH increases, the plating speed increases and the P content decreases.Also, in order to make the P content fall into the range of between 4 and 10 wt %,

2.4 CHAPTER TWO

TABLE 2.2 Electroless Ni-P-Immersion Au Deposition Process on Al Pads

Process Solution Time Thickness

Cleaning HNO3 50% 20 s N/A*

Zincate ZnO, NaOH 20 s N/Apretreatment

Acid dipping HNO3 50% 5 s N/A

Double ZnO, NaOH 20 s N/Azincate

Electroless Ni NiSO4⋅6H2O 20 min 5 µmplating NaPH2O2⋅6H2O

CH3COONaLactic acidThiourea

Immersion Au Immersion Auplating solution 20 min 0.2 µm

* N/A, not applicable.Source: Korea Advanced Institute of Science and Technology (KAIST).


FIGURE 2.3 Effects of complexing agent on P content and plating speed.

FIGURE 2.4 Effects of pH on P content and plating speed.

the pH must be 5 or less.Thus, KAIST has found an optimal plating combination: theplating rate is 0.25 µm/min, and the P content is 12.55 ± 0.40 at % (7.05 wt %). In thiscase, the average surface roughness is 437 ± 174 Å; the resistivity and hardness of theelectroless Ni-P are, respectively, 70 ± 10 µΩ-cm and 500 ± 50 HV (Vicker’s hard-ness).

2.2.2 Al-NiV-Cu UBM

Another popular UBM (Al-NiV-Cu) is given by Flip Chip Technology (FCT). Itwas originally developed for wafer-level redistribution flip chip packages (Fig.2.5), but it is applied to ordinary flip chip packages (Fig. 2.6) as well. The manufac-turing processes of the wafer-level redistribution flip chip package are (1) depositthe first layer of dielectric [benzocyclobutene (BCB1)] on the wafer; (2) open win-dows to expose the die bond pads; (3) sputter the wafer with layers of Al, NiV, andCu for UBM; and (4) pattern the UBM to form traces and bond pads for solderbumps.The Al-NiV-Cu UBM has been shown to be reliable and is suitable for lead-free solders.59

2.3 MICROBALL WAFER BUMPING WITH LEAD-FREE SOLDERS

Just like the solder ball mounting on the substrates of plastic ball grid array (PBGA)packages,65 the microball mounting on the chips54 of the wafer enjoys the sameadvantages, (e.g., high throughput, uniform bump height, and flexible solder alloyselection). The drawbacks are the tooling that is required and the availability ofmicroball mounters. In this section, we present Nippon Steel’s microball waferbumping of 93.5Sn-3.5Ag and 96.8Sn-2.6Ag-0.6Cu solders.

2.3.1 OVERVIEW OF MICROBALL WAFER BUMPING

The overall process of forming the UBM and lead-free solder bumps on a wafer bythe microball mounting is shown in Fig. 2.7. The UBM is formed by the electrolessNi-Au process with 5 µm of Ni and 0.05 µm of Au. The flux is applied on the UBMsand the micro-solder balls are then transferred to the UBMs, as shown in Fig. 2.7.Reflow the micro-solder balls in a nitrogen atmosphere furnace, and the flux residueis removed by cleaning.

2.3.2 MICROBALL PREPARATION

Figure 2.8 shows the SEM image of the 60-µm (in diameter) 96.6Sn-3.5Ag micro-solder balls. They are made from a bulk of Sn-Ag solid by cutting it into many unitsof specified mass that are heated and melted at a temperature that is much higherthan the material’s melting point. Due to the surface tension of liquid metal of thesolder, the balls can achieve a high level of sphericity. Figure 2.9 shows the diameter(size) and diameter (x and y directions) measurements of 50 of the Sn-Ag micro-solder balls. It can be seen that (1) the size (diameter) variations are less than ±3 µm,and (2) the tolerances of sphericity are controlled within ±3 µm.

2.6 CHAPTER TWO

2.7

FIGURE 2.5 FCT’s ultra-CSP. (a) BCB1 layer after being exposed anddeveloped, (b) thin-film UBM and redistribution layer (RDL) after beingetched, (c) BCB2 layer after being exposed and developed, and (d) ultra-CSP RDL and solder ball.

2.8 CHAPTER TWO

FIGURE 2.6 FCT-sputtered UBM and solder pastebumping process. (a) Sputter UBM; (b) apply photoresist,pattern and develop; (c) etch UBM; (d) print solder paste;(e) reflow solder.

FIGURE 2.7 Microball mounting process for UBMs and solder bumps.


FIGURE 2.8 SEM image of 60-µm micro-solder balls (96.5Sn-3.5Ag).

FIGURE 2.9 (a) 60-µm microball (96.5Sn-3.5Ag) size distribution, (b) 60-µm ball (96.5Sn-3.5Ag)diameter measured in x and y directions.

2.3.3 MICROBALL MANAGEMENT

Figure 2.10 shows the process flow of microball bumping with an emphasis onmicroball management. By vibrating the ball container and bringing the bondinghead with the arrangement (management) plate down and closer to the microballs,as shown in Fig. 2.10a, the microballs are retained in the suction holes by suction, asshown in Fig. 2.10b.

It should be pointed out that since the diameter of microballs is very small, excessmicroballs tend to adhere to portions other than the section holes of the manage-

ment plate, as shown in Fig. 2.11a. Also, since the weight of the microballs is verylight, a suction leak can occur through an extremely small clearance between a nor-mally attached ball and a suction hole. More than one microball could adherearound one suction hole (Fig. 2.11a). Furthermore, contamination and moisture con-tent on the microball surface may cause excess microballs to adhere to the manage-ment plate, as shown in Fig. 2.11a. To remove these excess microballs and keep thenormally attached ones in place, ultrasonic vibrations of the bonding head with themanagement plate are applied, as shown in Fig. 2.10b. The result is shown in Fig.2.11b. A schematic diagram of the microball mounter developed by Nippon Steel isshown in Fig. 2.12.

2.10 CHAPTER TWO

FIGURE 2.10 Microball mounting process.


FIGURE 2.11 Excess microballs (a) adhering to the management plate and (b) perfectly attached.Microball diameter: 100 µm.

FIGURE 2.12 Schematic diagram of the microball mounter.

(a) (b)

2.3.4 MICROBALL WAFER BUMPING

An 8-in test wafer containing 619 chips, each chip with 625 pads (25 × 25) on a 250-µm pitch, as shown in Fig. 2.13, is solder-bumped by the microball mounting method.The wafer bumping yield is 99.995 percent, and the cycle time is 5 min. The bumpheight is 67.56 ± 3.5 µm, as shown in Fig. 2.14; the bump shear strength is 56.23 ± 7 gf,as shown in Fig. 2.15. Typical cross sections of the chips are shown in Fig. 2.16. It canbe seen that voids are not visible. Figure 2.17 shows the top view of the 96.8Sn-2.6Ag-0.6Cu and 63Sn-37Pb solder bumps with an 80-µm diameter. They look almost thesame. It should be noted, however, that the IMC near the UBM is quite different forthese two solders, which will be discussed in detail in the subsequent sections of thischapter.

It should be emphasized that most of the wafer bumping methods60 cannot dosingle-point touch-up. However, just like the solder ball mounting method for thePBGA package, the microball mounting method can. By using single-ball mountequipment, the missing or damaged bumps can be repaired.54

2.4 Sn-Ag-Cu SOLDER BALL MOUNTING ON WAFERS

Hitachi also uses the microball mounting method to put the Sn-Ag-Cu solder bumpson an 8-in wafer, except that: (1) their solder ball diameter is 400 µm and larger; (2)they do wafer-level redistribution (WLR) before solder ball mounting; and (3) theyput a stress-relaxation layer on the wafer prior to WLR and mounting.56

Since the bump (ball) sizes are much larger for wafer-level chip-scale packages(WLCSPs), there are many ball mounters available to perform the wafer bumpingand reballing. As a matter of fact, most of the solder ball mounters (e.g., Shibuya,Vanguard, KME, Motorola, Fujitsu, Hitachi, and Toray) available today can place300-µm solder balls.

2.4.1 WLCSP

One of the unique features of most WLCSPs is the use of a metal layer to redistrib-ute the very fine pitch pads on the chip to much larger pitch area-arrayed pads withmuch bigger solder bumps, so that when the chip is attached on the PCB/substrate, itwill have taller solder joints. There are more than 30 different kinds of WLCSPs60,61;however, only Hitachi’s will be considered in this section because they use the ball-mounting method to bump the Sn-Ag-Cu solder balls on the wafer.

Figure 2.18 shows Hitachi’s 8-in wafer with about 300 chips.The chip size is about10 × 10 mm, and there are 54 pads. The original pads are along the centerline of thechip. After redistribution, they are in an area-array format with a minimum pitch of0.8 mm and with 400-µm solder bumps.

A schematic cross section of the WLCSP on a PCB is shown in Fig. 2.19, wherethe original chip pad, the WLR (interconnection) layer, and the new lands (pads)with solder bumps (joints) are visualized. Due to the very large thermal expansionmismatch between the silicon chip and the FR-4 epoxy PCB and the very little com-pliance of the solder joints, underfill may be needed for solder joint reliability.60–66

However, the underfill encapsulant not only increases the material cost and reducesthe manufacturing throughput, it also makes the assembly very difficult to reworkand recycle. Thus, alternatives are needed.

2.12 CHAPTER TWO


FIGURE 2.13 100-µm solder bumps (a) on the 8-in wafer (b).

2.14 CHAPTER TWO

FIGURE 2.14 Height variation of microball bumps on the 8-in wafer.

FIGURE 2.15 Shear-strength variation of microball bumps on the 8-in wafer.

2.4.2 WLCSP WITH STRESS-RELAXATION LAYER

To eliminate the underfill encapsulant and ensure the solder joint reliability,Hitachi adds a stress-relaxation layer on the wafer before it is redistributed andbumped, as shown in Fig. 2.20. It can be seen that the stress-relaxation layer isbetween the chip and the solder bumps, and its function is to increase the compli-ance of the interconnects and to reduce the thermal expansion mismatch betweenthe chip and the PCB. Also, the thick stress-relaxation layer will reduce the capaci-tance between the chip surface and the interconnections, which is favorable forhigh-frequency applications.


FIGURE 2.16 Cross section of 100-µm solder bumps on Ni-Au UBM.

2.16 CHAPTER TWO

FIGURE 2.17 SEM images of 80-µm Sn-Ag-Cu (a) and Sn-Pb (b) solder bumps.

2.17

FIGURE 2.18 Hitachi’s 8-in wafer with the WPP-2 chip.

The manufacturing process of Sn-Ag-Cu solder bump mounting on wafers with astress-relaxation layer is shown in Fig. 2.21. First, the wafer is spin-coated with a pho-tosensitive polyimide (P-PI), and the chip pads and dicing streets are opened up byphotolithography. Then, a stress-relaxation layer is formed by printing liquid resinon the wafer through a stencil mask with no openings on the pads and dicing streets.The resin will flow through the open edges and form smooth slopes.

To perform wafer redistribution (i.e., to add another layer of metal on the wafer),Hitachi sputters the seed metals (Cr and Cu) on the whole wafer, spin-coats a pho-toresist layer, and opens up the shapes of interconnections and new pads (in area-array format) by photolithography.Then, the interconnections and pads are formed byelectroplating the Cu and Ni.The photoresist and seed metals are removed by etching.

Now, the wafer is spin-coated with the P-PI again and the new chip pads and dic-ing streets are opened up by photolithography. This time, the P-PI acts like a solder

2.18 CHAPTER TWO

FIGURE 2.19 Hitachi’s wafer process package (WPP-1) attached on a PCB.

FIGURE 2.20 WLCSP with a stress-relaxation layer.


FIGURE 2.21 Process flow of WLCSP with a stress-relaxation layer.

mask.After fluxing, the Sn-Ag-Cu solder bumps are placed on the new land pads onthe wafer through a stencil mask with just about any solder ball mounters that areavailable today. Finally, the wafer is reflowed, cleaned, and diced. The effects of thestress-relaxation layer of the WLCSP on the solder joint reliability on the PCB willbe considered in Chap. 3, Sec. 3.2.

2.5 STENCIL PRINTING OF Sn-Ag SOLDER ON WAFERS WITH

Ni-Au UBM

By using the stencil-printing method, KAIST bumps the Ni-Au UBM wafer with the96.5Sn-3.5Ag solder.55 Their findings on the interfacial reactions between the elec-troless Ni and solders (96.5Sn-3.5Ag and 63Sn-37Pb) are presented here.

2.5.1 THE INTERFACE BETWEEN ELECTROLESS Ni AND SOLDERS

Figure 2.22 shows the cross section of a 96.5Sn-3.5Ag solder bumped by the stencil-printing method. The UBM is Ni-Au. The reflow time is 1 min at 250°C. It can beseen that there are four regions of interest. Region I is the 96.5Sn-3.5Ag bulk sol-der. Region II is the Ni3Sn4 IMC layer and has irregular shape. [There are threeIMCs in the Ni-Sn binary system (i.e., Ni3Sn4, Ni3Sn2, and Ni3Sn, and they all arestable at room temperature. However, only the Ni3Sn4 phase is present in theliquid-Sn–electroless-Ni interaction.] Region III is the P-rich Ni layer and isdarker than the rest of the electroless Ni. Region IV is the electroless Ni layer witha composition of 91.3 at % Ni and 8.7 at % P. The P content, which was 12.6 at %before solder reflow, decreases to 8.7 at % after soldering due to the diffusion of Patoms into the interface of the 96.5Sn-3.5Ag bulk solder. The sample is analyzedby backscattered electron image and energy-dispersive x-ray spectroscopy (EDS)and SEM.

The effects of reflow time on the interface between the electroless Ni and the63Sn-37Pb bulk solder is shown in Fig. 2.23. It can be seen that, for a very long(256 min) reflow time, discontinuous dots appear on the dark, thin, and continu-ous P-rich Ni layer (region III). The composition of this dot phase is 75 at %Ni–25 at % P (Ni3P, exactly 3 Ni to 1 P ratio). Before reflow, the P-rich Ni layerwas 73.6 at % Ni–19.1 at % P–7.3 at % Sn. Because of the consumption of Ni withSn, P is accumulated at the electroless Ni/IMC interface, resulting in a P-rich Nilayer.

What about the 7.3 at % Sn atom before reflow? KAIST concludes that the dis-continuous dot phase is the Ni3P crystalline phase, and that the thin, continuousdark layer is a metastable Ni-P phase containing Sn. Also, this discontinuous dotNi3P phase only appears during severe reflow conditions such as a high tempera-ture and a very long reflow time.Additional evidence of Sn diffusion into the P-richNi layer is the Kirkendall voids in the Ni3Sn4 IMC layer, which is just above the P-rich Ni layer. It can be seen from Figs. 2.23 and 2.24 that as the reflow timeincreases, the number and size of the Kirkendall voids increase, especially for theSn-Ag solder.

2.20 CHAPTER TWO

Usually, the Kirkendall voids are considered to be developed in the solder(above the IMC layer) due to the Sn diffusion into the IMC layer. However, theKirkendall voids in Figs. 2.23 and 2.24 are in the Ni3Sn4 IMC layer, which indicatesthat the Sn atom is a faster diffusion element than Ni or P during reflow reaction.Sn atoms are detected up to the bottom of the P-rich Ni layer and then disappear atthe electroless Ni layer. Thus, the electroless Ni layer can be a good Sn diffusionbarrier.


FIGURE 2.22 A typical interaction between the solderand electroless Ni-Au UBM.

2.5.2 GROWTH OF THE IMC AND P-RICH Ni LAYER

Again, the 96.5Sn-3.5Ag and 63Sn-37Pb solders on electroless Ni UBM are consid-ered, except that the reflow temperature is 250°C for 96.5Sn-3.5Ag solder and 210°Cfor 63Sn-37Pb solder. The focus is on the growth of the Ni3Sn4 IMC layer and the P-rich Ni layer for different reflow times. It has been known that the longer thereflow times, the larger the growth rate and that these layers can have an undesirableeffect, resulting in a serious degradation of solder joint reliability.

Figure 2.25 shows the growth rate of the Ni3Sn4 IMC layer and the P-rich Ni layerfor reflow times1/2. It can be seen that (1) for both solders, the thickness of the Ni3Sn4

IMC and P-rich Ni layers increases as the reflow time increases; (2) for both solders,the growth rate of the Ni3Sn4 IMC layer and the P-rich Ni layer is the largest duringthe first minute; (3) for both solders, the thickness of the Ni3Sn4 IMC is larger thanthat of the P-rich Ni layers; and (4) for the same reflow time, the thickness of the

2.22 CHAPTER TWO

FIGURE 2.23 Interfacial reactions at the electroless Ni-Au UBM and 63Sn-37Pb bulksolder at different reflow times. (a) 250°C, 1 min; (b) 250°C, 16 min; (c) 250°C, 256 min.

2.23

FIGURE 2.24 Kirkendall voids. (a) For Sn-Pb solder, reflow at 250°Cfor 4 min; (b) for Sn-Ag solder, reflow at 250°C for 256 min.

FIGURE 2.25 Growth of Ni3-Sn4 IMC and P-rich Ni layer as a function of reflow time1/2.

Ni3Sn4 IMC and P-rich Ni layers in the 96.5Sn-3.5Ag solder bump is larger than thatin the 63Sn-37Pb solder joint.The last item is expected since 96.5Sn-3.5Ag solder hasmore Sn content, and above all, it is reflowed at 40°C higher than the 63Sn-37Pb sol-der.Table 2.3 summarizes the thickness of the Ni3Sn4 IMC and P-rich Ni layers in the96.5Sn-3.5Ag and 63Sn-37Pb solders at 1 and 8 min.

2.5.3 BUMP SHEAR FRACTURE SURFACE

The solder bumps shown in Fig. 2.26 are subjected to shear tests. The effect ofreflow times (1 to 16 min) on the shear strength of the Sn-Ag and Sn-Pb solder

2.24 CHAPTER TWO

TABLE 2.3 Effects of Reflow Time on the IMC* and P-Rich Ni Layer

IMC P-rich Ni layer (µm)

Reflow time (min) Reflow time (min)

Solder 1 8 1 8

63Sn-37Pb 1.02 1.47 0.18 0.238

96.3Sn-3.7Ag 1.38 3.73 0.203 0.501

* IMC, intermetallic compound.

FIGURE 2.26 Screen-printed solder bumps with the electroless Ni-Au UBM.


FIGURE 2.27 Backscattered SEM images of a sheared Sn-Pb solderbump, reflowed at 250°C. (a) 1 min, (b) 4 min, (c) 16 min.

bumps is not significant since the fracture surface is in the bulk solder.Also, for the1-min reflow time (close to the real situation) as well as the reflow temperature(210°C for the 63Sn-37Pb solder bumps and 250°C for 93.5Sn-3.5Ag solderbumps), the shear fracture surfaces are in the bulk solder and nothing special hap-pens. However, for the 63Sn-37Pb solder bumps, reflowed at 250°C for the 16-minreflow time, then the brittle fracture occurs at the Ni3Sn4 IMC region, as shown inFig. 2.27. Also, the Kirkendall voids are found at the fractured Ni3Sn4 surface, asshown in Fig. 2.28. Thus, the growth of the IMCs, the P-rich Ni layer, and the Kirk-endall voids must be controlled in order to prevent the brittle fracture at the elec-troless Ni/solder bump. But this is not the real reflow condition.

2.26 CHAPTER TWO

FIGURE 2.28 (a) Magnified image of Fig. 2.27c, (b) top view of Fig. 2.27c.


2.6 STENCIL PRINTING OF Sn-Cu, Sn-Ag-Bi, AND Sn-Ag-Cu

SOLDERS ON WAFERS WITH Ni-Au UBM

Stencil printing of ultrafine mesh (Type 5, −500/+650) lead-free solders (e.g., 99.3Sn-0.7Cu, 96.5Sn-3.5Ag, 96Sn-2Ag-2Bi, and 95.5Sn-3.8Ag-0.7Cu) on wafers with electro-less Ni-Au and Ti-Cu or TiW-Cu UBM have been studied extensively by Motorola.Some of their useful results are presented in this and the following sections.

2.6.1 INTERFACE OF REFLOWED SOLDER BUMPS

By using their (Motorola’s) own optimized printing process,57, 58 some typical crosssections for the 99.3Sn-0.7Cu, 96.5Sn-3.5Ag, 96Sn-2Ag-2Bi, and 95.5Sn-3.8Ag-0.7Culead-free solder bumps are shown in Figs. 2.29 and 2.30. They are obtained after twotimes of reflow at 260°C. The microstructures are revealed using an etchant of 10percent HCl and 90 percent methanol for a few seconds. Detailed morphology of thesolder-UBM is revealed by using 4 parts glycerol + 1 part acetic acid + 1 part nitricacid at 80°C for a few seconds. This etchant removes the solder and leaves the IMCinterface that can be characterized by SEM and EDX.

Two types of intermetallics are of interest. One is at the solder-UBM interfaceand the other is in the bulk of the solder. Blocky-type interfacial intermetallicsformed between the solder and the electroless Ni-P UBM. In the bulk solder, larger

FIGURE 2.29 SEM images of various lead-free solder bumps (reflowed twice at 260°C)with electroless Ni-P-immersion Au UBM. (a) Sn-0.7Cu, (b) Sn-3.5Ag, (c) Sn-3.8Ag-0.7Cu,and (d) Sn-2Ag-2Bi.

intermetallic particles such as Ag3Sn and Cu6Sn5 are formed. Table 2.4 summarizesthe observed IMC. It can be seen that the composition of intermetallics is primarilydependent on the solder alloy composition. There are no large intermetallics ob-served in the Sn-2Ag-2Bi solder. However, there is a significant difference observedbetween the Cu-containing solder alloys (Figs. 2.29a and c, and 2.30a and c) andthose without Cu (Figs. 2.29b and d, and 2.30b and d). The Cu-containing solders(Sn-0.7Cu and Sn-3.8Ag-0.7Cu) have interfacial IMC with a Ni4Cu7Sn6 ternarycomposition. The solders without Cu (Sn-3.5Ag and Sn-2Ag-2Bi) have a binarycompound of Ni3Sn4, similar to that found in the soldering reaction between Sn-containing solder and electroless Ni UBM. The presence of Cu in the solder also

2.28 CHAPTER TWO

TABLE 2.4 IMC in Some Lead-Free Solders on Electroless Ni-P-Immersion Au UBM

Interfacial Large intermetallicsSolder alloys intermetallics inside the solder

99.3Sn-0.7Cu Sn-Cu-Ni Cu6Sn5

96.5Sn-3.5Ag Ni3Sn4 Ag3Sn

95.5Sn-3.8Ag-0.7Cu Sn-Cu-Ni Ag3Sn

96Sn-2Ag-2Bi Ni3Sn4 No large intermetallics

FIGURE 2.30 Magnified SEM images of Fig. 2.29. (a) Sn-0.7Cu, (b) Sn-3.5Ag, (c) Sn-3.8Ag-0.7Cu,and (d) Sn-2Ag-2Bi.


affected the adhesion of the interfacial IMC. In the Cu-containing solder alloybumps, the interfacial IMC (Sn-Cu-Ni) adhered well to the electroless Ni-P UBM,whereas those without Cu (the needle-type Ni3Sn4 compound) lost adhesion andspalled off into solder.

2.6.2 INTERFACE OF ANNEALED SOLDER BUMPS

Figure 2.31 shows the cross-sectional SEM micrographs of the 99.3Sn-0.7Cu,96.5Sn-3.5Ag, 96Sn-2Ag-2Bi, and 95.5Sn-3.8Ag-0.7Cu lead-free solder bumpsobtained by Motorola.57,58 They are obtained after two times of reflow at 260°Cand annealed at 150°C for 1000 h. For the Cu-containing solder alloys (Fig. 2.31aand c), the Ni-Cu-Sn IMCs grew to a small extent. The interfacial compound(Ni3Sn4) of the solders without Cu (Fig. 2.31b and d) did not spall into the solder.The large IMCs of Ag3Sn and Cu6Sn5 are unchanged during solid-state annealing.It appears that the interfacial IMCs in the Cu-containing solders are more uni-form and stable than those that form in alloys that contain no Cu. Figure 2.32shows the cross-sectional SEM images of the edge of a 95.5Sn-3.5Ag on a Ni-Pbump. The Ni3Sn4 IMC grows extensively near the edge of the UBM, and fractureoccurs at this location. The fracture is coincident with the region of excessive IMCgrowth.

FIGURE 2.31 SEM images of various lead-free solder bumps (reflowed twice at 260°C andannealed at 150°C for 1000 hours) with electroless Ni-P-immersion Au UBM. (a) Sn-0.7Cu, (b) Sn-3.5Ag, (c) Sn-3.8Ag-0.7Cu, and (d) Sn-2Ag-2Bi.

2.6.3 SHEAR STRENGTH OF SOLDER BUMPS

Shear tests are applied to solder bumps 115 to approximately 135 µm in height and130 to approximately 150 µm in diameter by Motorola.57,58 The shear blade tip is setto 30 µm above the land pads. Multiple reflows, from 2 to 10, and high-temperaturestorages at 125°C, 150°C, and 170°C are also performed. The shear tests are per-formed 7 days after reflow or temperature storage to minimize variation due to roomtemperature age-softening. Figure 2.33 shows the solder bump shear test results. Itcan be seen that both Sn-3.5Ag and Sn-3.8Ag-0.7Cu solder bump shear strengths arelower after the second thermal exposure but are not affected by subsequent multiplereflows. The Sn-37Pb and Sn-0.7Cu solder bump shear strengths are basicallyunchanged after either multiple reflows or high-temperature storage. There does

2.30 CHAPTER TWO

FIGURE 2.32 SEM images showing the breakage of the edge of electroless Ni-P-immersion Au UBM. (a) Sn-3.5Ag solder, 10 reflows at 260°C and annealed at 170°C for 1000h; (b) enlarged image of the circled region in (a).

FIGURE 2.33 Shear test force (strength) after multiple reflows and high-temperature storage.


not appear to be a correlation between multiple reflows and thermal annealing–induced IMCs and microstructural evolution and the solder bump shear strength.The shear fractures are predominantly through the bulk solder for all of the solderalloys.

2.7 STENCIL PRINTING OF Sn-Cu, Sn-Ag-Bi, AND Sn-Ag-Cu

SOLDERS ON WAFERS WITH Ti-Cu UBM

Today, most of the wafers are with Al conductor lines and land pads. However, semi-conductor leaders (e.g., IBM, Motorola, Intel,TSMC, UMC, and TI) are beginning touse Cu conductor lines and land pads.

2.7.1 INTERFACE OF REFLOWED SOLDER BUMPS

Again, some typical cross sections for the 99.3Sn-0.7Cu, 96.5Sn-3.5Ag, 95.5Sn-3.8Ag-0.7Cu, and 63Sn-37Pb solder bumps on the Ti-Cu or TiW-Cu (or simply Cu)UBM obtained by Motorola57,58 are shown in Fig. 2.34. They are obtained after twotimes of reflow at 260°C. The initial Cu UBM is 13 to 15 µm thick. The Cu6Sn5 IMChas good adhesion with the Cu UBM for all solders. However, a slightly differentmorphology with respect to solder compositions is observed. Table 2.5 summarizesthe IMC that is found in these solders on the electroplated Cu UBM. Similar to elec-troless Ni-P UBM, the IMCs are primarily dependent on the solder composition.The difference is that both Cu6Sn5 and Ag3Sn IMCs are present in Sn-3.8Ag-0.7Cusolder-Cu UBM, whereas there is only Ag3Sn IMC present in the Sn-3.8Ag-0.7Cusolder-Ni-P UBM. This is due to a greater supply of Cu from the Cu UBM duringreflow.

2.7.2 INTERFACE OF ANNEALED SOLDER BUMPS

Figure 2.35 shows the cross-sectional SEM micrographs of eutectic Sn-Pb and threelead-free solders on the electroplated Cu UBM after they have been reflowed twiceat 260°C and annealed at 150°C for 1000 h.The intermetallics grow extensively com-pared with the as-reflowed samples shown in Fig. 2.34. The intermetallics changefrom a blocky morphology to a rather planar type. This suggests that the IMCgrowth mechanism in the liquid state differs from that in the solid state. The blocky

TABLE 2.5 IMC in Some Lead-Free Solders on ElectroplatedCu UBM

Interfacial Large intermetallicsSolder alloys intermetallics inside the solder

Eutectic SnPb Cu6Sn5/Cu3Sn No large compound

99.3Sn0.7Cu Cu6Sn5/Cu3Sn Cu6Sn5

96.5Sn3.5Ag Cu6Sn5/Cu3Sn Ag3Sn

95.5Sn3.8Ag0.7Cu Cu6Sn5/Cu3Sn Ag3Sn & Cu6Sn5

2.32 CHAPTER TWO

FIGURE 2.35 SEM images of various solder bumps (reflowed twice at 260°C and annealed at150°C for 1000 hours) with Cu UBM. (a) Sn-37Pb, (b) Sn-0.7Cu, (c) Sn-3.5Ag, and (d) Sn-3.8Ag-0.7Cu.

FIGURE 2.34 SEM images of various solder bumps (reflowed twice at 260°C) with Cu UBM.(a) Sn-37Pb, (b) Sn-0.7Cu, (c) Sn-3.5Ag, and (d) Sn-3.8Ag-0.7Cu.

2.33

FIGURE 2.36 FCT’s 8-mil pitch full-array solder bumps. (a) Wafer, (b) close-up.

morphology may indicate that the growth of intermetallics can be achieved by aripening process as well as an interfacial reaction.57,58 The planar type of intermetal-lic may be a result of primarily interfacial reaction. This morphology change hasbeen reported in other lead-free solders, such as the Sn-Sb alloy.57,58

2.8 PASTE PRINTING OF SOLDERS ON WAFERS WITH

Al-NiV-Cu UBM

In Sec. 2.2.2, the first four steps showing how the Al-NiV-Cu UBM is made by theflip chip technology division of K & S have been presented. The next step is todeposit the second layer of dielectric to cover the UBM layer.59 Then, open windowsto define the solder bump attachment area. After depositing lead-free solder pasteof any kind on the land pads, reflow and clean the solder bumps. Figure 2.36 showsthe Sn-Ag-Cu solder bumps, and Fig. 2.37 shows a typical cross section of the lead-free solder-bumped flip chip on a substrate. Their solder joint reliability will be dis-cussed in Chap. 3.

ACKNOWLEDGMENTS

The authors would like to thank C. Kallmayer, H. Oppermann, S. Ankock, R.Azadeh, R.Aschenbrenner, and H. Reichl of the Technical University of Berlin andFraunhofer Institute IZM; J. Lin, A. De Silva, D. Frear, Y. Guo, J. Jang, L. Li, D.Mitchell, B. Yeung, C. Zhang, and Y. Rao of Motorola; Y. Jeon and K. Paik ofKAIST; K. Bok, W. Choi, and C. Cho of Samsung; P. Elenius, H. Balkan, D. Patter-son, G. Burgess, C. Carlson, M. Johnson, B. Rooney, J. Sanchez, D. Stepniak, andJ. Wood of Flip Chip Technology; E. Hashino, K. Shimokawa, Y. Yamamoto, andK. Tatsumi of Nippon Steel; and A. Kazama, T. Satch, Y. Yamaguchi, I. Anjoh, and

2.34 CHAPTER TWO

FIGURE 2.37 FCT’s ultra-CSP on a laminate substrate.

A. Nishimura of Hitachi for sharing their important and useful technology with theindustry.

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2.36 CHAPTER TWO

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2.38 CHAPTER TWO

CHAPTER 3WLCSP WITH LEAD-FREE

SOLDER BUMPS ONPCB/SUBSTRATE

3.1 INTRODUCTION

The solder joint reliability of various wafer-level chip-scale packages (WLCSPs) onorganic printed circuit board (PCB)/substrate is the focus of discussion in this chap-ter,1–117 since the compliance of lead-free solder joints is so small and the thermalexpansion mismatch between the silicon chip and epoxy PCB/substrate is so large.Specifically, the useful thermal cycling test and modeling results provided byHitachi, Motorola, and Flip Chip Technology are presented in this chapter. It shouldbe noted that the surface-mount technology (SMT) assembly of lead-free solder-bumped WLCSPs on PCB/substrate will be discussed in Chaps. 11 through 16. In thischapter, the solders on the chip before it is joined to the substrate are called solderbumps. After the solder bumps have been reflowed on the PCB/substrate, they arecalled solder joints.

3.2 SOLDER JOINT RELIABILITY OF SnAgCu WLCSP WITH

A STRESS-RELAXATION LAYER

The effects of WLCSP stress-relaxation layer on the SnAgCu solder joint reliabilityand high-frequency applications have been determined by Hitachi,114 whose usefulresults are presented in this section.

3.2.1 FINITE ELEMENT RESULTS

The finite element analysis model of Hitachi’s SnAgCu WLCSP with a stress-relaxation layer on a PCB (Fig. 2.20) is shown in Fig. 3.1. Due to double symmetries,only one-fourth of the structure is modeled. The distance from the neutral point(DNP) to a corner solder joint is 4.7 mm, as shown in Table 3.1, where informationon chip size, pin layout, and the PCB are also given. The thermal cycling is between−55 and +125°C.

Figure 3.2 shows the finite element analysis result in terms of the plastic straindistribution around the corner solder joint.The maximum plastic strain occurs insidethe corner solder joint near the interface of the solder joint and the land pad. Bychoosing the right material for the stress-relaxation layer (as shown in Fig. 3.3) andthe correct solder joint geometry (as shown in Fig. 3.4), an optimal structure can beachieved. Table 3.2 shows the material properties of three of the stress-relaxationlayers considered by Hitachi.

3.1


3.2.2 THERMAL CYCLING RESULTS

In order to determine the performance of these three materials for the stress-relaxation layer under thermal cycling condition, a simpler structure as shown in Fig.3.5 is constructed. Two different thicknesses of the stress-relaxation layer are con-sidered—75 and 100 µm—as shown in Table 3.3. Figure 3.6 and Table 3.3 show thethermal cycling (−55 and +125°C) test results for samples 1, 2, and 4. It can be seenthat sample 4 (resin A at 100 µm thick) gives the best result and sample 2 (resin B at

3.2 CHAPTER THREE

FIGURE 3.1 Three-dimensional finite element model of Hitachi’s WLCSP with a stress-relaxationlayer.

TABLE 3.1 Structural Parameters of Hitachi’s WLCSP

Chip Size 10 × 10 mm

Thickness 0.725 mm

Pin layout Number of bumps 54 (9 × 6)

Bump pitch 0.8 mm (minimum)

DNP 4.7 mm

Solder Material SnAgCu

Ball diameter 400 µm

Motherboard Material FR-4

Thickness 1.27 mm

75 µm thick) yields the worst. Even sample 1 (resin A at 75 µm thick) is not as goodas sample 4; however, it is quite close to sample 4 and gives reasonably good results.A typical failure mode at 2250 cycles is shown in Fig. 3.7. It can be seen that the fail-ure location is in the corner solder joint, near the interface between the SnAgCubulk solder and the stress-relaxation layer.

WLCSP WITH LEAD-FREE SOLDER BUMPS ON PCB/SUBSTRATE 3.3

FIGURE 3.2 Deformation of the corner solder joint.

FIGURE 3.3 Effects of stress-relaxation layer thickness andmodulus on the deformation of the corner solder joint.

Another thermal cycling (−55 and +125°C) test result shows that the solder jointsare not reliable if there is no underfill and no stress-relaxation layer, as shown in Fig.3.8. It can be seen that the joints without the underfill and stress-relaxation layerfailed at around 100 cycles. However, those with a 75-µm-thick stress-relaxationlayer lasted for more than 2000 cycles. Thus, with the stress-relaxation layer, the sol-der joints are reliable even without underfill encapsulant. This is because the stress-relaxation layer increases the compliance of the solder joints between the chip andPCB.

3.2.3 EFFECTS OF THE STRESS-RELAXATION LAYER

ON CAPACITANCE

Figure 3.9 shows a simple geometric model for the calculation of the capacitance ofthe (longest) interconnection and land pad with a stress-relaxation layer on thewafer. The results are shown in Fig. 3.10 for various thicknesses of the stress-relaxation layer. It can be seen that the capacitance drops from 1.4 pF (without astress-relaxation layer) to below 0.3 pF (with a 75-µm-thick stress-relaxation layer),which is ideal for high-frequency applications.

3.4 CHAPTER THREE

FIGURE 3.4 Effects of stress-relaxation layer thickness and padgeometry on the deformation of the corner solder joint.

TABLE 3.2 Mechanical Properties of Stress-Relaxation Layer

Young’s modulus MPa Glass transition Resin −55°C 25°C 125°C CTE (×10−6/°C) temperature (°C)

A 1200 900 800 92 191

B 2700 2000 1500 58 221

C 500 430 360 176 195

3.3 SOLDER JOINT RELIABILITY OF SnAg AND

SnAgCu WLCSPs WITH TiCu AND NiAu UBMs

The isothermal fatigue life and thermal cycling fatigue life of SnAg and SnAgCuWLCSPs with Cu and NiAu under-bump metallurgy (UBM) on PCB have beendetermined by Motorola,115,116 whose useful results are presented in this section.

3.3.1 ISOTHERMAL FATIGUE TEST RESULTS

For isothermal fatigue testing, the test specimen has 28 array solder bumps on a 3.6 × 4.2-mm silicon chip and is flip chip attached to a matching silicon chip. A testsystem with a displacement resolution of 0.1 µm is used to conduct the test. Fatiguetests are performed at room temperature with fully reversed cyclic loading at differ-ent displacements and frequencies ranging, respectively, from 2 to 4 µm and 0.1 to0.25 Hz. The total shear strain (= D/h) in the solder is determined from the cross-head displacement (D) and the solder joint height (h). The fatigue life (Nf 50%) isdetermined as the number of cycles to reach 50 percent load drop from peak load.


FIGURE 3.5 Cross section of the simplified WLCSP on PCB.

TABLE 3.3 Thermal Fatigue Life with Different Stress-Relaxation Layers

Stress-relaxation layer Resulting lifetime

Young’s modulus Thickness 50% fail 0.1% failSample no. Resin at −55°C (MPa) (µm) cycles cycles

1 A 1200 75 2600 1100

2 B 2700 75 1100 450

3 C 500 75 1700* —

4 A 1200 100 3200 1500

* Damaged at interconnection.

3.6 CHAPTER THREE

FIGURE 3.7 Tested samples at 2250 cycles (−55/125°C). Failure near the pad on the chip.

(a) Sample 1 (resin A)

(b) Sample 3 (resin C)

FIGURE 3.6 Solder joint life distributions under thermalcycling (−55/125°C).


FIGURE 3.8 Solder joint life distributions under thermal cycling (−55/125°C) with and without stress-relaxation layer.

FIGURE 3.9 Simple geometric model for capacitancecalculation.

FIGURE 3.10 Effects of the stress-relaxation layer thicknesson the capacitance.

The plastic strain range vs. isothermal fatigue life is plotted in Fig. 3.11 for sev-eral solder/UBM systems (SnAgCu on electroless Ni-P-immersion Au (for short,NiAu), SnPb on NiAu, SnPb on TiW/Cu (or in short, Cu), SnCu on Cu, SnCu onNiAu, and SnAg on Cu). It can be seen that the Sn0.7Cu on both Cu and NiAu hasthe best isothermal fatigue life of all the solder/UBM systems evaluated. Also, thedata indicate that the isothermal fatigue life for Sn0.7Cu is independent of thetwo UBMs and the failures are through the bulk of the Sn0.7Cu solder joint asshown in Fig. 3.12. However, the fracture surfaces of both Sn3.8Ag0.7Cu andSn37Pb solders appear to be related to the UBM, as shown in Figs. 3.13 and 3.14,respectively.

Also, Fig. 3.11 shows that Sn3.8Ag0.7Cu on NiAu UBM has the shortest isother-mal fatigue life. This is due to the majority of the failures occurring in the UBMintermetallic/solder interface as shown in Fig. 3.13(a). The Sn3.8Ag0.7Cu on CuUBM shows similar isothermal fatigue life to the Sn0.7Cu solder system and sharesa similar failure mode, as shown in Fig. 3.13(b). The Sn37Pb on Cu UBM has betterfatigue life than on NiAu UBM but a slightly shorter fatigue life than Sn0.7Cu andSn3.8Ag0.7Cu on Cu UBM, as shown in Fig. 3.11.Typical isothermal fatigue fracturesurfaces of Sn37Pb on NiAu and Cu UBMs are shown in Fig. 3.14.

3.3.2 THERMAL CYCLING FATIGUE TEST RESULTS

Air-to-air thermal cycling testing is used to study the thermal fatigue of several sol-der/UBM systems by Motorola. The test chip dimensions are 12.6 × 7.46 mm. Thechip has 137 pads in a mixed array of 300-µm pitch at the periphery and 225-µmpitch at the area array in the center, and is flip chip bonded to a 1-mm-thick organicsubstrate made of bismaleimide triazine (BT) resin and solder mask. The metal fin-ish of the substrate is either organic solderability preservative (OSP)/Cu orNiAu/Cu.All three lead-free solders (Sn0.7Cu, Sn3.8Ag0.7Cu, and Sn3.5Ag), as wellas the Sn37Pb solder, are prepared with Motorola’s custom-formulated flux(HBA2961 and HB2974)115,116 to achieve “void-free” solder joints. The paste/flux,pad finish, and reflow temperature profile used for the SMT assembly are summa-rized in Table 3.4. It should be noted that, in order to accelerate failures, there is nounderfill or stress-relaxation layer in any of the flip chip assemblies.

Figure 3.15 shows the life distribution (Weibull) plots of air-to-air thermal cycling(0 to 100°C) fatigue life for a variety of solder/UBMs. It can be seen that the SnCu0.7solder joints on both NiAu and Cu UBMs are the most uniform (largest Weibullslope) and have the best thermal fatigue life.Also, the characteristic lives of SnCu0.7on Cu UBM and SnCu0.7 on NiAu UBM are, respectively, 170 and 209 cycles. Thisvariation of characteristic life is due to the difference in the standoff height (chip-to-substrate) between these two bump/UBM structures. (The standoff height ofSnCu0.7 on NiAu UBM solder joints is taller.)

Figure 3.15 also shows that the SnAg3.8Cu0.7 on Cu UBM solder joints have verysimilar thermal fatigue characteristic lives compared to the eutectic SnPb on NiAuUBM solder joints.The SnAg3.5 on NiAu UBM solder joints have the worst thermalfatigue life. Figure 3.16 summarizes the typical thermal fatigue failure mechanisms ofthe Sn0.7Cu on NiAu UBM, Sn3.8Ag0.7Cu on Cu UBM, Sn3.5Ag on NiAu UBM,and Sn37Pb on NiAu UBM solder joints. It can be seen that the Sn0.7Cu solder jointfatigue failure mode is through the solder and the Sn37Pb solder joint fatigue cracksand propagates close to the solder-intermetallic interface. For Sn3.8Ag0.7Cu andSn3.5Ag solder joints, the cracks are at the solder-intermetallic interface, and in somecases, cracks occur through the intermetallics.

3.8 CHAPTER THREE


FIGURE 3.11 Isothermal fatigue life for various lead-free solders.

FIGURE 3.12 Typical isothermal fatigue failures: Sn0.7Cu on (a) Cu UBM and (b) NiPAu UBM.

(a)

(b)

3.10 CHAPTER THREE

FIGURE 3.13 Typical isothermal fatigue failures: Sn3.8Ag0.7Cu on (a) NiPAu UBM and (b) CuUBM.

FIGURE 3.14 Typical isothermal fatigue failures: Sn37Pb on (a) Ni-P-Au UBM and (b) Cu UBM.

(a)

(b)

(a)

(b)

Figure 3.17 shows the typical thermal fatigue failure mechanisms of the Sn37Pb onCu UBM, Sn3.5Ag on Cu UBM, and Sn0.7Cu on Cu UBM solder joints. It can beseen that the Sn37Pb on Cu UBM solder joints fail by crack formation and propaga-tion through heterogeneous coarsened bands near the solder/UBM interface. Thecracks are observed to form on the outer edge of the solder joint and propagate to thecenter of the joint. The surface of the solder joint remains smooth after thermalcycling, indicating that the damage is localized to the heterogeneous coarsened band.

The Sn0.7Cu on Cu UBM solder joints exhibit a failure mode that differs fromthose of the other solder alloys studied in this section.The initiation and propagation


TABLE 3.4 Paste/Flux, PCB Finishing, and Reflow Profile for Assembling Samples for Evaluation

Lot no. 001201 002107 001202 001203 001012

Solder/UBM SnCu/ SnCu/ SnCuAg/ SnAg/ SnPb/NiP-Au TiW-Cu TiW-Cu NiP-Au NiP-Au

Flux/paste HBA296L/ HB2974/ HBA296L/ HBA296L/ SMQ92/type SnCu0.7 SnCu0.7 SnAg3.8Cu0.7 SnAg3.5 SnPb37

Board pad OSP/Cu OSP/Cu OSP/Cu OSP/Cu NiAu/Cufinish

Reflow peak 247°C 247°C 245°C 245°C 220°Ctemp.

Time above 60 s/227°C 60 s/227°C 60 s/217°C 60 s/221°C 60 s/183°Cliquid temp.

FIGURE 3.15 Thermal fatigue life for various lead-free solders (0 to 100°C).

of fatigue cracks is through the grain boundaries in the bulk solder, as shown in Fig.3.17. The fatigue cracks initiate on the highest point of strain at the solder joint,closer to the edge of chip, then propagate across the middle of the joint toward thecenter of the chip. After thermal cycling testing, the surface of the solder joints is nolonger smooth. The Sn0.7Cu solder deforms by grain-boundary sliding and is socompliant in thermal fatigue that it undergoes massive deformation before failing bycrack propagation.

The thermal fatigue cracks in the Sn3.5Ag on Cu UBM solder joints initiate andpropagate through the intermetallics and at the intermetallics-solder interface. Thesurfaces of the solder joints also exhibit no deformation, with the damage concen-trated at the solder-intermetallic interface.The microstructure of the Sn3.5Ag solderjoint appears to have more and large Ag3Sn intermetallic compounds (IMCs)present, as shown in Fig. 3.17. Even the large Ag3Sn IMC may strengthen the solderbump, but it may reduce the compliance of the solder joint and thus shorten itsthermal fatigue life.

The thermal fatigue cracks in the Sn3.5Ag on NiAu UBM solder joints are shownin Figs. 3.18 and 3.19. It can be seen that the thermal fatigue failure mechanism ofSnAg3.5 solder joint is similar to that of SnAg0.8Cu0.7. Both solder joints have hadvery little plastic flow before failure. The thermal fatigue cracks initiate and propa-

3.12 CHAPTER THREE

FIGURE 3.16 Typical thermal fatigue failures of various solders (0 to 100°C).


FIGURE 3.17 Typical thermal fatigue failures of various solders on TiCuUBM (0 to 100°C).

gate through the intermetallics and intermetallics/solder interface. Fine colonies anddendrites are seen in the etched sample in Fig. 3.18(b). The SnAg3.5 solder jointshave more Ag3Sn intermetallics than the SnAg0.8Cu0.7 solder joints. This increasedamount of Ag3Sn intermetallics may have contributed to the shorter thermal fatiguelife of SnAg3.5 relative to SnAg0.8Cu0.7, as discussed earlier. Figure 3.19 showsmagnified photographs of fatigue crack profiles in the SnAg3.5-NiAu UBM inter-connect. Figure 3.19(a) shows that fatigue cracks initiate at the corner of the solderjoint and propagate through the interface of the solder and intermetallics. Figure3.19(b) shows how the interface of solder and Ag3Sn intermetallics can serve as afocal point for crack initiation and propagation.

It should be reemphasized that the results and insights presented herein are forthe case where there is no underfill or stress-relaxation layer between the chip andthe solder joints in the flip chip assembly. In real applications, either or both of thesewill ensure the WLCSP solder joint reliability.

3.14 CHAPTER THREE

FIGURE 3.18 Typical thermal fatigue failures of Sn3.5Ag on NiPAu UBM (a) before chemicaletching and (b) after chemical etching.

FIGURE 3.19 Magnified images of Fig. 3.18 (Sn3.5Ag on NiPAu UBM) (a) through intermetallicsnear UBM and (b) through interface of bulk Ag3Sn intermetallics/solder.

3.4 SOLDER JOINT RELIABILITY OF SnAg, SnAgCu, SnAgCuSb,

AND SnAgInCu WLCSPs WITH AlNiVCu UBM

The thermal fatigue life, high-temperature storage, and shear strength of variouslead-free WLCSPs on ceramic substrates and on organic substrates have been stud-ied by Flip Chip Technology,117 whose useful results are reported in this section.

3.4.1 THERMAL FATIGUE OF SnAg, SnAgCu, SnAgCuSb,

AND SnAgInCu WLCSPs ON CERAMIC SUBSTRATE

The thermal cycling test results of 96.3Sn3.5Ag, Sn3.5Ag0.7-1Cu, SnAgCuSb, andSnAgInCu WLCSPs with AlNiVCu UBM on ceramic substrate have been obtainedby Flip Chip Technology. There is no underfill encapsulant in the assemblies.

Figures 3.20 and 3.21 show the life distributions of the 96.3Sn3.5Ag, SnAgInCu,and Sn37Pb WLCSP solder joints. It can be seen that the thermal fatigue life of theSn37Pb solder joints is better than that of 96.3Sn3.5Ag solder joint. However, thethermal fatigue life of the SnAgInCu solder joints is better than that of the Sn37Pbsolder joints. Table 3.5 summarizes all the thermal cycling test results. It can be seenthat the WLCSP with the SnAgCuSb lead-free solder yields the best thermal fatiguelife on the ceramic substrate.

3.4.2 THERMAL FATIGUE OF SnAgCu WLCSP ON PCB

The PCB for the test specimen is made of the high glass transition temperatureFR-4 epoxy with Cu-OSP finish. The chip dimensions are 5.1 × 5.1 mm with 200-µm-pitch peripheral pads. The peak reflow temperature for the SnAgCu solder is257°C with time above 240°C at approximately 30 to 35 s and time above 217°C atapproximately 65 to 70 s. The underfill material is Namics 8437-2. The test condi-tions are shown in Table 3.6. It can be seen that the cycling condition is from −40to 125°C.

After 1000 cycles, there is no failure. The cross section of the tested specimensis shown in Fig. 3.22. It can be seen that there is no visible cracking. In order to seethe failure mode sooner, some specimens without underfill were tested. Asexpected, these specimens failed very early; a typical fracture surface is shown inFig. 3.23. It can be seen that the fracture plane for the SnAgCu solder joint isthrough the bulk solder near the UBM interface and is very similar to that ofSn37Pb solder joint.

3.4.3 HIGH-TEMPERATURE STORAGE OF SnAgCu WLCSP ON PCB

The effects of IMC formation and growth on SnAgCu solder joint reliability can bedetermined by high-temperature storage testing at 150°C for 1000 h as shown in


3.16 CHAPTER THREE

FIGURE 3.20 Life distributions of Sn37Pb and Sn3.5Ag solder joints on ceramic substrate.

FIGURE 3.21 Life distributions of Sn37Pb and SnAgInCu (LF-1) solder joints on ceramic substrate.


Table 3.6. After 1000 h of testing, none of the underfilled flip chip assemblies failed.A typical cross section of the tested assembly is shown in Fig. 3.24. It can be seen thatwhile the Sn37Pb solder joints display grain coarsening, the SnAgCu solder jointsare nearly unchanged.

A number of nonunderfilled flip chip assemblies were also subjected to 1000 h ofaging at 150°C. After the test, a pull test was performed. The fracture surface isshown in Fig. 3.25. It can be seen that the shear plane on the nonunderfilled flip chipassemblies is through the bulk solder above the IMC layer for both the SnPb andSnAgCu solder joints.

3.4.4 SHEAR STRENGTH OF SnAgCu WLCSP ON PCB

The solder joint shear force to failure (strength) of SnAgCu WLCSP on PCB can beobtained by the shear testing and is slightly higher than that of the SnPb assembly.The fracture surface, as shown in Fig. 3.26, is mostly through the bulk solder andabove the IMC layer on occasion, similar to that of SnPb solder joints.

A multiple reflow test was conducted on the wafer by Flip Chip Technology todemonstrate AlNiVCu UBM robustness of the SnAgCu WLCSP. It was found thatthe effects of multiple reflows is to slightly reduce the shear strength of theSnAgCu solder bumps for the first 6 reflows and level off by 10 reflow operations.The scanning electron microscope images of the Cu6Sn5 IMC at the UBM inter-face are shown in Fig. 3.27. It can be seen that: (1) the shear mode is ductile and noNiSn interaction is observed through the 10 reflow cycles; (2) The Cu6Sn5 IMC ismore columnar in the AnAgCu case; and (3) although Cu6Sn5 IMC is detected fol-lowing early reflow cycles in the SnPb solder joints, it is absent in the SnAgCu sol-der joints.

TABLE 3.5 Thermal Fatigue Life of Lead-Free Solder Joints on Ceramic Substrate

SnAg 63 SnPb CASTIN® SnAgCu

Weibull life (h) 166 208 550 774

Weibull slope 1.52 4.0 3.3 3.3

Relative reliability 0.8 1.0 2.6 3.7

TABLE 3.6 Reliability Test Conditions

Reliability Test Conditions Standard

Thermal cycle −40/125°C, 1000 cycles JEDEC Std. 22-A104-A

High-temp storage 150°C, 1000 h JEDEC Std. 22-A103-A

Die shear 2 mm/s, 10 Kg N/A

3.18 CHAPTER THREE

FIGURE 3.24 Typical cross section of (a) SnAgCu and (b) SnPb solder joints after 1000 h of high-temperature storage (150°C).

(a) (b)

FIGURE 3.23 Typical fracture surface (through the bulk solder) of both (a) SnAgCu and (b) SnPbsolder joints without underfill after thermal cycling (−40 to 125°C).

(a) (b)

FIGURE 3.22 Typical cross section of (a) SnAgCu and (b) SnPb solder joints after 1000 thermalcycles (−40 to 125°C).

(a) (b)


FIGURE 3.25 Typical pull test fracture surface (through bulk solder) of (a) SnAgCu and (b) SnPbsolder joints without underfill after 1000 h of high-temperature storage (150°C).

FIGURE 3.26 Typical shear test fracture surface (through bulk solder) of (a) SnAgCu and (b)SnPb solder joints without underfill.

(a) (b)

(a) (b)

FIGURE 3.27 Typical IMC (Cu6Sn5) at the interface for the SnAgCu solder bump: (a) no reflows,(b) 10 reflows.

(a) (b)

ACKNOWLEDGMENTS

The authors would like to thank A. Kazama, T. Satch, Y. Yamaguchi, I. Anjoh, and A.Nishimura of Hitachi; J. Lin,A. De Silva, D. Frear,Y. Guo, J. Jang, Li Li, D. Mitchell, B.Yeung, C. Zhang, and Y. Rao of Motorola; and P. Elenius, H. Balkan, D. Patterson, G.Burgess,C.Carlson,M.Johnson,B.Rooney,J.Sanchez,D.Stepniak,and J.Wood of FlipChip Technology for sharing their important and useful knowledge with the industry.

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38. Capote, M.A.,W. Johnson, S. Zhu, L. Zhou, and B. Gao,“Reflow-Curable Polymer Fluxesfor Flip Chip Encapsulation,” Proceedings of the International Conference on MultichipModules and High Density Packaging, pp. 41–46, Denver, CO, April 1998.

39. Vincent, M. B., and C. P. Wong, “Enhancement of Underfill Encapsulants for Flip-ChipTechnology,” Proceedings of Surface Mount International Conference, pp. 303–312, SanJose, CA, August 1998.

40. Vincent, M. B., L. Meyers, and C. P. Wong, “Enhancement of Underfill Performance forFlip-Chip Applications by Use of Silane Additives,” Proceedings of IEEE ElectronicComponents and Technology Conference, pp. 125–131, Seattle, WA, May 1998.

41. Wang, L., and C. P.Wong,“Novel Thermally Reworkable Underfill Encapsulants for Flip-Chip Applications,” Proceedings of IEEE Electronic Components and Technology Con-ference, pp. 92–100, Seattle, WA, May 1998.

42. Shi, S. H., and C. P. Wong, “Study of the Fluxing Agent Effects on the Properties of No-Flow Underfill Materials for Flip-Chip Applications,” Proceedings of IEEE ElectronicComponents and Technology Conference, pp. 117–124, Seattle, WA, May 1998.

43. Wong, C. P., D. Baldwin, M. B. Vincent, B. Fennell, L. J. Wang, and S. H. Shi, “Characteri-zation of a No-Flow Underfill Encapsulant During the Solder Reflow Process,” Proceed-ings of IEEE Electronic Components and Technology Conference, pp. 1253–1259, Seattle,WA, May 1998.

44. Ito, S., M. Mizutani, H. Noro, M. Kuwamura, and A. Prabhu, “A Novel Flip Chip Technol-ogy Using Non-Conductive Resin Sheet,” Proceedings of IEEE Electronic Componentsand Technology Conference, pp. 1047–1051, Seattle, WA, May 1998.

45. Gilleo, K., and D. Blumel, “The Great Underfill Race,” Proceedings of the InternationalSymposium on Microelectronics, pp. 701–706, San Diego, CA, November 1998.

46. Lau, J. H., C. Chang, T. Chen, D. Cheng, and E. Lao, “A Low-Cost Solder-Bumped ChipScale Package—NuCSP,” Circuit World, 24(3):11–25, April 1998.

47. Elshabini-Riad, A., and F. Barlow III, Thin Film Technology Handbook, McGraw-Hill,New York, 1998.

48. Garrou, P. E., and I. Turlik, Multichip Module Technology Handbook, McGraw-Hill, NewYork, 1998.

49. Lau, J. H., C. Chang, and O. Chien, “SMT Compatible No-Flow Underfill for SolderBumped Flip Chip on Low-Cost Substrates,” Journal of Electronics Manufacturing, 8(3and 4):151–164, December 1998.

3.22 CHAPTER THREE

50. Lau, J. H., and C. Chang, “Characterization of Underfill Materials for Functional SolderBumped Flip Chips on Board Applications,” IEEE Transactions on Components andPackaging Technology, Part A, 22(1):111–119, March 1999.

51. Thorpe, R., and D. F. Baldwin, “High Throughput Flip Chip Processing and ReliabilityAnalysis Using No-Flow Underfills,” Proceedings of IEEE Electronic Components andTechnology Conference, pp. 419–425, San Diego, CA, June 1999.

52. Qian, Z., M. Lu, W. Ren, and S. Liu, “Fatigue Life Prediction of Flip-Chips in Terms ofNonlinear Behaviors of Solder and Underfill,” Proceedings of IEEE Electronic Compo-nents and Technology Conference, pp. 141–148, San Diego, CA, June 1999.

53. Wang, L., and C. P.Wong,“Epoxy-Additive Interaction Studies of Thermally ReworkableUnderfills for Flip-Chip Applications,” Proceedings of IEEE Electronic Components andTechnology Conference, pp. 34–42, San Diego, CA, June 1999.

54. Lau, J. H., S.-W. Lee, C. Chang, and O. Chien, “Effects of Underfill Material Properties onthe Reliability of Solder Bumped Flip Chip on Board with Imperfect Underfill Encapsu-lants,” Proceedings of IEEE Electronic Components and Technology Conference, pp.571–582, San Diego, CA, June 1999.

55. Lau, J. H., C. Chang, and O. Chien, “No-Flow Underfill for Solder Bumped Flip Chip onLow-Cost Substrates,” Proceedings of NEPCON West, pp. 158–181, February 1999.

56. Tong, Q., A. Savoca, L. Nguyen, P. Fine, and B. Cobb, “Novel Fast Cure and ReworkableUnderfill Materials,” Proceedings of IEEE Electronic Components and Technology Con-ference, pp. 43–48, San Diego, CA, June 1999.

57. Benjamin, T. A., A. Chang, D. A. Dubois, M. Fan, D. L. Gelles, S. R. Iyer, S. Mohindra, P. N.Tutunjian, P. K. Wang, and W. J. Wright, “CARIVERSE Resin: A Thermally ReversibleNetwork Polymer for Electronic Applications,” Proceedings of IEEE Electronic Compo-nents and Technology Conference, pp. 49–55, San Diego, CA, June 1999.

58. Wada, M., “Development of Underfill Material with High Valued Performance,” Pro-ceedings of IEEE Electronic Components and Technology Conference, pp. 56–60, SanDiego, CA, June 1999.

59. Houston, P. N., D. F. Baldwin, M. Deladisma, L. N. Crane, and M. Konarski, “Low Cost FlipChip Processing and Reliability of Fast-Flow, Snap-Cure Underfills,” Proceedings of IEEEElectronic Components and Technology Conference, pp. 61–70, San Diego, CA, June 1999.

60. Kulojarvi, K., S. Pienimaa, and J. K. Kivilahti,“High Volume Capable Direct Chip Attach-ment Methods,” Proceedings of IEEE Electronic Components and Technology Confer-ence, pp. 441–445, San Diego, CA, June 1999.

61. Shi, S. H., and C. P. Wong, “Recent Advances in the Development of No-Flow UnderfillEncapsulants—A Practical Approach Towards the Actual Manufacturing Application,”Proceedings of IEEE Electronic Components and Technology Conference, pp. 770–776,San Diego, CA, June 1999.

62. Rao, Y., S. H. Shi, and C. P. Wong, “A Simple Evaluation Methodology of Young’s Modu-lus—Temperature Relationship for the Underfill Encapsulants,” Proceedings of IEEE Elec-tronic Components and Technology Conference, pp. 784–789, San Diego, CA, June 1999.

63. Fine, P., and L. Nguyen, “Flip Chip Underfill Flow Characteristics and Prediction,” Pro-ceedings of IEEE Electronic Components and Technology Conference, pp. 790–796, SanDiego, CA, June 1999.

64. Johnson, C. H., and D. F. Baldwin, “Wafer Scale Packaging Based on Underfill Applied atthe Wafer Level for Low-Cost Flip Chip Processing,” Proceedings of IEEE ElectronicComponents and Technology Conference, pp. 950–954, San Diego, CA, June 1999.

65. DeBarros, T., P. Neathway, and Q. Chu, “The No-Flow Fluxing Underfill Adhesive forLow Cost, High Reliability Flip Chip Assembly,” Proceedings of IEEE Electronic Com-ponents and Technology Conference, pp. 955–960, San Diego, CA, June 1999.

66. Shi, S. H., T. Yamashita, and C. P. Wong, “Development of the Wafer Level Compressive-Flow Underfill Process and Its Required Materials,” Proceedings of IEEE ElectronicComponents and Technology Conference, pp. 961–966, San Diego, CA, June 1999.


67. Chau, M. M., B. Ho, T. Herrington, and J. Bowen, “Novel Flip Chip Underfills,” Proceed-ings of IEEE Electronic Components and Technology Conference, pp. 967–974, SanDiego, CA, June 1999.

68. Feustel, F., and A. Eckebracht,“Influence of Flux Selection and Underfill Selection on theReliability of Flip Chips on FR-4,” Proceedings of IEEE Electronic Components andTechnology Conference, pp. 583–588, San Diego, CA, June 1999.

69. Okura, J. H., K. Drabha, S. Shetty, and A. Dasgupta, “Guidelines to Select Underfills forFlip Chip on Board Assemblies,” Proceedings of IEEE Electronic Components and Tech-nology Conference, pp. 589–594, San Diego, CA, June 1999.

70. Anderson, B., “Development Methodology for a High-Performance, Snap-Cure Flip-Chip Underfill,” Proceedings of NEPCON WEST, pp. 135–143, February 1999.

71. Wyllie, G., and B. Miquel, “Technical Advancements in Underfill Dispensing,” Proceed-ings of NEPCON WEST, pp. 152–157, February 1999.

72. Crane, L., A. Torres-Filho, E. Yager, M. Heuel, C. Ober, S. Yang, J. Chen, and R. Johnson,“Development of Reworkable Underfills, Materials, Reliability and Proceeding,” Pro-ceedings of NEPCON WEST, pp. 144–151, February 1999.

73. Gilleo, K.,“The Ultimate Flip Chip-Integrated Flux/Underfill,” Proceedings of NEPCONWEST, pp. 1477–1488, February 1999.

74. Miller, M., I. Mohammed, X. Dai, N. Jiang, and P. Ho, “Analysis of Flip-Chip PackagesUsing High Resolution Moire Interometry,” Proceedings of IEEE Electronic Compo-nents and Technology Conference, pp. 979–986, San Diego, CA, June 1999.

75. Hanna, C., S. Michaelides, P. Palaniappan, D. Baldwin, and S. Sitaraman, “Numerical andExperimental Study of the Evolution of Stresses in Flip Chip Assemblies During Assem-bly and Thermal Cycling,” Proceedings of IEEE Electronic Components and TechnologyConference, pp. 1001–1009, San Diego, CA, June 1999.

76. Emerson, J., and L. Adkins, “Techniques for Determining the Flow Properties of Under-fill Materials,” Proceedings of IEEE Electronic Components and Technology Conference,pp. 777–781, San Diego, CA, June 1999.

77. Guo, Y., G. Lehmann, T. Driscoll, and E. Cotts, “A Model of the Underfill Flow Process:Particle Distribution Effects,” Proceedings of IEEE Electronic Components and Technol-ogy Conference, pp. 71–76, San Diego, CA, June 1999.

78. Mercado, L.,V. Sarihan,Y. Guo, and A. Mawer,“Impact of Solder Pad Size on Solder JointReliability in Flip Chip PBGA Packages,” Proceedings of IEEE Electronic Componentsand Technology Conference, pp. 255–259, San Diego, CA, June 1999.

79. Qian, Z., M. Lu, W. Ren, and S. Liu, “Fatigue Life Prediction of Flip-Chips in Terms ofNonlinear Behaviors of Solder and Underfill,” Proceedings of IEEE Electronic Compo-nents and Technology Conference, pp. 141–148, San Diego, CA, June 1999.

80. Gektin, V., A. Bar-Cohen, and S. Witzman, “Thermo-Structural Behavior of UnderfilledFlip-Chips,” Proceedings of IEEE Electronic Components and Technology Conference,pp. 440–447, Orlando, FL, May 1996.

81. Wu, T. Y., Y. Tsukada, W. T. Chen, “Materials and Mechanics Issues in Flip-Chip OrganicPackaging,” Proceedings of IEEE Electronic Components and Technology Conference,pp. 524–534, Orlando, FL, May 1996.

82. Doot, R. K., “Motorola’s First DCA Product: The Gold Line Pen Pager,” Proceedings ofIEEE Electronic Components and Technology Conference, pp. 535–539, Orlando, FL, May1996.

83. Greer, S. T., “An Extended Eutectic Solder Bump for FCOB,” Proceedings of IEEE Elec-tronic Components and Technology Conference, pp. 546–551, Orlando, FL, May 1996.

84. Peterson, D. W., J. S. Sweet, S. N. Burchett, and A. Hsia, “Stresses From Flip-Chip Assem-bly and Underfill: Measurements with the ATC4.1 Assembly Test Chip and Analysis byFinite Element Method,” Proceedings of IEEE Electronic Components and TechnologyConference, pp. 134–143, San Jose, CA, May 1997.

3.24 CHAPTER THREE

85. Zhou, T., M. Hundt, C. Villa, R. Bond, and T. Lao, “Thermal Study for Flip Chip on FR-4Boards,” Proceedings of IEEE Electronic Components and Technology Conference, pp.879–884, San Jose, CA, May 1997.

86. Ni, G., M. H. Gordon, W. F. Schmidt, and R. P. Selvam, “Flow Properties of Liquid Under-fill Encapsulations and Underfill Process Considerations,” Proceedings of IEEE Elec-tronic Components and Technology Conference, pp. 101–108, Seattle, WA, May 1998.

87. Hoang, L., A. Murphy, and K. Desai, “Methodology for Screening High PerformanceUnderfill Materials,” Proceedings of IEEE Electronic Components and Technology Con-ference, pp. 111–116, Seattle, WA, May 1998.

88. Dai, X., M. V. Brillhart, and P. S. Ho, “Polymer Interfacial Adhesion in MicroelectronicAssemblies,” Proceedings of IEEE Electronic Components and Technology Conference,pp. 132–137, Seattle, WA, May 1998.

89. Zhao, J. X. Dai, and P. Ho, “Analysis and Modeling Verification for Thermal-MechanicalDeformation in Flip-Chip Packages,” Proceedings of IEEE Electronic Components andTechnology Conference, pp. 336–344, Seattle, WA, May 1998.

90. Matsushima, H., S. Baba, and Y. Tomita, “Thermally Enhanced Flip-Chip BGA withOrganic Substrate,” Proceedings of IEEE Electronic Components and Technology Con-ference, pp. 685–691, Seattle, WA, May 1998.

91. Gurumurthy, C., L. G. Norris, C. Hui, and E. Kramer, “Characterization of Underfill/Pas-sivation Interfacial Adhesion for Direct Chip Attach Assemblies Using Fracture Tough-ness and Hydro-Thermal Fatigue Measurements,” Proceedings of IEEE ElectronicComponents and Technology Conference, pp. 721–728, Seattle, WA, May 1998.

92. Palaniappan, P., P. Selman, D. Baldwin, J. Wu, and C. P. Wong, “Correlation of Flip ChipUnderfill Process Parameters and Material Properties with In-Process Stress Genera-tion,” Proceedings of IEEE Electronic Components and Technology Conference, pp.838–847, Seattle, WA, May 1998.

93. Qu, J., and C. P. Wong, “Effective Elastic Modulus of Underfill Material for Flip-ChipApplications,” Proceedings of IEEE Electronic Components and Technology Conference,pp. 848–850, Seattle, WA, May 1998.

94. Sylvester, M., D. Banks, R. Kern, and R. Pofahl, “Thermomechanical Reliability Assess-ment of Large Organic Flip-Chip Ball Grid Array Packages,” Proceedings of IEEE Elec-tronic Components and Technology Conference, pp. 851–860, Seattle, WA, May 1998.

95. Wiegele, S., P. Thompson, R. Lee, and E. Ramsland, “Reliability and Process Characteri-zation of Electroless Nickel-Gold/Solder Flip Chip Interconnect,” Proceedings of IEEEElectronic Components and Technology Conference, pp. 861–866, Seattle, WA, May 1998.

96. Caers, J., R. Oesterholt, R. Bressers, T. Mouthaan, J. Verweij, “Reliability of Flip Chip onBoard, First Order Model for the Effect on Contact Integrity of Moisture Penetration inthe Underfill,” Proceedings of IEEE Electronic Components and Technology Conference,pp. 867–871, Seattle, WA, May 1998.

97. Roesner, B., X. Baraton, K. Guttmann, and C. Samin, “Thermal Fatigue of Solder Flip-Chip Assemblies,” Proceedings of IEEE Electronic Components and Technology Confer-ence, pp. 872–877, Seattle, WA, May 1998.

98. Pang, J., T. Tan, and S. Sitaraman, “Thermo-Mechanical Analysis of Solder Joint Fatigueand Creep in a Flip Chip On Board Package Subjected to Temperature Cycling Loading,”Proceedings of IEEE Electronic Components and Technology Conference, pp. 878–883,Seattle, WA, May 1998.

99. Gopalakrishnan, L., M. Ranjan, Y. Sha, K. Srihari, and C. Woychik, “Encapsulant Materi-als for Flip-Chip Attach,” Proceedings of IEEE Electronic Components and TechnologyConference, pp. 1291–1297, Seattle, WA, May 1998.

100. Yang, H., S. Bayyuk,A. Krishnan,A. Przekwas, L. Nguyen, and P. Fine,“Computional Sim-ulation of Underfill Encapsulation of Flip-Chip ICs, Part I: Flow Modeling and Surface-Tension Effects,” Proceedings of IEEE Electronic Components and TechnologyConference, pp. 1311–1317, Seattle, WA, May 1998.


101. Liu, S., J.Wang, D. Zou, X. He, Z. Qian, and Y. Guo,“Resolving Displacement Field of Sol-der Ball in Flip-Chip Package by Both Phase Shifting Moire Interferometry and FEMModeling,” Proceedings of IEEE Electronic Components and Technology Conference, pp.1345–1353, Seattle, WA, May 1998.

102. Hong, B., and T. Yuan, “Integrated Flow—Thermomechanical and Reliability Analysis ofa Low Air Cooled Flip Chip-PBGA Package,” Proceedings of IEEE Electronic Compo-nents and Technology Conference, pp. 1354–1360, Seattle, WA, May 1998.

103. Wang, J., Z. Qian, D. Zou, and S. Liu, “Creep Behavior of a Flip-Chip Pacakge by BothFEM Modeling and Real Time Moire Interferometry,” Proceedings of IEEE ElectronicComponents and Technology Conference, pp. 1439–1445, Seattle, WA, May 1998.

104. Lau, J., C. Chang, C. Chen, R. Lee, T. Chen, D. Cheng, T. Tseng, and D. Lin, “Via-In-Pad(VIP) Substrates for Solder Bumped Flip Chip Applications,” Proceedings of SurfaceMount International Conference, pp. 128–136, September 1999.

105. Lau, J. H., “Critical Issues of WLCSP with Emphasis on Cost Analysis and Solder JointReliability,” IEEE Transactions on Electronics Packaging Manufacturing, 25(1):42–50,January 2002.

106. Lau, J. H., T. Chung, R. Lee, C. Chang, and C. Chen, “A Novel and Reliable Wafer-LevelChip Scale Package (WLCSP),” Proceedings of the Chip Scale International Conference,pp. H1–8, September 1999.

107. Lau, J. H., R. Lee, C. Chang, and C. Chen, “Solder Joint Reliability of Wafer Level ChipScale Packages (WLCSP): A Time-Temperature-Dependent Creep Analysis,” ASMEPaper No. 99-IMECE/EEP-5, International Mechanical Engineering Congress and Expo-sition, November 1999.

108. Lau, J. H., and R. Lee, “Effects of Printed Circuit Board Thickness on Solder Joint Reliabil-ity of Flip Chip Assemblies with Imperfect Underfill,”ASME Paper No. 99-IMECE/EEP-4,International Mechanical Engineering Congress and Exposition, November 1999.

109. Lau, J. H., C. Chang, and R. Lee, “Failure Analysis of Solder Bumped Flip Chip on Low-Cost Substrate,” Proceedings of the International Electronic Manufacturing TechnologySymposium, pp. 457–472, October 1999.

110. Lau, J. H., C. Chang, and C. Chen, “Characteristics and Reliability of No-Flow Underfillsfor Solder Bumped Flip Chips on Low Cost Substrates,” Proceedings of the InternationalSymposium on Microelectronics, pp. 592–598, October 1999.

111. Lau, J. H., and R. Lee,“Modeling and Analysis of 96.5Sn-3.5Ag Lead-Free Solder Joint ofWLCSP on Buildup Microvia Printed Circuit Board,” IEEE Transactions on ElectronicsPackaging Manufacturing, 25(1):51–58, January 2002.

112. Hashino, E., K. Shimokawa, Y. Yamamoto, and K. Tatsumi, “Micro-Ball Wafer Bumpingfor Flip Chip Interconnection,” IEEE Proceedings of Electronic Components and Tech-nology Conference, pp. 957–964, May 2001.

113. Jeon,Y., K. Paik, K. Bok,W. Choi, and C. Cho,“Studies on Ni-Sn Intermetallic Compoundand P-Rich Nie Layer at the Electroless Ni UBM—Solder Interface and Their Effects onFlip Chip Solder Joint Reliability,” IEEE Proceedings of Electronic Components andTechnology Conference, pp. 69–75, May 2001.

114. Kazama, A., T. Satoh, Y. Yamaguchi, I. Anjoh, A. Nishimura, “Development of Low-Costand Highly Reliable Wafer Process Package,” IEEE Proceedings of Electronic Compo-nents and Technology Conference, pp. 40–46, May 2001.

115. Zhang, C., J. Lin, and Li Li, “Thermal Fatigue Properties of Lead-free Solders on Cu andNiP Under Bump Metallurgies,” IEEE Proceedings of Electronic Components and Tech-nology Conference, pp. 463–470, May 2001.

116. Lin, J., A. De Silva, D. Frear, and Y. Guo, “Characterization of Lead-Free Solders andUnder Bump Metallurgies for Flip-Chip Package,” IEEE Proceedings of Electronic Com-ponents and Technology Conference, pp. 455–462, May 2001.

117. Balkan, H., D. Patterson, G. Burgess, C. Carlson, P. Elenius, M. Johnson, B. Rooney, J.Sanchez, D. Stepniak, and J. Wood, “Flip-Chip Reliability: Comparative Characterizationof Lead Free (Sn/Ag/Cu) and 63Sn/Pb Eutectic Solder,” IMAPs Flip Chip Workshop,October 2001.

3.26 CHAPTER THREE

CHAPTER 4CHIP (WAFER)-LEVEL

INTERCONNECTS WITHSOLDERLESS BUMPS

4.1 INTRODUCTION

The lead-free solder bumps discussed in Chaps. 2 and 3 are just one of many differ-ent kinds of chip (wafer)-level interconnects.1–19 In this chapter, solderless chip(wafer)-level interconnects such as electroless Ni-P-immersion Au bumps, electro-plated gold bumps, electroplated copper bumps, electroplated copper wires, wire-bonding gold wires (microsprings), wire-bonding gold stud bumps, and wire-bondingcopper stud bumps will be discussed.

4.2 WAFERS FOR ELECTROLESS Ni-Au, ELECTROPLATED Au,

AND ELECTROPLATED Cu BUMPS

In this section the wafer size for Ni-Au bumps, electroplated Au bumps, and electro-plated Cu bumps is 6 in. The chip is 0.5 in (12.7 mm) square and 25 mil (0.64 mm)thick. The street width between all the chips is 6 mil (0.15 mm). The chip has 8-mil(0.2-mm) square pads and 14-mil (0.36-mm) pitch.All of the pads are arranged sym-metrically around the perimeter of the chip and are interconnected via traces on thechip in an alternating pattern so as to provide daisy-chained connections when thechip is attached to the FR-4 printed circuit board (PCB).

The silicon wafer consists of a patterned aluminum layer on a layer of silicondioxide that is covered with a patterned silicon nitride passivation layer. The waferfabrication process flow starts with a 0.25-µm layer of silicon dioxide deposited byplasma enhanced chemical vapor deposition on a <111> silicon substrate. For themetal layer, a 0.85-µm layer of Al-1%Si-0.1%Ti alloy is sputtered over the silicondioxide.The metal pattern is then defined by coating with positive resist, exposed byprojection alignment, developed, and wet–chemical etched. After the photoresist isremoved by plasma stripping, the metal pattern is sintered at 450°C to remove filmstresses. For the passivation layer, a 0.75-µm layer of silicon nitride is deposited overthe entire surface of the wafer by plasma enhanced chemical vapor deposition. Thepad opening in the passivation layer is then defined by coating with positive resist,exposed by projection alignment, developed, and plasma-etched. Finally, the resist isremoved by plasma stripping, leaving the silicon nitride passivation layer to overlapthe perimeter of the Al pads by 10 µm.

4.3 ELECTROLESS Ni-P-IMMERSION Au BUMPS

One of the critical differences between electroless Ni-P-immersion Au (for short,Ni-Au)under-bumpmetallurgies (UBMs;Chap.2)andNi-Aubumps is theNi thickness.

4.1


Usually, the thickness of Ni-Au UBMs is 5 µm (but can be as high as 12 µm or as low as 3µm) and that of Ni-Au bumps is 20 µm (but can be as high as 25 µm or as low as 15 µm).

The advantages of the electroless Ni-Au UBMs are: (1) its costs are low; (2) it iscompatible with eutectic solders; (3) it has high solder wetability; (4) Al-Ni adhesion ismore than adequate for surface-mount technology applications; and (5) Ni-Sn inter-metallic growth is adequate for most of the solder joint thermal fatigue lives. However,the quality and uniformity of electroless Ni-Au bumps depend on the careful moni-toring of each process step and the tight contamination control (every 2 h) of the zin-cate, Ni, and Au solution tanks. The Al-Ni adhesion and electrical resistance at theinterface are strongly affected by the cleaning and activation processes.5 The advan-tages of electroless Ni-Au bumps are: (1) their costs are low and (2) they are suitablefor conductive adhesive material.

4.3.1 MATERIALS AND PROCESS

The materials and process flow of Ni-Au UBMs13 discussed in Chap. 2 can be used tofabricate the Ni-Au bumps except with much longer process time. However, in thissection, the process developed by PICOPAK in 1995 is presented as shown in thefollowing list (for a bump with 24-µm height and 80-gf shear force).

1. Visual inspection of wafer2. Test runs with diced wafer samples to find optimal process conditions3. Application of photoresist to cover possible ink dots, undesired openings in dic-

ing lanes, etc., using a standard photolithography process4. Application of photoresist on back side of wafer5. Plasma cleaning of exposed surfaces (<5 min at very low wafer temperature)6. First zincate (2 min at room temperature), rinse in deionized water7. Zinc strip (1 min at room temperature), rinse in deionized water8. Second zincate (2 min at room temperature), rinse in deionized water9. Nickel plating (1 h at <100°C), rinse in deionized water

10. Visual inspection of wafer11. Bump height measurements12. Immersion in gold (15 min at <100°C), rinse in deionized water13. Removal of photoresist in hot (100°C) acidic solution14. Visual inspection of wafer15. Bump shear test16. Bump wetting test (not necessary for anisotropic conductive film application)17. Documentation and shipping of goods

Figure 4.1 shows a cross section of the electroless Ni-Au bump on the 6-in Siwafer. It should be noted that for such a large Ni-Au bump (24-µm height), the Nicreates a large amount of stress that could crack the passivation.

4.3.2 PASSIVATION CRACKING

Figures 4.2 through 4.5 show some examples of passivation cracking of Ni-Aubumps. It can be seen that the cracks occur at the corner of the passivation due to

4.2 CHAPTER FOUR

FIGURE 4.1 A 24-µm electroless Ni-P-immersion Au bump.

FIGURE 4.2 Passivation crack in electroless Ni-P-immersion Au bump.

4.3



4.4


4.5

FIGURE 4.6 A perfect Ni-Au bump.

stress concentration of the passivation geometry stemming from the long durationof Ni plating. Thus, great care in monitoring of all process steps and tight contami-nation control of the Ni, Zn, and Au solution tanks must be taken in order to obtainthe Ni-Au bump shown in Figs. 4.1 and 4.6.

4.4 ELECTROPLATED Au BUMPS

Since their invention by Triggs and Byrns12 in 1971, gold bumps have been usedextensively for many applications, especially with tape automated bonding technol-ogy.14 Gold bumps usually contain two regions: the thin film adhesion layers to alu-minum metallization (UBM) and the main body of the Au bump.As shown in Chap.2, Table 2.1, there are many UBMs for Au bumps. It can be seen that the thin filmstructure consists of three layers: (1) an adhesion layer of Ti or Cr a few hundredangstroms thick; (2) a diffusion barrier of Cu, Pd, W, or Pt about 10,000 angstromsthick; and (3) the top capping layer, commonly Au, a few thousand angstroms thick.This capping layer provides an easy surface for plating the Au bump. Typical bumpheights range from 15 to 25 µm.


The 6-in wafer discussed in Sec.4.2 is Au-bumped by the electroplating process asshown in Fig. 4.7. The UBM of the wafers is titanium (Ti) and tungsten (W) sput-tered on the entire surface of the wafer: 0.1 to 0.2 µm of Ti first, followed by 0.3 to0.5 µm of W. A 20-µm layer of resist is then overlaid on the Ti-W and a bump maskis used to define the bump pattern. The openings in the resist are 7 to 10 µm widerthan the pad openings in the passivation layer. A 20-µm-thick layer of Au is thenplated over the Ti-W. The resist is then removed and the Ti-W is stripped off with ahydrogen peroxide etch. The final process is annealing of the bumps at an elevatedtemperature (such as 300°C) to obtain the desired hardness. The microhardness of awell-annealed gold bump is in the range of 50 to 60 on the Vickers scale, while thatof an as-deposited bump is about 120. Figure 4.8 shows a cross section of the elec-troplated Au bump on the 6-in Si wafer.

4.4.2 BUMP SPECIFICATIONS AND MEASUREMENT METHODS

For an Au bump with a height of 25 µm, the specification is ±3 µm. The bump heightuniformity should be ±1 µm within a chip and ±2 µm within a wafer.The sample sizesshould be five bumps per chip, five chips per wafer, and five wafers per lot.The shearforce should be 5.5 gf/mil2. Sample sizes for shearing test (6 µm from the chip pads)should be five bumps per chip, five chips per wafer, and three wafers per lot. Thehardness should be larger than 90 Knoop prior to annealing and 35 to 75 Knoopafter annealing. Sample sizes for hardness test should be five bumps per chip, fivechips per wafer, and two wafers per lot.

4.6 CHAPTER FOUR

FIGURE 4.7 Electroplated Au wafer-bumping process flow.

4.7

4.5 ELECTROPLATED Cu BUMPS

Because gold is very expensive and copper is much cheaper, copper has been con-sidered as an alternative to gold. In this section, the materials, process, and some spe-cial considerations about copper bumps are discussed.


The wafer-bumping process with Cu is almost the same as that with Au except forthe UBM, which is Ti (0.1 to 0.2 µm) and Cu (0.5 to 0.8 µm). On top of the UBM, theplated Cu thickness is 20 µm. The average microhardness of the as-deposited Cubumps is about 100 on the Vickers scale. Since the copper surface oxidizes and cor-rodes very easily, the Cu bumps are immersed with a very thin layer of Sn. Figure 4.9shows a cross section of the electroplated Cu bump on the 6-in Si wafer.

4.5.2 SPECIAL CONSIDERATIONS

In general, Cu bumps are cheaper than Au bumps. However, difficulty in bondingand additional process steps for the protective layer have prevented copper bumpsfrom gaining widespread use in tape automated bonding or chip on board technolo-gies. A high-purity copper bath (preferably copper sulfate solution) is required to

4.8 CHAPTER FOUR

FIGURE 4.8 A 20-µm Au bump.

produce soft copper bumps. Furthermore, with the aluminum conductor pads on thesilicon chip, the adhesion to the aluminum bond pads is a critical factor in determin-ing the fabrication yield and reliability of copper bumps.11 On the other hand, withthe increasing usage of copper conductor pads on the silicon chips nowadays, copperbumps may gain popularity in the near future.

4.6 ELECTROPLATED COPPER WIRES

Wire interconnect technology (WIT), which was invented by FCPT in 1994,15 pro-vides new opportunities for the integrated circuit designer unavailable from anyother chip-level interconnect method. In this section, WIT will be briefly discussed.

4.6.1 STRUCTURE

Structurally, WIT consists of a (copper) metal post (wire) approximately 10 µm indiameter and 50 µm long, as shown in Fig. 4.10. The free end of the copper wire canbe attached to a 25-µm-diameter copper pad with lead-free solder or conductiveadhesive on the substrate. The pad pitch can be as small as 50 µm. The copper wirecan be grown on either the silicon chip surface or the substrate. Due to the extremelyhigh compliance of the copper wire interconnect, the solder (or adhesive) joint isvery reliable under thermal cycling conditions.

CHIP (WAFER)-LEVEL INTERCONNECTS WITH SOLDERLESS BUMPS 4.9

FIGURE 4.9 A 20-µm Cu bump.

4.6.2 FABRICATION MATERIALS AND PROCESS

WIT is electroplated with copper on either the silicon chip or the substrate. High-aspect-ratio vias in a thick photoresist layer are patterned followed by a fine-grainplating process. The resulting copper wire has nearly the same properties as an-nealed pure copper. To allow multiple replacement of WIT-attached chips, a thinlayer of nickel is coated over the copper wires. The nickel acts as a diffusion barrierbetween the lead-free solder and copper materials, permitting multiple solderattachment and replacement cycles without degradation of the solder joints.

Figure 4.10 shows a scanning electron microscope photo of the copper wiresgrown on pads with 30-µm pitch. It can be seen that these particular copper wires areapproximately 47 µm tall, 10 µm in diameter on the base, and 20 µm in diameter onthe free end.To ensure proper assembly and high manufacturing yield,WIT must befabricated to within very tight height tolerances (for example, a 45-µm-tall WITshould be within ±2.5 percent over an area of 20 mm2 and the height uniformityacross a 6-in wafer should be ±5 percent.)

4.7 WIRE-BONDING MICROSPRINGS

The microspring developed by FormFactor is another chip (wafer)-level intercon-nect. Because of its special S shape, microspring could be even more compliant thanWIT. In this section, the material and process of FormFactor’s microspring arebriefly discussed.16

4.10 CHAPTER FOUR

FIGURE 4.10 FCPT’s electroplated copper wires.


The microspring contact is made of gold wire plated with nickel alloy and formed intoan S shape on a silicon wafer by a wire bonder as shown in Figs. 4.11 and 4.12. Form-

Factor called the technology microspringcontact on silicon technology (MOST). Itemploys a simple approach using conven-tional wire-bonding tools to provide thefoundation for the microspring contacts.Since there is no leadframe, no die attach,no molding, no UBM, and no bump, micro-spring technology has a significantly lowercost than the conventional chip (wafer)-level interconnect. Themicrosprings can beattached to the PCB or substrate with eitherlead-free solders or conductive adhesives.

4.7.2 SPECIAL CONSIDERATIONS

It should be emphasized that, due to thecompliance of the microspring contacts,unlike the solder-bumped flip chip wafer-level chip-scale package (WLCSP) on PCBsor organic substrates, underfill is not neededwith microsprings. Also, with FormFactor’swafer on wafer (WOW) process, WLCSPwith microspring contacts as shown in Fig.4.12 can be tested at high speed and burnedin at elevated temperatures. One concernabout this technology could be that there istoo much gold for the solder interconnect tolower its ductility.


FIGURE 4.11 FormFactor’s S-shaped mi-crospring.

FIGURE 4.12 FormFactor’s WLCSP with microsprings.

4.8 WIRE-BONDING Au STUD BUMPS

Gold stud bump bonding (SBB) technology was developed by Fujitsu, Matsushita,and others in Japan in the early 1990s17–19 and is now used in several variants in Japanand the European Union. It uses an adhesive to (electrically, mechanically, or both)glue the gold stud-bumped chip to the PCB or substrate.

Recently, this technology has been attracting a good deal of attention. This isdue to: (1) many critical issues such as cost and uncertainty caused by lead-freesoldering; (2) the availability of specially made high-throughput (∼15 bumps persecond) automatic production wire bonders, e.g., Panasert and Kulicke & Soffa;and (3) the fact that SBB technology is already in production. The chip (wafer)-level interconnect of the SBB technology is the Au stud bump, which is discussedin this section.


The Au stud bumps are formed on the Al bond pads on the chip or wafer by a mod-ified wire bonder. The wire is made of Au-1wt%Pd. The processes are illustrated inFig. 4.13. It can be seen that: (1) as in conventional wire bonding, a gold wire isbonded to the chip bond pads by either thermocompression or ultrasonic energy orboth, and (2) the capillary tube is withdrawn to form a loop path and breaks the wireon the top of the Au ball bond. After the Au stud bumping, a mechanical leveling(coining) for the coplanarity (within ±5 µm) of stud bumps is performed as shown inFig. 4.14. The leveling is usually executed by pressing the chip or wafer against anunyielding flat surface at about 50 gf per bump. Figure 4.15 shows a few differentstud bumps created by Kulicke & Soffa’s specially made wire bonder.

4.12 CHAPTER FOUR

FIGURE 4.13 Process flow for Au stud bumps.


FIGURE 4.14 Process flow for Au stud bumps on a wafer level.

FIGURE 4.15 Au stud bump samples by Kulicke & Soffa.

4.8.2 EQUIPMENT

As mentioned earlier, one of the reasons the SBB technology has attracted so muchattention recently is because of the availability of fully automatic equipment.Today,at least two companies—Panasert and Kulicke & Soffa—are providing this equip-ment.

Figure 4.16 shows Panasert’s STBS and Fig. 4.17 shows its operation sequence. Itcan be seen that this machine operates on chips in a waffle tray, and that the levelingand monitoring are optional. If the leveling option is included, then it will occursomewhere near the microscope and the heating stage (Fig. 4.16). Usually, Panasertuses its flip chip bonder (FCBII) to perform leveling, as shown in Chap. 5.

Figures 4.18 and 4.19 show Panasert’s STBW(1) and STBW(2), respectively. Theoperation sequence of these machines is shown in Fig. 4.20. It can be seen that (1) these machines operate on chips on a wafer; (2) STBW(1) is for wafers in a sin-

4.14 CHAPTER FOUR

FIGURE 4.16 Au stud bump bonder (STBS) by Panasert.

FIGURE 4.17 Operation sequence of STBS.

4.1

5

gle magazine; (3) STBW(2) is for wafers in two magazines; and (4) leveling is not anoption.

Figure 4.21 shows Panasert’s STBW2, and its operation sequence is shown in Fig.4.22. It can be seen that the biggest difference between STBW2 and STBW(2) isthere are three working stages for STBW2: one preheating unit, one bonding unit,and one postheating unit. Figure 4.23 shows a Au stud bump after leveling made byPanasert’s machines.

4.16 CHAPTER FOUR

FIGURE 4.18 Au stud bump bonder STBW(1) by Panasert.

Figure 4.24 shows Kulicke & Soffa’s dual bonder workcell (stud bump bonderson two sides of the wafer-handling unit).This unit can produce twice the throughputin only 1.6 times the space and saves 16 percent of the cost of their WaferPro studbump bonder with only one bonder.

4.9 WIRE-BONDING Cu STUD BUMPS

In general, application of Au stud bumps is performed with adhesives and not withsolders. This is due to the amount of Au in the stud bump, which will reduce the sol-der joint’s ductility. However, unlike solders (which self-align during reflow), adhe-sives require very high placement accuracy. Thus, in order to keep one of the unique


FIGURE 4.19 Au stud bump bonder STBW(2) by Panasert.

FIGURE 4.20 Operation sequence of STBW.

FIGURE 4.21 Au stud bump bonder STBW2 by Panasert.

4.18

FIGURE 4.22 Operation sequence of STBW2.

4.1

9

FIGURE 4.23 Leveled Au stud bump by Panasert.

FIGURE 4.24 Kulicke & Soffa dual bonder workcell (two bonders and one wafer-handling unit).

4.20

advantages of solders, Cu stud bumps are introduced.Also, with the increasing inter-est in copper conductor pads, Cu stud bumps could be very popular in the future. Inthis section, Furukawa’s Cu stud bumps are briefly discussed.19


Furukawa’s Cu wire is 25 µm in diameter and is made of 99.99 percent pure copper.In order to prevent the Cu bump from oxidizing, the Cu wire is liquefied in a reduc-tion atmosphere consisting of 5 percent H2 in N2 forming a ball 65 µm in diameter.The reduction gas is blown over the end of the copper wire at 1 l/min.

Figures 4.25 and 4.26 show, respectively, the Cu stud-bumping process flow andthe Cu ball. It can be seen that the Cu wire is bumped using ultrasonic power. Unlike

Au wires, Cu wires are not very malleable andcould contribute to chip pad cracking withrapid ultrasonic power. Thus, ultrasonic rampcontrol is very important for Cu stud bumpingto be successful.

Figure 4.27 and Table 4.1 show the Cu stud-bumping test results with various ultrasonicramp rates at 250°C and under controlledforces. (The initial ultrasonic voltage is mea-sured by an oscilloscope.) It can be seen that(1) the higher the ramp rates, the more the chippad cracks; and (2) at 129 V/s, there are no chippad cracks. Thus, the Cu stud bumps can bemade without cracking the chip pads if aproper ultrasonic ramp rate is used. Just aswith Au stud bumps, after the Cu stud bumpingon chips, the Cu studs are leveled to produceuniform bump heights. Figure 4.28 shows a Austud bump (a) and a Cu stud bump (b) afterleveling.


FIGURE 4.25 Cu stud bumping.

FIGURE 4.26 Cu stud copper ball.

4.22 CHAPTER FOUR

FIGURE 4.27 Initial voltage of ultrasonic power control.

FIGURE 4.28 Scanning electron microscope images of (a) Au stud and (b) Cu studbumps after leveling.

4.9.2 SHEAR STRENGTH

The shear strength (force) of Cu stud bumps, along with that of Au stud bumps, isshown in Fig. 4.29. It can be seen that the average shear strength (69 gf) of the Custud bumps is larger than that (58 gf) of the Au stud bumps. However, the fracturesurface is quite different for these two stud bumps. For the Au stud bumps, the frac-ture surface is at the bulk Au bump. However, for the Cu stud bumps, the fracturesurface is at the interface between the stud bump and the chip pad.This could be dueto the difference in hardness between these two materials; Au is softer than Cu.


TABLE 4.1 Under-Pad Crack Occurrence at VariousRamp Rates

Ramp rate Shear strength Number of cracks

No ramp 64.0 gf 8

194 V/s 83.4 gf 5

159 V/s 75.9 gf 2

129 V/s 68.3 gf 0

FIGURE 4.29 Typical shear test force of Au and Cu stud bumps.

ACKNOWLEDGMENTS

The authors would like to thank the friendly people of PICOPAK, Panasonic, andKulicke & Soffa; L. Moresco, D. Love, W. Chou, D. Horine, C. Wong, S. Beilin, and V.Holalkere of Fujitsu Computer Packaging Technologies; J. Healy of FormFactor; S.Zama, T. Hikami, and H. Murata of Furukawa; and D. Baldwin of the Georgia Insti-tute of Technology for providing very useful information to the industry.

REFERENCES

1. Tummala, R. R., Fundamentals of Microsystems Packaging, McGraw-Hill, New York, 2001.


3. Tummala, R. R., and E. Rymaszewski, Microelectronics Packaging Handbook, Van Nos-trand Reinhold, New York, 1989.

4. Lau, J. H., and S.W.R. Lee, Microvias for Low Cost High Density Interconnects, McGraw-Hill, New York, 2001.

5. Lau, J. H., Low Cost Flip Chip Technologies for DCA, WLCSP, and PBGA Assemblies,McGraw-Hill, New York, 2000.

6. Lau, J. H., and S.W.R. Lee, Chip Scale Package, Design, Materials, Process, Reliability, andApplications, McGraw-Hill, New York, 1999.





11. Lau, J. H., Chip On Board Technologies for Multichip Modules, Van Nostrand Reinhold,New York, NY, 1994.

12. Triggs, W., and C. Byrns Jr., U.S. Patent No. 3,599,060, 10 August, 1971.

13. Jeon, Y., K. Paik, K. Bok, W. Choi, and C. Cho, “Studies on Ni-Sn Intermetallic Compoundand P-rich Ni Layer at the Electroless Nickel UBM-Solder Interface and Their Effects onFlip Chip Solder Joint Reliability,” IEEE Proceedings of Electronic Components and Tech-nology Conference, pp. 69–75, May 2001.

14. Lau, J. H., Handbook of Tape Automated Bonding, Van Nostrand Reinhold, New York, 1992.

15. Moresco, L., D. Love, W. Chou, and V. Holalkere, “Wire Interconnect Technology: AnUltra-High-Density Flip Chip–Substrate Connection Method,” in Flip Chip Technology,Lau, J. H., ed., McGraw-Hill, New York, pp. 367–386, 1996.

16. Healy, J., “The Impact of Microsprings on Wafer Level Packaging of ICs,” Proceedings ofthe HDI EXPO, pp. 17–36, August 1999.

17. Tsunoi, K., T. Kusagaya, and H. Kira, “Flip Chip Mounting Using Stud Bumps and Adhe-sive for Encapsulation,” in Flip Chip Technology, Lau, J. H., ed., McGraw-Hill, New York,pp. 357–366, 1996.

18. Zakel, E., and H. Reichl, “Flip Chip Assembly Using Gold, Gold-Tin, and Nickel-GoldMetallurgy,” in Flip Chip Technology, Lau, J. H., ed., McGraw-Hill, New York, pp. 415–490,1996.

19. Zama, S., D. Baldwin, T. Hikami, and H. Murata, “Flip Chip Interconnect Systems UsingWire Stud Bumps and Lead Free Solder,” IEEE Proceedings of Electronic Componentsand Technology Conference, May 2000.

4.24 CHAPTER FOUR

CHAPTER 5WLCSPs WITH SOLDERLESSBUMPS ON PCB/SUBSTRATE

5.1 INTRODUCTION

Based on a full life-cycle analysis,1, 2 it is unclear whether lead-free solders are actu-ally more environmentally friendly. If materials and component availability, reliabil-ity uncertainty, increased processing difficulties, and end-of-life (EOL) issues areaccounted for, lead-free solders may not be a better choice. Ultimately, the best solu-tions may be completely new attachment technologies that do not use solder (i.e.,solderless, as discussed in Chap. 4). For example,1–46 flip chips with Cu stud bumps,Au stud bumps,Au bumps, Cu bumps, Ni-Au bumps, Cu wires, or Au wires on printedcircuit board (PCB)/substrate with isotropic conductive adhesive (ICA), anisotropicconductive paste (ACP), anisotropic conductive film (ACF), or nonconductiveadhesive (NCA). In this chapter, various of these technologies are discussed briefly.The novel development of these materials will be discussed in Chaps. 17 through 20.

5.2 DESIGN, MATERIALS, PROCESS, AND RELIABILITY OF WLCSPs

WITH Au BUMPS, Cu BUMPS, AND Ni-Au BUMPS ON PCB WITH ACF

In this section, direct chip attach (DCA) with solderless flip chip on board (FCOB)with ACF will be considered.4 The chip is bumped with three different metallugies(Ni-Au, Au, and Cu), as discussed in Chap. 4, Secs. 4.3 through 4.5, respectively. TheCu pads on the FR-4 epoxy PCB are with electroless Ni-immersion Au and areorganic-coated. Hitachi’s ACF is used for this study. The design, materials, andassembly process flow are shown in Fig. 5.1. In the following sections, some of themajor steps will be discussed. Also, some thermal cycling and surface insulationresistance (SIR) test results are presented.

5.2.1 PCB

A matching PCB is designed along with the chip discussed in Chap. 4, Sec. 4.2. TheCu pads are round (8-mil, or 0.2-mm, diameter) and in a 14-mil (0.36-mm) pitch. Inthis study, two Cu-pad finishings, electroless Ni-Au and organic-coated, are consid-ered (Fig. 5.2). Most of the pads are interconnected via traces on the PCB in an alter-nating pattern so as to provide daisy-chained connections with the chip.

5.2.2 ACF

The ACF used for this study is Hitachi Chemical’s double-layer ACF (Fig. 5.3). Itconsists of nonfilled thermal setting adhesive and Ni- (2 to approximately 5 µm)

5.1


conducting particle-filled thermal setting adhesive layers. Each layer is about 30 µmthick. Hitachi Chemical has shown that there are more conductive particles betweenthe chip bumps and the substrate pads if the conducting particle-filled thermal set-ting adhesive layer in the ACF faces toward the pad of the substrate (Fig. 5.4). Forthis study, the ACF is sandwiched by two layers of release paper.

5.2 CHAPTER FIVE

FIGURE 5.1 Flip chip on board/substrate with ACP/ACF process flow.

FIGURE 5.2 Au-bumped, Cu-bumped, and Ni-Au-bumped flip chip on aPCB with either Cu-Ni-Au or OCC finishes.

It should be pointed out that, unlike the underfills for the conventional solder-bumped flip chip on PCBs or substrates applications,28,34 the thermal setting adhesiveof the ACF doesn’t consist of any filler [which leads to very high thermal coefficientof expansion (TCE)] and has many voids in the bonded assembly. Since the compli-ance between the chip bumps and the pads on the PCB or substrate is very small,thermal fatigue reliability could be an issue, especially for telecommunication prod-ucts, which require 20 years of life. Thus, to lower the TCE, reduce the voids, andincrease the adhesion strength of the thermal setting adhesive of the ACF/ACP, someamount of nonconductive fillers should be incorporated into the ACF/ACP.

WLCSPS WITH SOLDERLESS BUMPS ON PCB/SUBSTRATE 5.3

FIGURE 5.3 Hitachi’s double-layer ACP.

FIGURE 5.4 Effect of Hitachi’s ACF arrangements on the number of conductive particles per bump.

5.2.3 FCOB ASSEMBLIES WITH ACF

More than 100 chips have been bonded on the Cu-Ni-Au and organic-coated copper(OCC) FR-4 PCB. These chips have three different kinds of bumps:

1. 20-µm Cu bumps2. 20-µm Au bumps3. 24-µm Ni-Au bumps

The assembly process of the ACF is verysimple and clean (fluxless). First of all,cut the ACF to a little larger than thesize of the chip and remove one of the release papers. Place the ACF on theFR-4 PCB with the other release paperfacing upward. This is called tacking.

The next step is to prepress the ACFunder a condition of 80°C and 5 s. This iscalled lamination. In the next step, alookup camera is used to read in some ofthe bumps of the chip. Remove therelease paper.Then a lookdown camera isused to read in the corresponding padlocations of the PCB. After the necessary

adjustment, the chip is placed on top of the ACF on the PCB. This is called pick andplace and is done on Hitachi’s aligning machine. Finally, the chip on board assembly istransported to the Hitachi bonder to do the bonding at a condition of 180°C,5 kg/mm2, and 20 s. Figure 5.5 shows a typical flip chip on PCB with ACF assembly. Atypical cross section of the assembly is shown in Fig. 5.6.There are many voids.

5.4 CHAPTER FIVE

FIGURE 5.5 Top view of a flip chip with anACF on a PCB.

FIGURE 5.6 Typical cross section of a flip chip with ACF on PCB (many voids).

Figures 5.7 through 5.10 show the scanning electron microscope (SEM) cross sec-tion of the ACF-bonded flip chip with Au bumps on a PCB with Cu-Ni-Au pads. Itcan be seen that there are a few Ni conducting particles. Since the electroplated Aubump on the chip is softer than the electroless Au-Ni on the PCB, these Ni conduct-ing particles penetrated more into the Au bump on the chip. One of the disadvan-tages of anisotropic conductive materials is to waste conductive particles, such as theone shown in Fig. 5.10.

Figure 5.11 shows the SEM cross section of the ACF-bonded flip chip with Cubumps on a PCB with Cu-Ni-Au pads. It can be seen that most of the Ni conductingparticles penetrated into the electroplated Cu (with a flesh of Sn) bump on the chip.Again, this is because the microhardness of the Au-Ni pads on the PCB is harderthan that of the Sn-Cu bumps on the chip.

Figure 5.12 shows the SEM cross section of the ACF-bonded flip chip with Ni-Aubumps on a PCB with Cu-Ni-Au pads. It can be seen that the penetration of the Niconducting particles into both the Ni (with a flesh of Au) bump on the chip and theAu-Ni pads on the PCB is small. As a matter of fact, some of the Ni conducting par-ticles have been badly deformed.

Figure 5.13 shows the SEM cross section of the ACF-bonded flip chip with Cubumps on a PCB with OCC pads. It can be seen that the Ni conducting particles pen-etrated into both the Cu (with a flesh of Sn) bump on the chip and the OCC pad onthe PCB.

In general, the ACF assembly yield is strongly affected by the kinds of bumps onthe chip and the flatness of the PCBs. In our cases, chips with Cu bumps have thehighest yield and chips with Ni-Au bumps have the lowest yield.This could be due tothe microhardness of the bumps, since the Cu bump is the smallest and the Ni-Aubump is the largest.Also, a PCB with OCC pads leads to a better assembly yield thanthat with Cu-Ni-Au pads. This could be due to the less-curve surface (more flatness)of the OCC finishing.


FIGURE 5.7 Au-bumped flip chip on Cu-Ni-Au PCB with Ni conductive particles.

5.6 CHAPTER FIVE

FIGURE 5.9 A closer look at the Au-bumped flip chip on Cu-Ni-Au PCB with Niconductive particles.

FIGURE 5.8 Magnified image of Au-bumped flip chip on Cu-Ni-Au PCB with Niconductive particles.


FIGURE 5.10 A nickel particle is wasted.

FIGURE 5.11 Cu-bumped flip chip on Cu-Ni-Au PCB with Ni conductive particles.

5.8 CHAPTER FIVE

FIGURE 5.12 Ni-Au-bumped flip chip on Cu-Ni-Au PCB with Ni conductive par-ticles.

FIGURE 5.13 Cu-bumped flip chip on OCC PCB with Ni conductive particles.

5.2.4 THERMAL CYCLING TEST OF FCOB ASSEMBLIES WITH ACF

Forty ACF-bonded flip chips (20 with Au bumps and 20 with Cu bumps) on a Cu-Ni-AuPCB are subjected to thermal cycling.The temperature profile is shown in Fig. 5.14. Itis from −20 to 110°C, and the cycle time is 1 h. The test results are shown in Fig. 5.15.It can be seen that after 1000 cycles, for both cases, there is no opening yet. However,the resistance of the FCOB assemblies with Cu bumps increases to about 29 percent.


FIGURE 5.14 Thermal cycling profile (−20 to 110°C, 15-min ramp and 15-mindwell).

FIGURE 5.15 Thermal cycling test results (−20 to 110°C, 15-min ramp and 15-mindwell).

On the other hand, the increase in resistance of the FCOB assemblies with Au bumpsis only 5.3 percent. This could be due to more degradation of the Cu-bumped FCOBthan that of the Au-bumped FCOB assemblies after thermal cycling. From the presentresults, it is expected that the Au-bumped FCOB with ACF assemblies should have abetter thermal fatigue life than the Cu-bumped FCOB with ACF assemblies.

5.2.5 SIR TEST RESULTS OF ACF FCOB ASSEMBLIES

Surface insulation resistance (SIR) testing is one of the most widely used techniquesto assess the electrical performance reliability in electronic packaging. Leakage cur-rents are monitored as a function of time at predetermined temperature, humidity,and bias voltage conditions. Leakage currents between closely spaced bumps andpads are sensitive indicators and are good signals of potential field risks. The SIRvalues of 100 MΩ or higher are acceptable for commercial and industrial applica-tions, and 500 MΩ is acceptable for military requirements. Figure 5.16 shows the SIRtest (85°C/85% RH at 10-V bias) results for the ACF-bonded FCOB assemblies. Itcan be seen that ACF is acceptable for all of the cases under consideration.

5.2.6 SUMMARY

The Au-bumped, Cu-bumped, and Ni-Au-bumped flip chips have been assembledon Cu-Ni-Au and OCC PCBs with ACF. Important parameters and process stepssuch as the wafer; wafer bumping with Au, Cu, and Ni-Au; PCB; ACF; tacking; lami-nation; pick and place; and bonding have been discussed. Furthermore, the ACF-bonded FCOB assemblies have been subjected to thermal cycling and SIR tests.Some important results are summarized as follows:

1. The ACF-bonded FCOB assembly yield is strongly affected by the kinds ofbumps (Au, Cu, and Ni-Au) on the chip. A flip chip with Cu bumps has the high-est assembly yield, and that with Ni-Au bumps has the lowest.

5.10 CHAPTER FIVE

FIGURE 5.16 SIR test results.

2. The ACF-bonded FCOB assembly yield is strongly affected by the flatness of thePCB. Also, in this study, a PCB with OCC pads leads to a better assembly yieldthan a PCB with Cu-Ni-Au pads.

3. There is no opening in the ACF-bonded flip chip on Cu-Ni-Au PCB assembliesafter 1000 temperature cycles (−20 to 110°C). The resistance change due to 1000temperature cycles of the Au-bumped flip chip assemblies (5.3 percent) is smallerthan that of Cu-bumped flip chip assemblies (29 percent).

4. In this study, the ACF-bonded FCOB assemblies meet the SIR test requirementsfor commercial, industrial, and military applications.

5.3 COPPER-WIRED WLCSP WITH SOLDERS OR ADHESIVES

ON SUBSTRATES

The electroplated copper-wired WLCSP, developed by Fujitsu Computer PackagingTechnologies, has been discussed in Chap. 4, Sec. 4.6.The assembly of this WLCSP onthe PCB or substrate has also been reported in Ref. 38. Figure 5.17 shows a typical


FIGURE 5.17 Electroplated copper wire soldered to a substrate.

cross section of the wire interconnect technology (WIT) assembly. In this case, thecopper-wired WLCSP is soldered on the substrate. However, the following twopoints should be emphasized:

1. Adhesives work as well as solders.2. The solder materials can be lead-free.

Usually, the solder or adhesive materials are stencil- or screen-printed on the sub-strate. The thermal and electrical performance, as well as the solder joint reliability,of this assembly have been demonstrated in Ref. 38.

5.4 MICROSPRING WLCSP WITH SOLDERS OR ADHESIVES

ON PCB/SUBSTRATE

The wire-bonding gold-wired WLCSP, developed by FormFactor, has been discussedin Chap. 4, Sec. 4.7. The assembly of this WLCSP on the PCB or substrate has alsobeen reported in Ref. 39. Figure 5.18 shows a typical photo of the microspring assem-bly. It can be seen that the S-shape microsprings connect to the PCB through a sol-der, which is printed on the PCB with a stencil and reflowed. If an adhesive is used,then it will be printed on the PCB with a screen and cured. The solder joint reliabil-ity of this assembly has been demonstrated in Ref. 39.

5.5 Au-STUD-BUMPED WLCSP WITH ICA ON PCB

The materials, process, and equipment of wire-bonding gold-stud-bumped WLCSPshave been discussed in Chap. 4, Sec. 4.8. In this section, the materials, process, andequipment of gold-stud-bumped WLCSP bonding (SBB) on PCB or substrate arebriefly discussed.

5.12 CHAPTER FIVE

FIGURE 5.18 Wire-bonding S-shape microsprings soldered to a substrate.

5.5.1 MATERIALS AND PROCESS FLOW

Figure 5.19 shows the process flow of the SBB technology.The following can be seen:

After leveling the Au-stud bumps on the chip (or on the wafer and after dicing), asdiscussed in Chap. 4, Sec. 4.8.1, the individual chip is flipped over and placed in arotating disk containing a shallow bath of ICA, such as the Ag-Pd paste.

Once the ICA is transferred to the stud bumps, the chip is mounted on top of thePCB or substrate with a load of 2 gf per bump.

This is followed by curing at a temperature ranging from 120 to 180°C.

The height of the ICA is usually controlled by a doctor blade and is about two-thirds of the height of the stud bump, including the leveled tail. Figure 5.20 shows aAu-stud bump after dipping in an ICA made by Panasert’s machines. It can be seenthat in order to avoid the shorting of the neighboring bumps, the ICA is only appliedon the top and the upper sides of the stud bump.

The final process step is to apply the underfill on one or two adjacent sides of thechip. Due to the capillary action, the underfill will flow and fill the gap between thechip and the PCB or substrate. After curing the underfill material, it will cement the chip on the PCB or substrate.

It should be noted that underfill encapsulants are very difficult, if not impossible,to rework. On the other hand, due to its thermoplastic nature (which offers only alimited adhesion strength), the ICA joint is very easy to rework. Therefore, most ofthe tests and rework should be done before the underfill process. In this case (andfor chips with peripheral pads), a small amount of nonconductive adhesive can beapplied to the chip center area on the PCB or substrate before chip placement and


FIGURE 5.19 Stud bump bonding (SBB) technology process flow.

bonding. This temporary adhesive is cured with the ICA adhesive at the same timeand is used to increase the adhesion strength between the chip and the PCB or sub-strate before underfilling.

5.5.2 EQUIPMENT FOR SBB TECHNOLOGY

As mentioned in Chap. 4, Sec. 4.8.2, there are at least two well-established com-panies (Panasert and Kulicke and Soffa) who are providing the SBB equipment.Figures 5.21 and 5.22 show the Panasert FCBII(1) and FCBII(2), respectively.FCBII(1) is the standard flip chip bonding for Panasert, and FCBII(2) is a high-throughput, automatic board handing, and IC flip-over feeding machine. Theiroperating sequences are shown in Fig. 5.23. It should be noted that thesemachines can be used for not only SBB technology but all other kinds of flip chiptechnologies. Kulicke and Soffa’s machines for SBB technology have been shownin Fig. 4.24.

5.6 Au-STUD-BUMPED WLCSP WITH ICA ON FLEX

Chip-on-flex (COF) has been used for a long time.36, 41 There are many differentkinds of COF [e.g., wire-bonding COF, tape automated bonding (TAB) COF, andflip COF (FCOF)], and their substrate is not rigid but flexible. In this section, thedesign, materials, process, and reliability of Matsushita’s Au-stud-bumped FCOFwith ICA is discussed.

5.14 CHAPTER FIVE

FIGURE 5.20 Leveled Au-stud bump after beingdipped in ICA.

A flexible substrate (or simply a type) is much thinner and lighter than the FR-4epoxy PCB, BT substrate, ceramic substrate, or other rigid substrates. Also, there isthe advantage of being able to fold a flexible substrate in order to pack it into a verysmall package’s case. Thus, with the flexible substrate, ultimately small and light-weight electronic products are possible.


FIGURE 5.21 Panasert’s flip chip bonder FCBII(1).


The materials and process of SBB technology with ICA on PCB have been discussedin Sec. 5.5 of this chapter. Figure 5.24 shows a schematic of Matsushita’s SBB tech-nology with ICA on a tape, and Fig. 5.25 shows the process flow.42 It can be seen thatthe Au-stud bumping and most of the processes are the same for both cases.The keydifference is the substrate (i.e., the PCB is rigid and the tape is flexible).

To keep the liquid crystal polymer (LCP) tape flat and fix during assembly, it isflattened on an aluminum plate with an adhesive sheet whose adhesive strength iseasily lost beyond 160°C. After the chip placement, the ICA and the temporary

5.16 CHAPTER FIVE

FIGURE 5.22 Panasert’s flip chip bonder FCBII(2).

adhesive are cured at 120°C for 2 h in a convection oven. Then, the underfill resin iscured at 150°C for 2 h in a convection oven.

Matsushita’s LCP tape for the Au-stud-bumped WLCSP is shown in Fig. 5.26.Thedimensions are 30 × 30 × 0.05 mm. There are 248 periphery pads and their pitch is0.15 mm. The dielectric constant, TCE, and modulus of the LCP tape are, respec-tively, 3 (at 1 MHz), 15 × 10−6/°C, and 6.86 GPa. Compared with those (4.8, 16 ×10−6/°C, and 21.4 GPa) of FR-4 epoxy PCB, it can be seen that the LCP is more suit-able for high-frequency applications (lower dielectric constant) and that it is moreeasily bent (lower modulus). The moisture absorption of LCP is 0.04 percent afterholding at 23°C for 24 h.


FIGURE 5.23 Operation sequence of Panasert’s flip chip bonder FCBII.

Matsushita’s ICA consists of an organic binder and a conductive filler. Thebisphenol-A type with high-molecular-weight epoxy resin, which is flexible over awide range of temperatures, is used as the organic binder. Flake Ag powder is usedas the conductive filler for high conductivity.The viscosity and volume resistivity are,respectively, 30 Pa·s and 1 × 10−4 Ωcm. It should be noted that this ICA is very easyto deform and thus relaxes the thermal stress due to thermal expansion mismatchbetween the chip and the tape.

The material properties of the temporary adhesive are TCE = 29 × 10−6/°C; mod-ulus = 8 GPa; Tg = 119°C; and viscosity = 42 Pa·s. One can see that

The viscosity is high enough not to form large voids during assembly. The glass transition temperature Tg is low enough to be cured quickly.

5.18 CHAPTER FIVE

FIGURE 5.24 Au-stud-bumped WLCSP on a flexiblesubstrate with an ICA.

FIGURE 5.25 Matsushita’s process flow of Au-stud-bumped WLCSP on a flexible substrate withan ICA.

The material properties of the underfill resin are TCE = 26 × 10−6/°C; modulus =11.4 GPa; Tg = 123°C; and viscosity = 5 Pa·s. It consists of an epoxy resin of acid anhy-deride type and 60 wt % of globular SiO2 fillers. Figure 5.27(a) shows a typical cross-section of the Au stud-bumped WLCSP on the LCP substrate with an ICA.

5.6.2 QUALIFICATION TESTS AND RESULTS

Matsushita’s Au-stud-bumped WLCSPs on the LCP substrate with the ICA are sub-jected to the multiple reflow test, pressure cooker test, and thermal shock test. Thetest conditions for the reflow soldering are as follows: after 168 h of 85°C/85% RH,three times reflow with 240°C for 5 s (JEDEC level 1). For the pressure cooker, thetest conditions are 100 h at 121°C and 2 atms. For thermal shock the test conditionsare −55 (5 min) +125 (5 min), liquid to liquid. The connection resistance per bump ismeasured using the four-point probe method.

These test results are shown in Figs. 5.27(b) through 5.30. It can be seen from Fig.5.27(b) that there is no crack of the Au-stud-ICA interconnect.Also, the chip thickness


FIGURE 5.26 Matsushita’s flexible substrate.

5.20 CHAPTER FIVE

FIGURE 5.27 Typical cross sections of Au-stud-bumped WLCSP on a flexible substrate with anICA after tests.

(a)

(b)


FIGURE 5.28 Contact resistance under JEDEC level 1 test.

FIGURE 5.29 Contact resistance under pressure cooker test.

is 0.4 mm; the LCP tape thickness is 0.05 mm; the underfill thickness is 0.07 mm; andthe ICA thickness is about 0.04 mm. Figures 5.28 through 5.30 show, respectively,that the connection resistance of the Au-stud-ICA joint on the LCP is stable underthe multiple reflow test, pressure cooker test, and thermal shock test.

5.7 Au-STUD-BUMPED WLCSP WITH ACP/ACF ON PCB

The material and process flow of Au-stud-bumped WLCSPs with ACF/ACP on the PCB or substrate are almost the same as those of Au-, Cu-, or Ni-Au-bumped WLCSPs with ACF/ACP on PCB, as discussed in Sec. 5.2 of this chapter, except forthe bump structure and material. As mentioned earlier, since the thermal settingadhesive in most of the ACP/ACF doesn’t contain nonconductive fillers, its TCE isvery large and the bonded assembly consists of many voids. To achieve the samelevel of reliability as the Au-stud-bumped WLCSP with ICA on the PCB with under-fill encapsulant, some amount of nonconductive fillers (in addition to the conductivefillers) are incorporated into the ACF/ACP. In this section, the effects of noncon-ductive fillers on the ACP/ACF joint reliability are presented.

5.7.1 ACF/ACP WITH NONCONDUCTIVE FILLERS

The Korea Advanced Institute of Science and Technology (KAIST)43 uses the Ni asthe conductive filler and silica as the nonconductive filler. They mix the silica (5 to45 wt %), the Ni, and the liquid epoxy to produce anisotropic conductive adhesives

5.22 CHAPTER FIVE

FIGURE 5.30 Contact resistance under thermal shock (liquid-to-liquid) test (−55 and+125°C).

(ACAs) of 10, 30, and 50 wt % total (conductive and nonconductive) filler content.Surface modification of fillers is performed to achieve uniform dispersion of fillerinside the epoxy matrix of the ACA composite.

The ACAs are formulated by mixing fillers, liquid epoxy resin, and a hardener.The mixtures are stirred and degassed under a vacuum for 3 h to eliminate the airthat is induced during stirring. A differential scanning calorimeter (DSC), thermo-mechanical analysis (TMA), dynamic mechanical analysis (DMA), and thermograv-itational analysis (TGA) are used to investigate the curing conditions and materialproperties of the modified ACAs.

5.7.2 DSC MEASUREMENT RESULTS

Figure 5.31a shows the effect of filler contents on the curing profiles, and Fig. 5.31bshows the effect of filler contents on Tg of the ACA composite materials. The fol-lowing can be seen:

The increase of filler contents slightly shifts the curing onset temperature andpeak temperature to the higher temperature.

The addition of Ni and silica fillers slightly modifies the shape of the DSC curvesand increases the Tg.

5.7.3 DMA MEASUREMENT RESULTS

Fig. 5.32, a and b, shows the effect of filler contents on the storage modulus and lossmodulus, respectively, of the ACA composite materials. One can see the following:

For all the filler contents, the higher the temperature the lower the storage modu-lus.

The higher the filler contents the higher the storage modulus, especially at roomtemperature.

The Tg (characterized by the knee in the loss modulus curve) increases as the fillercontent increases.

These behaviors could be due to the increasing interactions of polymer/filler inthe ACA composites.

5.7.4 TMA MEASUREMENT RESULTS

Figure 5.33 and Table 5.1 (α1 is the TCE below TgTMA, and α2 is the TCE above

TgTMA) show the effect of filler contents on the TCE and Tg

TMA of the ACA compos-ite materials. One can be see the following:

The higher the filler content the higher the TgTMA.

The higher the filler content the lower the α1. The filler content doesn’t affect α2 significantly.


5.24 CHAPTER FIVE

FIGURE 5.31 DSC curves of ACA specimens with silica and nickel fillers of various contents (10,30, and 50 wt %). (a) The curing profiles and (b) the glass transition curves.

(a)

(b)


FIGURE 5.32 DMA curves of ACA specimens with silica and nickel fillers of various contents (10,30, and 50 wt %). (a) The storage modulus and (b) the loss modulus.

(b)

(a)

Thus, higher filler contents are not usable for the thermal fatigue reliability of theACA joints.

5.7.5 TGA MEASUREMENT RESULTS

Figure 5.34 shows the effect of filler contents on the decomposition temperature andweight loss of ACA composites. The following can be seen:

The higher the filler content the lower the weight loss. The filler content doesn’t affect the decomposition temperature (393°C) of these

ACA composites.

5.7.6 85°C/85% RH TEST AND RESULTS

The test flip chip on the PCB (34 × 37 mm) as well as the Au stud bumps after level-ing are shown in Fig. 5.35.The dimensions of the chip are 5 × 5 mm. It has 48 pads (60µm in diameter) on a 130-µm pitch.The PCB is made of FR-4 epoxy with Ni-Au fin-

5.26 CHAPTER FIVE

FIGURE 5.33 TMA curves of ACA specimens with silica and nickel fillers of various contents(10, 30, and 50 wt %).

TABLE 5.1 TgTMA and CTE of ACA Composites Below and Above TgTMA

ACA composite TgTMA (°C) α1 (ppm/°C) α2 (ppm/°C)

ACA with 10 wt% filler 87.62 87.9 3960



ishing. Figure 5.36 shows the cross sections of the Au-stud-bumped WLCSP on aPCB with an ACA containing both conductive and nonconductive fillers. They arevery obvious. The contact resistance of a single interconnect is measured using afour-point probe method.

Figure 5.37 shows the contact resistance measurement results during the85°C/85% RH test (Fig. 5.37a) and 85°C/dry test (Fig. 5.37b). It should be notedthat no catastrophic failures are observed,43 and the following can be seen:

The contact resistance of the ACAs with 30 and 50 wt % filler contents is very sta-bilized up to 1000 h of 85°C/85% RH test.

The contact resistance of the ACA with 10 wt % filler contents is stabilized up to500 h of 85°C/85% RH test and then increases.

The contact resistance of the ACAs with 10, 30, and 50 wt % filler contents is verystabilized during all the 1000 h of 85°C/dry test.

5.7.7 THERMAL CYCLING TEST AND RESULTS

Figure 5.38 shows the contact resistance measurement results during the thermalcycling (−60 to 150°C, air-to-air). One can see the following:

The ACA with 10 wt % filler contents cannot pass 300 cycles. The ACA with 30 wt % filler contents cannot pass 400 cycles. The ACA with 50 wt % filler contents passed 700 cycles.


FIGURE 5.34 TGA curves of ACA specimens with silica and nickel fillers of various contents (10,30, and 50 wt %).

5.28 CHAPTER FIVE

FIGURE 5.35 KAIST’s Au-stud bumps (a) and test assembly (b).

(a)

These results show that filler contents are very important for the ACA joint reliabil-ity. Just like the underfill materials, higher filler contents will lead to lower TCE andhigher modulus ACAs and, consequently, higher thermal fatigue reliability of theACA joints.

5.8 Au-STUD-BUMPED WLCSP DIFFUSED ON Au-PLATED

PCB WITH NCA

The Au-stud bumps without leveling can be bonded on a Au-plated PCB with ther-mal compression. The interconnect long-term reliability can be assured by the non-conductive adhesive (NCA). In this section, the results obtained by Sharp arepresented.44


(b)

FIGURE 5.35 (Continued) KAIST’s Au-stud bumps (a) and test assembly (b).

5.30 CHAPTER FIVE

FIGURE 5.36 KAIST’s Au-stud bumps on a PCB with an ACA with both conductive and non-conductive fillers. (a) Optical view of cross section of flip chip assembly using an ACA; (b)enlarged view of the interconnection formed by Au stud bump and Ni particles between chip I/Oand substrate pad.

(b)

(a)


FIGURE 5.37 Contact resistance under (a) 85°C/85% RH test and (b) 85°C/dry test.

(a)

(a)


Figure 5.39 shows a Au-stud-bumped WLCSP on PCB with NCA assembly. Figure5.40 shows the Au-stud bumps (which can be made with the process discussed in Sec.5.5) without leveling. The bump diameter is 82 3 µm, bump height is 64 3 µm,and bump pitch is 110 µm. The bump shear strength is 0.44 0.04 N/bump.

The specifications of the chip and the PCB are shown in Table 5.2. It can be seenthat the chip dimensions are 10 × 10 × 0.2 mm. It has 316 pads on a 110-µm pitch.ThePCB is made of FR-5 glass epoxy, and the dimensions are 100 × 50 × 0.2 mm.The padfinishing is Cu-Ni-Au.

The NCA is made of silica particles and thermal setting epoxy resin.The materialproperties are as follows: modulus = 7.8 GPa; TCE = 28 × 10−6/°C; Tg = 150°C; andaverage silica filler size = 0.4 µm.

5.32 CHAPTER FIVE

FIGURE 5.38 Contact resistance under thermal cycling test (−60 to +150°C for 700cycles).

FIGURE 5.39 Au-stud-bumped WLCSP on Au-plated PCB with anNCA.

Sharp’s bonding process is very simple. First, dispense the NCA on the PCB.Next, place the Au-stud-bumped WLCSP, as shown in Fig. 5.40 face-down on thePCB. Finally, the NCA between the mounted chip and the PCB is hardened underheat and pressure using a flip chip bonder equipped with a constant-heat tool,which completes the process. Figure 5.41a shows a cross section of the Au-stud-bumped WLCSP with NCA on the Au-plated PCB assembly. The bonding condi-


FIGURE 5.40 Au stud bumps without leveling.

TABLE 5.2 Specifications of the Chip and the Substrate

Substrate Chip

FR-5 glass-epoxyMaterial (Tg: 180–190°C) Si

Size 100mm × 50mm 10mm × 10mm

Thickness 0.2 mm 0.2 mm

I/Os 316 316

Pitch 110 µm 110 µm

Electrode Flash-Au/Ni/Cu Au bump/Al Pad

5.34 CHAPTER FIVE

FIGURE 5.41 (a) Au stud bumps without leveling (gold-gold thermocompression) on the Au-plated PCB with an NCA. (b) Au-stud-bumped joints showing nonuniformity.

(b)

(a)

tions are as follows: bonding temperature = 337°C; bonding force = 300 N/chip; andbonding time = 5 s. It can be seen that the gap between the chip and the PCB isabout 20 µm.

One of the drawbacks of Au-Au metallic diffusion is joint nonuniformity, asshown in Fig. 5.41b. It can be seen that the left-hand joint is larger than the right-hand joint. Among other reasons, this is due to the variation of passivation openingof chip pads, which defines the bump deformation during bonding.

5.8.2 RELIABILITY

The effects of different bonding temperatures, bonding forces, and bonding times onthe performance (contact resistance) of the Au-stud-bumped WLCSP with NCA onthe Au-plated PCB assemblies subjected to the 85°C/85% RH test condition areshown in Figs. 5.42 through 5.44. To make a good Au-Au metallic diffusion bond:

A bonding force of 300 N/chip is required. The bonding time is not significant. The bonding temperature (337°C) is adequate.

Figure 5.45 shows the thermal cycling test (−40 to 125°C, 10-min dwells) results ofthe Au-stud-bumped WLCSP with NCA on the Au-plated PCB with a bonding tem-perature of 337°C, a bonding force of 300 N/chip, and a bonding time of 3.5 s. It canbe seen and confirmed that these process parameters will lead to sufficient reliableinterconnects for practical use.


FIGURE 5.42 Contact resistance under 85°C/85% RH test (200-N/chip bonding load and 5-s bonding time).

5.36 CHAPTER FIVE

FIGURE 5.43 Contact resistance under 85°C/85% RH test(300-N/chip bonding load and 5-s bonding time).

FIGURE 5.44 Contact resistance under 85°C/85% RH test(300-N/chip bonding load and 337°C bonding temperature).

5.9 Au-STUD-BUMPED WLCSP DIFFUSED ON Au-PLATED

FLEX WITH NCA

The Au-stud-bumped flip chip on Au-plated flexible substrate with NCA has beenstudied by NEC.45 Their results are presented in this section.


Figure 5.46 shows a schematic of NEC’s tape gold-gold gang bond BGA (T-G2BGA)package. Its assembly process flow is shown in Fig. 5.47. The focus of this section is


FIGURE 5.45 Contact resistance under thermal cycling test(−40 to +125°C, 10-min dwell). The bonding temperature is337°C, bonding force is 300 N/chip, and bonding time is 3.5 s.

FIGURE 5.46 NEC’s T-G2BGA structure.

5.38 CHAPTER FIVE

FIGURE 5.47 Process flow of NEC’s T-G2BGA package.

only on the materials and assembly process of the Au-stud-bumped WLCSP on theAu-plated tape substrate with NCA. The NEC Au-stud bump is shown in Fig. 5.48(very similar to that shown in Fig. 5.40).

The chip dimensions are 7.33 × 14.26 × 0.32 mm.There are 94 peripheral pads and64 center pads, and they are on a 150-µm pitch. The two-layer tape is made of poly-imide (50 µm thick), and the conductor thickness is 18-µm copper + 2 µm of electro-plated nickel + 0.5–1.5 µm of electroplated Au. The underfill material properties areshown in Table 5.3. It can be seen that the underfill consists of 62 wt % of filler con-tents and the resin is epoxy/amine.

Before assembly, the gold-plated pads on the tape are cleaned with argonplasma dry cleaner for good interconnection. Then, the flip chip is bonded on thetape substrate using thermocompression gold-gold interconnection technology.After the bonding, just like the conventional flip chip technology, the NCA (under-fill) is dispensed on one or two adjacent sides of the chip on the tape substrate andthen is cured.

5.9.2 RELIABILITY

The effects of bonding forces, electroplating thickness of Ni and Au on the pads ofthe tape substrate, and bonding time (the bonding temperature is 270°C) on theshear strength of the Au-stud-bumped assembly are shown in Figs. 5.49 through 5.52,in which the following can be seen:

The shear strength is not affected by the Ni and Au thickness (thus, a 2-µm thick-ness of Ni and a 0.5-µm thickness of Au are chosen).

The deformation of chip pads doesn’t increase significantly for the increase ofbonding force.

The shear strength increases as the bonding force increases. The bonding of 7.5 s yields the best interconnect.


FIGURE 5.48 Nonleveled (acute tail) Au stud bump.

TABLE 5.3 Underfill Resin Properties

Item Unit Data

Resin Epoxy/Amine

Filler content wt % 62

Tg °C 90

CTE* ppm <Tg 29

(TMA) >Tg 100

Bending modulus GPa 9.5

Bending strength MPa 130

Cl 0.5

Purity ppm Na+ <1

(after PCT 20 h) K+ <1

5.40 CHAPTER FIVE

FIGURE 5.49 Effects of bonding force and tape plating thickness on shear test force(strength), 94 bumps/chip.

FIGURE 5.50 Effects of bonding force and tape plating time on bondingdeformation.


FIGURE 5.51 Effects of bonding force and bonding time on shear test force (strength),64 bumps/chip.

FIGURE 5.52 Effects of bonding force and bonding time on bondingdeformation.

Thus, the optimal parameters for Au-Au thermocompression are as follows: bond-ing temperature = 270°C; bonding stage temperature = 70°C; bonding time = 7.5 s; andbonding force = 100 gf/bump. A closer look at the IMC (after dry-etching) of the Au-Au bonding is shown in Fig. 5.53.There is no crack and no clear boundary betweenthe Au stud bump and the gold-plated tape under the optimum bonding conditions.

5.42 CHAPTER FIVE

FIGURE 5.53 Interface between the Au stud bump and the flip chippad.

5.10 Cu-STUD-BUMPED WLCSP WITH LEAD-FREE SOLDERS ON PCB

As mentioned earlier, due to its rigidity (very little compliance), stud-bumped WLCSPs are usually attached to the PCB or substrate with adhesives cured at a tem-perature ranging from 120 to 180°C.Also, most of the adhesives for SBB technologyapplications are easy to deform; thus, they relax the thermal stresses due to the ther-mal expansion mismatch between the silicon chip and the organic substrate duringthe cooldown and operation of the assembly.

However, unlike solders (which have the self-alignment characteristic duringreflow), adhesives require very high placement accuracy (which will decreasethroughput and increase costs). Thus, stud-bumped WLCSPs on PCBs or substrateswith lead-free solder could be an alternative. Since Au stud bumps are too much forthe solder joints to remain ductile, Cu stud bumps are chosen by the Georgia Insti-tute of Technology (GIT) for their investigation.46


The lead-free solder paste studied by GIT is Sn-3.5Ag-0.5Cu, which consists of type4 solder particles and no-clean, low-solid flux. It is deposited onto the PCB by ametal stencil having 150-µm square apertures and a 50-µm thickness. The Cu studbumping developed by Furukawa has already been discussed in Chap. 4, Sec. 4.9.

By using a flip chip bonder, the Cu-stud-bumped WLCSP is placed on thepasted PCB. The reflow temperature profile is as follows: ramp rate = 1.7°C/s; soaktemperature = 130°C/150°C; soak time = 33 s; peak temperature = 242°C; timeabove the melting temperature = 43 s. Before dispensing the underfill, the PCB isbaked at 100°C for 2 h to reduce moisture contents. The underfill is dispensed at

90°C and cured at 150°C for 7 min. Figure 5.54, a and b, shows, respectively, the Au-stud-bumped and Cu-stud-bumped WLCSP on PCB with the lead-free Sn-3.5Ag-0.5Cu solder joint.

5.10.2 RELIABILITY

The Au-stud- and Cu-stud-bumped WLCSP on PCB with the lead-free solder areassembled (20 each) and are subjected to a thermal shock (liquid-to-liquid) test.Thetest condition is −50 to 125°C with 10-min cycles. The test results are shown in Fig.5.55. It can be seen that 30 percent of both of the Au-stud- and Cu-stud-bumpedWLCSP lead-free assemblies failed even before the test started. Also, more than 90percent of the Cu-stud- and Au-stud-bumped WLCSP lead-free assemblies failed,respectively, before 100 cycles and 500 cycles. Figure 5.56, a and b, shows, respec-tively, the cross section of the Au-stud- and Cu-stud-bumped WLCSP tested sam-ples. These could be due to the following:

The microhardness of Cu is harder than that of Au; thus, the Cu stud is less com-pliant.

The thermal expansion mismatch between the silicon chip and the organic PCBinitiates the crack near the interface between the chip pad and the bump duringthe cooldown of reflow soldering.

The pad area on the PCB is much larger than that on the chip; thus, for the sameshear force, there are more shear stresses on the chip side.

The following should be reemphasized:

The compliance of the stud-bumped WLCSP assemblies with solders is much lessthan that with adhesives.

The bonding temperature of the stud-bumped WLCSP assemblies with solders(especially lead-free) is much higher than that with adhesives.


FIGURE 5.54 (a) Au-stud- and (b) Cu-stud-bumped WLCSP on a PCB with Sn-3.5 Ag-0.5Cu lead-free solder joints.

5.44 CHAPTER FIVE

FIGURE 5.55 Thermal shock test results of Au-stud- and Cu-stud-bumped WLCSP on aPCB with Sn-3.5Ag-0.5Cu lead-free solder joints.

FIGURE 5.56 Failure modes of (a) Au-stud- and (b) Cu-stud-bumped WLCSP on a PCB with Sn-3.5Ag-0.5Cu lead-free solder joints after thermal shock.

The bonding time of the stud-bumped WLCSP assemblies with solders is muchlonger than that with adhesives.

Thus, to make SBB with lead-free solder technology work, the following are nec-essary:

To use less energy to make the Cu stud bumps To make the Cu stud bumps more compliant by annealing To better design the pad configuration on the chip and the PCB To use lower-melting-point lead-free solders To use less stiff (more compliance) lead-free solders To optimize the reflow temperature profile for this specific application

ACKNOWLEDGMENTS

The authors would like to thank L. Moresco, D. Love, W. Chou, D. Horine, C. Wong,S. Beilin, and V. Holalkere of Fujitsu Computer Packaging Technologies; J. Healy ofFormFactor; K. Tsunoi, T. Kusagaya, and H. Kira of Fujitsu; T. Garvin of Panasonic;P. Lin of K&S; Y. Kumano, Y. Tomura, M. Itagaki, and Y. Bessho of Matsushita; M.Yim and K. Paik of KAIST; Y. Sakamoto, H. Matsubara, K. Yamamura, and T. Nukiiof Sharp; S. Isozaki, T. Kimura, T. Shimada, and H. Nakajima of NEC; S. Zama, T.Hikami, and H. Murata of Furukawa; D. Baldwin of Georgia Institute of Technology(GIT); I. Watanabe, K. Takenura, N. Shiozawa, O. Watanabe, K. Kojima, A. Nagai,and T. Tanaka of Hitachi; and the people of PICOPAK for sharing their useful andimportant information with the industry.

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17. Hotta, Y., “Development of 0.025 mm Pitch Anisotropic Conductive Film,” Proceedings ofIEEE Electronic Components and Technology Conference, pp. 1042–1046, June 1998.

18. Dernevik, M., R. Sihlbom, K. Axelsson, Z. Lai, J. Liu, and P. Starski, “Electrically Conduc-tive Adhesives at Microwave Frequencies,” Proceedings of IEEE Electronic Componentsand Technology Conference, pp. 1026–1030, June 1998.

19. Kang, S. K., and S. Purushothaman, “Development of Low Cost, Low Temperature Con-ductive Adhesives,” Proceedings of IEEE Electronic Components and Technology Confer-ence, pp. 1031–1035, June 1998.

20. Yim, M., K. Paik,T. Kim, and Y. Kim,“Anisotropic Conductive Film (ACF) Interconnectionfor Display Packaging Applications,” Proceedings of IEEE Electronic Components andTechnology Conference, pp. 1036–1041, June 1998.

21. Wei,Y., and E. Sancaktar,“A Pressure Dependent Conduction Model for Electrically Con-ductive Adhesives,” Proceedings of International Symposium on Microelectronics, pp.231–236, 1955.


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26. Tummala, R. R., and E. Rymaszewski, Microelectronics Packaging Handbook, Van Nos-trand Reinhold, New York, 1989.

5.46 CHAPTER FIVE

27. Lau, J. H., and S. W. R. Lee, Microvias for Low-Cost High-Density Interconnects, McGraw-Hill, New York, 2001.

28. Lau, J. H., Low-Cost Flip Chip Technologies for DCA, WLCSP, and PBGA Assemblies,McGraw-Hill, New York, 2000.

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31. Lau, J. H., and Y.-H. Pao, Solder Joint Reliability of BGA, CSP, Flip Chip, and Fine-PitchSMT Assemblies, McGraw-Hill, New York, 1997.



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36. Lau, J. H., Handbook of Tape Automated Bonding,Van Nostrand Reinhold, New York, 1992.

37. Triggs, W., and C. Byrns, Jr., U.S. Patent No. 3,599,060, August 10, 1971.

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CHAPTER 6ENVIRONMENTALLY BENIGN

MOLDING COMPOUNDS FOR IC PACKAGES

6.1 INTRODUCTION

As discussed in Chap. 1, the Waste Electrical and Electronic Equipment and Reduc-tion of Hazardous Substances directives require electronic products to be halogen-freeand lead-free due to increasing awareness of environmental compatibility. Chip(wafer)-level interconnects with lead-free solder bumps have been discussed in Chap.2. Wafer-level chip-scale packages (WLCSPs) with lead-free solder bumps on printedcircuit boards (PCBs) or the substrate of flip chip in a package have been presented inChap.3.Chip (wafer)-level interconnects with solderless bumps have been discussed inChap.4 andWLCSPs with solderless bumps on PCBs, tapes,or the substrate of flip chipin a package have been reported in Chap. 5. In this chapter the halogen (Br or Sb)-freeflame-retardant molding compounds for plastic quad flat pack (PQFP), plastic ballgrid array (PBGA), and mold array PBGA (MAP-PBGA) packages are discussed.

To obtain halogen-free molding compound, it is necessary to maintain flameresistance, even when brominated (Br) epoxy and antimony (Sb) oxide (tradition-ally used as flame retardants) are eliminated.1–20 This can be achieved by either (1)adding novel flame retardants to the conventional resin, (2) mixing a compoundwith high filler content to give flame resistance, (3) changing resin construction toinflammable, or (4) combining any of (1), (2), and (3).

Unlike Sn-37Pb solder [melting temperature of 183°C and maximum reflow tem-perature of 220°C], most of the lead-free solders in the SnAgCuX family have melt-ing temperatures above 213°C. Thus, the maximum reflow temperature of thelead-free solders could be 260°C. With this increase of reflow temperature thedimensional deformation and vapor pressure (which are the driving forces for pack-age cracking, delamination, and popcorning)9–20 in the molding compound increaseas shown in Table 6.1 and Fig. 6.1. This is because (1) the thermal expansion mis-match within the package is a function of temperature and (2) the generated vaporpressure from moisture absorption of the molding compound is also temperaturedependent. Thus, novel epoxy molding compounds (EMCs) that are halogen freeand generate no defective products at 260°C of maximum reflow temperature aredesperately needed in the electronics industry.

6.2 ENVIRONMENTALLY BENIGN MOLDING COMPOUNDS

FOR PQFP PACKAGES

The major components of a conventional molding compound are epoxy resin, phe-nol resin–type hardener, spherical silica, triphenylphosphine as a catalyst, and other

6.1


ingredients such as a coupling agent, wax, and carbon black. In the study of noveladdition-type flame retardants by Sumitomo,13,14 flame-retarding agents were usedto replace spherical silica. The mixture was melted and kneaded after preliminarymixing at room temperature. The obtained material was formed into tablets afterbeing cooled and crushed, and was used for molding.

6.2.1 FLAME RESISTANCE SYSTEMS: ADDITION-TYPE

RETARDANTS

The new flame retardants studied by Sumitomo are shown in Table 6.2. It can beseen that three groups of materials are considered: an (inorganic) phosphorusgroup, an (aluminum or magnesium) metal hydrate group, and a (molybdenumand boric) metal compound group. The materials were considered individually aswell as in combination.

The test specimens used for flame resistance tests were molded by a low-pressuretransfer molding machine with a molding temperature of 175°C, a molding pressureof 7 MPa, and a curing time of 120 s (or 10 MPa for 90 s). The postmold curing timewas 8 h. Molded specimens of 125 × 13 × 1.6 mm and 125 × 13 × 3.2 mm were usedfor the UL-94 flammability test.

Measurements were taken by recording burning time of the specimens after theflame was removed following 10 s of ignition.This was repeated, again recording theburning time after removing the flame following 10 s of ignition. Five molded speci-mens were tested and were judged to be V-0 level when the maximum burning timewas within 10 s and the sum of burning time was within 50 s.

6.2 CHAPTER SIX

FIGURE 6.1 Influence of elevated reflow temperature and failure modes.

TABLE 6.1 Stress Increase Factors

Reflow temperature 240°C 260°C

Estimated maximum MPa 3.2 4.5 Saturated vapor vapor pressure Rate 100 141 pressure at reflow temp.

Thermal expansion % 0.30 0.41 Typical data of EMC of molding Rate 100 137 for SMDcompound

The test results for the single addition of novel addition-type flame retardants areshown in Table 6.3. It can be seen that the best flame resistance is in the inorganicphophorus group. Others are not adequate for single addition due to insufficientflame resistance and insufficient curing properties.

The test results for combinations of novel addition-type flame retardants areshown in Fig. 6.2. It can be seen that flame resistance is improved by the synergistic

ENVIRONMENTALLY BENIGN MOLDING COMPOUNDS FOR IC PACKAGES 6.3

TABLE 6.2 New Flame Retardants in Sumitomo Study

Flame retardant Assumed mechanism

Inorganic phosphorus Oxygen trap

Aluminum hydrate Discharge of water

Magnesium hydrate Discharge of water, buildup of char layer

Molybdenum compound Hydrogen extract, buildup of char layer

Boric compound Discharge of water

TABLE 6.3 Evaluation Results for Flame Retardants

Flame retardant Necessary quantity Defect

Inorganic phosphorus 1% None

Aluminum hydrate >15% Lack of flame resistance, curability drop

Magnesium hydrate 10% Curability drop

Molybdenum compound 10% Curability drop, water absorption increase

Boric compound 7% Curability drop, water absorption increase

Studied formulation: biphenyl epoxy/filler content 87%.Necessary quantity: minimum amount of flame retardant to pass UL-94 V-0/1.6 mmt.

FIGURE 6.2 Combination effects of two different flame-retardant compoundson UL-94 test.

effect when the molybdenum compound and the boric compound are used together,although they are not effective when used individually, especially for small amounts.

6.2.2 FLAME RESISTANCE SYSTEMS: NOVEL RESIN SYSTEMS

Various resins such as nitrogen-containing resins are proposed as flame-retardantresins; however, actual use is difficult due to insufficient moldability and moistureresistance. Since the foaming phenomenon is well known in the area of thermoplas-tic polymers, it was studied for a set of thermoset polymers as shown in Fig. 6.3. Theresults are shown in Table 6.4 and Fig. 6.4. It can be seen that the longer the distancebetween functional groups at cross-linked points in the hardener, the higher theflame resistance. Also, flame resistance is related to modulus at heated state, as seenin Fig. 6.4. An enlarged photo of a sample burned in the UL flammability test isshown in Fig. 6.5. It can be seen that the high-modulus compound shows carbonizedlayers and cracks, but the low-modulus compound with high flame resistance showscarbonized layers and foamed structure.

6.2.3 EFFECTS OF RAISED REFLOW TEMPERATURES

ON MOLDING COMPOUNDS

In order to study the lead-free reflow temperature resistance of the new compounds,two kinds of packages, 80-pin PQFP and 208-pin PQFP, were used for the evalua-tion.The body size, leadframe, and chip dimensions for the 80-pin QFP were, respec-

6.4 CHAPTER SIX

FIGURE 6.3 Resin structures studied by Sumitomo.


FIGURE 6.4 Effect of modulus of EMC on flammability.

tively, 14 × 20 × 2.7 mm, 42 alloy, and 7.5 × 7.5 mm (with passivation). The body size,leadframe, and chip dimensions for the 208-pin QFP were, respectively, 28 × 28 × 3.2mm, copper, and 10 × 10 mm (with passivation). The die attach material for the 42alloy leadframe was epoxy resin group Ag paste A with a curing condition of 200°Cfor 60 min; for the copper leadframe it was epoxy resin group Ag paste B with a cur-ing condition of 175°C for 30 mins as shown in Table 6.5.

These two PQFP packages were treated by the water-absorbing process of theJoint Electronic Device Engineering Council (JEDEC) Level 1 [85°C/85% relativehumidity (RH) for 168 h]. After drying, the packages were reflowed at 240°C and260°C maximum temperatures as shown in the reflow profiles in Fig. 6.6. Specimenswere subjected to three repetitions of reflow.

The weight of the packages was measured at the start and after moisture absorp-tion and reflow. Also, the amount of absorption before and after reflow wasobtained. Surface appearances were inspected visually and the internal elements ofthe package were studied by scanning acoustic microscopy.

In order to find out the influence of raised reflow temperature on the new mold-ing compounds, tests were preformed as shown in Table 6.6. It can be seen from theobjective of these tests that (1) Ag paste mounting without chips and without chip

TABLE 6.4 Evaluation Results for Various Resin Systems

Molding compound EMC-1 EMC-2 EMC-3 EMC-4 EMC-5

Epoxy Biphenyl epoxy

Formulation Hardener A B C D E

Flexural 240°C (MPa) 2000 2100 1200 600 1100modulus

Total flaming 1.6 mmt (s) Burned up 111 76 47 61time 3.2 mmt (s) 255 310 52 49 84

Studied filler content: 87%.

6.6 CHAPTER SIX

TABLE 6.5 Material Properties of Ag Pastes A and B

Ag paste

Item Condition Unit A B

Ag content wt% 70 75

CTE 1 (below Tg) TMA 10−5/°C 3.0 3.0

CTE 2 (above Tg) TMA 10−5/°C 8.0 8.0

Tg TMA °C 125 110

Die shear strength 200°C, 4 mm2 N 10.8 17.6

Flexural modulus RT MPa 6400 4900

Water absorption 85°C/85%, 72 h % 0.4 0.25

Note Application Alloy 42 Cu

islands, and (2) without moisture absorption, other than the normal level of chipmounting by Ag paste. Thus, these tests show the influence not only of reflow tem-perature but also of mounting materials and moisture absorption. The epoxy mold-ing compound is biphenyl-type epoxy resin (EMC-6), which is a mainstream insurface-mount technology packages in use today. The properties of this resin areshown in Table 6.7.

The measured amount of absorption and desorption after reflow is shown in Fig.6.7. It can be seen that the amount of desorption at a maximum 260°C reflow tem-perature is 1.4 times larger than that at 240°C. This means that the generated vaporpressure is higher at 260°C reflow due to a larger amount of vaporizing moisture.

The 80-pin PQFP package reflow resistance is shown in Table 6.8. It can be seenthat defects such as cracks and delamination increase when reflow temperature israised to 260°C from 240°C. Further, delamination of the lead is found in the samplelevel without chip islands, as shown in Fig. 6.8.

FIGURE 6.5 Failure surfaces after flame test.

In summary, the foaming phenomenon seen in a resin system is an effective wayto achieve flame resistance by selecting appropriate resins. A synergistic effect isbrought about by the different flame-retarding mechanisms of each; for example,molybdenum shares hydrogen abstraction and char formation and boric compoundsshare dehydration and heat absorption.

In summary, in order to resist the raised reflow temperature due to lead-free sol-dering, the new molding compound resin should decrease the moisture absorptionby 30 percent compared to conventional compounds. Also, improvement of theadhesion in the encapsulation resin seems to be important for resisting the raisedreflow temperature.


FIGURE 6.6 Two reflow temperature profiles for this study.

TABLE 6.6 Objectives and Test Conditions for Evaluating the Effects of Elevated Reflow Temperature

Evaluation content Main objective

No chip mounting Influence of chip and die mount material

No die pad Grasp of EMC performance without other composed materials

No water absorption Influence of moisture contenttreatment

Reflow temperature Comparison of 240 and 260°C reflow

TABLE 6.7 Material Properties of Molding Compound EMC-6

Item Unit RT 240°C 260°C

CTE 10−5/°C 1.4 5.4 5.4

Flexural strength MPa 178 22 20

Flexural modulus MPa 18,600 690 650

Adhesion strength Alloy 42 MPa 18.4 1.7 1.5

Cu MPa 19.0 1.6 1.4

6.2.4 HALOGEN-FREE MOLDING COMPOUNDS

FOR LEAD-FREE SOLDERING

Based on the studies in Secs. 6.2.1 through 6.2.3 and the guidelines presented in Fig.6.9 as well as pre-examination of various resin systems, the two resin systems shownin Fig. 6.10 were found by Sumitomo13,14 to be excellent. Their content and materialproperties are shown in Table 6.9.

The resin system with super low viscosity can improve reactions with high cross-link density and makes high filling possible. The super-flexible resin system has lowcross-link density; high flexibility, decreasing generated stress; and low moistureabsorption due to many aromatic rings in the resin skeleton.

6.8 CHAPTER SIX

FIGURE 6.7 Effect of reflow treatment on moisture absorption and dischargedmoisture.

TABLE 6.8 Test Results for 80-Pin PQFP

Test no. T-1 T-2 T-3 T-4 T-5

Package structure Die pad existence Yes Yes Yes No Yes

Chip mounting Yes Yes No No Yes

Water absorption 85°C/85%, 168 h Yes Yes Yes Yes No

Reflow temperature °C 240 260 260 260 260

Number of tested packages 10 10 10 10 10

External cracks 0 7 0 0 0

Internal cracks 0 10 7 0 0

Delamination on die surface None None — — None

Delamination on lead finger tips Small Large Large Large None

Delamination below die pad None Large Large — None

The test results for these novel resin 80-pin and 208-pin PQFP systems are shownin Figs. 6.11 and 6.12, respectively. In these figures, the failure point is calculated inaccordance with following crack points:

0: none20: small crack within half of length from die pad to lead finger tips50: middle-size crack within inner lead tips

100: large crack over inner lead tips


FIGURE 6.8 Scanning acoustic microscopyimage of 80-pin PQFP without chip pad and chipafter 260°C reflow.

FIGURE 6.9 Sumitomo’s strategy and key technology on environmentallyfriendly epoxy molding compounds.

as well as delamination points:

0: none10: delamination area <5 percent20: delamination area 5 to 25 percent50: delamination area 25 to 50 percent

100: delamination area >50 percent

It can be seen from Figs. 6.11 and 6.12 and Table 6.9 that the resin system with superlow viscosity can attain the UL V-0 flammability test results without flame retardantsif and only if the filler is mixed to 90 percent formulation. However, the super-flexibleresin system shows excellent flame resistance regardless of filler contents.

Also, both resin systems show higher solder crack resistance with higher fillercontents. Furthermore, they clear 260°C maximum reflow temperature with thefiller content of 90 percent for the resin system with super-low viscosity and with 84percent for the super-flexible resin system (Figs. 6.11 and 6.12).


FOR PBGA PACKAGES

One of critical differences between the PQFP and the PBGA packages is that PQFPuses a leadframe and PBGA uses an organic [usually bismaleimide triazine (BT)]substrate.11 Also, unlike PQFP, PBGA only molds the upper portion of the package.In this section, novel halogen-free molding compounds developed by Nitto Denkoare presented. These compounds pass the flammability test and allow minimumPBGA package warpages.

6.10 CHAPTER SIX

FIGURE 6.10 Sumitomo’s resin system for environmentally friendly epoxy moldingcompounds.

TABLE 6.9 Contents and Properties of New Resin System for Environmentally Friendly Molding Compounds

EMC-6 EMC-7 EMC-8 EMC-9 EMC-10 EMC-11 EMC-12

Resin system Unit Current Super Low Viscosity Super Flexible

Item filler content wt% 80 84 88 90 80 84 88

Spiral flow cm 85 >260 180 114 180 107 82

CTE 1 (<Tg) TMA 10−5/°C 1.4 1.2 0.8 0.7 1.6 1.3 0.9

CTE 2 (>Tg) TMA 10−5/°C 5.9 5.0 3.7 2.9 5.3 4.8 3.4

Tg TMA °C 150 130 130 130 135 135 135

Flexural strength RT MPa 168 185 175 184 177 179 185

240°C MPa 23 9 16 21 15 19 24

Flexural modulus RT MPa 19,900 23,700 24,500 28,800 18,200 23,400 22,800

240°C MPa 740 420 920 1,210 410 620 1,000

Adhesion strength at 240°C SiN MPa 5.7 3.8 5.6 6.8 6.5 7.0 6.5

Alloy 42 MPa 1.7 1.1 1.2 1.1 1.3 1.4 1.1

Cu MPa 1.1 3.4 3.4 3.0 1.2 1.8 2.4

Ag plating MPa 0.4 0.8 0.9 1.0 0.5 0.6 0.7

Water absorption Boil 24 h wt% 0.28 0.25 0.22 0.19 0.21 0.18 0.15

Flammability test UL-94 — Fail Fail Fail V-0 V-0 V-0 V-0

6.1

1

6.3.1 HALOGEN-FREE FLAME-RETARDANT AGENTS

Nitto Denko17 selected hydroxides as their halogen-free flame-retardant agents.Thematerials studied are aluminum hydroxide, magnesium hydroxide, and transitionmetal magnesium hydroxide complex (TMMHC). Their general material propertiesare shown in Table 6.10. The scanning electron microscopy images of Magnesium

6.12 CHAPTER SIX

FIGURE 6.12 Crack test results for the 208-pin PQFP after 260°C reflow.

FIGURE 6.11 Crack test results for the 90-pin PQFP after 260°C reflow.

Hydroxide and TMMHC are shown in Fig. 6.13(a) and (b), respectively. It can beseen that the particle size of TMMHC is much larger than that of magnesiumhydroxide.

The thermal properties and the particle sizes of these agents were measured bythermogravity analysis (TGA)/dynamic thermoanalysis and a laser particle distribu-tion analyzer, respectively.The compound flow viscosity was measured by a flow testeras a parameter to evaluate the potential damage due to narrow pad pitch and longinternal wires. The flammability performance was measured based on the UL-94 testand the passing level is V-0, and the dimensions of the samples were 127 × 12.7 ×1/16 mm.

Figure 6.14 shows the position for measuring PBGA package warpage and thedimensions of key elements (such as the chip, BT substrate, and molding compound)of the PBGA package. The package warpage was measured by the Micro DepthMeter from the top surface of the package in the diagonal direction.

Figures 6.15, 6.16, and 6.17 show, respectively, the TGA characteristics of magne-sium hydroxide, aluminum hydroxide, and non-flame-retardant material. It can beseen that (1) magnesium hydroxide, which has a higher reaction temperature, ismore effective than aluminum hydroxide; and (2) the heat decomposition of the


TABLE 6.10 General Properties of Hydroxide Flame Retardants

Mean of Decomposition DTG peakparticle size temperature temperature Weight loss

(µm) (°C) (°C) (wt%)

Aluminum hydroxide 15 230 300 34.6

Magnesium hydroxide 0.7 350 420 30.9

TMMHC 3 300–350 420 25–30

FIGURE 6.13 Scanning electron microscopy (a) images of magnesium hydroxide (×10,000) and (b) TMMHC (×10,000).

molding compound without flame-retardant agent starts from 300°C, and the DTGpeak appears at 480°C.

To achieve flame-retarding characteristics, the endothermic chemical reactiontemperature that generates water must be around the temperature of molding com-pound heat decomposition. In order to minimize the content of flame-retardantagent, TMMHC is introduced to accelerate the carbonization, which improvesflame-retarding performance.The flame-retarding mechanism of TMMHC is shownin Fig. 6.18. It can be seen that the hydroxide complex releases water underendothermic chemical reaction.Also, carbonization of the material is accelerated bythe transition metal. These then stop the oxygen supply.

6.14 CHAPTER SIX

FIGURE 6.14 Chip and package dimensions as well as position for measuring package warpage.

FIGURE 6.15 TGA curves for magnesium hydroxide.

The flammability test results are summarized in Table 6.11, and the TGA mea-surement results are shown in Figs. 6.19 and 6.20 for molding compounds containingmagnesium hydroxide and TMMHC, respectively. It can be seen that the DTG peakwithout flame-retardant agent is around 480°C (Fig. 6.17). The compound contain-ing magnesium hydroxide shows a DTG peak at 350°C and the shoulder is observed


FIGURE 6.16 TGA curves for aluminum hydroxide.

FIGURE 6.17 TGA curves for non-flame-retardant material.

from 400 to 470°C. On the other hand, the compound with the TMMHC shows aDTG peak at 350°C without a shoulder. This means that the TMMHC is most effec-tively working at a compound heat decomposition temperature and keeping tem-peratures lower.

6.3.2 PBGA PACKAGE WARPAGE CONTROLLED BY Tg DISPERSION

For conventional flame-retardant systems such as brominated epoxy and antimonytrioxide, the relationship between the package warpage and the total shrinkage ofmolding compound (at room temperature) is linear, as shown in Fig. 6.21. The totalshrinkage is defined as the sum of (1) curing shrinkage during molding and postmoldcuring; and (2) thermal shrinkage during cooling from curing temperature of themolding compound to room temperature. The extrapolation point, where the zerowarpage is given, is defined as the total shrinkage of molding compound equal to 0.2percent. (The value of 0.2 percent is equal to thermal shrinkage of BT substrate from175 to 25°C.)

In order to minimize the total shrinkage with the purpose of minimizing room-temperature warpage, most of the conventional PBGA compounds have a very highfiller content (over 90 wt%). These molding compounds have the advantage ofimproving flame retardancy due to the very low organic material content. However,

6.16 CHAPTER SIX

TABLE 6.11 Flammability Test Results

Flame retardant Magnesium hydroxide TMMHC

Content (%) 20 20

V-0 level No pass Pass

FIGURE 6.18 Flame-retarding mechanism ofTMMHC.

without flame-retardant agents, even with 92 wt% filler content, the molding com-pound cannot pass the UL-94 V-0 test.17

Other methods of minimizing molding compound total shrinkage include selec-tion of a compound with a high glass transition temperature Tg and small thermalshrinkage (a stress-reducing agent such as silicone modifier is sometimes introducedinto the molding compound to reduce the shrinkage); (2) selection of a low-Tg mate-rial with low viscosity characteristics (higher filler contents); and (3) controlling themodulus change around the Tg of molding compound (the concept of Tg dispersion).Their effects on PBGA package warpage from room temperature to 200°C areshown in Figs. 6.22 and 6.23. It can be seen that molding compounds with Tg disper-sion lead to the smallest PBGA package warpage, because the drastic change ofmodulus of molding compound around Tg has a great impact on the packagewarpage.


FIGURE 6.19 TGA curve of compound containing magnesium hydroxide.

FIGURE 6.20 TGA curve of compound containing TMMHC.

FIGURE 6.21 Effect of total shrinkage of molding compound on package warpage.

FIGURE 6.22 Effect of temperature as well as high-Tg and low-Tg compounds on packagewarpage.

6.18

Also, it can be seen from Fig. 6.22 that, for PBGA packages, the conventionaldesign concepts of high-Tg and low-Tg compounds are not the perfect solutions tocontrol package warpage. This is because of the modulus difference between themolding compound and the BT substrate. The modulus of the molding compound ison the same order as that of the BT substrate when it is below the Tg of the moldingcompound. However, the modulus of the molding compound is one or two orders ofmagnitude lower than that of the BT substrate when it is above the Tg. This drasticchange of modulus of the molding compound around the Tg is the driving force forPBGA package warpage and should be controlled.

The effect of viscosity (filler contents) on PBGA package warpage at room tem-perature is shown in Fig. 6.24. It can be seen that as the filler content increases (vis-cosity increases), the PBGA package warpage decreases.

6.3.3 PBGA PACKAGE WARPAGE CONTROLLED

BY STRESS-ABSORBING AGENTS

The effects of introducing a stress-absorbing agent on the PBGA package warpagefrom room temperature to 200°C are shown in Fig. 6.25. (During the cooling process,stress absorption is generated in the molding compound.) It can be seen that themolding compounds with both Tg dispersion and stress-absorbing agent yield signif-icant improvements in controlling the PBGA package warpage. By applying thestress-absorbing agent, a smaller package warpage with the same viscosity and thesame package warpage with a lower viscosity can be achieved, as shown in Fig. 6.26.


FIGURE 6.23 Effect of Tg-dispersed compound on package warpage.

6.20 CHAPTER SIX

FIGURE 6.24 Effect of viscosity (filler content) of Tg-dispersedcompound on package warpage.

FIGURE 6.25 Effect of Tg-dispersed and stress-absorption compound on package warpage.

The choice of Nitto Denko for their new TMMHC flame-retardant PBGA mold-ing compound system is shown in Fig. 6.27.They apply a halogen-free flame-retardantagent (15 wt%) into the PBGA molding compound which is designed by Tg disper-sion and the introduction of stress-absorbing agent. It passed the UL-94 V-0 test andyields reasonable package warpage.


FIGURE 6.26 Effect of viscosity (filler content) of Tg-dispersed with stress-absorptioncompound on package warpage.

FIGURE 6.27 Nitto Denko’s new TMMHC flame-retardant system PBGA com-pound.

6.22 CHAPTER SIX


FOR MAP-PBGA PACKAGES

Recently, the low-cost, minimum-space, and high-speed MAP-PBGA packages havebegun to play a very important role in mobile phones, notebook personal computers,personal digital assistants, and other wireless products and systems. Among all theMAP-type PBGA packages,Amkor’s ChipArrayBGA (CABGA) is the most popu-lar due to its cost, package size, and design, which provide ideal radio frequencyoperation (low inductance) for high-speed applications.

Since the CABGA package is singulated after overmolding the whole matrix,warpage of the matrix is very severe and has to be well controlled (<6 mil) at thecross-sectional direction of the 55.4 × 54-mm mold window size. Also, since theCABGA is near to the chip-scale package, moisture sensitivity level (MSL) ofJEDEC Level 3, 30°C/60% RH/192 h at a maximum reflow temperature of 260°C, isrequired for lead-free soldering.

An environmental friendly molding compound, which does not contain flameretardants with Br/Sb and which fits into the lead-free process, has been developedby Amkor and Kumgang Korea Chemical for the CABGA package with an excel-lent warpage performance and high reliability.18 Their results are presented in thenext section.

6.4.1 HALOGEN-FREE FLAME-RETARDANT RESINS

Four kinds of epoxy resins (biphenyl, naphthol, multifunctional, and orthocresolnovolac type) and the novolac hardener as shown in Fig. 6.28 have been applied tomolding compound for the CABGA packages.The formulation and the matrix resinsystems of molding compounds are shown in Tables 6.12 and 6.13, respectively. It canbe seen that all molding compounds under consideration do not contain bromine(Br) or antimony (Sb) components as the flame retardant. Instead, the modifiedphosphorus compound is used as the flame retardant. All raw materials are melt-mixed in a twin-screw kneader at 115 to 120°C. The mixed compound is cooled andcrushed into powder, which is pressed to form a pellet.

Basic properties of molding compounds such as spiral flow, gelation time, andminimum viscosity are measured at 175°C and their results are summarized in Table6.14. It can be seen that the highest gelation time and lowest viscosity are observedin sample A due to the nature of its biphenyl-type epoxy, which has the lowest vis-cosity of resin and lowest reactivity.

6.4.2 SAMPLE PREPARATION

Specimens for measurement of thermal properties such as Tg, coefficient of thermalexpansion (CTE), and shrinkage were prepared by transfer molding at 175°C andpostmold curing for 6 h at 175°C. Tg and CTE were measured by thermo-mechanicalanalyzer (TMA). The thermal (Tg, CTE, and shrinkage) and mechanical (flexuralstrength and modulus) test results of molding compounds are summarized in Table6.15. The moisture absorption was measured after soaking specimens at 85°C/85%RH for 48 h.

Total shrinkage of cured compounds was calculated by 100(%)(R1 − R2)/R1,where R1 is the inside diameter of the mold die at 175°C (disk type) and R2 is the out-


side diameter of the molded specimen at 25°C. Mechanical properties such as flex-ural strength and modulus were measured by a three-point bending test based onASTM D-790. Flammability of cured compounds (1-mm thickness) was tested bythe UL-94 method.

In order to evaluate their moldability properties such as void, coplanarity, andwire sweep, epoxy-type samples A, B, C, and D were assembled on Amkor’s

FIGURE 6.28 Chemical structure of epoxy and hardener for environmentally friendly moldingcompound.

6.24 CHAPTER SIX

TABLE 6.12 Formulation of New Molding Compound

Raw materials Parts by weight

Epoxy 100

Hardener See Fig. 6.28 52–62*

Accelerator Phosphonium salt 4

Filler Fused silica 1145

Flame retardant Modified phosphorus compound 0.7

Coupling agent γ-glycidoxypropyl trimethoxysilane 6

Releasing agent Carnauba wax 2.5

Colorant Carbon black 2.5

* Hardener content is controlled to be the equivalent of epoxy and phenol functionality (E/P = 1).

TABLE 6.13 Matrix Resin Systems to Be Considered

System Epoxy (ratio) Hardener (ratio)

A E4 (100) H1 (100)

B E4 (20)/E1 (80) H1 (100)

C E4 (20)/E2 (80) H1 (100)

D E4 (20)/E3 (80) H1 (100)

TABLE 6.14 Basic Properties of Molding Compounds

Item Unit A B C D

Spiral flow in 67 45 39 30

Gelation time s 26 22 22 21

Minimum viscosity P 49 98 120 160

CABGA assembly line. Figure 6.29(a) and (b) shows the top and bottom views,respectively, of the molded CABGA packages before singulation. It can be seen thateach strip is partitioned into four molded parts. Each mold window size is 55.4 × 54mm. After singulation, nine singulated CABGA packages 13 × 13 mm in size andhaving 144 pins were obtained from each molded part. The die size was 200 × 203 ×11.5 mil, the gold wire was 1 mil in diameter, and the maximum wire length was 69mil. The average values of coplanarity and wire sweep data from Amkor’s assemblyline are shown in Table 6.16.

6.4.3 EFFECTS OF Tg, SHRINKAGE, AND VISCOSITY

ON PACKAGE COPLANARITY

The effects of molding compound properties such as Tg and shrinkage on the copla-narity are shown in Fig. 6.30. It can be seen that the higher the Tg, the lower thecoplanarity. However, the higher the total shrinkage of the compound, the higher


TABLE 6.15 Physical and Mechanical Properties of Molding Compounds

Item Unit A B C D

Tg °C 141 171 203 165

CTE1 ppm/°C 9.1 11.4 11.5 10.3

CTE2 ppm/°C 33.4 33.9 31.4 31.1

Shrinkage % 0.333 0.288 0.272 0.272

Moisture absorption % 0.143 0.132 0.217 0.155

Flexural strength 25°C, kg/mm2 16.8 16.4 14.3 15.0

240°C,

kg/mm2 1.9 1.3 1.9 1.5

Flexural modulus 25°C, kg/mm2 2641 2845 2623 2695

240°C,

kg/mm2 223 175 348 274

Flammability — V-0 V-0 V-0 V-0

FIGURE 6.29 Amkor’s CABGA, (a) top view and (b) bottom view (before singulation).

the coplanarity. The total shrinkage is composed of thermal shrinkage in the rangefrom room temperature to molding temperature and cure shrinkage at moldingtemperature.

Dimensional change of cured compound from 25 to 175°C is measured by TMAand is used for thermal shrinkage calculation. Cure shrinkage is the differencebetween the total shrinkage and the thermal shrinkage. The thermal shrinkage andcure shrinkage of all samples are plotted as a function of coplanarity in Fig. 6.31. Itcan be seen that the coplanarity is linearly related to thermal shrinkage and pro-portional to the difference of the total shrinkage between the substrate and themolding compound. Thus, when the difference of shrinkage between the substrateand molding compound is minimized, the warpage or coplanarity of the package isminimized.

The relationship between the wire sweep of a package and the viscosity of amolding compound is well-known, and it is widely recognized that wire sweepdecreases with decreasing viscosity of molding compound.18 The same tendency isalso observed for halogen-free molding compound as shown in Fig. 6.32. It can beseen that the wire sweep of CABGA package decreases as the molding compoundviscosity decreases.

6.26 CHAPTER SIX

TABLE 6.16 Moldability Test Results for Molding Compounds

Item Unit A B C D

Coplanarity mil 4.02 3.14 2.40 2.89

Wire sweep % 3.78 3.89 5.21 7.15

FIGURE 6.30 Relationship between Tg, shrinkage, and coplanarity.

6.4.4 MOISTURE SENSITIVITY TESTS

Results of MSL tests are shown in Table 6.17. It can be seen that all the samples aresubjected to the JEDEC Level 3 condition (30°C/60% RH/192 h) with a maximumreflow temperature of 260°C, and the JEDEC Level 2 condition (85°C/60% RH/168h) at a maximum reflow temperature of 240°C. Also, it can be seen that all the sam-ples passed the test except sample C.


FIGURE 6.31 Relationship between coplanarity and thermal and cure shrinkage. Dot-ted line represents the thermal shrinkage of PCB substrate.

FIGURE 6.32 Effect of spiral flow and viscosity on wire sweep.

Under Level 3 conditions, there are external package cracks for all the testedunits (n = 22) of sample C.These cracks propagate to the top of the package after theoccurrence of the die-top delamination, which is scanned by scanning acousticmicroscopy. The typical external crack and delamination are shown in Fig. 6.33.

Under Level 2 conditions, the external package crack followed by the die-topdelamination are observed in 15 units of sample C. The failure mode is very similarto that under JEDEC Level 3 conditions.

There are many material properties of molding compounds, such as moistureabsorption, adhesion strength, flexural strength and modulus, and fracture toughness,that affect the package reliability. Among them, total amount of moisture absorbedcan be the most important factor in MSL performance at higher reflow temperature(260°C). Hence the worst MSL performance observed in sample C is assumed to bedue to the highest amount of moisture absorbed as shown in Table 6.15

6.28 CHAPTER SIX

TABLE 6.17 Reliability Test Results for Molding Compounds

Maximum reflowJEDEC level temperature A B C D

3 260°C Pass Pass Fail Pass

2 240°C Pass Pass Fail Pass

FIGURE 6.33 Photos of (a) delamination and (b) external cracking.

ACKNOWLEDGMENTS

The authors would like to thank S. Iwasaki, S. Ueda, T. Yagisawa, and H. Suzuki ofSumitomo; M. Yamaguchi, H. Shigyo, Y. Yamamoto, S. Sudo, and S. Ito of NittoDenko; B. Kong, H. Yun, and J. Lim of Kumgang Korea Chemical; and Y. Jung, D.Kim, and K. Chung of Amkor for sharing their useful and important knowledge withthe industry.

REFERENCES

1. Bergendahl, C. G., et al.,“Alternatives to Halogenated Flame Retardants in Electronic andElectrical Products: Results from a Conceptual Study,” IVF Research Publication 99824,1999.

2. Bergendahl, C. G., “Electronics Goes Halogen-Free: International Driving Forces and theAvailability and Potential of Halogen-Free Alternatives,” Proceedings of IEEE Interna-tional Symposium on Electronics and the Environment, pp. 54–58, 2000.

3. Hardy, M. L., “Toxicology of Commercial PBDPOs and TBBPA,” IPC Printed CircuitsExpo, April 5, 2000.

4. Hedemalm, P., et al., “Brominated Flame Retardants—An Overview of Toxicology andIndustrial Aspects,” Proceedings of IEEE International Symposium on Electronics and theEnvironment, pp. 203–208, 2000.

5. Hedemalm, P., et al., “Brominated and Phosphorus Flame Retardants—A Comparison ofHealth and Environmental Effects,” Proceedings of Electronics Goes Green 2000, pp.115–120, Berlin, Germany, 2000.

6. Hoffmann, M., et al., “Product Design Methodology,” Proceedings of Electronics GoesGreen 2000, pp. 217–222, Berlin, Germany, 2000.

7. Iji, M., et al., “New Environmentally Conscious Flame-Retarding Plastics for ElectronicProducts,” Proceedings of EcoDesign ’99, pp. 245–249, 1999.

8. Segerberg, T., et al., “Toxicological Aspects of Halogen Free Flame Retardants Based onDenitrification Inhibition Tests,” Proceedings of IEEE International Symposium on Elec-tronics and the Environment, pp. 69–74, 2000.

9. Lau, J. H., Lost Cost Flip Chip Technologies for DCA, WLCSP, and PBGA Assemblies,McGraw-Hill, New York, 2000.

10. Lau, J. H., C. P. Wong, J. Prince, and M. Nakayama, Electronic Packaging: Design, Materials,Process, and Reliability, McGraw-Hill, New York, 1998.


12. Lau, J. H., Thermal Stress and Strain in Microelectronics Packaging, Van Norstrand Rein-hold, New York, 1993.

13. Yagisawa, T., and H. Suzuki, “Development of the Environmentally Friendly Epoxy Mold-ing Compound,” IEEE Proceedings of Electronic Components and Technology Confer-ence, pp. 1737–1746, May 2000.

14. Iwasaki, S., and S. Ueda, “Development of Molding Compound for Non-Antimony andNon-Halogen,” IEEE Proceedings of Electronic Components and Technology Conference,pp. 1283–1288, May 1997.

15. Mogi, N., and H. Yasuda, “Development of High-Reliability Epoxy Molding Compoundsfor Surface-Mount Devices,” IEEE Proceedings of Electronic Components and TechnologyConference, pp. 1023–1029, May 1992.

16. Fujita, H., and N. Mogi, “High-Reliability Epoxy Compound for Surface Mount Devices,”IEEE Proceedings of Electronic Components and Technology Conference, pp. 735–741,May 1993.


17. Yamaguchi, M., H. Shigyo, Y. Yamamoto, S. Sudo, and S. Ito, “Non Halogen/AntimonyFlame Retardant System for High End IC Package,” IEEE Proceedings of ElectronicComponents and Technology Conference, pp. 1248–1253, May 1997.

18. Kong, B., H. Yim, J. Lim, Y. Jung, D. Kim, and K. Chung, “Highly Reliable and Environ-mentally Friendly Molding Compound for CABGA Packages,” IEEE Proceedings of Elec-tronic Components and Technology Conference, pp. 1393–1397, May 2001.

19. Oota, K., “Development of Molding Compounds for BGA,” IEEE Proceedings of Elec-tronic Components and Technology Conference, pp. 78–85, May 1995.

20. Ko, M., M. Kim, and D. Shin,“Investigation on the Effect of Molding Compounds on Pack-age Delamination,” IEEE Proceedings of Electronic Components and Technology Confer-ence, pp. 1242–1247, May 1997.

6.30 CHAPTER SIX

CHAPTER 7ENVIRONMENTALLY BENIGN

DIE ATTACH FILMS FOR IC PACKAGING

7.1 INTRODUCTION

As discussed in Chaps. 1 and 6, environmentally benign electronics manufacturing,especially in Japan and the European Union, wants to be lead-free. Today, the mostlikely solder candidate is the Sn-Ag-Cu family, with a melting point temperaturehigher than 213°C. Thus, the maximum reflow temperature of the lead-free soldertends to rise from 220 to 260°C or even higher. This trend leads to serious problemson current die attach materials for molded-plastic integrated circuit (IC) packages.In this chapter, a couple of new die attach films developed by Hitachi for a lead-freesoldering environment are presented.

For photonic and fiber-optic devices and packaging, lead-free solders [e.g.,indium (In) and tin-indium (Sn-In)] used as joining materials have become evermore popular due to their ductility. Since flux and high-temperature solder refloware not compatible with the optoelectronics packages, low-temperature, fluxlessbonding techniques are therefore very desirable. In this chapter, the low-temperature fluxless bonding technique using In-Sn lead-free solder, developed bythe University of California-Irvine, is discussed.

7.2 ENVIRONMENTALLY BENIGN DIE ATTACH FILMS

Silver pastes have been widely used for die attach materials for more than 40 years.However, due to lead-free soldering, most of the current silver pastes will crack anddelaminate the package (as shown in Fig. 7.1) during the higher-temperature reflowprofile.1–9 As discussed in Chap. 6, the popcorn phenomenon has been under controlthrough various formulation changes in epoxy molding compounds to reduce theamount of moisture absorption. However, die attach materials for lead-free solder-ing are still under development. Hitachi’s die attach films (DF-335-7 and DF-400)5

for lead-free soldering are discussed in the following two sections.

7.2.1 SILVER-FILLED FILM DF-335-7 FOR LEADFRAME

PQFP PACKAGES

Die attach materials for leadframe types of plastic quad flat pack (PQFP) packagesusually consist of a polyimide-base resin, a thermosetting adhesive resin, and a silverfiller. These materials should be low in cost and low in moisture absorption, as wellas high in peeling strength for package reliability.

7.1


Usually, polyimides absorb 1.3 wt % of moisture (type A), as shown in Table 7.1.To reduce its moisture absorption, hydrophobic chemical structures are introducedinto the polymer backbone (types B and C). It can be seen that because of itshydrophobic structure, the new polyimide type C shows significant low moistureabsorption (0.2 wt %). Therefore, it is chosen for a base resin of Hitachi’s new dieattach film DF-355-7.5–7

The effect of thermosetting resin contents on the peeling strength at 275°C isshown in Fig. 7.2. It can be seen that one unit of the thermosetting resin content

7.2 CHAPTER SEVEN

FIGURE 7.1 Plastic package cracking mechanism during solderreflow.

TABLE 7.1 Water Absorption ofPolyimides

Water absorption*Polyimide (wt%)

A 1.3

B 0.7

C 0.2

* Immersed for 24 h at room temperature(Ion exchange water).

leads to the maximum peeling strength, and cohesive failure is the fracture mode.For other thermosetting resin contents, interface failure is the peeling fracture mode.

The effects of thermosetting resin content on the modulus of the die attach filmand adhesion are shown in Fig. 7.3. The modulus is measured by dynamic mechani-cal analysis. Adhesion is calculated by the surface energies of a silicon chip, a lead-

ENVIRONMENTALLY BENIGN DIE ATTACH FILMS FOR IC PACKAGING 7.3

FIGURE 7.2 Effect of the thermosetting resin contenton peeling strength at 275°C.

FIGURE 7.3 Effect of the thermosetting resin content and adhesion’swork and modulus at 275°C.

frame, and the die attach film, which are measured by the contact angles of theirrespective surfaces. Note that the modulus of the die attach film reaches the maxi-mum value with one unit of thermosetting resin content. Also, adhesion’s work inboth the silicon chip–film and leadframe-film interfaces decreases with increasingthermosetting resin contents.

The effect of the die attach film’s silver filler content on the peeling strength at275°C is shown in Fig. 7.4. Note that the maximum peeling strength occurs at 40 wt% silver filler content, and that cohesive failure is the fracture mode. For other silverfiller contents (e.g., 20, 60, and 80 wt %), interface failure is the fracture mode (eithersilicon chip–film or leadframe-film interfaces).

7.4 CHAPTER SEVEN

FIGURE 7.4 Effect of silver filler content on peeling strength at275°C.

The effects of moisture absorption of both the die attach film and of adhesion’swork are shown in Fig. 7.5. It can be seen that the moisture absorption decreaseswith increasing silver filler content because silver fillers do not absorb moisture atall. However, adhesion’s work at the silicon chip–film and leadframe-film interfacesdecreases with increasing silver filler contents.

Figure 7.6 shows the relationship between peeling strength, moisture absorption,and package cracking resistance, as well as high-reliability (hatched) areas. Highpeeling strength and low moisture absorption are necessary to achieve Joint Elec-tronic Device Engineering Council (JEDEC) Level 1 test conditions.

Through all of the preceding investigations, a new die attach film DF-335-7 hasbeen developed by Hitachi,5–7 and its material properties are shown in Table 7.2.Note that DF-335-7 consists of a modified polyimide-base resin having hydrophobicstructure, a thermosetting resin of optimum content, and a silver filler of 40 wt %.


FIGURE 7.5 Effect of silver filler content on adhesion’s work andwater absorption.

FIGURE 7.6 Effect of water absorption on peeling strength andpackage cracking resistance.

The moisture absorption of DF-335-7 is only one-sixth that of a currently used silverpaste (Table 7.2). The peeling strength of DF-335-7 is eight times larger than that ofthe paste.

To evaluate the package cracking resistance of DF-335-7, the 14- × 20- × 1.4-mmlow-profile quad flat pack (LQFP) pack is used. The LQFP consists of a silicon chip(8 × 10 × 0.3 mm), a die attach film DF-335-7 (30 µm thick), a copper leadframe witha flat die stage, and an epoxy molding compound (CEL-9200 by Hitachi). The pack-age is exposed to moisture conditioning at 85°C/85% RH for 24 to 504 h and then istested at high temperature (265 to 275°C) during reflow soldering. No packagecracking is observed in packages using the DF-335-7 after these tests. Consequently,DF-335-7 indicates excellent package cracking resistance at high reflow tempera-tures (265 to 275°C) since its low-moisture-absoprtion and high-peeling-strengthcharacteristics prevent the popcorn phenomenon.

7.2.2 INSULATING FILM DF-400 FOR BT-SUBSTRATE

PBGA CSP PACKAGES

The requirements of die attach materials for bismaleimide triazine– (BT-) substratetypes of plastic ball grid array (PBGA) and chip-scale packages (CSPs) are higherthan for those of leadframe types of PQFP packages.Thus, in addition to lower mois-

7.6 CHAPTER SEVEN

TABLE 7.2 Characteristics of Hitachi’s DF-335-7 and Conventional Silver Paste

Silver TestItem Unit DF-335-7 paste condition

CompositionBase resin — Modified Epoxy resin —

polyimide+thermosettingresin

Silver filler content wt % 40 70 —

Die attach conditionTemperature °C 230 — —Pressure N/chip 0.5 1.0 —Time s 1.0 <1 —Cure °C-min 180-30 180-60 —

Package crack QFP:resistance 14 × 20 × 1.4 mm(85°C/85% RH) Chip size:

245°C — 504 h OK 24 h OK 8 × 10 mm265°C — 168 h OK 24 h NG Leadframe: Cu275°C — 168 h OK 24 h NG EMC: CEL-9200

Water vol % 0.2 1.2 RT for 24 habsorption

Peeling strength Chip size:245°C ×105 Pa 8.1 1.0 5 × 5 mm265°C ×105 Pa 8.1 1.0 Leadframe:275°C ×105 Pa 8.3 0.8 Cu

ture absorption and higher peeling strength, lower stress (which leads to less chipand substrate warpage) and lower glass transition temperature (Tg) (which leads tolower chip attachment temperature) are needed.

The effects of modulus of die attach film and Tg on chip warpage are shown inFigs. 7.7 and 7.8.The chip warpage is measured using a surface roughness meter witha scan distance of 11 mm. The test is performed on a silicon chip (5 × 13 × 0.4 mm),which is attached onto a copper leadframe (0.26 mm thick) using a die attach film (5 × 13 × 0.03 mm) at 230°C. The correlation coefficient between the chip warpageand the film’s modulus is 0.466, whereas that value between the chip warpage and Tg

of base resins is 0.969. Figures 7.7 and 7.8 show that the influence of Tg on chipwarpage is more remarkable than that of the film’s modulus. The chip warpagedecreases considerably with decreasing Tg of a base resin.That is to say, decreasing Tg

could effectively lower the stress in the chip, molding compound, and the substrate.The effects of die attach temperatures and Tg on the peeling strength of the die

attach materials are shown in Fig. 7.9. In the case of a film using a polyimide with arelatively high Tg (120°C), the peeling strength decreases below 220°C of the dieattach temperature. However, the peeling strength of a film using a polyimide with alow Tg (57°C) only decreases slightly, even at 180°C. From the results, it is clear thatthe low-Tg base resin leads to a lower die attach temperature.

Through all of the preceding investigations, a new die attach film DF-400 hasbeen developed by Hitachi5–7 for the BT-types of CSP and PBGA packages, and itsmaterial properties are shown in Table 7.3. DF-400 consists of a modified polyimide-base resin with a low Tg, a thermosetting resin of optimum content, and an insulat-ing filler. With these elements, DF-400 yields a relatively low attach temperature,low stress, and low chip warpage.

Figure 7.10 shows a dynamic mechanical analysis of DF-400. Note that as Tg ofthe base resin is relatively low (57°C), DF-400 melts and flows at high temperaturesbefore curing. This is one of the reasons for DF-400’s attachment ability at 180°C.The storage modulus of DF-400 in the rubber region at high temperatures (above


FIGURE 7.7 Effect of film’s modulus on chip warpage.

100°C) increases after curing because curing forms a polymer network. This shouldresult in its heat resistance and its good reliability.

Table 7.3 shows that the peeling strength of DF-400 is 4 to 5 times greater thanthat of the currently used die attach silver paste in the cases of both a polyimide sub-strate and a glass-epoxy substrate. The moisture absorption of DF-400 is only one-quarter that of the paste.

7.8 CHAPTER SEVEN

FIGURE 7.8 Effect of base resin’s Tg on chip warpage.

FIGURE 7.9 Effect of chip attach temperature onpeeling strength.

To evaluate DF-400’s package cracking resistance, the fine-pitch ball grid array(F-BGA) and stacked CSP are used.The F-BGA (18 × 18 × 0.8 mm) consists of a sil-icon chip, a DF-400 film (40 µm), a polyimide substrate having one electrical layerwithout a solder resist, and an epoxy molding compound. The stacked CSP (8 × 11 ×1.4 mm) consist of two silicon chips, two DF-400 films (25 µm), a polyimide substratehaving one electrical layer without a solder resist, and an epoxy molding compound.These packages are exposed to moisture conditioning at 85°C/60% RH for 168 hand then are tested at 245°C during reflow soldering. No package cracking isobserved in either package using DF-400 after the tests. Consequently, DF-400 isqualified for BT types of CSP and PBGA packages.


TABLE 7.3 Characteristics of DF-400 and Conventional Insulating Paste

Insulating TestItem Unit DF-400 paste condition

Base resin — Low-Tg modified Epoxy —polyimide+ resinthermosettingresin

Warpage of µm 20.0 40 Chip size:silicon chip 5 × 13 mm

Substrate: Cu

Die bondingcondition

Temperature °C 180.0 — —Pressure N/chip 1.0 1.0 —Time s 1.0 <1.0 —

Peeling strength(250°C)Polyimide ×105 Pa 2.35 0.46 Chip size:substrate 5 × 5 mm

without solderresist

Glass epoxy ×105 Pa 7.20 1.95 Chip size:substrate 5 × 5 mm

with solderresist

Water absorption vol % 0.2 0.85 RT for 24 h

Package cracking resistance (85°C/60% RH, 168 h)F-BGA OK/NG OK NG Package size:

18 × 18 × 0.8 mmwithout solderresist

Stacked CSP OK/NG OK — Package size:8 × 11 × 1.4 mmwithout solderresist

7.3 ENVIRONMENTALLY BENIGN In-Sn DIE ATTACH

BONDING TECHNIQUE

Indium has been used as a joining material for photonic devices, and has a relativelylow melting temperature of 156°C, which means that subsequent bonding opera-tions need a process with a bonding temperature that is lower than 156°C. Thus, thedesirable bonding temperature should be lower than 156°C but higher than the max-imum temperature of the solder during device operation. An In-Sn binary systemwith a eutectic temperature of 118°C is often chosen for this purpose.10–15

As a result of the low solidifying temperature, stresses in the bonded structuredue to thermal expansion mismatch are also reduced. It is known that the minimalstress or stress-free point in a conventional soldering process is at the temperaturewhere the solder solidifies. The chip, solder, and substrate contract at different ratesduring the cooldown process, and thus incur stresses that remain locked in the jointinterfaces. If the residual stresses are high, it can cause chip cracking and decreasesolder fatigue. Thus, a lower-temperature bonding process can reduce the residualstresses that are generated during chip attachment.

In conventional soldering processes, oxidation of the solder produces a solidoxide film on the surface of the molten solder. The solid oxide film has a very highmelting temperature and becomes a barrier that prevents molten solder from havingcontact with the parts to be joined. The base metal also gets oxidized easily and theoxide needs to be removed. As a result, the necessary chemical bonds between thesolder and the parts cannot be formed unless the oxide is broken up or removed.The most common method of removing the solder and base metal oxide is to applyacid rosin flux, which reduces the oxide and protects the solder and base metalagainst further oxidation. In applications where flux cannot be used (e.g., photonicand fiber-optic devices), a fluxless process is required.

7.10 CHAPTER SEVEN

FIGURE 7.10 Effect of temperature on the storage modulus.

A new composite solder made of ternary Au-In-Sn alloy has been developed byLee et al.10–15 This multilayer composite solder achieves a process temperature of140°C. Also, a fluxless, oxidation-free bonding technique that prohibits the initialoxidation of solder is reported by Lee et al.10–15 Their results will be reported in thissection.

7.3.1 In-Sn PHASE DIAGRAM

To explain the fluxless bonding principle, it is important to briefly review the In-Snphase diagram as displayed in Fig. 7.11.16 The most important application feature isthe 118°C eutectic point at composition of 51.7 at % In and 48.3 at % Sn. The equi-librium phases are terminal In and Sn solid solutions, two intermediate phases β andγ, and the eutectic between the two phases. There are no known intermetallic com-pounds that form in this system. The terminal In solid solution can contain as muchas 12 wt % Sn at room temperature and 11 wt % at 143°C.The two Sn terminal solidsolutions, metallic β-Sn and semiconducting α-Sn, can contain up to 7 wt % In. Thetransition between the two solid solutions occurs at 13°C, which is below the roomtemperature. The solid solution β-Sn exists all the way up to 224°C near the peritec-tic formation of γ at 224°C.

A eutectic composition of 51.7 at % In and 48.3 at % Sn is a mixture of twophases at room temperature, namely the In-rich β phase and the Sn-rich γ phase.Therange of composition of the β phase changes significantly with increasing tempera-ture.At room temperature the extent of Sn content in the β phase ranges from 15 to


FIGURE 7.11 In-Sn phase diagram.

7.12 CHAPTER SEVEN

28 at % only, while at high temperatures near the solidus curve it broadens tobetween 12 and 44 at % Sn. As the eutectic In-Sn is heated up, the amount of the βphase increases and, consequently, the amount of the γ phase decreases. However,the composition of β changes and becomes less In-rich.

At the eutectic temperature of 118°C, the mixture would completely melt andform a liquid phase. A composition that is slightly more In-rich compared with theeutectic composition would result in a mixture of the liquid phase with imbeddedsolid β grains at 118°C, and the mixture would completely melt at the liquidus tem-perature that is slightly higher than the eutectic melting point. Similarly, an alloy thatis Sn-rich in composition would be a mixture of liquid with imbedded γ grains untilit reaches the liquidus temperature, which is significantly higher.

7.3.2 DESIGN AND PROCESS OF In-Sn SOLDER JOINTS

The In-Sn multilayer composite design from UC-Irvine is shown in Fig. 7.12a. Thinlayers of Cr and Au are deposited on silicon substrate using E-beam evaporation in ahigh-vacuum system. On a separate piece of silicon wafer, Cr, Sn, In, and Au layers aredeposited sequentially using a high-vacuum thermal evaporator in one vacuum cycle.The initial Cr layer enhances the multilayer adhesion to the silicon wafer. The outerAu deposition leads to the immediate formation of AuIn2 intermetallic compound,which conveniently serves as a protective outer layer against inner In oxidation.AuIn2 initially forms at the In-Au interface almost immediately upon deposition, andcontinues to form along its grain boundaries.The Au-In phase diagram clearly showsthe AuIn2 compound phase.17 It has a diffusion coefficient of 6.05 × 10−4 m2/s and acti-vation energy of 0.97 e V.18 AuIn2 is thus a very stable compound and is known toremain unchanged even after many months at room temperature.18 Figure 7.12bshows the final multilayer composite after metallization and In-Au interaction.

During deposition, film thickness is monitored. On the substrate, 0.03 mm of Crand 0.05 mm of Au are deposited. On the die, 0.03 mm of Cr, 2.0 mm of Sn, 2.5 mmof In, and 0.05 mm of Au are deposited.Assuming that the reaction between Au andIn is complete, all of the Au is consumed, using up 0.154 mm of the In layer to formAuIn2. Remaining In is available to interact with Sn during bonding.

To fabricate the joint, the silicon substrate with Cr-Au metallization is cleavedinto square pieces with dimensions of 6 × 6 mm, and the silicon die with Cr-Sn-In-Aumetallization into 4 × 4-mm pieces.The In-Sn joint is achieved by heating up the twopieces held together while applying static pressure of 50 to 85 pounds per squareinch (psi).The assembly is placed in a furnace tube, purged with nitrogen, and heatedin hydrogen up to 140°C in 15 min and held at 140°C for 5 min. Once the bondingprocess is complete, the furnace is cooled down to room temperature. Cooling timeis approximately 20 min. A total of 54 specimens are bonded and evaluated.

The essential mechanism of the joint formation, shown in Fig. 7.13, is elaboratedas follows.When the furnace temperature reaches 118°C (the In-Sn eutectic meltingtemperature), the In would react with the Sn to form a thin liquid phase at the inter-face. Since a 140°C bonding temperature is lower than either the In melting point(157°C) or the Sn melting point (232°C), it appears that none of the multilayer com-posite would melt.

The fundamental concept is solid-state diffusion below the 118°C eutectic tem-perature. When the furnace temperature is moving toward 118°C, the In and Sn onthe interface interdiffuse in solid state to form a thin solid layer of eutectic alloy.When the temperature reaches 118°C, the solid eutectic layer melts and turns intoliquid. As the temperature moves beyond 118°C, the liquid eutectic phase dissolves


the adjacent In and Sn layers, and eventually turns the In-Sn composite into liquidphase.The molten In-Sn alloy wets and breaks up the thin AuIn2 layer to form a mix-ture of liquid with imbedded solid grains of AuIn2.The melting temperature of AuIn2

is 540.7°C, so it remains in the form of small solid grains during the entire bondingprocess. The mixture then comes in contact with the Au layer on the substrate andforms additional AuIn2. Once the bonding process is complete, the furnace is cooleddown to room temperature and the joint solidifies.

It should be noted that another new In-Sn bonding design and technique, whichleads to a wider process temperature window, has also been developed by Lee et al.10–15 The remelting temperature of the new In-Sn solder joints is in the rangefrom 175 to 200°C.

FIGURE 7.12 (a) In-Sn initial multilayer composite design and (b) In-Snmultilayer composite after AuIn2 formation.

(b)

(a)

7.14 CHAPTER SEVEN

FIGURE 7.13 Principles of the In-Sn fluxless bonding process. (a) Upon deposition, the AuIn2 layeris formed. (b) At 118°C, an In-Sn eutectic liquid phase is produced.

(a)

(b)

7.3.3 CHARACTERIZATION OF In-Sn SOLDER JOINTS

Scanning acoustic microscopy (SAM) is used to study the quality of the joints andto detect any voids inside the joint.10–15 For the samples studied, the contrast inacoustic images is caused mostly by the mismatch in acoustic impedance betweenthe joint material and the air, which is the void. A good sample would give a brightpicture, whereas a void would appear as a dark spot in the acoustic image.The oper-ating frequency is 140 MHz for this system, and the corresponding spatial resolu-tion at this frequency is approximately 25 µm. Therefore, it is able to detect voidsinside the joint that are 25 µm or larger with virtually no sample preparation. TheSAM images show that excellent bonding is consistently achieved and that voidsrarely occur inside the joint. Figure 7.14 displays a typical SAM image of a goodbonding.

To examine the thickness uniformity of the joint and to study the microstruc-ture of the joint, several specimens are cut and polished. Cross sections are exam-

ined under an optical microscope, a scanning electron microscope (SEM), and anenergy dispersion x-ray microanalyzer (EDX) system. Figure 7.15 exhibits anSEM image from the polished joint cross section. The thickness of the joint isfound to be very uniform throughout the cross section. The typical thickness is 5µm. It is somewhat difficult to analyze in detail the bonding mechanism and theresulting microstructure of the joint using only the SEM and the EDX. However,it is observed that the joint is composed of grains that are 2 to 5 µm in diameterand a surrounding matrix. The grains appear brighter than the matrix under thesecondary electron detector.

The results of the EDX on composition at several key locations are given in Table7.4. Locations 4 and 5 are from the matrix area, while the other three locations arefrom different grains. It is evident that there is a sharp difference in the compositionof the matrix and the grains. The matrix is mainly composed of Sn, while the visiblegrains contain a large amount of Au and In and very little Sn. It seems that during


FIGURE 7.13 (Continued) (c) During the 118- to-140°C range, additional liquid as well as AuIn2

are produced. (d) Solidification of the mixture completes the joint formation.

(c)

(d)

cooling, the alloy is divided into regions that are Sn-rich and regions that are In-rich.The overall composition of the joint calculated from the original multilayer struc-ture is 54.5 at % In, 42.1 at % Sn, and 3.4 at % Au. It appears that the matrix is com-posed of AuIn2 grains embedded in a solid solution of In in Sn. The most probablephase for this solid solution is β-Sn, which extends from 0 to 7 wt % of In at roomtemperature. Regarding the grains inside the joint, the exact compounds and phasesthat may be present are not as obvious. However, one possible explanation may be amixture of the Au-In γ phase, which exists in a narrow region around 30 at % In atroom temperature, and the intermetallic compound AuIn.18 Small amounts of one ormore In-Sn alloy phases may also be present. It is interesting to observe that noAuSn, AuSn2, or AuSn4 grain shown in the Au-Sn phase diagram18 is detected in thejoint.

Since the Au content in the joint is very small, it is logical to assume that themajority of the joint is made up of the In-Sn matrix rather than the grains. At first,the large amount of Au in the grains shown in the EDX results and the SEMimage seem inconsistent with the overall composition of the joint. However, oneshould keep in mind that the two-dimensional SEM image is misleading in theaccurate representation of the volume fractions of the materials in the joint. Theactual volume fraction of the grains is much smaller than it appears becausethe grains are spherical in shape, rather than cylindrical running through the entirejoint. Thus the total volume fraction of the grains inside the joint is significantly

7.16 CHAPTER SEVEN

FIGURE 7.14 SAM image of a perfectly bonded sample.

smaller than what appears in two-dimensional analysis. With this geometric con-sideration, the EDX results are quite consistent with the overall composition inthe joint.

A debonding test is performed on several samples. The overall remelting tem-perature of the joint is between 125 and 150°C. This indicates that, among the sam-ples tested, the In-Sn composition of the matrix in the joint ranges from neareutectic to Sn-rich. This result actually offers a very important advantage in that itincreases the maximum temperature that the device package can withstand after thebonding process.


FIGURE 7.15 SEM image of the cross section of a sample bonded with the fluxless Sn-In technol-ogy developed by UC-Irvine.

TABLE 7.4 Composition Determined by EDX at Locations Indicated in Fig. 7.15

Location In at % Sn at % Au at %

1 36.34 1.28 62.38

2 36.26 2.28 62.47

3 34.85 8.00 57.15

4 22.99 66.40 10.61

5 24.17 64.52 11.31

ACKNOWLEDGMENTS

The authors would like to thank S. Takeda and T. Masuko of Hitachi, and C. C. Lee,R. Chuang, S. Choe, and W. So of the University of California-Irvine for sharing theirimportant and useful technologies with the industry.

REFERENCES

1. Lau, J. H., Lost-Cost Flip Chip Technologies for DCA, WLCSP, and PBGA Assemblies,McGraw-Hill, New York, 2000.

2. Lau, J. H., C. P. Wong, J. Prince, and M. Nakayama, Electronic Packaging: Design, Materials,Process, and Reliability, McGraw-Hill, New York, 1998.

3. Lau, J. H., Ball Grid Array Technology, McGraw-Hill, New York, 1995.4. Lau, J. H., Thermal Stress and Strain in Microelectronics Packaging, Van Nostrand Rein-

hold, New York, 1993.5. Takeda, S., and T. Masuko, “Novel Die Attach Films Having High Reliability Performance

for Lead-free Solder and CSP,” IEEE Proceedings of Electronic Components and Technol-ogy Conferences, pp. 1616–1622, May 2000.

6. Takeda, S.,“A Novel Die Attach Films Having High Reliability Performance,” Proceedingsof the 9th Micro Electronic Symposium, Osaka, Japan, pp. 249–252, October 1999.

7. Takeda, S., T. Masuko, Y. Miyaders, M. Yamazaki, and I. Mackawa, “A Novel Die BondingAdhesive-Silver Filled Film,” IEEE Proceedings of Electronic Components and Technol-ogy Conferences, pp. 518–524, May 1997.

8. Harads, M., “X-ray Analysis of the Package Cracking During Reflow Soldering,” IEEEProceedings of International Reliability Physics Symposium, pp. 182–187, 1992.

9. Lau, J. H., R. Chen, and C. Chang,“Real-Time Popcorn Analysis of Plastic Ball Grid ArrayPackage During Solder Reflow,” IEEE Proceedings of International Electronics Manufac-turing Technology Symposium, pp. 455–463, October 1998.

10. Choe, S., W. So, and C. Lee, “Low Temperature Fluxless Bonding Technique Using In-SnComposite,” IEEE Proceedings of Electronic Components and Technology Conferences,pp. 114–118, May 2000.

11. Chung, R., S. Choe, and C. Lee, “A Fluxless Sn-In Bonding Process Achieving High Re-Melting Temperatures,” IEEE Proceedings of Electronic Components and TechnologyConferences, pp. 671–674, May 2001.

12. Lee, C., C. Wang, and G. Matijasevic, “A New Bonding Technology Using Gold and TinMultilayer Composite Structures,” IEEE Transactions on Components, Hybrids, and Man-ufacturing Technology, vol. 14, pp. 407–412, June 1991.

13. Lee, C. Wang, and G. Matijasevic, “Advances in Bonding Technology for Electronic Pack-aging,” ASME Transactions, Journal of Electronic Packaging, 115:201–207, June 1993.

14. Chen, Y., W. So, and C. Lee, “A Fluxless Bonding Technology Using Indium-Silver Multi-layer Composites,” IEEE Transactions on Components, Hybrids, and Manufacturing Tech-nology, 20:46–51, March 1997.

15. Matijasevic, G., and C. Lee, “Void-Free Au-Sn Eutectic Bonding of GaAs Dice and ItsCharacterization Using Scanning Acoustic Microscopy,” Journal of Electronic Materials,18:327–337, March 1989.

16. Okamoto, H., and T. Massalki, Binary Alloy Phase Diagram, vol. 3, ASM International,Metals Park, OH, pp. 2295–2296, 1990.

17. Okamoto, H., and T. Massalki, Binary Alloy Phase Diagram, vol. 3, ASM International,Metals Park, OH, pp. 381–383, 1990.

18. Moffat, W., The Handbook of Binary Phase Diagrams, Genium Publishing Corporation,Schenectady, NY, 1995.

7.18 CHAPTER SEVEN

CHAPTER 8ENVIRONMENTAL ISSUES FOR

CONVENTIONAL PCBs

8.1 INTRODUCTION

Electronics manufacturing and disposal of electronics products are perturbing theequilibrium of the atmosphere-land-ocean system on earth. While microelectronicsimproves the global standard of living, its environmental impact poses a majorthreat to the quality and existence of life on earth. In the past century, the major con-cern was pollution from the automobile industry, the steel industry, and energy pro-duction using coal. The harmful effects of combustion engine emission made theproblem even worse. This is further aggravated by the present impact from the elec-tronics manufacturing industry. These cumulative effects have brought the environ-ment to such a level that clean water and air can no longer be guaranteed.

Regarding environmental concerns about microelectronics, in addition to the useof lead in solder materials, which has been a well-known issue, the other significantaspect is the use of bromine and antimony-containing flame retardants in printedcircuit boards (PCBs) and in area-array package substrates. These flame retardantsare based on a combination of bromine and antimony oxide. Halogens, includingbromine, are only weak fire retardants, and antimony oxide by itself is not a fireretardant; however, when combined, they become very effective. The combinationretards flames in two ways. During burning, antimony oxide promotes the formationof char (essentially, carbon), which reduces the formation of volatile gases. At thesame time, the heat of initial combustion promotes a cross-linking between theorganic compound and the antimony, which results in a more stable thermoset poly-mer. In addition, at temperatures above 315°C, bromine forms hydrobromic acid,which reacts with the antimony oxide to form antimony trihalides and oxyhalidesthat trap free radicals, inhibiting ignition and pyrolysis.1

The objection to bromine-antimony flame retardants is what happens at the endof a product’s life.According to the European Union (EU) directive on Waste Elec-trical and Electronic Equipment (WEEE), when electronics that are treated withbromine flame retardants are recycled, they can generate dioxins and furans. (Diox-ins and furans are based on a common chemical skeleton, onto which one to eightchlorine atoms can be attached in a variety of positions. The different combinationsyield 75 distinct dioxins and 135 different furans.)

In the mid-1980s, studies showed that toxic polybrominated dibenzofurans(PBDFs) and polybrominated dibenzodioxins (PBDDs) were formed during theextruding process, which is part of the plastic recycling process. There is also evi-dence that polybrominated diphenyl-ethers (PBDEs) might act as endocrine dis-rupters, and high concentrations of PBDEs have been found in the blood of workersin recycling plants. Moreover, polybrominated biphenyls (PBBs) have been found inArctic seal samples, which indicate a wide geographical distribution from both PBBmanufacturing and waste dumps. Once PBBs have been released into the environ-ment, they can reach the food chain.

8.1


On the packaging side, some qualified suppliers may be able to produce sub-strates that do not contain halides or antimony, so there is no problem meeting theEuropean Community (EC) requirements. In the larger world of complete systems,it is not so clear whether there are easy substitutes for bromine and antimony.A con-troversy arises because in the United States, the State Fire Marshals Association hasindicated that it will not accept consumer electronics that contain fiberglass-reinforced circuit boards not protected with brominated flame retardants. In arecent press release distributed to European publications, the association indicatedthat it will call for a trade embargo on any consumer electronics manufactured with-out brominated flame retardants. It remains to be seen how this will be resolved.2

8.2 INFLUENCE OF ELECTRONIC PRODUCTS

Until the early 1980s, electronics manufacturing was still considered a clean processin the public eye. Due to increasing environmental awareness, more detailed inves-tigations on electronics manufacturing have led to a belief that the operation is nolonger clean. Figure 8.1 illustrates six aspects of the life cycle of an electronic prod-uct, which includes the following:3

Electrical, mechanical, and chemical design Raw materials, production of integrated circuits (ICs) and passive components,

and organic board fabrication, involving a variety of chemicals Packaging of components using a variety of harmful materials Transportation of the end products to the customers Usage and consumption of products Disposal and recycling

From an environmental standpoint, the preceding factors fall into several categories,a detailed discussion of which follows.

8.2.1 MAJOR ENVIRONMENTAL CONCERNS4, 5

8.2.1.1 Global Warming. Global warming is, by far, the most important environ-mental concern today. Global warming is caused by excessive shielding gases such asCO2, CH4, N2O, and Freon, which mostly come from human activities (e.g., energygeneration by burning fossil fuels).These gases act as a barrier to prevent the releaseof ground heat to the universe, resulting in higher temperatures on earth. In fact, thepresence of these gases and the ozone is also required to some extent to shield theharmful radiation from the sun. Otherwise, the atmosphere would provide no insula-tion to heat radiation in and out of the earth, and the earth would turn into a frigidairless moon.The moon absorbs four times more solar heat than the earth. But its sur-face is, on the average, 63°F colder than the earth’s due to the absence of atmosphere.

According to the Intergovernmental Panel on Climate Change (IPCC) report,the concentration of CO2 in 2100 will be twice that of 1990, as shown in Fig. 8.2. Thiswill cause an increase in temperature by 2°C and a rise in the sea level by 50 cm. It isbelieved that 1 billion people will be submerged and that there will be a grain andfood shortage, as well as abnormal weather, because of the increased carbon dioxidelevel. To prevent this situation, a treaty for lowering the emission of warming gaseswas proposed in 1994, as illustrated in Table 8.1.

8.2 CHAPTER EIGHT

ENVIRONMENTAL ISSUES FOR CONVENTIONAL PCBs 8.3

FIGURE 8.1 Life cycle of electronic products and environmental concerns.

FIGURE 8.2 Production of CO2 in developing and advanced nations.

8.2.1.2 Depletion of Natural Resources. Minerals are precious to human beingsliving in this highly industrialized society.The minerals are available in limited quan-tities, which were produced during the earth’s creation over 46 billion years ago.Human beings have continually been consuming minerals, especially since thebeginning of the industrial revolution. If the population continues to use them at thispace, most of the minerals will be depleted within the next 100 years, as shown inTable 8.2.

8.2.1.3 Ozone Hole, Acid Rain, and Pollution. Decomposition of Freon gas inthe stratosphere generates chlorine atoms. These atoms destroy the ozone layer andresult in an ozone hole. In the presence of the ozone hole, ultraviolet rays directlyreach the ground, having a negative influence on people’s health and the ecosystem.The ozone hole is becoming larger and larger every year.

Rain with pH below 5.6 is called acid rain. Sulfur oxides (SO2) and nitrogenoxides (NO2) are gases generated from the use of fossil fuels, which acidify rain. Theacidification of soil, rivers, lakes, and marshes badly damages the forests and ecosys-tem.

Pollution is caused by discarding harmful industrial waste into the soil or sea.Typical harmful substances include heavy metals and plastic materials. For instance,the daily waste generated in the United States was around 65 million tons in 2001.On a worldwide scale, the amount of waste going into landfills is gigantic. Pollutionalso occurs from the toxic gases that are released when harmful industrial wastematerials are incinerated.

8.4 CHAPTER EIGHT

TABLE 8.1 Targeted Reduction of ObjectionableGases (from 2008 to 2012 as a Percentage of 1990 Levels)

The objectionable gases (CO2, CH4, N2O, HF, PFC, SF6)

United States 7%

Japan 6%

European Union 8%

TABLE 8.2 Depletion of Critical Materials

Total amount Remaining amount Remaining duration

Oil (barrels) 2 trillion 900 billion 45 years

Natural gases (m3) 200 trillion 100 trillion 56 years

Coal (tons) 10 trillion 1 trillion 300 years

Uranium (tons) — 4.36 million 72 years

Silver (tons) 280,000 15,000 19 years

Gold (tons) 42,000 1,800 23 years

Titanium (tons) 173 million 6.45 million 27 years

Copper (tons) 352 million 9 million 39 years

Nickel (tons) 49 million 0.87 million 36 years

Steel (tons) 66 billion 0.982 billion 67 years

8.2.1.4 Decrease of Tropical Rain Forests and Increase of Desert Areas. Thetropical rain forest area has been continuously reduced by burning forests to gaincultivable land, by cutting trees to produce lumber, and by acid rain. It is estimatedthat 17 million hectares of rain forests are disappearing from the surface of the eartheach year. When the tropical rain forests decrease, the ability to consume CO2 fromthe atmosphere decreases, thus accelerating the global warming process. On aver-age, the desert area increases by 6 million hectares worldwide every year as a resultof drought, overcultivation, and the use of pesticides. This influences the productionof food and leads to changes in climate.

8.2.1.5 Transfer of Harmful Industrial Waste. Advanced industrial nations usu-ally have strict regulations and high costs for the disposal of industrial waste. As aconsequence, harmful industrial waste is often transferred from countries where dis-posal costs are high to countries where the disposal costs are low. Some countries areprone to severe environmental damage, because waste is left undisposed due tolenient local laws and economic constraints. To control the border transgression ofharmful industrial waste, the Basel Treaty was adopted in 1989.

It should be noted that the aforementioned environmental issues do not standalone; instead, they influence one another, as shown in Fig. 8.3. Furthermore, sincethe population will increase from 6 billion people in 2000 to 12 billion by 2050, it isevident that environmental issues are the most significant issues facing humanbeings today.

8.2.2 ENERGY ISSUES6

About 90 percent of the primary energy demand in the world is supplied by fossilfuels (e.g., coal, oil, and gas). Electricity is by far the largest energy source requiredduring electronics manufacturing and operation of end products and systems.Approximately one-half of the world’s electricity production comes from burningfossil fuels, which cost about 20 percent of total fossil energy resources. The otherhalf of electricity comes from nuclear power and hydropower, both of which are con-sidered to have less of an environmental impact than the fossil-based electricity pro-duction, although they present their own set of problems.

The consumption of fossil energy resources may have several significant envi-ronmental effects. One direct impact is the depletion of fossil fuels, and another isthe higher concentration of polluting gases such as CO2, nitrogen oxides (NO2), andsulfur oxides (SO2). These gases lead to global warming and acid rain as mentionedbefore. Nitrogen oxide and different hydrocarbon emissions are also significantsources of ground-level ozone, which damages agricultural production and con-tributes to health hazards.

In general, there are two approaches by which energy consumption can be con-trolled. First, consumers can choose suppliers that use better energy productionmethods or better fuel qualities. Second, energy consumption can be loweredthrough improved system design that consumes the energy in a more effective way.For instance, reduction in standby consumption of electricity in ac/dc devices couldbe very important for products that are plugged in permanently but are not in realuse. During long-term operation, more energy is consumed in the standby modethan in the actual use. By switching off the device when not in use, the actual use-phase energy consumption can be improved dramatically. For example, consider thecharging of a mobile phone. If the charger is plugged in permanently instead ofcharging it only when needed, the energy consumption during a year of usage can be


8.6 CHAPTER EIGHT

FIGURE 8.3 Mutual relationship among various environmental concerns.

20 times higher. To avoid this kind of energy consumption, one option is to designsmart charging devices that can shut themselves down. Most of the system opera-tions are controlled by software. Hence, software design can play a very importantrole in reducing power consumption, thereby helping the environment.

The daylighting system has been proven to be able to provide energy savings innumerous cases. A leading aviation company in the United States has retrofittedsuch a lighting system in its design and manufacturing areas, and was able to cutalmost 90 percent of the lighting energy being used. The Eco Store with the day-lighting system helped to increase the productivity of employees and also boostsales. Examples like these indicate that there are untapped sources of alternativeenergy. Conventional alternative energy such as wind power and solar energy can beutilized in many tropical developing countries as a source for household energy. Inthe United States, California receives 9 percent of its energy from renewable sourcesother than hydroelectricity. In the energy sector, the world’s fastest-growing tech-nologies are wind power and photovoltaic solar cells.

8.2.3 CHEMICAL ISSUES7

8.2.3.1 Risk Substances in Products. Figure 8.4 illustrates the most commonprocesses for electronics production, starting from an IC to a final product such as aPC or a cellular phone. The relevant materials with environmental concerns andtheir potential solutions are also given for reference.

Lead (Pb). Lead is mainly used in the form of solder alloys for connection ofcomponents to the printed circuit boards (PCBs), surface treatment of components,PCB finishes, and lead batteries. Lead-containing solders, particularly tin-lead eutec-tic solder, have been embraced by the electronics manufacturing industry due totheir combined benefits of low cost, good soldering properties (such as wettability,adequate melting temperature range, and high oxidation resistance), and desiredphysical, mechanical, electrical, and metallurgical properties.

Lead causes severe health problems, including irreversible brain damage andinjury to blood-forming systems, even at relatively low levels in the body. The factthat modern technologies and living habits are significantly contributing to lead con-centrations in humans can be illustrated using the following data:

Prehistoric man: 3 × 10−4 g Pb Current Americans: 1500 × 10−4 g Pb Minimum lead poisoning level: 6000 × 10−4 g

Lead enters the body mostly by ingestion. Even after kidney excretion, it remains inthe bones with an average life of 3 to 5 years.

Cadmium (Cd). Cd is used in nickel-cadmium batteries, chemicals for surfacetreatments, pigments for paints, and in plastics. Cadmium is a highly toxic metal, andit tends to accumulate in the body.

Halogenated Materials. Fluorine (F), chlorine (Cl), and bromine (Br) are veryreactive, and are the key elements in many toxic compounds. In the periodic table,they belong to the highly reactive group of elements called halogens. A halogenatedmaterial contains one or more of these elements.

Halogenated materials are mainly used as flame retardants in electronic prod-ucts. Bromine is the most common halogen used as a flame retardant. To appreciate


the function of a halogen, the mechanism of polymer combustion has to be under-stood. The mechanism of polymer combustion has been found to be as follows:

Polymer decomposes by heat, resulting in the generation of a hydrocarbon. Hydrocarbon is converted to a radical (OH radical) by oxidation. The OH radical repeats the exothermic oxidation of CO while releasing a lot of

heat.

If a halogenated flame retardant is present in the polymer, the heat generated causesthe halogenated materials to decompose and form a halogenate gas (HX), which cantrap the OH radical and end the radical reaction (combustion).

The requirement for flame retardants in the electronics industry goes back to itsinception in the late 1940s and 1950s, when it became apparent that high voltages,current, and heat could cause fires. Brominated flame retardants are largely used inplastic frames, housings, cable sets, and PCBs. The current focus is on the use ofbromine in PCB epoxy resin (Fig. 8.5) to meet the Underwriters Laboratories (UL),Inc., 94 V0 specifications, which define the self-extinguishing capability of materials.

The acceptance of brominated materials became a concern in the late 1970s whenstudies in Great Britain and Germany revealed that two-thirds of the deaths fromaccidental fires were caused by toxic gases rather than by direct fire burns. Further

8.8 CHAPTER EIGHT

FIGURE 8.4 Environmental concerns and potential solutions.


FIGURE 8.5 Brominated epoxy resin.

research showed that, under specialburning conditions (700 to 900°C),brominated plastics can produce diben-zodioxins and dibenzofuranes (Fig. 8.6),which are carcinogenic gases and 10,000times more harmful than cyanate gas.

The preceding concerns resulted inmany proposed legislations but weredefeated because of the lack of avail-ability of a good, low-cost alternative.The WEEE of the EC drafted the mostrecent legislation. This proposal wouldeliminate brominated material by 2004from consumer electronic products.Other countries, including Germany,have already enforced laws that limit theproducts that are allowed to containbrominated flame retardants.

8.2.3.2 Risky Substances Used in the Manufacturing Process. Besides the sub-stances described in the previous sections, there are other materials that are usedspecifically in manufacturing processes, but are not included in the final products.Examples of such materials include the following:

Solvents used in the PCB manufacturing process [e.g., methyl ethyl ketone(MEK), dimethyl formamide (DMF), etc.]

Chemicals used for flux cleaning during solder connection (Freon) Chemicals used in the etching process to print copper lines or circuits on PWBs

(e.g., strong acids, oxidizers) Wastewater Volatile organic compounds (VOC) used in flux, solder paste, and so on

Restrictions have been imposed on the use and disposal of chemicals for flux clean-ing and etching. Solvents that include VOCs and wastewater are a major concern,

FIGURE 8.6 Structure for (a) bromodibenzo-dioxin and (b) bromodibenzofuran.

(b)

(a)

even in today’s PCB manufacturing. Solvents used in industry are regarded as harm-ful and as a source of ecological pollution because they can directly, or indirectly,lead to the following problems:

Poisons to the human body Destruction of the ozone layer in the stratosphere Bad odor Global warming Acidification of rain by generation of photochemical oxidants

Wastewater is another source of ecological pollution. The water from a PCB plantcontains metals (Cu, Sn, Pb, Fe, Ni, etc.), anions (cyanides, fluoroborates, nitrates, sul-fates, etc.), dissolved solids, and suspended solids.The pH is usually between 6 and 9.Ecological pollution can be controlled by implementing legal restrictions. However,it should be recognized that not everything can be controlled or implemented bylaws. A commonsense approach to protect health should prevail in such circum-stances.

As an example of harmful solvents used in industry, the process and chemicalsused in PCB manufacturing will be discussed briefly. A PCB is a composite materialconsisting of reinforcement, such as glass cloth or mat, and a resin such as epoxy.Themajority of the epoxy resins used in the process are room-temperature solids. Theseresins are diluted with a solvent to make a solution of suitable viscosity for impreg-nation into the glass cloth. These toxic solvents evaporate into the atmosphere aftertheir use. A significant amount of heat is also wasted in evaporating these solvents.During this drying process, the resin is partially reacted to reach higher molecularweight, resulting in an adequate melt viscosity. These partially cured materials arecalled prepregs, and the intermediate state of the resin is called B-stage resin. In thisstage, molecules are still linear and the resin can be melted by heat. Prepregs arepiled on each other to reach the desired thickness. They are generally covered withcopper foils on one or both sides and laminated under heat and pressure. During thisprocess, the resin melts, and the piled-up sheets and copper foil adhere to each otherto form a laminated structure. The final product results from the subsequent curingby cross-linking reaction.This fully cured state of matrix resin is called C-stage resin.

8.2.3.3 Replacement of Lead-Based Solders. Lead-based solders are usedextensively in microsystems assembly.Two types of alternate materials, namely, con-ductive adhesives and lead-free solders, are being developed to replace lead-basedsolders.

Isotropic Conductive Adhesives. Isotropic conductive adhesives were devel-oped to replace eutectic solder (63% Sn, 37% Pb). The main advantages of usingpolymeric materials are low die stress and low processing temperatures. Most of theconductive adhesives are silver-filled epoxy with more than 65 wt % filler loading.Silver flakes are used because of their high conductivity. Although these adhesivesoffer electrical conductivity close to that of solder (<5 mΩ/4 × 4 mil bump), and bet-ter stencil yields, their usage has been limited to niche applications such as opticaldevices that are assembled at low temperatures.The deficiency of these adhesives toreplace conventional solders for SMT assembly arises from poor reliability andassembly yields. Silver-filled epoxies are not favorable from an economic point ofview. These materials cost between $2.50 to $7.00/cc compared with $0.55/cc for sol-ders. In addition, silver-filled epoxy is susceptible to brittle fracture, due to the highfiller loading. Unlike lead solders, they do not have self-alignment capability due to

8.10 CHAPTER EIGHT

their low surface energy. Other disadvantages arise from nonreworkability and sil-ver migration failure between tightly pitched leads.

Anisotropic Conductive Adhesives. Anisotropic adhesives also have nicheapplications where low assembly temperature is required. These are mostly used toattach LCD display drivers, because the high solder reflow temperature may destroythe device. Anistropic conductive adhesives may never replace the volume of solderinterconnects because of the high pressure required for their assembly. They alsosuffer from low yield and reliability. Coplanarity is extremely critical and affects theassembly yield.The cured epoxy is also vulnerable to failure by moisture absorption,cracking, and swelling.

Lead-Free Solder Alloys. Replacement of eutectic tin-lead solders is still underdevelopment. Process parameters (e.g., reflow temperature, preheat temperature,flux type, oven gas, or product bake-out times and temperatures) may require mod-ifications. Although there are successful products assembled with lead-free solderstoday, acceptable yield is still the bottleneck for many electronic products. Effortsare still under way to find a suitable replacement. All proposed alternative alloys totin-lead solders use tin as the major constituent, since tin is readily available at a lowcost, has a low melting temperature, a high oxidation resistance, and good solderingcharacteristics such as wettability. Recent studies have been focused on formulatinga suitable element that can be alloyed with tin.The search is limited to about a dozenelements. The acceptance or rejection criteria involve toxicity, cost, performance,and availability. Since 13,500 metric tons of lead are consumed in electronic soldersworldwide, the availability of a substitute element at a low cost is important.

8.2.3.4 Elimination of Halogenated Flame Retardants. There is an ongoingdebate on the replacement of halogenated materials because of the lack of a suitablealternative. For years, many U.S.-based companies and commissions debated numer-ous concerns (Table 8.3). These include phosphorus- or nitrogen-modified epoxyresins, hydrated alumina, magnesium hydroxide, and high-density polymers. Each ofthese, however, presents its own challenges. Phosphorus, for example, is expensiveand presents electrical leakage when exposed to humidity. Hydrated alumina andmagnesium hydroxide, on the other hand, present rheological problems. In spite ofall of the difficulties for halogen replacement, many companies, primarily the Japa-nese consumer electronic manufacturers, are now producing halogen-free laminates


TABLE 8.3 Nonhalogenated Electronic Resin Products

Alternative technology Concern

Phosphorus- and nitrogen-modified Phosphorus is also toxic when burned, is moreEpoxy resin microencapsulated expensive to process (+30%), and will resultPhosphorus in electrical leakage with moisture.

Hydrated alumina Need very high filler loading because flameMagnesium hydroxide retardability is relatively low. Difficult to process

when polymer is loaded with high percentage of weight because of high viscosity; these flame retardants release water at relatively lower temperatures, resulting in lower heat resistance.

High-carbon-ring-density polymers Completely different polymer chemistry that (naturally flame-retardant) is not easily compatible with existing

laminate processing.

and PCBs. These are incorporated into several consumer products that are targetedfor the European market. Japanese companies believe that this initiative will gainfavorable attention from the EC, which, in turn, will boost the depressed Japaneseexport market.

In 1998, a Japanese company announced the production of the world’s first PCmade with a halogen-free motherboard.The resin contains nitrogen- and phosphorus-based flame retardants. They are seeking additional suppliers for their resins andboards. Other companies in Europe and the United States are also planning toimplement halogen-free PCBs for cellular phones. It is estimated that the initialprices will be 10 to 20 percent higher compared with the conventional brominatedPCBs. It is expected that the prices for halogen-free boards will fall as higher vol-umes are manufactured. Other electronic equipment manufacturers also desire tomeet the pending environmental laws, which have generated many requests for thenonhalogenated boards and, consequently, has spurred increased development andproduction of nonhalogenated resins.

Halogenated flame retardants are generally replaced by phosphorus, nitrogen,inorganic hydroxide (e.g., hydrated alumina), and magnesium hydroxide. Usuallythese materials are combined. The combination of phosphorus- and nitrogen-modified epoxy resins is most common.At elevated temperatures, phosphorus in thepolymer is converted to phosphoric acid. This strong acid reacts with the polymerand absorb the water molecules inside, thus converting it to a carbonlike structure(char). This process of polymer condensation by removal of hydroxyl groups isreferred to as carbonization. The carbonized material offers high thermal resistanceand acts as a barrier to the diffusion of O2 into the polymer, thus preventing furthercombustion of the polymer. Nitrogen in polymers forms inert gases during combus-tion and dilutes the radicals generated during the combustion. Phosphorus andnitrogen have a synergistic effect on the termination of the combustion. Inorganichydroxides release the water of crystallization when heated above 200 to 400°C.Theincombustible water can also absorb heat, dilute the radicals, and hence prevent thepolymer combustion.

The combination of phosphorus- and nitrogen-modified epoxy resins has somedrawbacks, although it is the most common halogen-free technology known today. Itis predicted that there will be widespread phosphorus regulation in the near futurebecause of its toxicity level. Hence, the emerging trend is to eliminate both halogensand phosphorus from electronic materials.Accordingly, new flame-retardant epoxieshave been developed from a recent collaboration between a Japanese and a German-based company.These materials do not have halogens or phosphorus sources but areinherently flame-retardant. This compound consists of an aromatic epoxy resin anda phenol derivative hardener. Because of the highly aromatic structure, this resinprovides high heat resistance. On the contrary, the resin is not very rigid, because ithas lower cross-link density after cure. When a polymer that has both high heatresistance and low stiffness is exposed to flames, the combustible gases (hydrocar-bons) generated from the thermal degradation convert the surface into a layer offoam. This layer prevents the diffusion of oxygen and heat into the resin and stopsthe combustion.

8.2.3.5 Reduction of Toxic Solvents. Adopting the new PCB process with nodischarge of any solvent to the environment can prevent toxic solvent evaporation.A PCB manufacturer in Japan has developed this process and has already utilized itin their production line. Figure 8.7 shows a schematic diagram of this process. Unlikethe conventional process, the solvent in this process acts not only as a diluent, butalso as a hardener. It means that 100 percent of the solvent is consumed into the

8.12 CHAPTER EIGHT

resin network structure during curing, thus eliminating the discharge to atmosphere.Furthermore, the resin is directly converted to the C stage without passing throughthe B stage (Fig. 8.8). This new process can save energy in comparison with the con-ventional process, by avoiding heating for solvent evaporation. Since the process iscontinuous, heating and cooling steps are eliminated during lamination. Therefore,this process has a much lower environmental impact in terms of solvent emissionand thermal energy efficiency.

The other way to prevent toxic solvent evaporation in the PCB manufacturingprocess is to use a direct melting process with no added solvents. It should be notedthat most epoxy resins used for PCBs are solids at room temperature. Epoxies canbe formulated with a melting temperature ranging from 80 to 150°C. Therefore, bythermal treatment, epoxies can be melted to attain adequate viscosity for impregna-tion. In this process, the liquid epoxy resin is directly impregnated into the rein-forcement such as glass cloth. When utilizing this process, toxic solvent can beeliminated. Some companies have already demonstrated this process in their pro-duction line. Furthermore, collaboration between industry and academia in theUnited States has resulted in a novel recyclable and solvent-free epoxy resin systemfor PCBs. This novel resin is a lignin-based epoxy, which can be diluted with water.


FIGURE 8.7 Comparison of PCB fabrication processes. (a) Solvent usage in conventional processand (b) new process without emission of solvent. (Courtesy of Matsushita Electric Works.)

(b)

(a)

Lignin is one of the constituents of wood and is produced in pulp mills in large vol-umes, comparable with the production of paper. It is usually handled as a residualwaste material. Thus, in addition to environmental friendliness, PCBs made withlignin offer other advantages (e.g., cost reduction and efficient usage of naturalresources).

8.2.4 DISPOSAL AND RECYCLING

8.2.4.1 Current State of Disposal and Recycling. When an electronic product isscrapped at the end of its life, an electronic recycler, who specializes in dismountingthe products and processing the parts and components, will start to take action. Risksubstances can influence the environment during recycling and disposal.

Electronic products can be divided into four different scrap categories:

1. Cable sets2. Printed board assemblies (PBAs)3. Components and parts that need to be separated for reuse or special waste

treatment4. Structures or housing used for mechanical protection of the product

8.14 CHAPTER EIGHT

FIGURE 8.8 Schematic diagram for the epoxy reaction.

In the following section, the environmental concerns that arise from the treatmentof a product during its disposal, and the current state of recycling for each of thescrap categories, will be discussed.

Cable Sets. The primary environmental concern with dismantling cables andconnector frames is the presence of halogenated plastics. A prominent halogenatedmaterial is polyvinyl chloride (PVC) with chlorinated and brominated flame retar-dants. Chemicals that are dissolved in the product during surface treatment of cablesare also of concern during recycling. From the current state of recycling, it isobserved that all accessible cables are dismounted by the electronic recycler andsent to a special cable recycling facility. Some cables and connectors may have to fol-low the PBAs into the PBA recycling process.The end results of these two processesare virtually the same: the copper and precious metals are recycled, and the plasticsare incinerated or sent to a landfill.

PBAs. Brominated flame retardants and the presence of lead components inthe soldering system are of special concern today. More toxic substances like mer-cury have been eliminated by previous regulations. A significant waste fractionarises from PBAs. The recycling of PBAs is very difficult because it is a combinationof a variety of materials. The use of halogenated materials and other environmentalrisk substances in the PCBs and components makes recycling even more difficult.Furthermore, PBAs are made of thermoset resins in order to provide sufficient heatresistance to sustain the high-temperature assembly process. Efforts are being madeto use thermoplastic resins for PCBs and encapsulants. However, these technologieshave not yet been established. Many PBAs follow their host products into landfillsor are incinerated as a part of a big waste stream. Since PBAs contain costly and pre-cious metals, there is a strong interest in their recovery. Metal recovery from PBAscan lead to emission of pollutants if done without proper waste treatment. Specialtreatment facilities control emissions through extensive cleaning of the waste.

Components and Parts Requiring Special Treatment. A wide variety of elec-tronic components contain one or several environmental risk substances that are notsuitable for the normal waste disposal. For example, toxic metals such as cadmiumand lead present in batteries have a high environmental impact if they are not recy-cled. Therefore, such components need special treatment. All types of batteriesshould be dismounted and sent to special facilities for material recovery. Some com-ponents and parts, such as computer memories, should be separated for reuse. Itshould also be made a common practice to reuse old components as spare parts ifthey are in short supply.

Structures or Housing of Electronic Systems. As discussed previously, halo-genated materials within plastics raise environmental concerns. Surface coatingsmade of chromium(VI) and other mixtures are also considered hazardous.The recy-cling potential is high for metal structures made of steel and aluminum.The mechan-ical structures are sent to the producers of raw metals and alloys.Alloys and metalliccoatings may not be included in the recycled materials, depending on whether therecovery of individual elements from these mixtures is economical. Some metalsthat are lost in the recovery process may have to be replaced.The recycling potentialfor plastics is low, because it comprises a mixture of materials and additives. Energyrecovery should be considered during the incineration of plastics.

8.2.4.2 Recycling Potential and Future Trends. In principle, recycling of elec-tronics is limited to copper and other precious metals. The recycling potential ofplastics is low compared with metals. Plastics are incinerated or sent to the landfilleven though they contain risk substances like halogenated flame retardants or


leaded alloys. If such plastics are landfilled, the risk substances can leach into the soiland pollute the groundwater, subterranean and sea. Acid rain may accelerate thisprocess. The incineration of plastics also leads to pollution of the environment.When halogenated plastics are incinerated in the temperature range of 700 to 900°C,carcinogenic gases such as dibenzodioxin and dibenzofuran are released. Anotherconcern is the transfer of harmful industrial waste from advanced countries to devel-oping countries.

In spite of the significant increase in the variety of products over the past severalyears, the number of products being handled by an electronic recycler is low. Themajority of scrapped electronic parts is dumped in landfills or incinerated.A fractionof these products goes through another harmful scrapping process. The environ-mental risks increase with the increasing number and volume of products. Improve-ments are necessary to the electronic waste system. Landfilling, along withprevention of incineration of harmful chemicals, should be made mandatory in thefuture. It is time to implement suitable electronics recycling systems. Electronicsrecycling must be geared toward automatic shredding of the products into individ-ual material types. Recycling processes should be standardized in order to furtherreduce the use of harmful substances. If a recycling system had been established inthe past, the use of lead would have already been minimized. High-end productmanufacturers should be scrutinized and the practice of recycling should be strictlyreinforced. Low-end consumer products should use fewer risk substances, because itis difficult to control the recycling of those products.

8.2.5 DESIGN FOR ENVIRONMENT

To compare materials, systems, and products from an environmental standpoint,total environmental impact (from the components to the system-level manufac-turing, use, and disposal of products) should be considered. Similarly, the total costshould be estimated when comparing materials, systems, and products. This shouldinclude the cost of purchase, use, and disposal. Life cycle assessment (LCA) is atool for estimating the impact of a product on the environment during its lifetime,which spans from raw material usage during production to its service untilscrapped or recycled. Therefore, an LCA reviews the entire life cycle of a productcomprehensively.

As a demonstration, an LCA study for the new PCB process is compared with theconventional process. The new process can eliminate the emission of solvents andcan increase the energy efficiency, as discussed before (Fig. 8.9). Figure 8.10 indicatesthe results of inventory analysis for energy consumption and CO2 emission. Theanalysis was done considering raw materials, production, transfer, use, and disposal.As seen in Fig. 8.10, the new PCB process can reduce energy consumption by 77 per-cent and CO2 emission by 74 percent during production. Furthermore, the LCAanalysis helps in understanding that the environmental impact of raw materials isdominant. As a next step toward improvement, one must focus on the selection ofraw materials, and choose raw materials that have lower environmental impact.From the LCA study, the importance of reducing the environmental impactthroughout the life cycle of a product can be understood. Accomplishing this reduc-tion, however, requires implementation at the beginning of the life cycle. This iscalled a design for the environment (DfE).

In the twenty-first century, customers demand more information regarding theproduct, process, and its environmental compatibility. Currently, limited informationis available to customers regarding the material content of products, process-related

8.16 CHAPTER EIGHT

chemicals, and the energy that is consumed in the process. Hence, the customers can-not assess environmental performance while making a purchasing decision.An envi-ronmental measure using just one symbol provides limited information to supportthese kinds of decisions. More information is needed in order to be able to evaluatethe use of energy and environmental risk substances. With this knowledge, cus-tomers may be able to influence manufacturers to choose environmentally friendlyprocesses and products.


FIGURE 8.9 Environmental performance profile.

FIGURE 8.10 Energy consumption carbon dioxide emission. (a) Energy consumption in mega-joules per sheet of PWB and (b) CO2 emission in kilograms per sheet of PWB. (Courtesy of Mat-sushita Electric Works.)

(a)

(b)

Another DfE requirement is that data regarding the environmental impact of aproduct should be collected during the design phase itself. The design requirementsfrom an environmental point of view should be quantified, so that the informationcan be shared among different customers. In the future, manufacturers should beprepared to provide more information and answer more questions pertaining to theenvironmental impact of their products.

The future of “green” electronics seems to be extremely bright. However, as longas human beings do not regard environmental issues as their own concerns, naturalresources will keep decreasing, industrial output and pollution will keep increasing,and finally, catastrophe in the middle of the twenty-first century will occur. Evenlegal enforcements may not provide a good solution in such situations. By develop-ing a consciousness of saying no to industrial waste and other types of pollutions, andinvesting in nature’s capital, people can significantly help to reduce the depletion ofnature’s resources and to balance the ecosystem on this planet. Each person musttackle the current environmental issues head-on and follow the path that leads to asustainable scenario.8–11

8.3 ENVIRONMENTAL RESEARCH FOR THE PCB INDUSTRY

The global concern over environmental and health issues has resulted in a pull fromconsumers for green technology and products, and a push from regulatory agenciesat both the national and international levels. This results in a shift from end-of-pipesolutions (e.g., waste disposal and remediation) to new emphasis on DfE, where thefocus is to reduce energy and consumables in the manufacturing of products, and todesign products to minimize environmental impact so that they can be disassem-bled, reused, and/or recycled. This trend may increasingly affect the manufacturingand design of computer products, their technology development, plant locations, andmarketing strategies. As the computer industry proceeds in the twenty-first century,further research efforts are required to provide creative solutions to address thoseissues that are needed to minimize environmental impact, enhance global competi-tiveness, and address regulatory issues without an impact on quality, productivity,and cost.12–15

In 1994, IBM Research, in partnership with its manufacturing divisions and sup-ported by funding from SEMATECH, developed a diluted cleaning solution usedfor silicon wafer processing.This replaced the typical cleaning solution that has beenused throughout the industry for the past 25 years, and resulted not only in a lowercost due to a one-third reduction in chemical consumption and a saving of 3 milliongallons of water per year, but also an increase in yield. This process has now beenimplemented worldwide in manufacturing facilities. It is an excellent exampledemonstrating that examining legacy manufacturing processes to reduce resourceconsumption can also result in enhanced performance and lower costs, and ulti-mately competitive advantages.16–21

Recently, technologies are being developed to improve materials and processesfor PCB fabrication. In this industry, it is a common practice to use high volumesof solvent to coat epoxy resins on the glass fabric that makes up the bulk of thePCB. In the meanwhile, substantial electrical power is needed to dry and cure theresin on the cloth. Currently the PCBs are not recycled, because the removal ofsoldered and encapsulated subassemblies is too costly. In general, these boards areincinerated using high-cost scrubbers, and the residual ash (approximately 30 per-

8.18 CHAPTER EIGHT

cent by weight of PCB) must be buried in hazardous-waste landfills due to its leadcontent.

To address these concerns, various efforts from the government, academia, aswell as private industries are consolidating to develop green card technologies. Suchemerging trends are aimed at the following objectives22:

To develop microwave technology to eliminate the solvents and reduce the energythat is required to fabricate PCB prepreg

To develop new resin systems such as (1) water-based epoxies to eliminate solventhazards and (2) resins derived from renewable resources (plants or micro-organisms) to reduce the usage of oil-based resins

To develop alternatives to lead-based solders To develop new, reworkable encapsulation materials to enable the reclamation of

devices and packages, and to address disassembly issues

8.3.1 ENERGY AND SOLVENT REDUCTION

In PCB manufacturing, prepreg fabrication has the greatest impact on environmen-tal emissions, since the resin (usually an epoxy resin) is first dissolved in a low-boiling solvent, such as MEK or acetone to control viscosity and ensure efficientwetting of the resin on the glass for uniform coating. The glass cloth is then dippedthrough a tank containing the solvent resin and passed up a treater tower, whichconsists of a number of ovens or hot-air jets heating the web to remove the solventand partially cure the resin (B stage) in a continuous operation. A schematic dia-gram of the process is illustrated in Fig. 8.11.After removal from the web, the solventis then vented to the outside or incinerated, emitting up to 600 lb CO2/h or about 5million lb/year for a medium-sized facility, depending on the environmental regula-tions at the location of the treater tower.This accounts for about 20 million lb of sol-vent per year industry-wide, or about 4 percent of the total U.S. production of MEK.In addition to the environmental hazard, there is also the risk of explosion or firewithin the treater facility, as well as the risk of operators being exposed to the sol-vent. The latter not only drives the cost of new installations, but places a severe lim-itation on the types of solvents that can be used.

The expected, more stringent environmental laws will limit the continued useof solvent coating. Some alternatives include melt, powder, and emulsion coatingin which the resin is carried by water. For the manufacture of prepreg, the bestalternative is in water-based resin systems, which still permit a liquid phase tocarry the resin into the closely woven cross-ply glass bundles. A new FR-4 epoxy-water emulsion has been developed for prepregging. Initial tests indicate that afterwater removal, the prepreg has essentially identical properties as prepreg madefrom organic solvents. Laminates fabricated from the water-based resin have sim-ilar electrical, physical, and mechanical properties as those from solvent-basedresins.

However, effective processing of water-based resins might require new technol-ogy and tooling. The limiting factor in the speed of production of prepreg today liesin the heat transfer rate to the resin and fiber in the tower. With the use of micro-wave radiation drying and curing, the energy savings for a typical tower operationresults in the heat utilization being reduced from 1 MW to 20 to 30 kW, which is asubstantial reduction. The use of microwave curing to dry and cure resin-coated


glass fabric is particularly attractive because microwaves will transfer their energydirectly to the resin and solvent without losing heat to convection losses, whichwastefully heats the surrounding air or equipment. While microwave technology isbroadly used in the food industry, it has been slow to impact applications that call foruniform and controllable heating. Extending microwave drying and curing to a wideweb of epoxy-coated glass fabric requires the coupling of the fundamental under-standing of processes and materials, with the design and engineering of newmicrowave applicators.As microwaves couple very efficiently with water, the devel-opment of microwave technology would enable the use of water-based resins. Thesematerials would eliminate the environmental and safety concerns of solvents, andprovide low-cost materials. Microwave technology has great potential to eliminatethe solvents that are used in PCB manufacturing, reduce energy consumption, andprovide lower cost, more compact tooling, and enable the use of water-basedresins.23

8.20 CHAPTER EIGHT

FIGURE 8.11 Application of microwave prepregging for heat reduction.


8.3.2 RENEWABLE RESINS FOR PCB

Another area of interest is the use of biopolymers (materials that are derived fromplants and micro-organisms) to replace the oil-based epoxy resins that are typicallyused in the industry. Of these materials, lignin is of particular interest. Lignin is essen-tially the glue that ties the cellulose together in plants and trees. It is produced in largevolume (∼100 tons/day) as a by-product of paper manufacturing, and its molecularstructure provides the thermal stability and chemical resistance necessary for PCBs.Although research is in the initial stages, results of testing at IBM and Sandia NationalLaboratories have shown that PCBs fabricated using a formulation of epoxy resinswith 50 percent lignin content can provide a PCB with equivalent or better thermaland electrical performance than current high-volume PCBs. In addition, the cost ofresin materials can be significantly lowered and the dependence on fossil fuels is sub-stantially reduced. Furthermore, the waste stream in paper mills is minimized.24 Table8.4 compares the material properties of lignin laminates with typical FR-4 materials.

TABLE 8.4 Material Properties of Lignin Resin Laminates

IPC Standards for FR-4 Lignin laminate FR-4 control

Glass transition, min (°C) >110 124–136 128

Coefficient of thermal expansion (µm/m°C)Below Tg

X 15.4 24.2Y 19.7 24.2Z 27.1 60.6

Above Tg

X 7.11 14.6Y 8.43 11.3Z 342 430

Decomposition temperature (°C) 314 313

Moisture absorption max. (wt %)24 h room-temperature water 0.80 0.3285 0.346216 h boiling water 1.7386 2.17601 h pressure cooker 1.5553 1.7233

Copper peel strength of 1 oz. 8 7.3 11.3Cu, min (lb/in)

Permittivity, max (at 1 MHz) 5.4 4.1 4.0

Dissipation factor, man (at 1 MHz) 0.035 0.023 0.015

Volume resistivity, min (MΩ-cm) 106 1010*

Surface resistivity, min (MΩ-cm) 104 1010*

Dielectric breakdown, min (kV) 40 >45*

Electrical strength, min (V/mil) 7.50 9.00*

Arc resistance, min (sec) >60 66*

Flammability UL V-0 (using UL V-0brominated epoxy)

* Testing performed by Trace Laboratories on laboratory laminate samples.

8.3.3 REWORKABLE ENCAPSULANTS FOR DISASSEMBLY

Over the past 30 years, epoxy thermosetting polymers have improved to the pointwhere they are the mainstay for electronic packaging, providing high reliability atlow cost. They are easily processed prior to curing because of their low viscositywithout the addition of solvents. However, after curing, these materials becomehighly cross-linked and, hence, insoluble and infusible. While they are excellentencapsulants, it is not possible to recover part-good assemblies, or to remove andrecycle chips and components. To address these concerns, a new thermosettingepoxy composition has been developed that functions like a typical encapsulant,but can be removed for chip rework and component disassembly.25–27 This newepoxy incorporates a chemically cleavable link into the polymer network, asshown in Fig. 8.12, which allows the material to be completely dissolved in spe-cially designed, water-based, mildly acidic systems. The material meets all of thestringent requirements of a typical epoxy, including curing speed, temperature and

8.22 CHAPTER EIGHT

FIGURE 8.12 New formulation allows encapsulation to be dissolved in water.


humidity stability, ionic purity, and age testing under actual-use conditions. Thematerial is easily synthesized and has been scaled up at a commercial source.Actual assemblies have been fabricated and tested, and the system is undergoingqualification.

The development and rapid implementation of the preceding innovative tech-nologies have been effective because of partnerships with industry, academia, andgovernment agencies, as well as a concurrent focus on cost, performance, and pro-ductivity. While these new technologies are aimed at the electronics industry, it isbelieved that they may impact a much broader technology base.

8.4 INTERNATIONAL DRIVING FORCES

FOR HALOGEN-FREE ALTERNATIVES

The term halogenated refers to the addition of halogenated organic compounds to apolymer in order to achieve a flame retardancy function. Halogenated flame retar-dants (HFRs) can be used in a number of applications within electronic and electri-cal products. Halogen additives [such as polyterafluoroethylene (PTFE)] may beadded to prevent dripping of a polymer in the case of fire. PVC may be used inenclosures, and Teflon/PTFE (a fluoro-based polymer) substrates may be used toachieve good high-frequency properties. Trace amounts of halogens may also bepresent in epoxy resins due to limits to the ability to purify chemicals.

The electronics manufacturing industries are facing an increasing need to findhalogen-free alternatives for flame retardancy in their products due to the legisla-tive actions and market pressure. To support development of corporate actions, theInternational Project on Flame Retardancy in Electronics-Conceptual Study hasbeen carried out with the collaboration among major electronics manufacturers andmaterials suppliers from Europe and Japan.28 The idea behind the conceptualapproach was to gain a comprehensive view of state-of-the-art halogen-free alterna-tives and of the mechanism governing the design for flame retardancy.

8.4.1 BACKGROUND AND CHALLENGE

In the past decade, there has been an increasing concern about the potential effectson the environment from the use of HFRs. The demands for reducing the usage ofHFRs have turned into legal actions. The major electronics manufacturers perceivenowadays that they will be judged by the market if their products contain HFRs. Inthe long run, all electronics should be free from HFRs. This is a transformation thatrequires an extensive technical development and thorough assessments of new solu-tions in terms of technical, economic, and environmental performance, as well as in-field reliability and considerations concerning timing and sourcing.29

The question of flame retardancy (or fire safety) includes at least three majoraspects that must be considered:

1. Electrical condition. Electronic products contain energy sources making them apotential source of fire.

2. Origin of fire. The objective of design for fire safety in electronic products is toavoid fire being caused by the product itself (internal fire safety) and to avoidelectronic products contributing to and/or enhancing fires originating fromsources in the vicinity of the products (external fire safety).

8.24 CHAPTER EIGHT

3. Trade-off condition. To find a balance between flame retardancy (fire safetylevel), environmental impacts, technical performance, and life cycle economics.

8.4.2 DRIVING FORCES

The trend toward halogen-free alternatives is created by a number of driving forces.Brief descriptions of the major ones are summarized as follows.

8.4.2.1 Legal Actions. In May 1998, the first draft proposal for an EC directiveon WEEE was presented, followed by a second draft in July 1998. In July 1999, adraft proposal for a European Parliament and Council Directive on WEEE was pre-sented. This draft calls for a ban of PBB and PBDE from 2004.

In March 1999, the Swedish chemical inspectorate (KemI) forwarded a report tothe Swedish government concerning a prohibition of flame retardants. KemI pro-posed that Sweden introduce a prohibition against marketing and use of PBDE andPBB and prohibit the marketing of goods containing these substances. Swedenshould also work for a prohibition within the EU and make additional efforts toachieve a change on the international market. It is unclear what the final outcome ofthese and other legal actions will be. In any case, they highlight the use of HFRs.

8.4.2.2 Environmental Labeling Schemes. Environmental schemes such asTCO’95, TCO’99, Nordic Swan, and Blue Angel do not allow the use of HFRs. It istrue that the requirement is restricted to plastic parts with a certain minimum weight(i.e., PCBs and component encapsulants are not included).The schemes have a clearimpact on the market. More than 1000 display units (models) have received theTCO label, and it is estimated that more than 100 million display units with the TCOlabel are in use worldwide. Environmental schemes will probably gain increasingrecognition in the future, and the requirements on flame retardants will probablybecome more severe.

8.4.2.3 Japanese Dioxin Case. Dioxin emissions have become a major environ-mental issue in Japan in recent years. In March 1997, local authorities in TokorozawaCity detected dioxin from Japanese incineration facilities due to too low processtemperatures. A local regulation is now in place and legislative actions are beingconsidered by the Japanese government. This has caused Japanese electronics man-ufacturers to develop halogen-free solutions and to implement halogen-free materi-als in products.

8.4.2.4 Customer Requirements and Public Perception. Large companies andinstitutional buyers increasingly ask for material declarations when buying officeequipment. Halogenated flame retardants are one of the major substances that areconsidered when evaluating quotations. Scientific studies have shown occurrence ofHFRs in human blood and breast milk. Halogenated flame retardants are now arecurrent subject in Nordic newspapers and magazines and are increasingly per-ceived as an environmental problem by the public.

8.4.2.5 Corporate Actions. An increasing number of companies are today eval-uating halogen-free PCB laminates and IC encapsulation materials. Several largeJapanese electronics manufacturers have implemented halogen-free PCBs in prod-ucts such as laptop computers.

8.4.3 MATERIAL AVAILABILITY

To assess the availability of halogen-free materials and components, a large numberof polymer, PCB laminate, and cable manufacturers, as well as semiconductor man-ufacturers, were surveyed. The results are summarized as follows:

At least four halogen-free FR-4-grade laminates were commercially available. Substantial work has been done in developing halogen-free encapsulants for semi-

conductors by a number of companies. From 1999, some of these materials arecommercially available. Quite a wide range of halogen-free polymers is availablefor other applications such as connectors, transformers, relays, and so forth.

A wide range of halogen-free polymers are available for enclosure applications. There is a large number of compound suppliers for cables and wires. Most of them

are currently developing or producing halogen-free materials.

The International Conference on Halogen-Free Materials for Electronic and Elec-trical Products, held in September 1999, showed that additional halogen-free PCBlaminates are available on the market.30

8.4.4 DESIGN MEASURES AND PERFORMANCE

In addition to the survey of halogen-free materials, design measures reducing oreliminating the need for flame retardants were identified and assessed. The identifi-cation of design measures was based on the fundamental conditions for a fire to takeplace. The outbreak of fire in a piece of electronic equipment is dependent on thesupply of fuel, heat, and oxygen. For ignition to occur, all three components areneeded. In the case of electronic equipment, the fuel will initially consist of poly-mers, possibly dust, and other combustible material in the equipment.

Keeping in mind the conditions for a fire to take place, the following potentialdesign options are available:

Eliminate or reduce the amount of fuel (e.g., by using materials with a low rate ofcombustion and a high glass transition temperature).

Reduce heat and/or energy levels (e.g., by using fuses, keeping down normal oper-ating temperatures, protecting inputs from transients, and using materials that willconduct heat away from hot spot).

Reduce or eliminate oxygen (e.g., by restricting air supply).

The challenge of “Design for Environmentally Conscious Fire Safety” has beenaddressed in a new project, called FIRESEL, which began in early spring 2000. Theaim of this project has been to investigate the science of fire safety in electronic sys-tems to enable reliable, cost-effective, and environmentally sound solutions to firesafety.

REFERENCES

1. Feldman, K.,“Suitability of Thermoplastic Base Materials for PCBs,” Proceedings PCWC7,Basel, Switzerland, pp. 12–18, 1996.


2. Goldberg, L. H., and W. Middleton, eds., Green Electronics/Green Bottom Line: Environ-mentally Responsible Engineering, Newnes, Boston, 2000.

3. Tummula, R., Fundamentals of Microsystems Packaging, McGraw-Hill, New York, 2001.

4. Beckel, L., ed., The Atlas of Global Change, Simon & Schuster Macmillan, New York, 1998.

5. Mannion, M., and S. R. Bowlby, eds., Environmental Issues, John Wiley & Sons, New York,1992.

6. Mendicino, L., and L. Simpson, eds., Proceedings of the Second International Symposiumon Environmental Issues in the Electronics and Semiconductor Industries, ElectrochemicalSociety, Seattle, WA, 1999.

7. Murphy, J., The Additive for Plastics Handbook, Elsevier Technology, New York, pp.324–325, 1996.

8. Shaw, J. M., S. L. Buchwalter, J. C. Hedrick, S. K. Kang, L. L. Kosbar, and J. D. Gelorme,“BigBlue Goes Green,” Printed Circuit Fabrication, 19(11):38–44, 1996.

9. Shirrcl, C. D., W. H. Christiansen, S. R. Lyer, and L. D. Bravence, “Prepregging for theTwenty First Century: A Solventless Prepregging Process,” Proceedings IPC Printed Cir-cuits Expo., 1996.

10. Simmons, G., Earth, Air, Water: Resources and Environment in the Late 20th Century,Edward Arnold, New York, 1991.

11. Simpson, C. R., et al., eds., Environmental Issues in the Electronics/Semiconductor Indus-tries and Electrochemical/Photochemical Methods for Pollution Abatement, Electrochemi-cal Society, NJ, 1998.

12. Balzhiser, R. E., Technology and the Environment, National Academy of Engineering,Washington, DC, 1989.

13. U.S. Congress, Office of Technology Assessment, Green Products by Design Choices for aCleaner Environment, OTA-E541, U.S. Govt. Printing Office, Washington, DC, October1992.

14. MCC Task Force, Environmental Consciousness:A Strategic Competitive Issue for the Elec-tronics and Computer Industry, MCC Task Force, March 1993.

15. The National Science and Technology Council, Technology for a Sustainable Future, TheNational Science and Technology Council, Washington, DC, 1994.

16. Cohen, S., et al.,“Minimizing Chemical Consumption for Semiconductor Wet Wafer Clean-ing Processes,” Microcontamination Conference, San Jose, CA, 1993.

17. Lewis, D. A., S. Whitehair, A. Viehbeck, and J. M. Shaw, MRS Proceedings, Spring 1996.

18. Rosbar, L., and J. D. Gelorme, Bio-Based Resins for the Manufacture of PCBs, SPEANTEC, May 1996.

19. Saraf, R. F., J. M. Roldan, C. J. Sambucetti, M.A. Gaynes, and R. Lewis,“High PerformanceIsotropic Polymer/Metal Composite for Interconnect Technology,” Japan InternationalElectronic Manufacturing Technology Symposium, IEEE/CPMT, Omiya, Japan, Decem-ber 1995.

20. Saraf, R. F., J. M. Roldan, R. Jagannathan, C. Sambucetti, J. Marino, and C. Jahnes, “Poly-mer/Metal Composite for Interconnection Technology,” 45th Annual Electronic Compo-nents and Technology Conference, IEEE, Las Vegas, NV, May 1995.

21. Brusic,V., G. S. Frankel, J. Roldan, and R. Saraf,“Corrosion and Protection of a ConductiveSilver Paste,” J. Electrochem. Soc., 142:2591, 1995.

22. Gaynes, M.A., R. H. Lewis, R. F. Saraf, and J. M. Roldan,“Evaluation of Contact Resistancefor Isotropic Electrically Conductive Adhesives,” IEEE Trans., Comp. Pkg., Mfg. Tech.,18:299, 1995.

23. Gaynes, M.A., R. H. Lewis, R. F. Saraf, and J. M. Roldan,“Evaluation of Contact Resistancefor Isotropic Electrically Conductive Adhesives,” 1st Intl. Conf. on Adhesive Joining Tech.in Electronics Mfg., Berlin, Germany, November 1994.

8.26 CHAPTER EIGHT

24. Kang, S., T. Graham, S. Purushothaman, J. Roldan, and R. Saraf, “Lead-Free ConductingAdhesives,” Proc. IEEE International Symposium on Electronics and the Environment, pp.177–181, Orlando, FL, May 1–3, 1995.

25. Kang, S., R. Rai, and S. Purushothaman, “Development of High Conductivity Lead (Pb)-Free Conducting Adhesives,” Proc. 46th Electronic Components and Technology Conf., pp.568–570, Orlando, FL, May 1996.

26. Buchwalter, S. L., and J. D. Gelorme, “Reworkable Encapsulation,” Proc. IEEE Interna-tional Symposium on Electronics and the Environment, pp. 81–82, Orlando, FL, May 1–3,1995.

27. Pompao, F. L., A. J. Call, J. T. Coffin, and S. L. Buchwalter, “Reworkable Encapsulation forFlip-Chip Packaging,” Advances in Electronic Packaging,ASME 1995, EEP, 10(2):781–787,1995.

28. Bergendahl, C. G., et al., “Alternatives to halogenated flame retardants in electronic andelectrical products. Results from a conceptual study,” IVF Research Publication 99824,TheSwedish Institute of Production Engineering Research, 1999.

29. Conference documentation from the “International Conference on Halogen-free Materi-als for Electronic and Electrical Products,” 27–28 September 1999, IVF Research Publica-tion 99828, The Swedish Institute of Production Engineering Research, 1999.

30. Bergendahl, C. G., “Electronics Goes Halogen-Free: International Driving Forces and theAvailability and Potential of Halogen-Free Alternatives,” Proc. IEEE International Sym-posium on Electronics and the Environment, pp. 54–58, Orlando, FL, May 1–3, 1995.


CHAPTER 9HALOGENATED AND

HALOGEN-FREE MATERIALSFOR FLAME RETARDATION

9.1 INTRODUCTION

In many cases some form of flame retardant is needed if polymeric materials are tobe used. In modern society, flame retardants save lives by preventing fire initiationand propagation, which in a wider sense is also good for the environment. In electri-cal and electronic equipment, flame retardants based on bromine are dominant.Some of the brominated flame retardants, such as polybrominated biphenyls(PBBs), have been shown to have serious long-term health and environmentaleffects. Others, such as hexabromocyclododecane (HBCD), are suspected of havingsimilar effects. Some brominated flame retardants, especially reactive types, may bemore or less harmless. Thus, brominated flame retardants constitute a broad groupof substances that have a wide range of properties with regard to toxicity and envi-ronmental aspects.

The acute toxicity of the majority of the brominated flame retardants is low orvery low. This is also the case for many of the breakdown products. The importantrisks of brominated flame retardants are therefore mainly connected to long-termeffects. Long-term effects are only relevant if the substance or its breakdown prod-ucts may bioaccumulate. A substance that is fat soluble and stable and that has aroute of exposure to a specific organism may bioaccumulate. Substances that arewater soluble or unstable, or that have no route of exposure, will not bioaccumulate.Other factors, such as molecule size and metabolism, may also affect the rate ofbioaccumulation.

Among the large group of brominated flame retardants, there are substances andbreakdown products that have the potential to bioaccumulate, but also many withlow or negligible potential to do so. Breakdown products of specific interest due totheir high toxicity and potential for bioaccumulation are polybrominated dibenzo-dioxins (PBDDs) and dibenzofurans (PBDFs). These may form under certain con-ditions in combustion of some brominated flame retardants. PBDFs may also formin photolysis (i.e., degradation in sunlight) of polybrominated diphenyl ether(PBDE.)

PBBs and PBDE will easily form brominated dibenzodioxins in combustion. Sev-eral other flame retardants are likely to have a high or moderate potential of form-ing these compounds. However, there is also a large group for which the potential offorming brominated dibenzodioxins and dibenzofurans will be low or very low. Noteven the high-volume brominated flame retardants are well understood in terms ofenvironmental effects. Much needs to be investigated in order to perform riskassessments based on the whole life cycle of these substances.1

9.1


9.2 BROMINATED FLAME RETARDANTS

9.2.1 PRODUCTION ASPECTS

The total use of brominated flame retardants is increasing, and the world productionin 1998 was 250,000 to 300,000 tons.2 At least eight types of brominated flame retar-dants are produced in volumes exceeding 5,000 tons per year:

Tetrabromobisphenol A (TBBA) PBDEs HBCD Tetrabromophtalimide Tribromophenol and derivatives TBBA-polycarbonate oligomer TBBA-epoxyoligomer Brominated polystyrene

TBBA is by far the most common of the brominated flame retardants. TBBA andTBBA derivatives are used for epoxy and polycarbonate plastics and can be found inpractically every electrical and electronic product.A comprehensive survey shows thatmore than 60 commercial flame retardants may be produced in a year (Table 9.1).

9.2 CHAPTER NINE

TABLE 9.1 Commercially Available Brominated Flame Retardants

Chemical group Chemical name CAS number

PBBs Decabromobiphenyl (deca-BB) 13654-09-6

PBDEs Decabromodiphenyl ether (deca-BDE) 1163-19-5

Octabromodiphenyl ether (octa-BDE) 32536-52-0

Pentabromodiphenyl ether (penta-BDE) 32534-81-9

Substances similar to PBDEs Poly(2,6-dibromophenylene oxide) 69882-11-7

Tetradecabromodiphenoxybenzene 58965-66-5

1,2-bis(2,4,6-tribromophenoxy) ethane 37853-59-1

TBBA and derivatives 3,5,3′,5′-tetrabromobisphenol A (TBBA) 79-94-7

TBBA, unspecified 30496-13-0

TBBA-epichlorhydrin oligomer 40039-93-8

TBBA-TBBA-diglycidylether oligomer 70682-74-5

TBBA carbonate oligomer 28906-13-0

TBBA carbonate oligomer, phenoxy end capped 94334-64-2

TBBA carbonate oligomer,2,4,6-tribromophenol terminated 71342-77-3

TBBA-bisphenol A-phosgene polymer 32844-27-2

TBBA-bis-(2,3-dibromopropyl ether) 21850-44-2

TBBA-bis-(2-hydroxyethyl ether) 4162-45-2

TBBA-bis-(allyl ether) 25327-89-3

TBBA-dimethyl ether 37853-61-5

HALOGENATED AND HALOGEN-FREE MATERIALS FOR FLAME RETARDATION 9.3

TABLE 9.1 Commercially Available Brominated Flame Retardants (Continued)

Tetrabromobisphenol S (TBBS) TBBS 39635-79-5and derivatives TBBS-bis-(2,3-dibromopropyl ether) 42757-55-1

Bromophenols and derivatives 2,4-dibromophenol 615-58-7

2,4,6-tribromophenol 118-79-6

Pentabromophenol 608-71-9

2,4,6-tribromophenylallyl ether 3278-89-5

Tribromophenylallyl ether, unspecified 26762-91-4

Cycloaliphatic brominated HBCD, unspecified 25637-99-4flame retardants 1,2,5,9,10-hexabromocyclododecane (HBCD) 3194-55-6

Tetrabromocyclooctane 31454-48-5

1,2-dibromo-4-(1,2 dibromomethyl)-cyclohexane 3322-93-8

Tetrabromophthalic acid (TBPA) and TBPA Na salt 25357-79-3derivatives Tetrabromophthalic anhydride 632-79-1

Bis(methyl) tetrabromophtalate 55481-60-2

Bis(2-ethylhexyl) tetrabromophtalate 26040-51-7

2-hydroxy-propyl-2-(2-hydroxy-ethoxy)-ethyl-TBP 20566-35-2

TBPA, glycol, and propylene oxide esters 75790-69-1

Phthalimides and related substances N,N′-ethylene-bis-(tetrabromophthalimide) 32588-76-4

Ethylene-bis(5,6-dibromonorbornane-2, 52907-07-03-dicarboximide)

Bromine-containing alcohols 2,3-dibromo-2-butene-1,4-diol 3234-02-4and polyols Dibromoneopentyl glycol 3296-90-0

Dibromopropanol 96-13-9

Tribromoneopentyl alcohol 36483-57-5

Brominated polystyrene Polytribromostyrene 57137-10-7

Tribromostyrene 61368-34-1

Dibromostyrene grafted PP 171091-06-8

Polydibromostyrene 31780-26-4

Brominated alkanes and alkenes Bromo/chloro paraffins 68955-41-9

Bromo/chloro alpha olefin 82600-56-4

Vinyl bromide 593-60-2

Brominated cyanurate derivatives Tris-(2,3-dibromopropyl)-isocyanurate 52434-90-9

Bromine- and phosphate-containing Tris(2,4-dibromophenyl) phosphate 49690-63-3flame retardants Tris(tribromoneopentyl) phosphate 19186-97-1

Chlorinated and brominated phosphate ester 125997-20-8

Brominated toluenes Pentabromotoluene 87-83-2

Pentabromobenzyl bromide 38521-51-6

Other brominated flame retardants 1,3-butadiene homopolymer brominated 68441-46-3

Pentabromobenzyl acrylate, monomer 59447-55-1

Pentabromobenzyl acrylate, polymer 59447-57-3

Decabromodiphenyl ethane 61262-53-1

Tribromobisphenyl maleinimide 59789-51-4

9.2.2 CLASSIFICATION

Flame retardants can be divided into reactive, additive, and oligomeric flame categories.3,4

9.2.2.1 Reactive Flame Retardants. This category refers to flame retardantsthat are chemically reacted into the polymer, effectively becoming a part of a poly-mer molecule. Reactive flame retardants are mainly used in thermoset plastics andresins, especially in epoxy, polyesters, and polyurethanes.

The chemical bond between the flame retardant and the polymer makes it diffi-cult for the flame retardant to escape from the polymer in its original form, e.g., incombustion or a waste deposit. However, it is important to keep in mind that chem-ical reactions are seldom complete, and traces of the original, unreacted flame retar-dant will therefore usually be found.

Reactive flame retardants will affect the physical and chemical properties of theplastic, and it is therefore more technically demanding and more expensive to usereactive flame retardants than additive flame retardants. Reactive flame retardantshave no plasticizing effect, and usually small effects on the thermal stability of thepolymer.

Reactive flame retardants are designed to be highly reactive in order to bond tothe polymer. Therefore, they are often irritating, allergenic, or toxic in their freeform. However, properly reacted into the polymer matrix, the reactivity will be lost,and they cannot easily leach or bleed out of the material. Consequently, the finalpolymer will be without risk to handle and use, and also less toxic than additiveflame retardants, at least until a fire decomposes the polymeric structure. If theflame retardants escape, some of the breakdown products may have a certain solu-bility in water. The most common reactive brominated flame retardant, TBBA, isoften used in epoxy and polycarbonate plastics. TBBA derivatives are used in poly-esters and polyurethanes.

9.2.2.2 Additive Flame Retardants. This category refers to flame retardants thatdo not react into the polymer molecule.This means that the additive may escape in itsoriginal form from the polymer, e.g., in combustion, in sunlight, or in a waste deposit.Additive flame retardants are usually fat soluble, with a low solubility in water.

There is a risk that the additive flame retardant may escape in the use phase viasurface evaporation. When the surface layer has been depleted of substances thatcan evaporate, there may be a risk that additional additive flame retardants may besupplied to the surface through migration of material from the bulk of the polymer.The manufacturing cost of polymers containing additive flame retardants is usuallylower than that of polymers containing reactive flame retardants. Additive flameretardants are mainly used in thermoplastics, textiles, and rubber, e.g., polyolefins,polyvinyl chloride, polystyrene, polyurethane, polyesters, and polyamides. If they arecompatible with the plastic and soluble in the polymer bulk, they act as plasticizers;otherwise, they are considered fillers. They are sometimes volatile or tend to bleed,so the flame retardation of the plastic may gradually be reduced. If they are com-patible, the plastic may be transparent, and the mechanical properties are littleaffected by the flame retardant. If they are not compatible, the plastic will not betransparent, and the chemical properties will be affected.

Additive brominated flame retardants usually consist of relatively small mole-cules that easily migrate through the material, especially when the material meltsduring fire. Thus, additive flame retardants are often more efficient as flame retar-dants than reactive flame retardants. The most common types of additive bromi-

9.4 CHAPTER NINE

nated flame retardants, PBDEs and HBCD, are used in polymers such as acrylonitrile-butadine-styrene (ABS).

9.2.2.3 Oligomeric Flame Retardants. This category has properties betweenthose of additive and reactive flame retardants. Oligomers are not chemically boundto the polymer matrix. These molecules are bigger and do not leach out as easily asadditive flame retardants. The oligomeric molecules are normally too big to effi-ciently penetrate animal or human body tissue, though they may decompose tosmaller molecules in a fire or other degradation processes, e.g., in human tissue.

A common type of oligomeric flame retardants are oligomers of TBBA, with orwithout terminal groups of brominated paraffins. Other common oligomeric flameretardants are oligomers of 2-6-dibromophenol and oligomers of dibromo- and tri-bromostyrene.

9.2.3 RISK ASSESSMENT

9.2.3.1 PBBs. PBBs are aromatic brominated flame retardants, and are similarto polychlorinated biphenyls (PCBs), differing only in the type of halogen atoms inthe chemical structure. However, the bromine atom is much bigger and heavier thanthe chlorine atom. Bromine is also more loosely attached to carbon than chlorine.This means that the physical, chemical, and toxicological behaviors of PBBs will dif-fer somewhat from those of PCBs.

Both PCBs and PBBs are stable compounds, due to so-called resonance stability(which is due to the fact that electrons can easily move around in the whole mole-cule).Thus, they break down very slowly in nature. PBBs are slightly less stable thanPCBs and thus bioaccumulate slightly less readily than PCBs. PBBs are generallyless soluble than PCBs in fat.5,6 Neither PCBs nor PBBs are soluble in water. Theonly isomer of PBBs in current use is deca-BDE, though commercial mixtures mayalso contain smaller amounts of nona- and octa-BDE.

From the structure of PBBs it can be concluded that brominated dibenzofuransand dibenzodioxins can be formed in combustion. This takes place via a smalldecomposition that forms radicals, followed by a slight oxidation or a rearrange-ment. Therefore, PBBs readily form brominated dibenzodioxins and dibenzofuransin uncontrolled combustion.

There has also been a report that mixed chlorine- and bromine-containing diben-zodioxins and dibenzofurans can be formed from PBBs and chlorine-containingmaterial.7 Bromine can also be substituted with chlorine. Health and environmentaleffects of mixed bromine- and chlorine-containing dibenzodioxins are not known.PBBs have been shown to induce chronic toxicity and cancer in animals. The acutetoxicity is low, but cancer has been induced at a daily dose of merely 0.5 mg/kg bodyweight. A number of chronic toxic effects have been observed in experimental ani-mals at long-term exposure doses of around 1 mg/kg body weight per day.

The long-term toxicities of PCBs and PBBs are similar, though PCBs are slightlymore toxic than PBBs. The obvious risk that the effects of PCBs and PBBs may beadditive or even multiplicative cannot be neglected. With few exceptions, the causeof death in laboratory animals exposed to halogenated aromatic compounds cannotusually be attributed to a single organ or system. On exposure, the first signs arebody weight loss, followed by weakness, debilitation, and finally death. Some authorsuse the term metabolic death. Other diseases often complicate the picture, and maymake it difficult to attribute the body response to a certain substance.


9.2.3.2 PBDEs. PBDEs are a group of aromatic brominated flame retardants.At first glance, it may seem that the only difference between PBBs and PBDEs is theintroduction of an oxygen atom between the aromatic rings, and that PBBs andPBDEs may show a similar toxicity pattern. However, this is not the case. Severalanimal tests show clearly that PBDEs are much less toxic than PBBs. This is mainlydue to their totally different three-dimensional structures.8

PBDEs are stable compounds, but their stability is significantly lower than thatof PCBs and PBBs. The lower stability of PBDEs is due to the oxygen atombetween the phenyl rings. The oxygen atom will restrict the mobility of electronsbetween the two phenyl rings and thereby give a lower stability to the compoundcompared to PCBs and PBBs. From the structure of PBDEs it can be concludedthat brominated dibenzofurans and dibenzodioxins can be formed in combustion.This will take place via a small decomposition that forms radicals, followed by aslight oxidation or a rearrangement. There have also been reports that polybromi-nated dibenzofurans may form from lower brominated PBDEs via photolysis(sunlight).9

The acute toxicity of PBDEs is low, and only long-term effects are a concern. Anexample is the oral lethal dose 50 percent (LD50) of penta-BDE, which is high (5.0 to7.4 g/kg body weight, comparable to the toxicity of ethanol). The biological activityof PBDEs is significantly lower than that of PBBs. Deca-BDE may give rise totumors in the liver, and may exert negative effects on reproduction. Deca-BDE islisted by the Environmental Protection Agency as a suspected carcinogen and devel-opmental toxicant. Deca-BDE can easily be debrominated in sunlight to lowerbrominated congeners.

Penta- and tetra-BDE are persistent, bioaccumulating, and toxic to aquatic life.Toxic effects are mainly observed in the liver, but thyroid hormone and neurotoxiceffects can also occur. An investigation has shown that neonatal exposure to tetra-BDE and penta-BDE can induce neurotoxic effects in the adult animal. Both tetra-BDE and penta-BDE induced permanent abberations in spontaneous motorbehavior, a disruption that also worsened with age. Neonatal exposure to penta-BDE also affected learning and memory functions in the adult animal. Similar neu-rotoxic effects have been reported with certain types of PCBs.10

9.2.3.3 TBBA. TBBA is usually used as an aromatic brominated flame retar-dant, either in a reactive form or as an additive oligomer. Thus, assessment of theproperties of the TBBA monomer is only of limited value in predicting the environ-mental properties of the final polymer. TBBA bound in a polymer matrix is severalorders of magnitude less dangerous to health and the environment than PBDEs,which in turn are an order of magnitude less dangerous than PBBs. Degradationproducts of TBBA fixed in a polymer matrix are several orders of magnitude lessdangerous to health and the environment than degradation products from PBDEsor PBBs.

The formation of brominated dibenzodioxins and dibenzofurans from TBBAtakes place only under highly unusual conditions. When TBBA is reacted into apolymer, the properties of the TBBA monomer are no longer relevant to the fin-ished polymer. However, the study of the degradation processes of the monomerTBBA may give some insight into a subset of the chemicals that may be formed indegradation of the TBBA-containing polymer. The TBBA monomer is photolyti-cally decomposed when exposed to ultraviolet light, both in the absence and thepresence of hydroxyl radicals. The main product is 2,4,6-tribromophenol. A numberof other decomposition products are also found, and some of these have been tenta-tively identified:11

9.6 CHAPTER NINE

Di- and tribromobisphenol A Dibromophenol 2,6-dibromo-4-(bromoisopropylene)phenol 2,6-dibromo-4-(dibromoisopropylene)phenol 2,6-dibromo-1,4-dihydroxybenzene

Lower brominated bromophenols are soluble in water and therefore cannot bioac-cumulate, but they are toxic.

It can be expected that many of the degradation products of the epoxy-TBBA orpolycarbonate-TBBA will also be water soluble, and thus have no long-term effectson the environment. TBBA itself has low solubility in water, but good solubility inother polar solvents. Its solubility in aromatic solvents is moderate to low and its sol-ubility in fat is moderate to low.12

Studies of TBBA in sediments have shown that a dimethylated derivate is alsofound that is not present in the product itself. It is not completely understood howthis methylated TBBA is formed, but one hypothesis is that TBBA is methylated bymicroorganisms in the sediment.13 TBBA, when reacted into a polymer matrix, willnot interact to any appreciable degree with biological matter. However, if TBBA hasnot been reacted properly into the matrix, several superficial health effects mayoccur, such as sensitization to allergy or photoallergy, skin irritation, eye irritation,etc. It has also been discussed whether hormonal effects may be caused by TBBA inits free form, but no proofs of this have been presented.

It seems reasonable for the industry not to use materials with proven negativeeffects, such as PBBs. It also seems reasonable to investigate possible long-termeffects of any suspected chemicals used in products, including an assessment of pos-sible degradation products. Bromine in itself is not an environmental toxin. How-ever, bromine is included in some synthetic substances that are stable and maybioaccumulate or that have the potential to form toxins in combustion. Nonhalogenflame retardants are not yet proven to be better for the environment than halo-genated flame retardants. There is little basis, from an environmental perspective, tophase out or prohibit the use of all halogenated flame retardants.14

9.3 TOXICOLOGICAL ASPECTS

OF HALOGEN-FREE FLAME RETARDANTS

The change from brominated flame retardants to nonhalogenated alternatives hasstarted in the electronics industry. Information about toxicological aspects concerningthe involved chemicals is limited, however. This study shows that one nonhalogenatedalternative has lower toxicity than the brominated flame retardant TBBA when testedfor inhibition of denitrification. The other nonhalogenated alternative showed highertoxicity, but was more soluble in water than TBBA, indicating less risk for bioaccumu-lation. Denitrification is a bacterial process that is of great importance in soil and waterenvironments.15

9.3.1 FUNDAMENTALS

9.3.1.1 General Trends. The change from brominated flame retardants tononhalogenated alternatives has started due to environmental concern. One sig-


nificant role is played by the discussion about toxicity of the brominated flameretardants.

Brominated flame retardants are one of 15 chemicals listed for priority action bythe OSPAR Convention (1992), i.e., the Convention for the Protection of the MarineEnvironment of the Northeast Atlantic, which entered into effect in 1998. The con-vention’s statement says that discharges, emissions, and losses of hazardous sub-stances (i.e., toxic, persistent, and liable to bioaccumulate) shall be reduced.

The third draft of the European Parliament and Council Directive on WasteElectrical and Electronic Equipment was presented in 1999. It requires that memberstates phase out the use of PBBs and PBDEs by January 1, 2004. The second draftrequired all halogenated flame retardants to be phased out.

9.3.1.2 Use of Flame Retardants. PCBs have been used as a flame retardant andsoftener in plastics—mainly cables and rubber—in electronic products, but this usewas limited to the 1950s and 1960s. PBBs are assumed to have been used in protec-tion shields and buttons in electronic products. It is unlikely that they have beenused in printed circuit boards.

PBDEs are used as an additive flame retardant in thermoplastics or rubber.Theyexist in TV and personal computer housings but seem to have disappeared startingin the mid 1990s. They are also present in some printed circuit boards. PBDEs havebeen used in higher quantities than PBBs. Commercially, penta-, octa-, and deca-BDE are used. Octa- and deca-BDE are the most usual forms in electronics,although penta-BDE is primarily used in paper phenol printed circuit boards.

TBBA is presently used in printed circuit boards of FR-4 type. It was the mostused brominated flame retardant in the mid-1990s.

9.3.1.3 Toxicity. A substance may have a toxic (harmful) effect on an organismin the environment. This effect may consist of an inhibition of special enzymes,growth, or reproduction, or it may lead to death of the organism. Toxicity may bereduced by adsorption to dead and inert organic and inorganic matter as well as bychemical binding and chelation.

TBBA is expected to become associated with soil and sediment, and is notexpected to evaporate. The compound is suspected to be nonbiodegradable, tobioaccumulate in organisms, and to be very toxic to water-living organisms. It has alow toxicity to mammals and is not suspected to be mutagenic. The regional riskcharacterization ratios (EUSES method) indicate that neither the environment norhuman beings in the regional area are at risk from TBBA. Available data indicatethat TBBA, when used as a reactive flame retardant, can be expected to be at least10 times less dangerous to mammals than PBBs and PBDEs.

9.3.1.4 Exposure. PBDEs have been detected in white-beaked dolphins (>7ppm) and harbor seals (>1 ppm) feeding in the North Sea and the Wadden Sea. It hasbeen found in herring, gray seals, and ringed seals along the Swedish coastline of theBaltic Sea. Studies suggest that PBDEs biomagnify, because lower levels were foundin fish than in seals.

The level of PBDEs in breast milk of native Swedish mothers living in the Stock-holm region has increased from less than 0.2 to 4.0 ng/g lipid between 1972 and 1997.The levels are strongly correlated with the fat content of the milk. However, thelevel of organochlorine compounds (which were banned in the early 1970s) hasdecreased. In 1997 the level of PCBs was still approximately 350 ng/g lipid. TBBAhas a certain solubility in water and can, due to its chemical structure, bond to sul-phate and gluconic acid via the hydroxy groups. It can be secreted via the gallblad-der and leave the body with feces.

9.8 CHAPTER NINE

9.3.1.5 Bromine Industry Activities. A theory launched by the Bromine Scienceand Environmental Forum (BSEF) argues that some individual cases resulted inhigh emissions of PBDEs, which would explain the present findings in biota. Thecases are oil industry offshore drilling in the North Sea in the early 1990s and coalmining industry (Germany and Sweden) hydraulic fluids during the 1980s. HigherPBDE congeners (octa- and deca-PBDE) are very stable products and do notdegrade in the environment. Commercial penta-PBDE is manufactured and used invery low volumes today. Therefore, the quantities of lower PBDE congeners foundrecently cannot be explained by brominated flame retardants.

TBBA has minimal toxicity on acute and repeated dosing. In aquatic systems,toxicity and bioconcentration are dependent on species. The relatively high biocon-centration factor in some species is balanced by rapid elimination. Therefore, thereis little potential for bioaccumulation.

9.3.2 DENITRIFICATION

9.3.2.1 Interest in Denitrification. Interest in denitritication exists for severalreasons. First, it is a major mechanism of loss of fertilizer nitrogen resulting indecreased efficiency of fertilizer use. Second, it is of great potential application inthe removal of nitrogen from high-nitrogen waste materials such as animalresidues. Third, denitrification is an important process, contributing N2O to theatmosphere, where it is involved in stratospheric reactions, which result in thedepletion of ozone. Fourth, it is the mechanism by which the global nitrogen cycleis balanced.

The Swedish government, for example, introduced requirements for nitrogenreduction in wastewater treatment plants in the early 1990s, aiming to reduce thenitrogen discharges to the Baltic and Kattegatt in order to prevent eutrophica-tion.16

9.3.2.2 Definition of Denitrification. Denitrification refers to the dissimilatoryreduction, by essentially aerobic bacteria, of one or both of the nitrogen oxides,nitrate (NO3

−) and nitrite (NO2−) to nitrogen gas (N2) via the gaseous oxides [nitric

oxide (NO) and nitrous oxide (N2O)] as shown in Fig. 9.1.The nitrogen oxides act asterminal electron acceptors in the absence of oxygen.

The reduction is catalyzed by the enzyme systems nitrate reductase, nitrite reduc-tase, nitric oxide reductase, and nitrous oxide reductase. Here the inhibition ofnitrite reductase is measured, since it is the key enzyme step in denitrification.17


nitratereductase

NO2–

N2O

NO3–

N2

NO

nitritereductase

nitrous oxidereductase

nitric oxidereductase

FIGURE 9.1 Enzymes for denitrification.

9.3.3 BIOASSAY PROCEDURES

9.3.3.1 Bacterium. The ability to denitrify is a property possessed by a diversecategory of bacteria. The present toxicity tests are based on a pure culture of a de-nitrifying bacterium in order to achieve good reproducibility and comparableresults. The bacterium is isolated from wastewater treatment sludge. It has beenthoroughly characterized and is a member of the Comamonas group.

9.10 CHAPTER NINE

FIGURE 9.2 Typical results of denitrification inhibition tests: (a) TBBA, (b) OAP,(c) LMB 6129.

(a)

(b)

(c)

9.3.3.2 Denitrification Inhibition Test. The Comamonas bacteria were grown onnutrient agar plates and kept at 30°C overnight. A bacterial suspension containing0.01 g (wet mass) of bacteria and 1 ml of nutrient broth was mixed.The substrate, NO2,was added as a NaNO2 solution, to give a final concentration of 150 mg NO2/l. Theexperiments were carried out in microtiter plates, where each well contains 250 µl intotal.

None of the flame retardants were completely soluble in water. Several solvents,with decreasing dielectric constants, were tested. TBBA and OAP were dissolved inmethanol, while LMB 6129 was dissolved in acetone. All experiments were done asduplicates, and some typical test results are presented in Fig. 9.2.

The nitrite consumption rate was followed over time as samples of 5 µl weretaken out at 15-min intervals.The nitrite concentrations were measured colorimetri-cally using a spectrophotometer at a wavelength of 595 nm. This test is a substrateutilization method, which measures the decrease in a substrate (nitrite). A colorcomplex is formed between the unreacted substrate and the reagent. The colorweakens as the substrate level decreases. Inhibition of the bacteria is measured as adecrease in the rate at which the substrate is consumed. If the bacteria are inhibited,the rate is lower than for a reference sample.18

9.3.3.3 Water Solution Test. The fact that the flame retardants have poor water sol-ubility made it interesting to perform a long-term solubility test. The flame retardantswere placed in water for 1 month, after which the saturated water solution was testedfor toxicity. In all other aspects this test was performed like the preceding experiment.

9.3.3.4 Bacterium Growth Test. The pure culture of the denitrfying bacteriumwas streaked out on a nutrient agar plate. A small amount of each flame retardantwas then added in the middle of the plate. The agar plates were incubated overnightat 30°C and then analyzed by visual inspection for bacterial growth. A circle of nogrowth around the flame retardant indicates inhibition of growth.The bigger the cir-cle, the more toxic the tested compound.

9.4 ENVIRONMENTALLY CONSCIOUS

FLAME-RETARDING PLASTICS

In electronic products, thermoplastics, which include polycarbonate, acrylonitrile-butadien-styrene copolymer, and polystyrene, are mainly used in housings. Further-more, thermosetting plastics—mainly epoxy resin compounds consisting of epoxyresin, hardener, and additives—are used as insulating materials for electronic parts. Inorder to prevent fire from originating in electronic products, these plastics containflame-retarding additives, most commonly in the form of organic halogen compoundssuch as brominated aromatic compounds. There is, however, a serious problem withsuch halogen compounds: during burning, they generate toxic substances that caninjure people and contaminate the environment. In addition to the fire-related dan-gers, the treatment and recycling of the waste materials is also made extremely difficult.

While attempts have begun to replace these halogen compounds with saferphosphorus compounds that generate almost no toxic gas, phosphorus compoundsthemselves are somewhat toxic and can leak out of waste plastics. Furthermore,most phosphorus compounds have the disadvantage of reducing the moldabilityand humidity resistance of the plastics. New flame-retardant plastics have beendeveloped for electronic products in order to overcome the environmental hazardsoriginating from current plastics containing toxic flame-retarding additives.19


9.4.1 FLAME-RETARDANT POLYCARBONATE RESIN

9.4.1.1 New Silicone Flame-Retarding Additive. Silicone compounds havebeen studied as candidates for safer flame-retarding additives because of their highheat resistance, nontoxicity, and lack of generation of toxic gases during combustion.Previous attempts mainly include studies to improve the flame resistance of plasticsas a whole by adding silicones. That is, in plastics, heat resistance networks formedwith polydimethylsiloxans (methyl silicones) containing reactive functional groupswere studied. Also, addition of cross-linked methyl silicone powders with high heatresistance to plastics was attempted. However, their flame-retardant properties werenot good enough because they did not retard flaming on the plastics’ surfaces, wherecombustion actually occurs.

A new flame-retarding siliconecompound, as illustrated in Fig. 9.3, wasdeveloped for polycarbonate resin.From the comparison presented in Fig.9.4, the new silicone shows far higherflame-retardant effectiveness for poly-carbonate resin than previous siliconesand can perfectly replace the currentflame-retarding additives includinghalogen (bromine) compounds andphosphorus compounds. The new sili-cone has a special structure, which is abranched chain containing aromaticgroups and nonreactive terminals. Itincorporates a new flame-retardingmechanism, as shown in Fig. 9.5: the sil-icone finely disperses in the polycar-

bonate resin, moves from inside to the polycarbonate’s surface during combustion,and then forms a flame-resistant barrier on the surface.

As shown in Fig. 9.6, in polycarbonate resin, dispersion of the new silicone isextremely fine. This can be mainly credited to its high solubility in the polycar-bonate resin, which is due to the aromatic groups in the chain. Methyl silicone(branch type and linear type) showed low solubility in the polycarbonate resin dueto the lack of aromatic groups. Branch silicone derivatives with reactive groups inthe terminals, whether containing aromatic groups in the chain or not, showedpoor dispersion because of their gelation during mixing with the polycarbonateresin.20

Figure 9.7 indicates the concentration (ratio of Si to C) of the new silicone at thesurface of the molded polycarbonate resin before and after combustion, and showsmovement of the silicone from the inside of the polycarbonate resin to its surfaceduring combustion. This movement can result from differences in solubility and vis-cosity between the silicone and the polycarbonate resin at high temperatures duringburning.

Figure 9.8 shows the flame resistance of silicone derivatives themselves. The newsilicone showed far higher flame resistance than the polycarbonate resin alone orwith methyl silicones. This is mainly due to its aromatic groups, which can form con-densed aromatic compounds known as precursors to highly flame-resistant char(carbonaceous substances), and also due to its branch structure, which shows higherheat resistance than a linear structure. Therefore, the new silicone can form a flame-resistant barrier after moving to the polycarbonate’s surface during combustion.We

9.12 CHAPTER NINE

FIGURE 9.3 Structure of a new flame-retardingsilicone compound for polycarbonate resin.

believe that during combustion the new silicone and the polycarbonate synergisti-cally form a highly flame-retardant complex substance on the polycarbonate’s sur-face because the polycarbonate alone can form similar condensed aromaticcompounds on the surface during combustion.

9.4.1.2 General Properties of the Polycarbonate Resin with the New Silicone.The polycarbonate resin containing the new silicone achieved far higher safety


FIGURE 9.4 Flame-retardant effectiveness of silicones for polycarbonate resin.

FIGURE 9.5 Flame-retarding mechanism of new silicone for poly-carbonate resin.

9.14 CHAPTER NINE

FIGURE 9.6 Dispersion of silicones in polycarbon-ate resin.

FIGURE 9.7 Movement of new silicone to the surface of poly-carbonate resin during combustion.


FIGURE 9.8 Flame resistance of silicones.

TABLE 9.2 Properties of Polycarbonate Resin with Flame Retardants

With phenyl With newProperties With TBBA phosphorus ester silicone

Impact strength 12 4 45(kgf/cm/cm, 1⁄8 in)

Flexural strength 980 1080 920(kgf/cm2)

Flexural modulus 230 270 230(kgf/mm2)

HDT (°C) 134 106 133

Melt flow (g/10 min) 22 47 22

Flame resistance V-0 V-0 V-0(UL-94, 1⁄16 in)

Recycling properties (100%)*

Impact strength 11 4 38(kgf/cm/cm)

Flame resistance V-1 V-0 V-0(UL-94, 1⁄16 in)

* Reextruding and retabulating.

9.16 CHAPTER NINE

FIGURE 9.9 Self-extinguishing network formed by new epoxy resin compound.

FIGURE 9.10 Flame resistance of epoxy resin compounds.


when burned and when disposed of than current plastics containing halogen orphosphorus-type flame-retarding additives. The polycarbonate resin meets a highflame-retarding standard (UL 94V-0) and has other good important properties,such as strength, moldability (contamination of mold, melt flow characteristic inmold), surface hardness, and heat resistance (Table 9.2). In particular, the impactstrength is better than that of polycarbonate containing a bromine compound anda phosphorus compound and the heat resistance is higher than that with phospho-rus compounds. Furthermore, these characteristics are maintained when the prod-uct is recycled. NEC Corporation has started to use the new plastic in housings ofliquid crystal display monitors and projectors, and also in battery packs for note-book computers.21

FIGURE 9.11 Cross-section of molded epoxy resin compounds.

9.4.2 SELF-EXTINGUISHING EPOXY RESIN COMPOUND

A new self-extinguishing epoxy resin compound was developed as a flame retardantfor electronic products. It contains no toxic flame-retarding additives and can beused as a high-quality molding resin for electronic parts.As illustrated in Fig. 9.9, thecompound contains an aromatic epoxy resin and a phenol derivative hardener, bothof which contain multiaromatic groups in their main chains and also additives (silicaparticle filler, releasing agent, etc). From the comparison presented in Fig. 9.10, theresin compound shows far higher flame retardation than current epoxy resin com-pounds because of its new flame-retarding mechanism: the formation of a foamlayer on the resin surface during combustion (see Fig. 9.11), which can retard oxygen

9.18 CHAPTER NINE

TABLE 9.3 Characteristics of Newly Developed Molding Resin

Newly Currentdeveloped molding

Characteristics molding resin resin*

BromineComposition Flame-retarding additive None compound

and Sb2O3

Filler Silica >80% Silica >80%

Specific gravity (g/cm3) 1.96 1.91Water resistance Water absorption ratio (%) (boiling for 24 h) 0.17 0.22Strength Flexural strength at room temperature (N/mm2) 172 167

Flexural modulus at room temperature (N/mm2) 21,360 18,130Flexural strength at 240°C (N/mm2) 15.9 19.6Flexural modulus at 240°C (N/mm2) 480 510

Heat resistance Glass transition temperature (°C) 132 145Insulation Volume resistance (×1011 Ω/cm) at 150°C 104 42Flow behavior Spiral flow length (cm) 106 85Flame retardance UL-94 test (1.6 mmt) V-0 V-0

Humidity treatment time (h) before soldering 96 168 96 168

Resistance to soldering heat Delamination between chip and compound 0 S 0 S(number of failures Package cracking 0 0 0 0in 6 packages) 80-pin QFP (package size = 20 × 14 × 2 mm, silicone chip size = 9.0 × 9.0 × 0.35 mm)

85°C, 85% RH, IR at 240°C for 10 s (3 times), S figure: small delamination

PCT time (h) 120 0 0Pressure cooker bias test 160 0 2(number of failures 200 0 2in 15 packages) 240 1 —

16-pin DIP (package size = 19 × 5 × 4 mm, silicone chip size = 3.0 × 3.5 × 0.35 mm)125°C, 100% RH, applied voltage = 20 V

Pressure cooker test PCT time (h) 204 0 0after IR reflow (number 318 0 0of failures in 6 packages) 514 0 0

80-pin QFP (package size = 20 × 14 × 2 mm, silicone chip size = 3.0 × 3.5 × 0.35 mm)85°C, 85% RH for 48 h and IR at 240°C for 10 s, → 121°C, 100% RH

Thermal cycle test 700 cycles 0 0(number of failure 80-pin QFP (package size = 20 × 14 × 2 mm, silicone chip size = 9.0 × 9.0 × 0.35 mm)in 12 packages) −65 ∼ +150°C for 10 min

* Biphenyl-type epoxy resin and phenol resin hardener. QFP, quad flat pack; IR, infrared; PCT, pressure cooker test; DIP,dual inline package; RH, relative humidity.

passage and heat transfer. After a curing reaction between the epoxy resin and thehardener, the resin compound forms a special network structure with a low cross-link density and high resistance to thermal decomposition due to the multiaromaticgroups in the main chain. Because of the compound’s elasticity at high temperatureresulting from this low cross-link density, gaslike substances generated from theinside of the resin compound by thermal degradation during combustion form thesurface material into a foam layer. Furthermore, the higher resistance to thermaldecomposition plays an important role in the stability of the foam layer during com-bustion.

The epoxy resin compound has good characteristics as a high-quality moldingresin as well as high flame resistance (Table 9.3). In fact, its resistance to solder heat-ing and thermal cycles is as high as that for current high-quality molding resins con-taining bromine-type flame-retarding additives used for LSIs. Its resistance tohumidity is even better. Other general properties, such as insulation, heat resistance,strength, and moldability are good enough for practical use.22

Environmentally friendly flame-retardant plastics containing no toxic flame-retarding additives such as halogen (bromine) compounds and phosphorus com-pounds have been developed for electronic products. A polycarbonate resincontaining a new silicone flame-retarding additive has been developed for use inhousings. Furthermore, a self-extinguishing epoxy resin compound containing noflame-retarding additives was developed as a high-quality molding resin for elec-tronic parts. These plastics show good general properties as well as high flame retar-dation.

REFERENCES

1. Hedemalm, P., A. Eklund, R. Bloom, and J. Haggstrom, “Brominated Flame Retardants—An Overview of Toxicology and Industrial Aspects,” Proc. IEEE International Symposiumon Electronics and the Environment, pp. 203–208, San Francisco, CA, May 8–10, 2000.

2. Troitzsch, J. H., International Plastics Flammability Handbook: Principles, Regulations,Test-ing and Approval, 2d ed., Munich, Germany: Hanser, 1990.

3. IPCS International Programme on Chemical Safety: Environmental Health Criteria 152:“Polybrominated Biphenyls,” World Health Organization, Geneva, 1994.

4. IPCS International Programme on Chemical Safety: Environmental Health Criteria 140:“Polychlorinated Biphenyls and Terphenyls,” World Health Organization, Geneva, 1993.

5. IPCS International Programme on Chemical Safety: Environmental Health Criteria 162:“Brominated Diphenyl Ethers,” World Health Organization, Geneva, 1994.

6. Watanabe, I., and R. Satsukawa, “Formation of Brominated Dibenzofurans from the Pho-tolysis of Flame Retardant Decabromodiphenyl Ether in Hexane Solution by UV andSunlight,” Bull. Environ. Cona. Toxicol., 39, 953–959, 1987.

7. Eriksson, P., E. Jakobsson, and A. Fredriksson, Organohalogen Compounds, 35:375–377,1998.

8. Eriksson, J., and E. Iakobsson, “Decomposition of Tetrabromobisphenol A in Presence ofUV Light and Hydroxyl Radicals,” Organohalogen Compounds, 35:419–422, 1998.

9. Flick, E. W., Plastics Additives, an Industrial Guide, 2d ed., Noyes Publications, 1993.

10. Segerberg, T., L. Gumaelius, H. Hessle, and E. Ostensson, “Toxicological Aspects ofHalogen-Free Flame Retardants Based on Denitrification Inhibition Tests,” Proc. IEEEInternational Symposium on Electronics and the Environment, pp. 69–74, San Francisco,CA May 8–10, 2000.


11. Grunditz, C.,“Bioassays for the Determination of Nitrification Inhibition,” Royal Instituteof Technology, Department of Biotechnology, 1999.

12. Gumaelius, L., and G. Dalhammar, “Development of a Biosensor for Denitrification Inhi-bition,” Royal Institute of Technology, Department of Biotechnology, 1998. [in Swedish]

13. Hardy, M. L., “Tetrabromobisphenol A: Toxicology and Environmental Effects Evaluationby the World Health Organization under the International Programme on ChemicalSafety,” CMA BFRIP Brominated Flame Retardants Workshop, 1995.

14. Hessle, H., and E. Ostensson, “Toxicological Characterization of Laminate Flame Retar-dancy Systems,” Royal Institute of Technology, Engineers School, 1999.

15. Noren, K., and D. Meironyte,“Contaminants in Swedish Human Milk—Decreasing Levelsof Organochlorine and Increasing Levels of Organobromine Compounds,” Dioxin ’98,Organohalogen Compounds, 38:1–4, 1998.

16. Camino, G., and L. Costa, Polymer Degradation and Stability, 20:271–294, 1988.

17. Cullis, C. F., Journal of Analytical and Pyrolysis, 11:451–463, 1987.

18. Dumler, R., H. Thoma, and O. Hutzinger, Chemosphere, 19(1–6):305–308, 1989.

19. Luijk, P., H.A.J. Govers, G. B. Eijkel, and J. J. Boon, Journal of Applied Pyrolysis,20:303–319, 1991.

20. Iji, M., S. Serizawa, and Y. Kiuchi, “New Environmentally Conscious Flame-RetardingPlastics for Electronics Products,” Proceedings of the First International Symposium onEnvironmentally Conscious Design and Inverse Manufacturing, pp. 245–249, 1999.

21. Iji, M., and S. Serizawa, Polymers for Advanced Technologies, 9:1–8, 1998.

22. Bush, B., Plastics Engineering, 42(2):29–32, 1986.

9.20 CHAPTER NINE

CHAPTER 10FABRICATION OF

ENVIRONMENTALLY FRIENDLY PCB

10.1 INTRODUCTION

The integration of factors concerning environmental impact during manufacturing,use, and disposition of product is an emerging issue in design of electronics. Thedrivers for this in electronics include certification standards such as ISO 14000 andBritish Standard 7750; ecolabeling programs such as Energy Star, Blue Angel, andTCO; increasing internalization of waste mitigation and disposal costs into theproduct cost; and an increasing need for environmental accountability in global sup-ply networks. One important aspect of product design for the environment (DfE) inelectronics is consideration of environmental impact during the manufacturingstage. In particular, the fabrication and assembly stages of printed circuit board(PCB) are significant contributors to life cycle environmental impacts. Process mod-els can be used as an analytical tool to develop environmental performance indica-tors for products. These process models, while modeling the manufacturing wastestreams, also implicitly model the product parameters and can be used convenientlyfor product design optimization.1

In the following sections, techniques based on process modeling and productoptimization will be introduced. These techniques may not only be implemented inPCB assembly design to minimize environmental impact, but also can be easilyapplied to other product optimization problems. First the process models for PCBfabrication are briefly described. A waste stream weighting scheme, which is veryuseful for comparing two or more dissimilar waste streams, is then introduced. Sub-sequently an optimization algorithm will be discussed for seeking a balance betweenthe various board design parameters, such as board area, number of layers, and num-ber of boards on a panel, in order to come up with the physical board design with theminimum manufacturing waste per board.A case study will be presented to demon-strate the implementation of these models and principles. Furthermore, specificissues such as how to incorporate various design constraints and how the analyses fitinto the overall life cycle assessment of a product will be reviewed.

10.2 PCB DfE

10.2.1 PROCESS MODELING

Manufacturing a PCB assembly consists of three parts: (1) semiconductor fabrica-tion, (2) electronic packaging, and (3) bare-board manufacturing and componentassembly. Despite a tremendous flexibility in process selection in semiconductor

10.1


manufacturing, there is somewhat limited room for product optimization to achievebetter environmental results. In electronic packaging, one can choose from a widevariety of packages. However, in most cases, the functional requirements veryquickly narrow the choice to a few types.

For bare-board fabrication, about 7 percent of the materials used actually go intothe product and the remaining 93 percent are emitted as process waste.2 Thus, mini-mizing the process waste is the most logical way to minimize the environmentalimpact of circuit boards. The process steps include laminate core fabrication, resistcoating, exposure, development, copper etching, resist stripping, oxide treatment,lamination, drilling, desmearing, copper plating, and solder masking, as shown inFig. 10.1.

In general, the thermochemical and thermomechanical behaviors of variousprocess steps involved in fabrication may be modeled to predict the waste streams.For instance, the waste streams from the etching operation can be modeled in termsof the board parameters as

mCu = ρCu (1 − KCu) Acore tCu (10.1)

metc = mCu (MCuCl2/MCu) (10.2)

mCu2Cl2 = mCu (MCu2Cl2/MCu) (10.3)

where m represents the mass, M represents the molecular weight, A represents thearea, t represents the thickness, and K represents the fraction of copper retained onthe board after etching for a particular layer. Similar models can be formulated forcomponent assembly operations such as stenciling, component placement, reflow,wave soldering, and board cleaning.3,4 These models must be validated using theactual waste stream data collected at the fabrication site. Figure 10.2 shows themodel estimates for the reflow and wave-soldering processes.

10.2.2 HEALTH HAZARD ASSESSMENT

It is crucial to be able to compare the raw mass of waste streams. This can beachieved using some kind of health hazard assessment method such as the healthhazard scoring system.5 This system computes a scalar weighting factor called thehealth hazard score (HHS), which is calculated for a particular waste stream using itshealth hazard potential data (carcinogenicity, reactivity, flammability, dermal irri-tability, inhalation toxicity, oral toxicity, and eye irritation), its phase (solid, solid par-ticulate, liquid, vapor, or aerosol), and the safety practices on site. This numberessentially brings two dissimilar waste streams to the same level of hazard for thepurpose of mutual comparison.

The categorical hazard score H is determined based on information from varioussources such as the American Conference of Governmental and Industrial Hygien-ists threshold limit value data and the Registry of Toxic Effects of Chemical Sub-stances database.To introduce phase effects with chemical hazard subscores, a phasematrix P can be constructed to partition the hazard share for each factor among thedifferent physical phases using the Kepner-Tregoe method, where each coefficient isassigned a fractional value from 0 to 1.

The final factor to consider is site modeling. Since each site in which manufactur-ing occurs is different in terms of facilities design, equipment, and work practices,these site-specific factors have a significant influence on waste stream environmen-tal impact. However, these factors are largely qualitative in nature (e.g., wearing pro-

10.2 CHAPTER TEN

FIGURE 10.1 Typical process steps in PCB fabrication.

10.3

FIGURE 10.2 Process model estimates for reflow and wave-soldering processes: (a) solder paste waste, (b) dross waste.

10.4

tective garments, continuous monitoring of the process). A major challenge, there-fore, is to reduce qualitative site information to a quantitative form, which can beevaluated along with chemical hazards and phase.

One approach is to construct a set of pairwise evaluations comparing toxicity(oral and inhaled), carcinogenesis, irritation (dermal and eye), reactivity, and flam-mability using the analytic hierarchy process (AHP), a method that ranks pairwisecomparisons between various factors. Successive comparisons set up a matrix. Ini-tially, a subjective set of priorities is elicited from the user regarding the differentfactors (such as toxicity, reactivity, and flammability). These priorities then areplaced into an AHP matrix. The schematic of the HHS method is shown in Fig. 10.3.

FABRICATION OF ENVIRONMENTALLY FRIENDLY PCB 10.5

FIGURE 10.3 HHS method.

Examples of categorical hazard scores are shown in Table 10.1. A sample phasematrix is shown in Table 10.2, and a sample AHP matrix is shown in Table 10.3.Based on this matrix, an l-x-7 fate and transport column vector F can be calculated.A rank value is determined with the following equation:

Rl = k

j = lXij

l/k(10.4)

where Xij are the elements of the k-x-k prioritization matrix X. Here, k = 7, the num-ber of effects of interest. The elements of F then are determined by a simple nor-malization:

F (i) = Ri /k

i = lRi, i = l,......k (10.5)

For the matrix in Table 10.3, the F vector is

F = [0.01 0.05 0.07 0.43 0.03 0.21 0.20]T (10.6)

The environmental impact index, or HHS for the ith waste stream and the jth phase(HHSij) can be calculated as

HHSij = HiPij ⋅ F (10.7)

where Hi is the l-x-7 vector of the raw score [0, I, E,......, F] for the ith waste stream,Pij is the transposition of the jth column of the phase matrix for the ith waste stream,and HiPij is an l-x-7 vector equal to the element-by-element product of Hi and Pij.Once derived, an overall HHS index can be written for chemical species with multi-phase pathways in terms of the mass fractions of the different phases as

HHSi = j

HHSijMj /j

Mj (10.8)

10.6 CHAPTER TEN

TABLE 10.2 Sample Phase Matrix

Phase

Hazard Solid Liquid Aerosol Vapor Solid particles

Oral toxicity 0.3 0.4 0 0 0.3

Inhalation toxicity 0 0 0.5 0.3 0.2

Eye irritation 0 0 0.4 0.4 0.2

Dermal irritation 0.2 0.5 0 0 0.3

Carcinogenicity 0 0.3 0.3 0.3 0.1

Reactivity 0 0.5 0.2 0.2 0.1

Flammability 0.1 0.6 0.1 0.1 0.1

TABLE 10.3 Sample AHP Matrix for Site-Specific Prioritization

X = F

1⁄20

1⁄51⁄331⁄811

R1⁄20

1⁄51⁄421⁄511

C1⁄222

15158

D1⁄30

1⁄10

1⁄61

1⁄15

1⁄21⁄3

E1⁄10

1161⁄243

I1⁄511

101⁄255

O.15

10302

2020

OI

EDCRF

TABLE 10.1 Score Hi for Reactivity

Score Reacts with

9 Metals, oxidizing agents, acids, bases, moist air, water, etc.

8 Metals, moist air

7 Metals

6 Moist air

4–5 Oxidizing agents

1–3 Acids or bases

0 No known substance (inert)

10.2.3 BOARD OPTIMIZATION

10.2.3.1 Deciding the Variables of the Problem. Once equipped with a set ofprocess models and hazard assessments, we can analyze a particular board design.Keeping the functionality of the board untouched, we can change its physicaldesign and observe the change in the waste stream generation impact. First, weintuitively decide which factors will have the most significant impact on the wastestream. For circuit boards, we observe from the process models that the number oflayers, number of boards tiled on a single panel, and board area are the key designparameters. We cannot vary the number of pins or components at this pointbecause that will hamper the functionality of the board and our focus is not ondevices on the board.

10.2.3.2 Deriving Interparameter Relationships. Changing certain designparameters affects other design parameters. For example, changing the dimensionsof the board requires recomputing the copper fraction for every layer.Thus we needto derive relationships between various parameters.These relationships serve as theequality constraints of the optimization problem. In the following example of boarddesign, there are three major formulas to be derived.

Copper Fraction of the Signal Layers. In a previous study,6 the relationship forthe average length of copper traces on a board has been derived as

Lavg = (A + 1) [1 + 2As /Nl(As2 +1)] (10.9)

where As is the aspect ratio, Ar is the routable area of the board, Nl is the total num-ber of component pins, and n is the average net size. From this formula, the copperfraction of signal layers can be written as

KCu−board = (LavgNintttrace)/Ab (10.10)

where Nint = Nl (n − 1)/n.Number of Boards per Panel. Boards can be arranged on a rectangular panel in

two possible orientations, as shown in Fig. 10.4. If we denote the length and breadthof the board and panel by Lb, Wb, Lp, and Wp, respectively, then the number ofboards for the two orientations may be expressed as follows:

Orientation 1: (10.11)

Orientation 2: (10.12)

We must calculate the number of boards per panel using these formulas and choosethe orientation that gives the maximum number of boards per panel. In this waymore of the material will go into the product and less into the manufacturing waste.

Total Number of Signal Layers. The number of signal layers can be estimatedgiven the routable board area Ar and the number of “reference components” Nref

using the density approach.6 The number of reference components is equivalent tothe number of components when all components are weighted with reference to a

(if GW > Wb)(otherwise)

Nb = Int(Lp/Lb) Int(Wp/Wb) + Int(Wp/Lb)Nb = Int(Lp/Lb) Int(Wp/Wb)

(if GL > Wb)(otherwise)

Nb = Int(Lp/Wb) Int(Wp/Lb) + Int(Lp/Lb)Nb = Int(Lp/Wb) Int(Wp/Lb)

Ar6(n − 1)As


10.8 CHAPTER TEN

TABLE 10.4 Signal Layer Estimation

Ar /Nref (in2/14-pin DIP) Ns-layers

Above 1.0 2

0.8–1.0 2

0.6–0.8 4

0.42–0.6 6

0.35–0.42 8

0.2–0.35 10

0.0–0.2 10+

14-pin DIP component as one unit. The categorical functional dependence is givenin Table 10.4.

10.2.3.3 Optimization. Once we derive the process models and the precedingrelationships, we can follow a procedure like the one depicted in Fig. 10.5 to optimizethe board design parameters. The results of such an optimization are shown in Fig.10.6. The plot shows the weighted mass of the waste streams on a per-board basis asa function of the board dimensions. From this plot, we can choose the dimensions ofthe board corresponding to the minimum waste.

We also can observe the effect of relaxing a constraint or imposing an additionalconstraint on the optimization problem. These constraints can easily be imposed orremoved during waste stream calculation from process models. Several useful con-clusions can be drawn from these plots:

The minimum waste does not necessarily occur for the smallest or thinnest (i.e.,fewest layers) board size.

A large variation (more than 100 percent in the plots shown) in the amount ofwaste per board occurs as the dimensions and the number of layers of the boardare varied. Thus the scope for waste minimization through design optimization istremendous.

The choice of panels also makes a difference in waste generation. Thus, whenevera variety of panels is available, we must calculate which to choose.

FIGURE 10.4 Layout of two panels: (a) orientation 1, (b) orientation 2.

FIGURE 10.5 Board design optimization procedure.

10.9

FIGURE 10.6 Waste mass as a function of board dimensions for two different panel sizes: (a) 18 × 24 in, (b) 21 × 27 in.

10.1

0

10.2.4 LIFE CYCLE ASSESSMENT (LCA)

A bare board is only one component of an overall PCB assembly. Thus, it is impor-tant to perform a similar optimization analysis on every component of the assembly.Also, design variation in one component sometimes necessitates design variation inanother. For example, making the board smaller may allow the use of flip chip ballgrid array components, which may not be as environmentally benign as quad flatpackages because of the potential worker exposure hazard in the solder-bumpingprocess. Therefore, the optimization procedure eventually can be extended to incor-porate selection of component packaging. Electronic packaging presents discretechoices of package types. We can formulate the process models for each packagetype as a function of a functional parameter, such as the pin count. For semiconduc-tor or die manufacturing, environmental issues are largely a function of waste miti-gation in the manufacturing process; currently, limited opportunity is available toeffect environmental decisions through design changes. However, a change in tech-nology brings about major changes in process steps, chemicals used, and processmechanics and a resulting significant change in process waste. Therefore, processmodels for semiconductor manufacturing must predict the waste per die as a func-tion of the yield (which is a function of die size and wafer size) and the technologyused.

So far, we have examined mainly design optimization of a product for minimumprocess waste. One aspect neglected in this analysis is the amount of materialgoing into the product itself and its eventual fate. Although manufacturing wastesrepresent the dominant life cycle impact for PCBs, we cannot neglect the environ-mental impacts of use and disposition phases of the life cycle for other compo-nents of a circuit board or computer. LCA examines a product from the time itsraw material is mined to the time the product is disposed back into the environ-ment. It looks at material and energy flows in mining, material refinement, manu-facturing, use, consumption, and disposal (which includes recycling, reuse,remanufacturing, incineration, deposit in a landfill, etc.) associated with a particu-lar product. While LCA conceptually is straightforward, to implement such analy-ses requires a great deal of data that often are unavailable or of low quality. LCAis extremely data intensive, and while results can be found for some comparisons,often the data are so poor that little can be learned from them. Furthermore, fewhave agreed on how the multiple health and environmental impacts of a product’slife cycle can be compared to those of another product, making comparisonsbetween products difficult.

LCA is ultimately an attempt to draw a quantitative connection between theexistence of a product (its manufacture, use, and disposition) and environmentalimpact. DfE is an attempt to incorporate this information (i.e., connections, analy-ses) to minimize environmental impacts of design decisions.

Just as more traditional engineering models help designers predict the perform-ance of their designs in terms of speed, weight, energy consumption, and other morestandard measures of performance, LCA is a model that predicts for designers theenvironmental performance of their designs.

10.3 IMPLEMENTING GREEN PCB MANUFACTURING

The U.S. PCB industry has significantly improved its environmental performanceover the past 20 years. This industry, which has the potential to be a major polluter,


has consciously altered its processes and practices to minimize its toxic output.While PCB manufacturers once viewed the effects of environmental regulations asa threat to their long-term growth, they have reversed this position. One of the bestexamples of this remarkable turnaround is how they actively became partners withregulatory agency programs such as the U.S. Environmental Protection Agency(EPA) Design for the Environment program. The industry continues to work withthese agencies to assess the performance, cost, and environmental impact of alterna-tives to traditional PCB manufacturing methods.7 Despite the great advances withinthe PCB industry, environmentally conscious designers can further reduce theimpact of the products they design by being aware of the consequences of design onprocesses and material.

The concept of connecting design decisions to environmental consequences fur-ther down the supply chain is referred to as DfE. It means, for example, that requir-ing a solder surface on finished PCBs to be shipped to an assembler has moreenvironmental consequences than allowing the PCB manufacturer to use an alter-native finish such as Sn. The proposed concept and the procedure for making suchcircuit boards are further reviewed in the subsequent sections.

10.3.1 BASIC PROCESSES

There are several types of PCBs, depending on use, operational environment, andcost constraints.The simplest type of board is single-sided, with no holes for connec-tions to other layers; these typically are made out of inexpensive laminate materialand used for consumer products where there is little electronic sophistication andcost is the driver. A copper-clad dielectric is coated with a resist material, which ispatterned to protect the areas where circuitry will be formed and the remainingcopper etched away. This is referred to as the print and etch process. The chiefwaste products are etchant, consumed resist, and scrap or excess board material.The dielectric material can be either a rigid plastic (typically paper phenolic orglass/epoxy) or flexible plastic (e.g., certain nylons or polyesters).

The second type of board is double sided, with plated through-holes. This boardalso is in common use. Because of the need to plate the dielectric on the walls of theholes connecting both sides, either electroless copper or, more recently, direct plat-ing chemistries are used to make the surface conductive. Once the surface is con-ductive, the walls are typically plated using standard electroplating copper baths.This means that the surface of the board also is plated either as a sheet of copper(panel plating) or as resist-defined circuit areas (pattern plating). In either case, thecopper between circuit lines must be removed by etching. In pattern plating, ofcourse, the background copper is much thinner than the circuit lines. The dielectricagain can be rigid or flexible.

The most sophisticated boards, requiring complicated circuit routing to accom-modate interconnecting integrated circuit chips, are usually multilayer boards.Theseboards traditionally have been manufactured by building innerlayers using the printand etch process. These innerlayers then are stacked in register and laminatedtogether with partially polymerized dielectric between the layers along with outerlayers of plain copper sheet. The outer layer is then typically drilled with through-holes to connect the various layers. The outer layers are either panel or patternplated after the hole walls are coated using electroless or direct metallization.Againthe dielectric can be rigid, flexible, or a combination of the two.

The various processes used to make these types of boards have undergonechanges during the past few years, the most significant of which are listed in the next

10.12 CHAPTER TEN

section. All these changes have had a significant effect in reducing the environmen-tal impact of this industry.

10.3.2 PROCESS MODIFICATIONS

10.3.2.1 Switch from Chromic-Based Etchants. Beginning in the late 1970s andcontinuing through the early 1990s, alternatives were developed to replace chromic-based etchants. This happened because chromic-based etchants were not easy toregenerate, etched at a slow rate, and had a low limit of dissolved copper.8 They werealso being regulated by environmental and health and safety agencies due to thehexavalent species, which was considered carcinogenic. The copper chloride andammoniacal etchants, which have now replaced chromic etchants in the PCB indus-try, overcame all these disadvantages and were cheaper. The changes also signifi-cantly reduced the volume of etchant used by the industry and, therefore, itsgeneration of spent etchant. Unlike chromic-based etchants, spent ammoniacal andcupric chloride etchants can be regenerated, reclaimed, or reused in other manufac-turing operations.9

10.3.2.2 Elimination of Chlorinated Solvents. With the invention of solvent-developable dry film photoresists in the late 1960s, the PCB industry used significantamounts of trichloroethane and methylene chloride to develop and strip them. Inthe late 1970s and early 1980s, aqueous and semiaqueous processable resists weredeveloped, using either bicarbonate/hydroxide or butyl carbitol/cellosolve as devel-opers or strippers. Trichloroethane continued to be used as a cleaning solvent until1990, when it was found to contribute to stratospheric ozone depletion. During theensuing decade, it was virtually eliminated from use when the EPA adopted a phase-out program for all ozone-depleting substances in Title VI of the Clean Air Actamendments.10 Other cleaning solvents categorized as ozone-depleting substances,for example HCFC-141(b), also were eliminated when the industry switched toalternatives, such as citrus-based terpenes and aqueous-based cleaners.

10.3.2.3 Improved Process Control. Some of the industry’s most successfulenvironmental improvements include the widespread adoption of simple house-keeping measures that minimize waste generation and improve process control.Examples of such improvements include taking steps to reduce chemical loss,increase process bath life, and recover materials that otherwise would be disposed.11

These steps often included better bath control through improved use of sensorsand control equipment. In addition, total quality management (TQM) systems canproduce improved environmental as well as economic results. Adoption of a totalquality management program can reduce facility scrap rates and result in processchanges that reduce chemical losses or conserve process baths.

10.3.2.4 Reduced Use of Sn-Pb as Etch Resist. For many years, electroplatedSn-Pb was the most common PCB metal etch resist used in pattern plating. Its wide-spread use also is due to the requirement of many upstream customers to be able touse reflowed Sn or Pb as the PCB surface finish of choice. The industry began toswitch to Pb-free etch metal resists due to technical constraints associated with Sn-Pb etch resists. In addition, the industry saw widespread use of hot-air solder lev-eling (HASL) as the predominant PCB surface finish prior to adding components.Typically, any Sn-Pb plating had to be stripped off prior to the HASL process,


thereby generating a Pb-containing hazardous waste. Facilities that switched toHASL began to look for an etch resist that, when stripped, would not be an envi-ronmental hazard. Sn was the most logical alternative.As an etch resist, Sn performsjust as well as Sn-Pb; however, Sn is not considered a hazardous waste and does notrepresent the worker safety issues associated with Pb.12

10.3.2.5 Increased Use of an Alternative Metal-Bonding Surface Finish. To beeffective, a PCB surface finish must prevent copper oxidization, facilitate solderability,and prevent defects during assembly. For many years, the reflowed Sn-Pb surface fin-ish, which applies Sn-Pb solder to all exposed PCB traces, was the predominant PCBsurface finish. Currently, HASL, which applies Sn-Pb solder only to PCB throughholes and pads, has significantly reduced the industry’s use of Pb. Despite its advan-tages, HASL still poses several drawbacks; for instance, unless its waste solder dross isrecycled, it must be managed as a hazardous waste. HASL also results in domed sol-der surfaces, whereas new and more complex packaging designs require flat surfaces.13

Among its other problems, HASL does not effectively cover the electrolessnickel/immersion gold process, which has grown in use due to the ease of bondingwire semiconductors to it. Also, it has difficulty in adapting to the increased minia-turization of component attachment points. The Institute for Printed Circuits (IPC)is currently assessing Pb-free alternatives to HASL through the EPA DfE PCBProject.The project will assess the economic, environmental, and performance char-acteristics of the following HASL alternatives; organic solder protectorates, im-mersion Sn and Ag, electroless nickel/immersion gold, immersion palladium, andelectroless nickel/immersion palladium.

10.3.2.6 Improved End-of-Pipe Pollution Control Practices. In the 1970s, con-ventional metal precipitation systems were the most common type of wastewatertreatment utilized by the industry. Although precipitation remains very common,some facilities are supplementing or replacing such systems with ion exchange andelectrowinnowing technologies. For most facilities, copper, Pb, and nickel, all ofwhich are amenable to ion exchange and electrowinnowing, are the only metal ionspresent in significant concentrations. Furthermore, these techniques result in salableforms of metals, which can produce economic revenue for the company as well as a“cleaner” waste treatment sludge that may not be subject to Resource Conservationand Recovery Act (RCRA) hazardous waste regulations.

The use of nonchelated process chemistries also has reduced the generation ofwastewater treatment sludge. Reduced sludge generation means cost savings, sincethe avoided cost of managing this sludge, which the EPA considers a listed haz-ardous waste (e.g., F006) in most cases, can be significant.

10.3.2.7 Increased Reuse and Recycling of Manufacturing By-Products. Thecircuit formation processes noted already are subtractive and, in many cases, themost reliable and cost-effective ways to manufacture PCBs. The subtractive methodgenerates large amounts of copper-bearing waste streams. Approximately 60 per-cent of the copper is removed in the typical etching process, resulting in a significantamount of copper leaving the facility as waste.

In general, the industry recycles a majority of its manufacturing by-products (e.g.,wastewater treatment sludge, etchant, off-specification boards, frames, and solderdross). Recycling extracts copper for reuse, reducing the need for virgin copper oreto be mined and reducing potential groundwater contamination (which could occurif copper-containing waste or incinerator ash is left in a landfill). PCB manufacturerscan use on-site recycling methods, such as electrowinnowing, to remove metallic ions

10.14 CHAPTER TEN

from spent process solutions, ion exchange regenerant, and concentrated rinsewaters; or they can send their manufacturing by-products off site to facilities wherethe valuable constituents are reused or reclaimed.

Currently, the following materials are not subject to the RCRA hazardous wasterestrictions and rules when they are reclaimed: scrap boards, scrap trim, router dust,solder baths and dumps, and solder dross. In addition, some states have ruled thatspent etchant, when shipped to specific facilities that use the etchant as a direct feedstock in manufacturing operations, is not subject to RCRA.

10.3.2.8 Increased Use of Direct Metallization. As noted, the electroless copperprocess has been used to make drilled through-holes conductive. Electroless copperuses large quantities of water, formaldehyde, and chelators, such as ethylenediaminetetraacetic acid (EDTA). These chelators also chelate metal waste streams,complicating their treatment.

The EPA and the IPC, under a DfE project, assessed a number of direct metal-lization alternatives—carbon, graphite, palladium, and conductive polymers—andfound that all of them cost less, perform as well, and have less environmental impact(no formaldehyde or EDTA and less water use) than electroless copper. In addition,facilities that switched to alternatives have found that these alternatives often areless hazardous to use; increase production flow; decrease maintenance require-ments; and reduce cycle time, operating costs, and water usage, increasing the facili-ties’ bottom lines.

10.3.2.9 Increased Water Reuse and Recycling. The PCB industry is dependenton the use of large quantities of high-quality water, which is used primarily to rinsecircuit boards between process steps. PCB facilities now increasingly employ waterreuse and recycling technologies to extend process bath life and decrease theirreliance on municipal water, which may be costly and, in some cases, of poor quality.Facilities located in areas where water supplies are scarce may face flow restrictionsfrom municipal water authorities.

The installation of on-site water recycling systems and the use of simple water usereduction methods (e.g., flow restrictors, conductivity controls, flow meters, counter-flow rinse tanks, and spray rinse systems) have resulted in large water use reductionsfor a number of PCB facilities. Fortunately, the adoption of water conservation prac-tices often results in economic gain.

10.3.2.10 Future Opportunities for Pollution Prevention. There are additionalopportunities for improved environmental impact. These might include:

One hundred percent beneficial reuse and recycling of all hazardous by-products Use of raw materials from sustainable sources (e.g., copper foil from recycled cop-

per, laminate from bio-based or recycled plastic sources) Zero-water-discharge manufacturing processes Systematic management of environmental health safety (EHS) compliance and

performance [ISO 1400 I and Environmental Management Systems (EMAS)] Development of technically acceptable Pb-free solders Standardization and implementation of design for reuse and disassembly practices Development and implementation of energy-efficient manufacturing operations Integration of environmental cost and activity-based cost accounting tools into

traditional accounting methods


10.3.3 ENVIRONMENTAL IMPACT

10.3.3.1 Current Technology Trends. Since the industry is being driven to pro-duce lighter, denser, cheaper circuits, designers must take advantage of these tech-nologies. Many of the newer processes utilize less material, produce less waste, andare more energy efficient. For example, the two major approaches to microvias uti-lize either laser to make vias without drilling or photodielectric materials. Whileboth these approaches produce vias that connect only layer to layer where physi-cally needed, this immediately increases the circuit density and reduces materialusage. Photodielectrics can also produce circuit “channels” as well as vias, which incombination with additive metallization would further significantly reduce waste.

In addition to being used to make vias, lasers also are being utilized more seri-ously to create pattern resists used in making the circuit traces. Laser direct imagingallows the circuit design to go directly from the digital output of the design to theboard itself without utilizing a phototool.This eliminates the phototool and its wastestreams. In the case of a photodielectric, it may even be possible to image the viasand channels directly into the dielectric.

10.3.3.2 Electrical Design Effects. As can be seen from the improvements atboth the chemical usage and the technology levels, it is possible to utilize materialsand processes to significantly reduce the environmental impact of a circuit board.However, this can happen only if the PCB manufacturer is allowed to pick the bestprocess and materials and not be hampered by outmoded specifications that mightcall for some specific chemical usage. For example, if electroless copper in the vias orholes is specified, then the manufacturer cannot utilize direct metallizationprocesses to do the same process. Thinner copper and utilization of polymer thickfilm conductors for via metallization, shielding, and even conductor metallizationcan save much waste, since either less copper is used or, in the case of polymer thickfilms, the process is strictly additive.

In addition, newer technology should be implemented where possible. Forexample, microvia technologies can be used to significantly reduce layer count orboard size. This freedom to trade off layer count for board size can be significant,especially if the original board dimensions do not fit well into the standard panelsizes used in the industry. Unused panel areas that end up as edge trim scrap typi-cally must go through all the same process steps as the final board. This means thatall that processing and material is wasted and accounts for a significant portion ofthe scrap. Using mixed flex-rigid boards, thinner flexible materials can be used inplace of rigid materials to not only carry circuitry but also function as an interboardconnector.

The key to utilizing the right materials and processes is to work closely with thePCB manufacturers. Decisions can be arrived at that both meet technical specifica-tions and have the least environmental impact. Many people currently are workingon methods to incorporate DfE in design algorithms, but direct discussions with sup-pliers will often serve the same function. Many PCB fabricators also now have in-house design capabilities and can suggest alternatives to a given design.

The PCB industry has made great strides in reducing its environmental impact bystepping up its pollution prevention efforts. Additional improvements may be madeby optimizing the utilized processes and materials. Still other ongoing efforts mayreduce the environmental burden even further.While the manufacturers themselvescan implement much of this, cooperation along the electronic supply chain is critical.Those asking for specific materials and design considerations must be aware of theconsequences of those design decisions. Despite the number of efforts to implement

10.16 CHAPTER TEN

DfE mechanisms to make this process easier, it is just as important to work directlywith suppliers to understand the constraints that design decisions impose and theopportunities for improvement should other choices be made.14

10.4 CONFORMAL COATING WITH ENVIRONMENTAL SAFETY

10.4.1 FUNDAMENTALS

A conformal coating is a thin layer of insulating material applied to the surface of aPCB to protect sensitive components from thermal shock, moisture, humidity, cor-rosion, dust, dirt, and other damaging elements. When properly applied, these coat-ings provide a high degree of insulative protection and are usually resistant to manytypes of solvents and harsh environments. Coatings also provide excellent dielectricresistance. Demanding applications where conformal coatings are critical includeautomotive products, consumer electronics and appliances, industrial controls, mili-tary/aerospace systems, and medical devices.15

In the past, because of the cost of conformal coating materials and applicationprocesses, only the most expensive boards or those with special reliability require-ments were coated. Recent advances in application technology and process abilityhave improved the economics of conformal coating use. Additionally, as circuit sizediminishes and components become more delicate, protective barrier coatings aregrowing in importance.

While many available conformal coatings are still solvent based, the market forsolvent-free coatings in North America and other parts of the world is growing rap-idly. In 1970, the U.S. government passed the Clean Air Act, which gave the EPA theauthority to set national air quality standards to protect against common pollutants,including materials that release volatile organic compounds and ozone-depletingchemicals. EPA and Occupational Health and Safety Administration standards,stringent state and local regulations, and emerging environmental awareness havecombined to encourage coatings formulators and electronics manufacturers to usesolvent-free and low–ozone-depleting chemical/volatile organic compound coatingswherever possible in their manufacturing operations.

Although solvent-free coatings are more expensive on a per-unit basis than solvent-based materials, much less volume is used. Because solvent-free coatings are100 percent solids, they do not evaporate as part of the curing process. Solvent-basedmaterials are typically 60 to 70 percent solvents, all of which are wasted during evap-oration.

10.4.2 COATING SELECTION

Conformal coatings are generally classified according to the molecular structure oftheir polymer backbone. There are five traditional conformal coating chemistries:acrylic, epoxy, urethane and parylene (commonly grouped together as organics), andsilicone (an inorganic). All except parylene were solvent based until a decade ago,when increased environmental concerns and resulting government regulations dic-tated the reformulation of conformal coatings to solvent-free, low–ozone-depletingchemical/volatile organic compound materials and processes. Many environmen-tally acceptable coatings are hybrid formulations that combine two or more coatingchemistries (e.g., urethane acrylate and acrylic functional silicones) to improve per-formance properties, wetting, adhesion, and cure requirements.


10.18 CHAPTER TEN

Solvent-free organic coatings are typically tough, abrasion-resistant materialsthat offer improved moisture and chemical resistance and operate at temperaturesranging from −40 to 125°C. Typical organic coating dielectric strength is 1000 V/mil.Organic coatings in general, and acrylics and urethanes in particular, are resistant toa broad range of solvents. However, acrylics and urethanes may not be the best coat-ing chemistries for environments exposed to wide fluctuations in temperature overshort periods of time, as they tend to crack under thermal stress. Rework on acrylicsand urethanes can be handled using mechanical abrasion or microsand blasting.Epoxies tend to be the least popular conformal coatings because of rework issues.Because most board substrates are made of epoxy, the manufacturer may destroythe board by removing the coating.

Silicone coatings are soft, flexible materials with a high coefficient of thermalexpansion, which allows them to absorb expansion and contraction stress withoutharming protected components and to function well in environments with extremetemperature cycling from −40 to 204°C. Silicones are very forgiving materials inproduction because they coat and adhere to just about any surface found on a PCBand offer good resistance to polar solvents, an attribute that makes them ideal forautomotive electronics applications. Silicone’s dielectric strength is typically 500V/mil.

Parylene coatings are deposited onto PCBs using gas-phase polymerization toprovide a very thin uniform coating. Boards coated with parylene must be processedin a batch operation using special high-vacuum equipment. An adhesion promotionprocess using silane and isopropyl alcohol followed by a rinse and bake-out step isgenerally required as a pretreatment for most electronic components bound forparylene deposition. Because this coating can find its way into gaps as small as 0.001in, airtight masking of interconnects is required to prevent leakage. Parylene isapplied in the cured state during the chemical vapor deposition process once the rawdimer material is sublimated.

10.4.3 CURING METHODS

There are various methods available to achieve rapid cure or solidification of con-formal coatings, including two-component mixing, heat, moisture, and ultraviolet(UV) light exposure. Each of these methods is appropriate for specific coatingchemistries and has distinct advantages and disadvantages.

Traditional acrylic, urethane, and epoxy coatings can cure or solidify in minutesusing heat or two-component technology, which involves a room-temperature chem-ical reaction. Silicone coatings may be cured by exposure to heat, UV light, or ambi-ent moisture. Hybrid coating formulations, which incorporate multiple coatingchemistries, are designed either to be UV curable or to rely upon dual-cure mecha-nisms such as UV light, heat, or ambient catalyzation to enhance cure efficiency andincrease in-line cure speeds.

Every cure method has its own set of advantages and disadvantages. Two-component mixing offers wide latitude in adjusting a coating’s cure speed and potlife. However, this technology is often considered undesirable because it requires theuser to inventory and mix, in the proper ratio, two different materials. Catalyzedcoatings use two-component room-temperature cure. Properly formulated, thesematerials have a 1:1 mix ratio and a pot life of 8 to 10 h (one shift) or up to 5 daysdepending on the chemistry, which makes them good candidates for robotic applica-tions. These coatings wet and adhere well in the no-clean process, and have a veryeffective shadow cure. Recent advances in static mixing and meter mix equipment


make it possible to mate two-component materials with both atomized spray andselective application equipment.

While heat cure improves wetting and lowers viscosity, some heat-cure coatings,particularly platinum-catalyzed silicones, are subject to cure inhibition. This occurswhen the coating comes into contact with various sulfur, amine, or organometalliccompounds that are sometimes found on boards as residual contaminates from theintegrated circuit chip demolding or solder flux process.Additionally, achieving veryrapid cure (less than 2 min) requires a temperature in excess of 150°C, which may behigh enough to damage some components.

Moisture-cure coatings solidify rapidly on exposure to ambient or induced mois-ture. Extremely moisture-sensitive materials may cure inconveniently (e.g., in thefeed line, on the surface of the supply reservoir, or at the dispense nozzle). To con-trol a potential rise in viscosity, the coating should be exposed to as little moisture aspossible prior to application.

UV light cure is an efficient process. UV light cure materials contain a photoini-tiator that cures the coating in seconds when exposed to the proper UV light wave-length. One major problem encountered with UV materials is their inability to curein areas not exposed to UV light.To overcome this problem, UV coatings have beenformulated with a secondary cure mechanism to ensure full cure in areas that are notdirectly exposed to UV energy. Full cure in shadowed areas is extremely importantfor board performance with all coating chemistries, as elevated operating or testtemperatures may cause uncured material to expand, rupturing the coating fillet andcracking solder joints or integrated circuit packages.

UV light curing materials also are subject to oxygen inhibition, a process thatoccurs at the coating-air interface when oxygen reacts competitively with free radicalsgenerated during UV light exposure. Oxygen inhibition can be overcome by increas-ing UV light intensity or by reducing oxygen concentration with a nitrogen blanket.

Prior to selecting a coating chemistry, the end-use environment of the PCBshould be reviewed, assessing the potential for exposure to solvents, temperatureextremes, dramatic temperature gradients, and physical stress. No one coating isright for all boards and conditions. Working closely with reputable coating materialsuppliers that team up with equipment suppliers is the best way to ensure the selec-tion of an appropriate material and process.

10.4.4 DISPENSING METHODS

Normally the final manufacturing step on a PCB assembly line, conformal coatingscan be applied manually or with semi- or fully automated techniques. Preparation forconformal coating is a four-step process. First, board cleanliness is established and, ifnecessary, the board is cleaned. Next, connectors on the PCB are masked off as nec-essary to protect interconnects from the coating process. Coatings are then applied tothe board and cured. Finally, any protective masks on the board are removed.

Conformal coating dispensing techniques can be selective or nonselective. Non-selective systems apply the coating uniformly at a very fast pace, but add substantialtime and cost in material waste and manual masking/demasking operations. Exam-ples of nonselective dispensing techniques are dip, atomized spray, brush, and waveor flow coating.

Dip. Boards are immersed into liquid conformal coatings and withdrawn. Becausemost dipped coatings will not penetrate very narrow gaps, the dipping process isdecreasing in popularity.

10.20 CHAPTER TEN

Atomized spray. Performed with a standard automotive paint spraying gun, atom-ized spray is the most popular method of manual coating as it provides fast, uni-form coverage.

Brush. Typically used for small boards or localized repair and touch-up coatingapplication, brushing is the least used application method and is best suited forlow-volume production. Uniform thickness and bubbling are the main problemswith brush application.

Wave or flow coating. Boards are indexed over a wave or pumped tide of coatingmaterial that is applied to one side at a time.

Computer-controlled selective coating systems apply coatings to designated areas ofthe PCB. Because coatings are precisely dispensed in defined areas, masking anddemasking operations as well as other off-line batch activities are significantlyreduced or eliminated. Selective coating systems offer substantial savings in resinconservation and off-line masking and demasking labor operations.

Some popular selective coating systems are as follows:

An airless process using a nozzle to dispense a shaped curtain of coating material,designed for use with solvent-based materials. Evaporation helps control curedfilm thickness and migration of wet material.

A Venturi-type air-assisted dispensing head designed to dispense high-viscosity,solvent-free materials.This device produces a wide (≥1 in) coating curtain from thehead and stretches it over the board’s surface. Overspray is dependent on the vis-cosity of the material being applied and the on/off times programmed into the dis-pensing profile.

A dispensing head shaped like a probe. During selective dispensing, an air assist“twists” the coating stream into a bead, corkscrew, or conical mist pattern onthe substrate. Though designed to dispense low-viscosity coatings, this head is not as sensitive to viscosity or chemistry as other technologies, and offersthree different dispensing patterns that can be programmed for specific boardgeometries.

Selective atomized spray head systems. These have been found to work well witha wide range of coating viscosities. Multiple tools and spray heads can be incorpo-rated to perform varied dispensing tasks.

All selective systems can be configured with indexers, board inverters, and variouscure systems to create an automated process that can be added to a conventionalassembly line.

10.4.5 PROCESS ISSUES

The degree of solder mask cure is important to conformal coating performance. If notcured completely, ingredients such as glycols, bromides, and ionic compounds can exitthe film during subsequent solder excursions, leaving residues that may ultimatelyaffect the wetting characteristics and adhesion of the conformal coating. Some ofthese residues can also contribute to electrochemical migration and subsequent den-drite formation. Also, if the solder mask materials are not applied to clean, unconta-minated substrates, many problems can occur during conformal coating. Whilecontaminated boards fail less quickly with a conformal coating, they can still failbecause of trapped ionic and corrosive species between the substrate and the coating.

Not long ago, board substrates were G-10 or FR-4 materials using RMA fluxchemistries and CFC-113 solvent-cleaning methods. These limited variables madetracking compatibility problems relatively simple. Today’s wide selection of soldermasks, flux chemistries, solder processes, and alternative cleaning processes hasgreatly complicated compatibility issues.

By performing cleanliness and adhesion testing, board manufacturers can assessthe baseline contamination of the PCB and determine whether further cleaning is nec-essary to ensure conformal coating adhesion, integrity, and reliability. Two effectivemethods of testing boards are ionic cleanliness testing, which determines levels ofchloride per square inch, and surface insulation, an electrical test of solder joints.Although conventional ionic cleanliness testing methods are not as accurate as ionchromatography, they are excellent cleanliness monitors for the production environ-ment.

If contamination levels are higher than recommended (5.7 µg/NaCl/cm2), boardmanufacturers have a number of choices for cleaning the assembly. However, thereis still no single, easy, environmentally safe answer to board cleaning. When consid-ering replacement cleaning chemistries, manufacturers should note the cleaningcapability, evaporation times, residue, and odor of the process.With any of the avail-able cleaning processes, a chemical reaction may take place that could affect boardperformance and reliability.

No-clean/low-solids fluxes are not usually removed by cleaning prior to coat-ing application. The type and level of remaining residue is dependent on theboard design and the solder profile. Inadequate preheat temperatures and dwelltime duration can affect the success of a no-clean process. If the profile is notdone correctly, ionic and corrosive species from the flux can cause a variety ofperformance problems. The flux application method and soldering environmentalso have an impact on how much unvolatized flux species will remain on theassembly.

Because a conformal coating can retard corrosion but not prevent it, manufac-turers using these materials must benchmark and maintain proper processes to pre-vent corrosion and ensure reliability. Excessive residues can be coated, butentrapment of the ionic and corrosive species will cause a variety of problems overtime. Conventional ionic testing is not routinely performed on low solids becauseevaporation of the isopropyl alcohol in the test solution causes a visible whiteresidue to form around the solder fillets, creating aesthetic concerns.

Water-soluble fluxes work well when used in conjunction with conformal coatingoperations. As these materials are very aggressive, they must be washed off withinminutes of soldering operations. As a result, board surfaces are generally wellcleaned and few coating problems are encountered. However, water-soluble fluxescan outgas residual absorbed contaminants and water during heat excursion, causingdendrite formation or coating blisters in areas of flux stains.The drying process mustbe as controlled as the actual cleaning process. Again, proper process controls willgreatly reduce potential problems.

For effective, long-term conformal coating solutions, board manufacturers shouldwork closely with conformal coating formulators to determine effective board clean-ing methods. Although this was once the sole responsibility of board manufacturers,coating suppliers will now assess and test process-ready assemblies for cleanlinessand coat them with application-appropriate coatings to determine their effective-ness in the assembly process. Coating companies will also work closely with equip-ment manufacturers to prequalify adhesives and dispense equipment, ensuring thatmaterials and the application systems will run smoothly on the customer’s produc-tion line.


REFERENCES

1. Siddhaye, S., and P. Sheng, “Design for Environment: A Printed Circuit Board AssemblyExample,” in Green Electronics, Green Bottom Line, Goldberg, L. H., and W. Middleton,eds., Newnes, Boston, pp. 113–122, 2000.

2. Allen, D., “Life Cycle Assessment and Design for the Environment,” Tutorial Notes, IEEESymposium on Electronics and the Environment, San Francisco, CA, May 1997.

3. Siddhaye, S., and P. Sheng, “Integration of Environmental Factors for Process Modeling ofPrinted Circuit Board Manufacturing—II. Fabrication,” Proceedings of the IEEE Inter-national Symposium on Electronics and the Environment, pp. 226–233, San Francisco, CA,May 1997.

4. Worhach, P., and P. Sheng, “Integration of Environmental Factors for Process Modeling ofPrinted Circuit Board Manufacturing—I. Assembly,” Proceedings of the IEEE Inter-national Symposium on Electronics and the Environment, pp. 218–225, San Francisco, CA,May 1997.

5. Srinivasan, M.,T.Wu, and P. Sheng,“Development of a Scoring Index for the Evaluation ofEnvironmental Factors in Machining Processes: Part I, Formulation,” Transactions ofNAMR, 23:115–121, 1995.

6. Balakrishnan, S., and M. Pecht, Placement and Routing of Electronic Modules, pp. 59–96,Dekker, New York, 1993.

7. www.epa.gov/opptintr/dfe/pwb/pwb.html.

8. Coombs, C. F., Printed Circuits Handbook, McGraw-Hill, New York, 1996.

9. PWB Project Case Study 2, On-Site Etchant Regeneration, EPA744-F-95005, July 1995.

10. FR 42 USC Section 7671(c). 5. PWB Project Case Study 1, Pollution Prevention WorkPractices, EPA 744F-95-004.

11. U.S. EPA, “Printed Wiring Board Industry and Use Cluster Profile,” EPA 744-R-95-005,pp. 2–38, September 1995.

12. IPC Surface Mount Council White Paper, “PWB Surface Finishes,” SMCWP-005, April1997.

13. U.S. EPA, “Implementing Cleaner Technologies in the Printed Wiring Board Industry:Making Holes Conductive,” EPA 744-R-97-001, February 1997.

14. Evans, H., and J. W. Lott, “Implementing Green Printed Wiring Board Manufacturing,” inGreen Electronics, Green Bottom Line, Goldberg, L. H., and W. Middleton, eds., Newnes,Boston, pp. 153–160, 2000.

15. Ritchie, B., and L. Bennington, “New Conformal Coatings Combine Protection with Envi-ronmental Safety,” SMT, 14(4):44–48, April 2000.

10.22 CHAPTER TEN

www.epa.gov/opptintr/dfe/pwb/pwb.html

CHAPTER 11GLOBAL STATUS OF

LEAD-FREE SOLDERING

11.1 INTRODUCTION

Lead (Pb) has been widely used in the industry for a long time. Of the approximately5 million tons of lead consumed globally every year, 81 percent is used in the storagebatteries, with ammunition and lead oxides together accounting for about 10 per-cent, as shown in Table 11.1.1 However, despite the longtime acceptance of lead byhuman society, lead poisoning is now well recognized as a health threat. The com-mon clinical types of lead poisoning may be classified according to their clinicalpicture as (a) alimentary, (b) neuromotor, and (c) encephalic.2 Lead poisoning com-monly occurs following prolonged exposure to lead or lead compounds.The damageoften is induced slowly, but definitely. Some historians even speculate that the fall ofthe Roman Empire could be related to the use of lead in the pipelines that carrieddrinking water to Roman cities.

Due to the profound evidence of toxicity, the use of lead chemicals in paint andgasoline has been prohibited for several years. Storage batteries, due to their almost100 percent recycling levels, do not contribute to pollution or contamination andthus pose no immediate issue. On the other hand, although solder is only a smallpercentage by weight of electronic products (TVs, refrigerators, PCs, phones, etc.),these devices often end up in landfills after being disposed, and the lead could leachout into the water supply. For instance, in Japan the lead elution environmentalstandard in landfills is 0.3 mg/l. In the toxic materials detection tests recently per-formed by the Japanese Environmental Agency, it was confirmed that the amount oflead leaching from the pulverized remains of TV tubes and printed substrates forPCs and pachinko machines far exceeds the environmental standard.3 In the USA,the regulatory limit for lead in drinking water is 0.015 mg/l per EPA40 CFR141. Thelimit is 5 mg/l if the test is done by Toxicity Characteristics Leaching Procedure perEPA40 CFR261.A recent study4 demonstrates that the lead leached out from soldercan be several hundred times higher than the limit.

This concern about lead is a natural result of the growing global concern aboutthe environment. This environmental awareness can be demonstrated by the “Ger-man Blue Bird” system, which has been widely used in the European community forsome time. Consumers in Germany call it the “Blue Eco Angel,” but its official nameis Environmental Label. This is a special logo for products with positive environ-mental features on the German market and has been in use for two decades. As oftoday, over 4000 products bear the mark.5

11.2 INITIAL ACTIVITIES

The attempt to ban lead from electronic solder was initiated by the U.S. Congress. In1990, Reid S2638—subsequently modified to S729—proposed banning of all lead-

11.1


bearing alloys, including electronic solders, and instituting a tax of $1.69 per kg on pri-mary lead and $0.83 per kg on secondary lead used in the industry. However, lead sol-ders were removed from the bills after intense lobbying by the U.S. electronics industry.

In 1994, Denmark, Sweden, Norway, Finland and Iceland signed a statement tophase out Pb in long run. On June 16, 1997, a press release from the Swedish Gov-ernment identified lead as one of the elements to be eliminated from products overthe following 10 years.The Sweden Environmental Quality Objectives direct that anynew products, including batteries, introduced in Sweden should be largely free fromPb by 2020. Swedish manufacturers are also under a voluntary ban effective in 2000.6

At about the same time frame, recycling laws were proposed in various Asiancountries. In October 1996, the Discard Processing/Resource Reclamation Commit-tee of the Industrial Structure Council in the Japanese Ministry of InternationalTrade and Industry (MITI) announced goals for recycling discarded automobiles.Also in 1996, the Japanese Automobile Industrial Association set up a self-managedenvironmental program.The Pb used in new automobiles is to be cut in half by 2000(excluding Pb used in batteries) and to one-third of 1996 values by 2005. Most of thePb usage in Japanese vehicles now is in paint and radiators.

11.3 RECENT ACTIVITIES

There are pending producer responsibility laws for electronic and electrical equip-ment in a number of European countries. Laws were passed in Holland and Switzer-land before 1999 involving producer responsibility. Norway followed in 1999 andSweden in 2000. In some cases producer responsibility may involve the manufac-turer, importer, or reseller taking responsibility for the take-back of products andproper end-of-life treatment. Threshold limits for recycling of specified types ofmaterials may be included also. Denmark has proposed its own Pb ban, but cathoderay tubes and electronics are not included.7

In 1998 the European Union (EU) introduced a draft directive (law) called theWaste from Electrical and Electronic Equipment Directive (WEEE), which calls for

11.2 CHAPTER ELEVEN

TABLE 11.1 Lead Consumption by Product

Product Consumption (%)

Storage batteries 80.81

Other oxides (paint, glass and ceramic products, pigments, and chemicals) 4.78

Ammunition 4.69

Sheet lead 1.79

Cable covering 1.40

Casting metals 1.13

Brass and bronze billets and ingots 0.72

Pipes, traps, and other extruded products 0.72

Solder (excluding electronic solder) 0.70

Electronic solder 0.49

Miscellaneous 2.77

a ban on lead in all electronics (except automotive) by January 1, 2004. WEEE,which intends to ban the selling and/or importing of electrical/electronic equipmentcontaining lead interconnects, encountered objection from many European elec-tronics trade bodies including EUROBIT (information technology), ECTEL(telecommunications), the Printed Circuit Industry Federation, the Federation ofElectronics Industries, etc. On June 13, 2000, the EU Commission officially adoptedthe WEEE proposals as two separate but associated draft directives for submissionto the European Parliament—WEEE and Reduction of Hazardous Substances(ROHS). The ROHS proposals required replacement of lead and various otherheavy metals and brominated flame retardants beginning January 1, 2008.

On April 24, 2001, the Environmental Committee of the European Parliamentadopted a number of amendments to the two pending EU waste directives andadvanced their progress through the EU legislative process.8 The Committee agreedto exempt electronic applications where high-temperature melting solder is used,thereby alleviating the pressure in identifying lead-free high-temperature solderalternatives.The deadline for the proposed chemical ban was advanced from 2008 to2006. Other results from the vote included: (1) an amendment to include servers andstorage equipment in the ROHS chemical ban exemption failed; (2) consumables(e.g., printer cartridges) are now covered by WEEE, with producers responsible forcollection; (3) collection targets have also been increased from 4 to 6 kg/head/year;(4) recovery targets increased by 5 to 10 percent; (5) the deadline for implementingcollection and recycling schemes advanced from 5 years to 30 months after imple-mentation, and (6) the costs of historic waste may be financed collectively by pro-ducers through a visible fee.

The European Parliament voted on May 15, 2001, to adopt proposals to amendthe date for the hazardous material ban in the WEEE/ROHS draft to 2006. Mate-rial and components where substitution is “impossible” are exempt from the ban,including lead in server, storage, voice and data transmission, and networkingequipment. The Council of Ministers from each European state government dis-cussed the proposals on April 10, 2002. Their opinion aligns with the parliamentproposals in setting a target date of January 2006 for a ban on hazardous materialsincluding lead. This opinion of the council will be discussed and voted on by theEuropean Parliament in the next few months in order to allow the directives to befinalized.The list of hazardous materials is due to be reviewed in 2003, with the pos-sibility of extending the ban to, e.g., polyvinyl chloride and other halogenated flameretardants, etc.5

The move toward Pb-free processes raised the attention of some major manufac-turers. Nortel Networks is one of the lead-free pioneers in Europe. They initiated alead-free program in 1991, selected Sn99.3/Cu0.7 in 1994, built 500 lead-free phonesin 1998, and targeted meeting the second WEEE in 2001.9

In Korea, Samsung Group declared its commitment to protecting the environ-ment in 1994, and completed development of a “green” semiconductor product thatuses no halogen compounds (which contain such toxic substances as lead, chlorine,and bromine). Company officials say that the new-concept device will go into massproduction in the second half of 2001. The new concept is initially being applied toSamsung’s 128-Mb synchronous dynamic random access memory package and 256-Mbyte module for PC-133 systems. Samsung has replaced the tin-lead compoundused to plate the package terminals with a tin-bismuth compound. Conventional tin-lead solder paste has also been replaced by a tin-silver-copper solder. Moreover,external packages and printed circuit boards no longer contain halogens such aschlorine, bromine, or antimony. Examples of these green products include conven-tional packages, ball grid array (BGA) packages, and memory modules.10

GLOBAL STATUS OF LEAD-FREE SOLDERING 11.3

In Japan, the only legislative activities deal with the reclamation and recycling ofelectronics. The Home Electronics Recycling Law came into force on April 1, 2001,and applies only to TVs, refrigerators, and similar items.

On January 30, 1998, the Japanese Electronic Industry Development Association(JEIDA) and the Japanese Institute of Electronics Packaging (JIEP) presented areport entitled “Challenges and Efforts Toward Commercialization of Lead-FreeSolder—Roadmap 2000 for Commercialization of Lead-Free Solder.” This report,which includes a survey of 132 companies, offers the Japanese perspective on lead-free electronics. In this report, JEIDA proposes the following roadmap for leadelimination:

First adoption of lead-free solders in mass-produced goods: 1999 Adoption of lead-free components: 2000 Adoption of lead-free solders in wave soldering: 2000 Expansion of use of lead-free components: 2001 Expansion of use of lead-free solders in new products: 2001 General use of lead-free solders in new products: 2002 Full use of lead-free solders in all new products: 2003 Lead-containing solders used only exceptionally: 2005

The roadmap presented by JIEP is fairly similar to that of JEIDA, as shown in thefollowing list:1

Mass production using Pb-free solder: 1999 to 2000 Adoption of Pb-free components: 1999 Increased adoption of Pb-free components: 2000 to 2001 Full-scale recycling of assembly boards: 2001 to 2002 Pb-free solder used for new products preferentially: 2003 Pb-containing solder used only exceptionally: 2005 to 2010 Elimination of Pb solders: 2010

Some major Japanese original equipment manufacturers (OEMs) have begun tojointly develop recycling processes for electronic products. A number of major Jap-anese companies, e.g., Sony,Toshiba, Matsushita, Hitachi, and NEC, have made com-mitments to go lead free by 2001. This is in advance of Japanese legislation ontake-back due to come into force in April 2001.The Pb-free advancement roadmapsof those companies are detailed in the following list:

Matsushita (Panasonic) has been shipping 40,000 MiniDisc players per month withlead-free solder since October 1, 1998, and plans to eliminate all lead intercon-nects in four products by April 2001. Matsushita’s market share of its lead-freeMiniDisc player jumped from 4.6 percent to 15 percent in 6 months in Japan. Thislead-free product was reported to be introduced into Europe in March, 1999. Mat-sushita is using SnAgBiIn and SnCu.5 Currently only disc players and TVs areusing lead-free solder. Matsushita has also indicated it will begin marketing lead-free products in the U.S. in 2000. Each division of Matsushita is charged with usinga lead-free solder for at least one electronic product by March 2002.

Sony reduced its lead usage in 1999 by half of that used in 1996, and it plans tocompletely eliminate lead from all products except high-density packaging by

11.4 CHAPTER ELEVEN

2001. Sony’s suppliers have been instructed to provide only lead-free materialsand parts. Sony uses SnAgBiCu and possibly also Sn93.4/Ag2/Bi4/Cu0.5/Ge0.1solder for assembly. By 2001, all lead will be eliminated except in high-densityelectronics packaging. Akikazu Shibata of Sony predicted in 1999 that the com-pany would be 50 percent lead free in 1 to 2 years, and more than 75 percent leadfree in 5 years. Sony aims to introduce lead-free solder for all models produced inJapan and overseas by March 2001 and March 2002, respectively.

Fujitsu has announced the following lead-free roadmap: Complete lineup of lead-free LSI products to be available by October 2000. Half of all printed circuit boards used in Fujitsu products to be lead free by

December 2001. Total elimination of lead from all Fujitsu products by December 2002. This ini-

tiative includes not only components internally produced at Fujitsu, but alsoparts supplied by outside vendors.

Toshiba eliminated Pb from refrigerators, TVs, cleaners, PCs, and other majorproducts by December 2000 and plans to eliminate Pb solder in mobile phones by2003. The company possibly uses SnAgCu.5

Hitachi cut lead usage by 50 percent by March 2000 from 1997 levels. Half of itsdomestic products were lead free in 2000, with Pb having been eliminated fromrefrigerators, air conditioners, TVs, VCRs, and PCs since 1999. The company willeliminate inner Pb interconnects by March 2002 and Pb will be completely phasedout by March 2004. Hitachi uses SnAgBi and SnAgCu and is currently investing1.2 billion yen ($11.2 million) to expand production of lead-free solder.5

NEC launched the world’s first three notebook computers with lead-free mother-boards, manufactured with SnZn. It plans to install lead-free motherboards indesktop PCs next. NEC will reduce lead use by 50 percent by fiscal 2003 (versus1998) and is currently using lead-free semiconductors, which began shipping inJanuary 2001. NEC is labeling lead-free products to differentiate them from thosethat contain lead. The company uses SnAgCu, SnZn, SnCu, and SnZnBi.5

Mitsubishi plans to cut Pb usage to 50 percent by 2004 and to eliminate it entirelyby 2005 for four major products.5

NTT has announced it will use no Pb or Cd in newly purchased equipment.11

11.4 IMPACT OF JAPANESE ACTIVITIES

By 2001 the leading Japanese OEMs will have introduced products that contain noPb in their interconnect systems. This will allow the Japanese to be positioned toexclude products from Japan that do not meet these environmental standards. Fur-thermore, existence of Japanese products will justify European legislation requiringPb reduction and highly recyclable electronic products by 2007,12 thereby furtherincreasing the pressure on the rest of the world to convert to Pb-free processes.

11.5 U.S. REACTION

Since the initial attempt in Congress in early 1990s, very little activity has been seenin the U.S. until recently. The automotive segment is probably the only one with sus-


tained interest.There is no legislation pending.The Lead Industry Association, Elec-tronic Industries Alliance (EIA), Institute for Printed Circuits (IPC), and NationalElectrical Manufacturers Association all have been active in lobbying against lead-free legislation. The only activities under way in the U.S. are at the state level anddeal primarily with electronics recycling rather than reduction in the use of toxic ele-ments.

Obviously, the message from offshore is loud and clear: either work on lead-freesoldering now or forget about doing business. Thus the National Electronics Manu-facturing Initiative (NEMI) called for a “Lead-Free Initiative Meeting” in February1999 to review the situation, and since then has rolled out a series of action items toestablish a Pb-free direction for the U.S. electronics industry. The second NEMImeeting, held on May 26 to 27, 1999, effectively motivated many manufacturers toget involved in Pb-free development.

In April 1999 the IPC board of directors announced the following positionstatement:

The US electronics interconnection industry, represented by the IPC, uses less than 2percent of the world’s annual lead consumption. Furthermore, all available scientificevidence and US government reports indicate that the lead used in US printed wiringboard (PWB) manufacturing and electronic assembly produces no significant envi-ronmental or health hazards. Nonetheless, in the opinion of IPC, the pressure toeliminate lead in electronic interconnections will continue in the future from both the legislative and competitive sides. IPC encourages and supports research anddevelopment of lead-free materials and technologies. These new technologies shouldprovide product integrity, performance and reliability equivalent to lead-containingproducts without introducing new environmental risks or health hazards. IPC prefersglobal rather than regional solutions to this issue, and is encouraging a coordinatedapproach to the voluntary reduction or elimination of lead by the electronics inter-connection industry.

The IPC statement probably fairly truly reflects the opinion of most of U.S. indus-try: “Pb in electronics is not perceived as a health issue, but government and com-mercial drivers will push for its adoption anyway.Thus IPC will facilitate activities toenable it to happen.”13

To serve the industry by helping with lead-free initiatives, the IPC developed andmaintained the IPC lead-free e-mail forum. In addition, the IPC also organized aconference, IPC Works ’99, held in October 1999, with major emphasis on the Pb-free issue. It was at this conference that the IPC presented the industry with a firstdraft of the IPC Roadmap for Electronics Assemblies.The impact of this conferenceis that it was industry talking, including customers, suppliers, and competitors.

The HAL User Group, an organization composed of manufacturers of printedcircuit boards (PCBs), original equipment manufacturers (OEMs), contract manu-facturers (CMs), chemical suppliers, and equipment manufacturers, also met inAugust, 1999 to address Pb-free surface finishes as a response to the pressure for Pb-free soldering.

On January 17, 2001, the Environmental Protection Agency (EPA) lowered thethreshold reporting level for lead starting with calendar year 2001 in its ToxicRelease Inventory Rule. Therefore, any company that manufactures, processes, orotherwise uses lead or lead-containing products in quantities of 100 lb or more mustfile a report. This change will require many U.S. manufacturers who had never metthe previous requirement of 10,000/25,000 pounds to file reports.The first report forthe new requirements will be due on July 1, 2002.14

11.6 CHAPTER ELEVEN

To meet this pressing lead-free challenge from offshore, a number of Americancompanies also engaged in lead-free programs, as reported by the IPC:5

Boeing. Currently evaluating and developing reliability data on Pb-free finishes ChipPAC. Has qualified Pb-free BGAs and is scheduled to go into high-volume

production in the fourth quarter of 2000 Delphi Delco. Conducting developmental activities, 2- to 3-year window Lucent Technologies. Aligning with industry through consortium activities Motorola SPS. Currently evaluating and running pilot plant production with lead-

free soldering Shipley Co. LLC. Currently offering/developing Pb-free finishes for components

and connectors as well as PWB final finish applications Sun Microsystems. Participating in industry consortia and monitoring the situation Texas Instruments. Introduced NiPd finish for components in 1989; believes it is in

a leadership position Hadco. Investigating processes to replace SnPb board finishes IBM. Interim strategy developed in 1999; plans to stay ahead of the industry Honeywell. Has formed a team to formulate activities on Pb-free processes Visasystems. Has a patent on an organic solderability preservative (OSP)

However, a second opinion also exists on the aggressive move toward lead-freesolder. On April 10, 2001, the IPC and 35 other trade associations sued the EPA.Thissuit is directed against the EPA ruling that reduces reporting thresholds of lead andchallenges the PBT label for metals. As reported by Harvey Miller at InfraFOCUS,“PBT stands for persistent, bioaccumulative, and toxic; that is the combination thatcharacterizes another class of toxins, often confused with metals—persistent organicpollutants, such as polychlorinated biphenyls, DDT, and other synthetic creations ofthe last 50 years that are truly dangerous to life. These chemicals cause cancer, dis-rupt metabolism, and hormonal signals. It is very difficult to eliminate them orreverse their damage. Metals, even essential ones, can present toxic effects whenexcessive amounts are present, but on every other score, they are comparativelybenign.” This suit is supported by the numerous scientific reports in the ExpertWorkshop that was cosponsored by the EPA and was held in January 2000.

11.6 WHAT ARE LEAD-FREE INTERCONNECTS?

Pb may be present in metals, such as tin, as an impurity at a level of <0.1 percent byweight. Obviously this impurity will carry over into Pb-free alloys. In addition, it isdifficult to have all components converted to Pb-free finishes in a given amount oftime. It was therefore proposed by the High Density Packages User Group(HDPUG), a nonprofit U.S. trade organization, that a target Pb content of 1 percentby weight in the interconnect NOW would be reasonable with a level of <0.1 percentin several years. When considering the presence of Pb per weight of product, the Pbconcentration might be around 100 ppm.6

Besides the HDPUG proposal, the top three European semiconductor manufac-turers—Infineon Technologies, Philips Semiconductors, and STMicroelectronics—also unveiled on July 12, 2001, their proposal for the world’s first standard for


defining and evaluating lead-free semiconductor devices. This proposal contains anupper limit for lead-free components of 0.1 percent related to the individual mate-rial, not to the whole package or component. The proposal is a result of an initiativeto eliminate lead from semiconductors, aiming at accelerating the use of lead-freetechnologies. The three companies will be able to introduce their lead-free productsfar in advance to the legislative deadlines. Fully qualified lead-free components willbe available by the end of 2001.15

11.7 CRITERIA FOR LEAD-FREE SOLDER

The criteria used for screening candidate Pb-free alloys can be categorized as fol-lows:

Nontoxic Available and affordable Narrow plastic range Acceptable wetting Material manufacturable Acceptable processing temperature Form reliable joints

11.8 VIABLE LEAD-FREE ALLOYS

The following alloys are considered representative of viable candidates for replacingeutectic SnPb systems. Many of the systems are based on adding small quantities ofthird or fourth elements to binary alloy systems in order to lower the melting pointand increase the wetting and reliability. It is reported that with increasing amountsof additive elements, (1) the melting point of the system decreases; (2) the bondstrength first rapidly decreases, then almost levels off, then decreases again; and (3)the wettability first increases rapidly, reaching the maximum at a composition corre-sponding to the midpoint of the plateau of bond strength, then decreases.16

11.8.1 Sn96.5/Ag3.5

Sn96.5/Ag3.5 (221°C) is one of most promising alloys used by the National Centerfor Manufacturing Sciences (NCMS), Ford, Motorola, and TI Japan. A Germanstudy has suggested that it is one of the most suitable alloys.There is long experiencewith using this alloy. Indium Corp. reported it to have the poorest wetting for reflowsoldering among high-Sn alloys.17

11.8.2 Sn99.3/Cu0.7

Sn99.3/Cu0.7 (227°C) is reported by Nortel to have soldering quality equal to that ofeutectic SnPb in telephone manufacturing. In air reflow the wettability is reduced

11.8 CHAPTER ELEVEN

and fillet exhibits a rough and textured appearance.This alloy is probably the “poor-est” in mechanical properties of all Pb-free solders. It is preferably used for wave sol-dering because of the low cost of materials and of inerting of waves.

11.8.3 SnAgCu

SnAgCu is a ternary eutectic (217°C), although the exact composition needs to beclarified. Cu is added to SnAg in order to slow the Cu dissolution; lower the meltingtemperature; and improve wettability, creep, and thermal fatigue characteristics.Nokia and Multicore have found yields and reliability comparable to or better thanthose for eutectic SnPb alloy. The Brite-Euram project reported better reliabilityand solderability than SnAg and SnCu, and recommended this alloy for general-purpose use. The following compositions are examples:

Sn93.6/Ag4.7/Cu1.7 (216 to 218°C, AMES Labs, patent USP5527628 covers anyalloy containing 3.5 to 7.7 percent Ag, 1 to 4 percent Cu, 0 to 10 percent Bi, 0 to 1percent Zn, balance Sn)

Sn95/Ag4.0/Cu1 (217 to 219°C, AMES Labs) 96.5Sn/3.0Ag/0.5Cu (Harris Brazing Co.) Sn95.5/Ag4.0/Cu0.5 (217 to 219°C, published 50 years ago,18 unpatentable) Sn95.5/Ag3.8/Cu0.7 (217 to 219°C) Sn96.3/Ag3.2/Cu0.5 (217 to 218°C) Sn95.75/Ag3.5/Cu0.75 (Senju, patent JP5050286 covers 3 to 5 percent Ag, 0.5 to 3

percent Cu, 0 to 5 percent Sb, balance Sn)

11.8.4 SnAgCuX

Sn96.2/Ag2.5/Cu0.8/Sb0.5 (213 to 218°C,AIM, Castin Alloy) is reported by Interna-tional Tin Research Institute (ITRI), Lucent, Ford, and Sandia Labs to have greaterfatigue performance than eutectic SnPb alloy. The Brite-Euram project reportedthat addition of 0.5 percent Sb may strengthen the alloy further. Sn97/Cu2/Sb0.8/Ag0.2 (226 to 228°C, Kester, SAF-A-LLOY) may be considered for wave- and hand-soldering applications. SnAgCuIn (Tamura) may also be promising.

11.8.5 SnAgBiX

Addition of ≤5 percent Bi lowers the melting point and improves wettability ofSnAg systems. Solderability is the best among a range of Pb-free alloys, as confirmedby Indium Corp.17 and Matsushita.The NCMS observed fillet lifting at through-holejoints as a concern for wave soldering, although other alloys such as Sn96.5/Ag3.5also suffer fillet lifting to a lesser extent. Fillet lifting is caused by mismatch in ther-mal coefficient of expansion between solder and PCB materials and is aggravated bysolders with a pasty range. It can be altered by addition of other elements. Additionof Cu and/or Ge results in strength improvement and possibly wettability improve-ment. Adding Pb to SnBi alloys can cause a 96°C ternary eutectic, Bi52/Pb32/Sn16,to form. Calculations predict that at a fixed 6 percent Pb, even alloys with ≤4.8 per-cent Bi can have this eutectic liquid form, and thus SnPb surface finishes should beavoided. The Japan Electronic Industry Promotion Association recommends both


SnAgCu and SnAgBi. Some examples are shown in the following list. There are nounpatented compositions.

Sn91.8/Ag3.4/Bi4.8 (202 to 215°C, Sandia Labs Patent USP5439639, covers Ag 3 to4 percent, Bi 3 to 5 percent, Sn balance): considered by the NCMS to be mostpromising alloys, along with eutectic SnAg and eutectic Sn/Bi

Sn93.5/Ag3.5/Bi3 (210 to 217°C, Nihon Handa) Sn90.5/Bi7.5/Ag2 (191 to 210°C, Tamura Kaken) SnAgBi (Matsushita) Sn94/Ag3/Bi3 (213°C) Sn92/Ag3/Bi5 (210°C) Sn92.7/Ag3.2/Bi3/Cu1.1/Ge (Japan Solder) Sn93/Ag3.5/Bi0.5/In3 (Harima, Mitsui Metals)

Addition of a large amount (about 5 to 20 percent) of Bi lowers the melting pointto that of eutectic SnPb solders but sacrifices the good properties of eutectic SnAgsystems. Moreover, low-temperature eutectic Bi58/Sn42, which has a low partialmelting point (138°C), may be created. And there are reliability problems such asinterfacial problems with plating containing Pb on the electrodes of electronic com-ponents. It is attractive for low-cost manufacturing. Examples are shown in the fol-lowing list:

Sn/Ag2/Bi7.5/Cu0.5 (Alloy H, Alpha Metals, developed at ITRI) Sn/Ag2.0–2.8/Bi13–17/Cu0–1 (Hitachi) Sn/Ag2.8/Bi10/Cu0.6 (Ono) Sn/Ag/Bi3 (210°C, Matsushita) Sn/Ag/Bi6 (220°C, Matsushita) Sn/Ag/Bi10 (205°C, Matsushita) Sn/Ag/Bi15 (209°C, Matsushita)

11.8.6 SnSb

Sn95/Sb5 (232 to 240°C) has poor wetting, although better than Sn96.5/Ag3.5, andthe liquidus temperature is too high.

11.8.7 SnZnX

Sn91/Zn9 (eutectic 199°C) is fairly reactive, since Zn causes oxidation and corro-sion, and reacts with flux to form a hardened paste. Japanese home electronics man-ufacturers are interested in Sn89/Zn8/Bi3. Bi replaces Zn to reduce Zn corrosion inhumid conditions. SnZnBi alloys can have melting points close to that of eutecticSnPb. These alloys were developed primarily by home electronics manufacturerstargeting low-cost products.

Sn90/Zn9/In1 (AT&T) Sn89/Zn8/Bi3 (191 to 195°, Matsushita, Senju) SnZnBiX (Hitachi Harima, Tamura)

11.10 CHAPTER ELEVEN

11.8.8 SnBi

Bi58/Sn42 (138°C) is recommended by the NCMS as a promising replacement.Eutectic Bi58/Sn42 is unusually resistant to coarsening. It is reported by Hewlett-Packard to have properties better than or equivalent to those of eutectic SnPb andis promising for low-temperature applications or some consumer products.Additionof 1 percent Cu dramatically slows coarsening of eutectic SnBi. The concerns arethat (1) eutectic Bi52/Pb32/Sn16 (96°C) is formed on Pb surface finishes, (2) Bi is by-product of Pb mining, and (3) there is difficulty in separating Bi from Cu at recy-cling.

11.9 COST

The cost of solder bar is dictated by the raw materials cost (see Table 11.2).19 How-ever, for fabricated products such as solder pastes, the processing cost of manufac-turing can become a dominant factor, and the difference between SnPb and Pb-freematerials becomes very small.


TABLE 11.2 Relative Cost of Lead-Free Solder Materials

Relative Relativebar cost paste cost

Solder alloy ($/kg) ($/kg)

Sn63/Pb37 1 1

Sn96.5/Ag3.5 2.29 1.07

Sn95/Ag3/Bi2 2.17 1.06

Sn96.1/Ag2.6/Cu0.8/Sb0.5 2.06 1.05

Sn91.8/Ag3.4/Bi4.8 2.26 1.06

Sn95/Ag3.5/Cu0.5/Zn1 2.27 1.06

Sn93.6/Ag4.7/Cu1.7 2.56 1.08

Sn96.1/Ag3.2/Cu0.7 2.21 1.06

Sn95.2/Ag3.5/Cu1.3 2.28 1.06

Relative cost of selected metals: Pb, 1; Zn, 1.7; Cu, 3; Sb, 3.9; Bi, 8.6;Sn, 11; Ag, 260; Au, 15,000.

11.10 PCB FINISHES

Pb-free surface finishes for PCB are readily available. Some, such as NiAu and OSPs,have long use history. Shown below are some more promising options:

OSPs, such as benzotriazole and benzimidazole. Low-temperature processes maynot remove OSPs, and high-temperature processes may remove them and allowoxidation, particularly for multiple passes.

Immersion Ag (organic Ag, Alpha Level)

Immersion Au/Electroless Ni Hot-air solder leveling SnCu SnBi Electroless Pd/Electroless Ni Electroless PdCu Sn (pure, whiskerless varieties)

11.11 COMPONENTS

Pb can exist in components in three different forms. Among those, the second andthe third categories are related to soldering.

1. Lead used in functional materials in piezoelectric elements, capacitors, glass,fuses, etc.

2. Lead in solder used in internal connections within components3. Lead in solder-plating surface finishes on the leads of components

In general, it is relatively easy to eliminate Pb from surface finishes of the leadsof components. Examples of alternatives include Sn, PdNi, Au, Ag, NiPd, NiAu,AgPt, AgPd, PtPdAg, NiAuCu, Pd, and Ni. As one of the pioneers, ASAT installedpure matte tin for plating leadframes.20 Sony chooses NiPdAu or SnBi plating forreplacing SnPb surface-mount device (SMD) or through-hole device (THD) surfacefinishes.21 Among those options, Pd plating is difficult when the leads are made outof iron Alloy 42.4,6 Also, AgPd can cause voids due to Ag diffusion into solder.

As to the Pb in solder used in internal connections within the components, suchas flip chip in package, the first-level interconnection solder melting temperature(around 300°C) is normally considerably higher than the second-level interconnec-tion solder melting temperature (about 180°C). If the melting temperature for thelatter interconnection is set at around 220°C, then the first-level interconnectionneeds to have a melting point preferred to be above 260 to 270°C in order to avoidremelt during subsequent reflow processes. There are only few alloys that fall intothat category, such as Au80/Sn20 (280°C), which is very expensive. 95Sn/5Sb (232 to240°C) and Sn65/Ag25/Sb10 (233°C) might also be considered. The later, known asJ alloy, exhibits very low ductility and unacceptable thermal and mechanical fatiguelife for die attach.3 Both alloys are also too close in melting temperature to SnAgCuto be good candidates for internal connections. Technologically it will be even moredifficult to substitute for the Pb used in functional materials in components.

11.12 THERMAL DAMAGE

There is more to ponder besides finding solder alloy alternatives to phase out lead.Since most of the promising alloy alternatives require a higher processing tempera-ture, whether the components or substrates used can sustain the process becomes abig question mark.6 For instance, electrolytic capacitors are highly susceptible tohigh-temperature damage.Wound components, such as relays, are also susceptible tohigh-temperature damage and are considered likely to experience an increased ten-


dency toward the popcorn effect from plastic-encapsulated integrated circuits neartheir expiry date. In addition, a parametric damage to memory integrated circuitsprocessed around 250°C is possible.As mentioned earlier, PCB and BGA polymericsubstrates and solder masks may also suffer from higher processing temperatures.The plastic insulation of connectors may also become distorted. All these situationspose a great challenge to material scientists and design engineers as far as findingsolutions.

11.13 OTHER CONCERNS

The corrosion and electromigration tendencies of the new alloys need to be meas-ured. Pb-free solder on Pb solder, Pb-free solder on Pb-free solder, pastes, and fluxesneed to be evaluated. Since solder without Pb is different in appearance and is moredifficult to monitor via x-ray, new standards for visual and x-ray inspection areneeded.

11.14 CONSORTIUM ACTIVITY

There are many coordinated efforts addressing the lead-free challenge. In NorthAmerica, the NCMS has dedicated $10 million over four years, with reports releasedin August 1997. The National Institute of Standards and Technology is also active inparticipating in Pb-free programs. Currently, NEMI is most active in leading theindustry in finalizing the options for Pb-free soldering processes.

In Japan, the New Industry and Industrial Technology Development Organiza-tion has committed 350 million yen over two years to find the answers, while inEurope, Improved Design Life and Environmentally Aware Manufacture of Elec-tronic Assemblies by Lead-Free Soldering, a six-member European collaborationsupported by the Brite-Euram project of the European Commission, is scheduledfor a 3-year project (May 1996 to April 1999). ITRI has been involved in developingPb-free solder options since the early 1990s.

11.15 OPINIONS OF CONSORTIA

In the United States, the NCMS recommends three alloys: Sn96.5/Ag3.5,Sn91.7/Ag3.5/Bi4.8, and Bi58/Sn42. NEMI maintains that SnAgCu without Bi (217to 221°C) is the most reliable in the presence of Pb contamination. The highest-meltalloy with Bi is more manufacturable but the 96°C PbSnBi phase is an issue. TheBrite-Euram project recommends Sn95.5/Ag3.8/Cu0.7 for general-purpose solder-ing. Other alloys with potential are Sn99.3/Cu0.7, Sn96.5/Ag3.5, and SnAgBi. In theUK, the Department of Trade and Industry perceives that the favored option variesdepending on applications: high professional group (automotive, military)—SnAgCu(Sb); medium professional group (industrial, telecoms)—SnAgCu, SnAg;general consumer and low professional group (TV, audio-video, office equipment)—SnAgCu(Sb), SnAg, SnCu, SnAgBi. JEIDA favors SnAgCu (before Pb-free compo-nents are available) and SnAgBi (after Pb-free components are available). InGermany, the favored alloys appear to be Sn96.5/Ag3.5 and Sn99/Cu1.


11.16 WHAT ARE THE SELECTIONS

OF PIONEERS?

Following is the list of the selections or seriously considered candidates of some Pb-free pioneering companies. Since new data are being generated rapidly, the favoredoptions may change with time.

Nortel. Sn99.3/Cu0.7 (N2) wave and reflow Motorola. Sn95.5/Ag3.8/Cu0.7 and Sn96.5/Ag3.5 (most likely) Ford. Sn96.5/Ag3.5 Texas Instruments. SnAgCuSb (NiPd finish) Delco. SnAgCu (probably) Nokia. Sn95.5/Ag3.8/Cu0.7 Ericsson. Sn95.5/Ag3.8/Cu0.7 Hitachi. Sn91.75/Ag3.5/Bi5/Cu0.7 NEC. Sn94.25/Ag2/Bi3/Cu0.75, Sn97.25/Ag2/Cu0.75, and SnZnBi Matsushita. Sn90.5/Ag3.5/Bi6 and SnAgBiX series Fujitsu. Sn42.9/Bi57/Ag0.1 Toshiba. SnAgCu Sony. Sn93.4/Ag2/Bi4/Cu0.5/Ge0.1 (claimed to have five times the reliability of

SnPb) Solectron. May end up with a high-temperature and low-temperature alloy, but

would prefer only one for bar, paste, and rework

11.17 POSSIBLE PATH

The goal of NEMI is to have North American companies capable of producing Pb-free products by 2001 with a total elimination of Pb-based products by 2004 (partic-ipating companies will determine the actual timing of deployment). The possiblepath to Pb-free soldering according to NEMI7 can be shown in the following list.However, this sequence may vary by manufacturer.

SMT solder pastes and rework Board finishes Component metallization Wave soldering Internal component interconnects

The implementation sequence expected by IPC is as follows, with no time schedule:

Phase 1. Wave solder/solder paste used to manufacture products are Pb free.Phase 2. PWB finishes are Pb free and phase 1 is implemented.Phase 3. Pb-free component finishes are in place and phases 1 and 2 are imple-mented.


Most likely the last industries that will be affected by this issue will be militaryand aerospace, which rely on unique, high-reliability products. On the other hand,consumer electronics, which usually carry a 2- to 3-year life expectancy, will easilybecome the primary focus in the initial stage. The experience accumulated at thisfirst stage will be applied for future products that may have more stringent reliabil-ity requirements.

It should be realized that lead-free implementation most likely will not be a con-certed process, and overlap between phases should be expected. As a result, thechoice of materials and processes for any phase should be compatible with those forother phases.

As reported at the NEMI meeting,12 the Japanese scenario appears to be takinga cautious step, as shown in the following list:

Initial implementation of SnAgBiX in low-tier products; convert high-tier prod-ucts when Pb-free components are available or utilize SnAgX alloys.

Lower-temperature components usable vs. SnAgX (5 to 10°C higher).

11.18 IS Pb-FREE SAFE?

While the industry is moving quickly toward Pb-free soldering processes, and whileeverything seems to fall into place as far as supporting a green world, an odd ques-tion is asked:“Is Pb-free solder environmentally safe?”According to a recent study,4

five Pb-free solders—Sn96.3/Ag3.2/Cu0.5, Sn96.5/Ag3.5, Sn98/Ag2, Sn99.3/Cu0.7,and Sn95/Sb5—were leached using EPA methods designed to simulate waste dis-posal and groundwater contact. The results indicate that Sb and Ag alloys failedevery test. SnCu has least environmental impact. Sn did not leach significantly, dueto low solubility of Sn salt in water. Since both Sb and Ag elements—particularlyAg—are very likely to be included in the Pb-free alloy alternatives, these data defi-nitely demonstrate that the road to a Pb-free soldering world may be bumpier thananticipated.

11.19 SUMMARY

Lead-free soldering for the electronics industry is a segment of the global trendtoward a lead-free environment. Although initiated in the U.S. in early 1990s, itadvanced much more rapidly in Japan and Europe. This differentiation in Pb-freeprogress triggered great concerns among users of Pb-containing solders about main-taining business opportunities, therefore further expediting the advancement of Pb-free soldering programs.The favored Pb-free solder alternatives vary from region toregion. However, in general, high-tin alloys are preferred, including SnAg, SnCu,SnAgCu, SnAgBi, and various versions of those alloys with additions of smallamounts of other elements, such as Sb. SnAgBi systems are used in some Japaneseproducts already. However, SnAgCu systems are more tolerant toward Pb contami-nation than Bi-containing systems and therefore are more compatible with existinginfrastructures for the transition stage. Pb-free surface finishes for PCBs includeOSPs, immersion Ag, immersion Au/electroless Ni, hot-air solder leveling SnCu,SnBi, electroless Pd/electroless Ni, electroless Pd/Cu, and Sn. The challenge forcomponents is greater than for solder materials or PCBs. Although some Pb-free


surface finishes for components exist, such as Sn, PdNi, Au, Ag, NiPd, NiAu, AgPt,AgPd, PtPdAg, NiAuCu, Pd, and Ni, their performance remains to be verified. Inaddition, options for higher-melting-temperature solder are still not available forhigh-temperature applications, including first-level interconnect within the compo-nents. Thermal damage can be a concern for both PCBs and components.

11.20 INFORMATION RESOURCES

11.20.1 Legislation

Europe

Waste Electrical and Electronic Equipment Directive (WEEE)www.itri.co.uk/WEEE2.htmwww.smtuk.demon.co.uk/www.itri.co.uk/index.htm

AsiaNo legislation

Americas

EPA information on Toxic Release Inventory Rule—www.epa.gov/tri/tri_pb_rule.htm

EPA information on elements—www.epa.gov/opptintr/pbt

11.20.2 INITIATIVES FROM INDEPENDENT CORPORATIONS

AND ELECTRONICS INDUSTRY ORGANIZATIONS

Europe

Department of Trade and Industry (DTI) Industry Evaluation of Lead-Free Soldering (IDEAL) International Tin Research Institute (ITRI)—www.itri.co.uk

Asia

Japanese Ministry of International Trade and Industry (MITI)—www.miti.go.jp/index-e.html

Japan Electronic Industry Development Association (JEIDA)www.jeida.or.jp/guide/gaiyou/index-e.htmlwww.jeida.or.jp/document/geppou/etc/9802narmari.html

Japanese Institute of Electronic Packaging (JIEP)—www3.famille.ne.jp/∼jiep/index.html

Japan Printed Circuit Association (JPCA)—www.ipc.org/html/nr7042.htm New Industry and Industrial Technology Development Organization (NEDO)—

www.nedo.go.jp/index-e.html Matsushita (Panasonic)

www.panasonic.co.jp/environment/98e/09e.htmwww.panasonic.co.jp/environment/98e/13ae.htm


www.itri.co.uk/WEEE2.htm

www.smtuk.demon.co.uk/

www.itri.co.uk/index.htm

www.epa.gov/tri/tri_pb_rule.htm


www.epa.gov/opptintr/pbt

www.itri.co.uk

www.miti.go.jp/index-e.html

www.miti.go.jp/index-e.html

www.jeida.or.jp/guide/gaiyou/index-e.html

www.jeida.or.jp/document/geppou/etc/9802narmari.html

www3.famille.ne.jp/~jiep/index.html

www3.famille.ne.jp/~iep/

www.ipc.org/html/nr7042.htm

www.nedo.go.jp/index-e.html

www.panasonic.co.jp/environment/98e/09e.htm

www.panasonic.co.jp/environment/98e/13ae.htm

www3.famille.ne.jp/~jiep/index.html

Sony—www.sel.sony.com/semi/PDF/LeadFreePkg.pdf Toshiba—www.toshiba.com NEC—www.nec.co.jp/english/profile/kan/action/action.html Fujitsu—www.fujitsu.co.jp

Americas

National Center for Manufacturing Sciences (NCMS)—www.ncms.org National Institute of Standards and Technology (NIST)—www.nist.gov National Electronics Manufacturing Initiative (NEMI)—www.nemi.org IPC

www.ipc.org/html/nr9071.htmwww.leadfree.org

Lehigh University—www.lehigh.edu/∼dj10/Research1.html High Density Packaging User Group International, Inc.—www.hdpug.org

11.20.3 VIABLE ALLOYS UNDER CONSIDERATION

SnAgCu (patents—www.patents.ibm.com/patlist?icnt=US&patent_number=5527628, JP5050286, Japan 08-215880)

REFERENCES

1. National Center for Manufacturing Sciences, “Lead and the Electronic Industry: A Proac-tive Approach,” May 1995.

2. Sax, N. I., Dangerous Properties of Industrial Materials, 6th ed., Van Nostrand Reinhold,New York, 1984.

3. JEIDA, “Lead-Free Solder Roadmap—A Scenario for Commercial Application,”www.jeida.or.jp/document/geppou/etc/9802namari.html, February 3, 1998.

4. Smith, E. B. III, and L. K. Swanger, “Are Lead-Free Solders Really EnvironmentalFriendly?” SMT, 64–66, March 1999.

5. IPC Leadfree Website, www.leadfree.org, August 2001.

6. HDP User Group International, Inc., “Lead Free Soldering 1,” doc number Proj032, RevA, June 1999.

7. NEMI, Lead Free Task Meeting, Northbrook, IL, May 26, 1999.

8. Evans, H., “EU Parliament’s Environment Committee Amends Waste Directives,” 703-907-7576; [email protected].

9. Gibbs, F., “Pb-Free Interconnect,” NEMI Lead Free Meeting, Chicago, May 25, 1999.

10. Samsung Electronics, “Samsung Electronics Develops Environmentally-Friendly MemoryModule,” M2 Communications, May 9, 2001.

11. www.nec.co.jp/english/profile/kan/action/action.html

12. Bradley, E.,“Overview of No-Lead Solder Issue,” NEMI meeting,Anaheim, CA, February23, 1999.

13. Buetow, M., “The Latest on the Lead-Free Issue,” Technical Source, IPC 1999 Spring/Summer Catalog.


www.sel.sony.com/semi/PDF/LeadFreePkg.pdf

www.toshiba.com

www.nec.co.jp/english/profile/kan/action/action.html

www.fujitsu.co.jp

www.ipc.org/html/nr9071.htm

www.leadfree.org

www.hdpug.org

www.ncms.org

www.nist.gov

www.nemi.org

www.lehigh.edu/~dj10/Research1.html

www.patents.ibm.com/patlist?icnt=US&patent_number=5527628

www.patents.ibm.com/patlist?icnt=US&patent_number=5527628

www.jeida.or.jp/document/geppou/etc/9802namari.html

www.leadfree.org

www.nec.co.jp/english/profile/kan/action/action.html

14. www.epa.gov/tri/tri_pb_rule.htm

15. Business Wire, July 12, 2001.

16. Furusawa, A., K. Suetsugu, A. Yamaguchi, and H. Taketomo, “Thermoset Pb-Free SolderUsing Heat-Resistant Sn-Ag Paste,” National Technical Report, 43(1), February 1997.

17. Huang, B. L., and N. C. Lee, “Prospects of Lead Free Alternatives For Reflow Soldering,”Proceedings of IMAPS’99, Chicago, October 28, 1999.

18. Beghardt, E., and G. Petzow, “Ueber den Aufbau des Systems Silber-Kupfer-Zinn,”Zeitschrift fuer Metallkunde, 50:597–605, 1959.

19. Handwerker, C., “NCMS Lead Free Solder Project: A National Program,” NEMI LeadFree Solder Meeting, Chicago, May 25, 1999.

20. www.e-insite.net/epp/index.asp?layout=article&articleId=CA84263, e-mail from E. Bradley,June 21, 2001.

21. www.sel.sony.com/semi/PDF/LeadFreePkg.pdf, Suganuma, K., “Japan Leadfree 2001,”ISIR, Osaka University.



www.e-insite.net/epp/index.asp?layout=article&articleId=CA84263

www.sel.sony.com/semi/PDF/LeadFreePkg.pdf

CHAPTER 12DEVELOPMENT OF LEAD-FREE

SOLDER ALLOYS

12.1 CRITERIA

There are many lead-free solders available in the industry. However, most of themare not considered viable options. The criteria for an acceptable lead-free solderalternative are as follows:

Nontoxic Available and affordable Acceptable processing temperature Acceptable wetting Forms reliable joints Material manufacturable

These criteria are key points of commonsense consideration. Quantified corre-sponding criteria primarily based on National Center for Manufacturing Sciences(NCMS) Pb-Free Solder Project consideration are shown in Table 12.1.1 Since 240°Cis considered the maximum acceptable component temperature during assembly, andsince soldering temperature typically is at least 15°C above liquidus temperature, themaximum acceptable liquidus temperature should accordingly be 225°C. On theother hand, a narrow pasty range is considered essential for preventing rupture dur-ing wave soldering as well as for achieving good wetting. Both wetability and area ofcoverage reflect the wetting ability of solders. Methods for determining wetabilitycan include either reflow spread factor testing or wetting balance testing. The lattermethod is more meaningful for wave soldering applications. Thermomechanicalfatigue, coefficient of thermal expansion, creep, and elongation provide good insightinto the reliability of solder joints. Elongation also provides a clue on ductility, whichis critical to the manufacturability of solders, such as solder wire or solder preforms.

12.2 TOXICITY

Toxicity information for elements that are potential solder constituents is shown inTable 12.2.1,2 Due to incomplete data, direct comparison among elements is oftendifficult. In addition, depending on the type of toxicity investigated, rankings maychange. Rankings may also be affected by perceptions as a result of historical usagein human society. The ranking of elements in terms of increasing toxicity accordingto various criteria is shown in the following list.

OSHA PEL and ACGIH TLV.Bi < Zn oxide fume < Sn (inorganic) < Cu (dust) < Sb (and compounds) < Sn(organic) < Cu (fume) < In (and compounds) < Ag (dust and fume) < Ag (andsoluble compounds) < Pb (inorganic)

12.1


Chronic toxicity.Bi < In < Zn (oxide) < Cu < Ag < Sn < Sb < Pb

Surface Mount Council report.3

Bi < Zn < In < Sn < Cu < Sb < Ag < Pb

Table 12.2 shows toxicity ranking of elements.1,2 Pb is considered highly toxicdue to the long history of human Pb toxicity problems and its well-documentedimpact on the young and elderly, although its LDLo value is higher than those ofmost other elements in Table 12.2. Cd, which is not listed in Table 12.2, is regardedas a carcinogen by the International Agency for Research on Cancer and is notconsidered an option. Sb has the same LDLo value as Cd and is highly toxic wheninhaled or ingested; therefore, its use in solder should be minimized. Ag and Cuare probably similar, although Ag may be slightly higher in toxicity. Sn exhibits a moderately low toxicity, while Bi, Zn, and In are considered to cause low or notoxicity.

12.2 CHAPTER TWELVE

TABLE 12.1 Criteria for Alloy Selection

Property Acceptable levels Notes

Toxicity Considerably lower than that of Pb

Supply Sufficient supply for 80%conversion

Cost <$10/lb in bulk form

Solidus temperature >Operating temperature −55 to +100°C for consumerconsidered electronics and

telecommunications−55 to +125°C for military

electronics−55 to +180°C for aerospace and

automotive electronics

Liquidus temperature <225°C To prevent component thermaldamage

Pasty range <30°C To prevent rupture during wavesoldering

Wetability Comparable to eutectic Sn/Pb

Area of coverage >85% On Cu in dip test

Thermomechanical fatigue >75% Of eutectic Sn/Pb

Coefficient of thermal <29 ppm/°C Prevents local stress on solder expansion joints

Creep >500 psi To cause failure in 10,000 minat room temperature

Elongation >>10% Under uniaxial tension at roomtemperature; also important for fabricating solder wires and preforms

DEVELOPMENT OF LEAD-FREE SOLDER ALLOYS 12.3

TABLE 12.2 Toxicity Ranking of Elements Presented in Order of Increasing OccupationalChronic Toxicity

LDLo OSHA PEL and (mg/kg Acute Chronic ACGIH TLV*

Element body weight) Effects toxicity toxicity (mg/m3)

Bi 15 Moderate None None Nonetoxicity: heart,liver, lungs

In 10† High toxicity: Irritation None, related 0.1 (anylungs, solely to indium compound)gastrointestinal metaltract

Zn 124‡ Minimal toxicity: Irritation Metal fume fever 5 (oxide fume)skin (oxide) (oxide)

Cu 0.12‡ High toxicity: Irritation Irritation, metal 1 (dust)reproductive (dust, mist) fume feverorgans, possiblecarcinogen andteratogen

Ag 1§ High toxicity: None Permanent 0.1 (dust andskin discoloration fume); 0.01 (Ag

of skin, eyes, and solublemucous compounds)membranes;irritation; metalfume fever

Sn No value Moderate to low Irritation Difficulty 2 (inorganic);found toxicity: breathing 0.1 (organic)

gastrointestinaltract

Sb 15 High toxicity Irritation Emphysema, 0.5 (Sb and(oral): heart, pulmonary compounds)liver, lungs edema

Pb 450‡ High toxicity: None Nervous system 0.05 (inorganic)nervous system, effects, anemia,carcinogen of kidney damage;lungs, suspected reproductiveteratogen and

developmentaleffects

* For a 5-day workweek with 8-h workdays.† LDLo based on animal studies; high toxicity when injected, low toxicity when inhaled.‡ Toxic dose, lower limit.§ Toxic concentration, lower limit (mg/m3).ACGIH, American Conference of Government Industrial Hygienists; LDLo, lethal dose, lower limit;OSHA, Occupational Safety and Health Administration; PEL, Permissible Exposure Limit; TLV, thresholdlimit value.

However, the selection or elimination of an element for solder should not only con-sider the toxicity of the element itself, but also the overall quantity of the element tobe used in an electronic product. Figure 12.1 shows the chemical content of a printedcircuit board for a mobile product.The estimated overall contribution of metallic ele-ments on toxic potential can be roughly ranked in decreasing order as shown here:

Pb > Cu > Ni > Ag > Al > Sn > Au

Many of these metallic elements, such as Cu and Ag, are not primarily used as sol-der. Cu is used as the circuitry conductor or ground plane, while Ag may be used asa thick film material or a surface finish.As long as Cu and Ag continue to be used forthose applications, eliminating them from solder materials due to toxicity consider-ations is virtually meaningless.

12.3 COST AND AVAILABILITY

The cost and availability of potential elements to be used in solders are shown inTable 12.3.5 With Pb being the cheapest element, all Pb-free alternatives are destinedto be more expensive than eutectic Sn-Pb solder. Zn, Cu, and Sb are relatively low incost; however, it is questionable that they can serve as essential constituents. Thehigh cost of Ag and In suggests that those elements should not compose more thana small percentage of the solder. The limited availability of Bi and In imparts con-straints on using them as significant constituents.

12.4 DEVELOPMENT OF LEAD-FREE ALLOYS

12.4.1 Existing Alloys

The first group of Pb-free solder candidates is the existing Pb-free alloys. Thisincludes (1) binary eutectic Sn-containing alloys, such as Sn-Ag, Sn-Au, Sn-Cu,Sn-Bi, Sn-In, Sn-Sb, and Sn-Zn, (2) noneutectic binary Sn-containing alloys,6 such as97.5Sn-2.5Ag (melting range 221 to 226°C), 95Sn-5Ag (221 to 240°C), 90Sn-10Ag(221 to 295°C), 97Sn-3Cu (227 to 300°C), and 60Sn-40Bi (138 to 170°C), (3) Pb-free

12.4 CHAPTER TWELVE

FIGURE 12.1 Chemical content of a printed circuit board of a typical mobile product.4 Lightcolumns: material mass. Dark columns: material assessment by means of toxic potential indicator(TPI).

alloys without tin,6 such as 97In-3Ag (143°C), 99.3In-0.7Ga (150°C), 95In-5Bi (125to 150°C), 67Bi-33In (109°C), and 88Au-12Ge (356°C), and (4) other Pb-free alloys,6

such as 65Sn-25Ag-10Sb (233°C), etc. Many of the materials in this group, particu-larly the binary eutectic Sn-containing alloys, have been used by the electronicsindustry for many years, and their properties and performance are well understood.

12.4.2 MODIFICATION

The second group of candidates is newly modified preexisting alloys. Modification isoften accomplished by addition of a small amount of additional elements, such asAg, Cu, Bi, In, Sb, Ge, P, Ni, Fe, Au, Ga, or Co, with the goals of improving thewetability, bond strength, oxidation resistance, and impurity tolerance level; reduc-ing the melting temperature; refining the grain structure, and so on.

12.4.2.1 Wetting. The wetting process is favored by a low-surface-energy solder,which often can be regulated by addition of impurities. The general rule is that asmall amount of surface-active impurity, usually low in surface energy, can producea marked decrease in surface energy, while similar amounts of a surface-inactiveimpurity do not produce more than a very small rise in surface energy. It follows thatthe effects of surface-inactive impurities on solder should be too small to have anysignificant effect on wetting behavior.7 Figure 12.2 shows the effect of impurities onsurface energies for some relevant binary systems with tin.

The effect of the surface energy of additives on the surface energy of alloys canbe further illustrated by examining the 60Sn-40Pb system. Surface tension isother-mals (250°C) for 60Sn-40Pb with 0 to 4 percent Bi or 0 to 5 percent Sb show a non-linear fall with increasing ternary addition, which may be explained by the lowsurface tension of the third elements. Surface tension isotherms for 60Sn-40Pb with0 to 2 percent Ag (215 and 250°C) or 0 to 0.6 percent Cu (250°C) indicate higher val-ues with increasing ternary addition, which may be explained by the higher surfacetension of the third elements. However, there are exceptions to this rule; for exam-ple, increasing ternary addition of low-surface-tension P at 0 to 0.013 percent resultsin a higher surface tension.8


TABLE 12.3 Cost and Availability of Elements

Annual U.S.Cost ($/lb, as of consumption

Element Feb. 3, 1999) Density (lb/in3) (millions of lb)* Availability

Pb $0.45 0.41 7040 Available

Zn $0.50 0.258 1560 Available

Cu $0.65 0.324 4900 Available

Sb $0.80 0.239 100 Available

Bi $3.40 0.354 9 Limited

Sn $3.50 0.264 180 Available

Ag $84.20 0.379 3.5 Limited

In $125.00 0.264 0.2 Scarce

* As defined by the U.S. Bureau of Mines.

Another driving force for wetting is the rate of formation of intermetallic com-pounds. Since formation of intermetallics involves reaction between solder and basemetal, this inevitably results in more spreading or more wetting. Accordingly, ahigher formation rate typically results in a better wetting. The ability of Sn to easilyform intermetallics with many metals is the primary reason it is the essential con-stituent in many solder alloys. Addition of other elements may affect the formationrate of intermetallics, thus affecting the wetting.Table 12.4 shows the common inter-metallic compounds encountered in the electronics industry.

12.4.2.2 Melting Temperature and Bond Strength. Since tin, which is often thepreferred primary constituent, exhibits a melting point of 232°C, additives that canlower the alloy melting temperature are often desired in order to minimize thermaldamage to both components and boards. Although several elements such as Hg andCd are capable of lowering the melting temperature of alloys, Bi and In are the twomost commonly utilized elements for Pb-free solder applications due to their benignnature.

Furusawa et al.9 reported that the addition of small amounts of some additive ele-ments would reduce the melting temperature and the bond strength, but would ini-tially increase the wetting of solders, then reach a maximum, then decrease wetting,as shown in Fig. 12.3. The wetting phenomenon observed in this case suggests thatthe relationship between wetting and surface energy may only be a secondary effect.

The effect of additives on melting temperature seems to be applicable to Bi and In as additives, and is supported by the studies on the effect of Bi addition onSn-Zn,10 Sn-3.5Ag,11 and Sn-Ag-Cu10 systems, including Sn-3.5Ag-1Cu,12 as shown inFigs. 12.4 through 12.6. It is also supported by the effect of In addition on Sn-3.5Ag,12

as shown in Fig. 12.5. Besides Bi and In, a number of other elements, such as Mg,Ag,Cu,Al, Ga, and Zn, also exhibit a melting temperature depression effect, as shown inTable 12.5.13

The effect of additives on bond strength reported by Furusawa also appears to beapplicable to Bi and In as additives. Therefore, the effect of Bi content on the pullstrength of the Sn-3.5Ag-Bi system (see Fig. 12.6) and the effect of addition of In andBi on the tensile strength of the Sn-3.5Ag-1Cu alloy12 (see Fig. 12.7) all exhibit adecrease in mechanical strength with increasing content of additives.

12.6 CHAPTER TWELVE

FIGURE 12.2 Effect of impurities on surface energies.

It should be pointed out that Furusawa’s observation on the relation betweenaddition and wetting is not well supported by the work on Bi. Zhao et al.12 reportedthat for the Sn-3.5Ag-1Cu system, wetting improves with increasing addition of Inbut deteriorates with increasing addition of Bi, as shown in Fig. 12.8. The adverseeffect of Bi on wetting cannot be explained by its reduced surface tension, as shownin Fig. 12.2. Presumably this can be attributed to the poor wetting ability of Bi itself.


TABLE 12.4 Common Intermetallic Compounds Encountered in the Electronics Industry

Solder Substrate metallization Intermetallic compound

Au-based Cu-based Au3Cu, AuCu, AuCu3

Au-based Pb-based AuPb2, Au2Pb

Bi-based Au-based Au2Bi

Bi-based In-based BiIn, Bi3In5, BiIn2

In-based Cu Cu11In9, Cu4In, Cu2In

In-based Au-based Au7In, Au4In, Au3In, Au7In3, Au3In2, AuIn, AuIn2

In-based Ag-based Ag3In, Ag2In, AgIn2

In-based Ni Ni3In2, Ni3In, NiIn, Ni2In3

In-based Sn solder coating In3Sn, InSn4

Sb-based Brass or Zn coating ZnSb, Zn3Sb2

Sb-based Cu-based Cu3Sb, Cu5.5Sb, Cu4.5Sb, Cu3Sb, Cu3.3Sb, Cu2Sb

Sb-based Ag-based AgSb, Ag3Sb

Sb-based Au-based AuSb2

Sn-based Cu Cu6Sn5, Cu3Sn

Sn-based Au-based AuSn, AuSn2, AuSn4

Sn-based Ag-based Ag3Sn

Sn-based Pd-based PdSn4

Sn-based Ni Ni3Sn4

FIGURE 12.3 Effect of additive amount on solder melting point, joint bond strength, and wetting(spread factor).

(b)(a)

It has been reported that, based on the wetting study on a series of eutectic binarysolder alloys, the ability to promote spreading for several elements can be ranked asfollows: Sn > Pb > Ag > In > Bi.14

Ackroyd et al.15 studied the effect of additives on the wetting of 60Sn-40Pb.Results indicate that Al, Sb, As, Cd, P, S, and Zn all cause a decrease in spread area.Bi shows adverse effects on steel and brass but no effect on Cu. On the other hand,Cu addition causes a decrease in spread area on steel but a slight increase on brass.

12.4.2.3 Oxidation Resistance. Elements such as P are sometimes used as deox-idants in the production of Cu and solders from secondary metals.15 At levels of 0.01percent, P significantly reduced the oxidation of 60Sn-40Pb at all temperatures inthe stirring tests.14

12.8 CHAPTER TWELVE

FIGURE 12.4 Effect of Bi content on the melting range of Sn-Ag-Cu-Bi and Sn-Zn-Bi determined by differential scanning calorimeter.

FIGURE 12.5 Effect of addition of In or Bi on the melting temperature of Sn-3.5Ag-1Cu alloy.

12.4.2.4 Grain Structure. Creep is deformation of materials with time under agiven tension or shear load. Creep occurs via thermally activated processes. It isimportant when the service temperature exceeds half the melting temperature (indegrees kelvin) of solder. Creep is the most important deformation mechanism ofsolder.16

Depending on the stress level, the deformation mechanism can be divided intothree phases.17 With increasing stress τ, the creep mechanisms shift from dislocationclimb-controlled bulk creep to grain boundary slide-controlled intergranular creepto dislocation glide-controlled creep.

At high stress, the deformation mechanism undergoes transition to tertiary creepand elongation to failure. The creep is sensitive to microstructure, and the mecha-nisms include (1) onset of cavitation damage at grain boundaries and (2) plasticinstability leading to inhomogeneous deformation. Morris et al.18 reported that cav-itation is responsible for tertiary creep in bulk solder samples tested in tension. Cav-


FIGURE 12.6 Effect of Bi content on Sn-3.5Ag-Bi alloys.

TABLE 12.5 Effect of Additive Elements in Depressing Melting Temperature of Sn-Binary Alloys

Melting temperature depression (°C/wt%)

Additive element 160–183°C 184–199°C 200–230°C

In 2.3 2.1 1.8

Bi 1.7 1.7 1.7

Mg — — 16.0

Ag — — 3.1 (above 221°C)

Cu — — 7.1 (above 227°C)

Al — — 7.4 (above 228°C)

Ga 2.6 2.5 2.4

Zn — 3.8 (above 198°C) —

ities nucleate primarily at three- or four-grain junctions. They grow with strain andmerge to form larger voids to cause failure.This process is aggravated by (1) increasein grain size, which enhances the stress concentration at grain junctions; (2) irregulargrain shapes, which introduce sites of unusual stress concentration; and (3) (possi-bly) intergranular precipitates, which constrain deformation at grain boundaries,thus resulting in uneven stress distribution. Plastic instability mainly incurs at shearbands, which often follow planes of microstructural weakness, such as phase bound-aries and colony boundaries in eutectic materials.19,20 The development of shearbands is particularly pronounced in solders exhibiting unstable, eutectic microstruc-tures that are easily recrystallizable, such as eutectic Sn-Pb. In these solders, theincipient shear bands cause development of the well-defined recrystallized bandsfor joints that are crept or fatigued in shear. Such a localized recrystallized material,usually observed near an intermetallic layer, accelerates damage processes andshortens the fatigue life of solder joints.

Since a larger grain size results in a higher creep and failure rate, a microstructurewith a refined grain structure is typically desired. In general, this is considered one

12.10 CHAPTER TWELVE

FIGURE 12.8 Effect of addition of In or Bi on the wetting angle of Sn-3.5Ag-1Cu alloys.

FIGURE 12.7 Effect of addition of In and Bi on the tensile strength of Sn-3.5Ag-1Cualloy.

of the most effective methods for improving the reliability of solder alloys, and isoften accomplished through the addition of a small amount of elements such as Cu,Zn, As, Fe, or Ag into the alloy. These elements often precipitate at the grain bound-ary, thus retarding the further growth or recrystallization of the grains in the solder.

For example, addition of 1 percent Cu dramatically slows coarsening of eutecticSn-Bi.21–23 On the other hand, addition of insoluble dispersoid Fe particles using amagnetic distribution technique forms a three-dimensional network of finely dis-persed iron particles in a Bi-43Sn eutectic solder. These iron particles reduce thecoarsening and the onset of tertiary creep of 57Bi-43Sn, thus improving themicrostructural stability, raising the service temperature, and resulting in a fivefoldincrease in creep resistance at 100°C.24–27

The addition of 1 percent Zn significantly improves the mechanical strength of95.5Sn-3.5Ag alloy by as much as 48 percent while maintaining the same level ofductility. It also significantly improves creep resistance. The high-temperature creepresistance of Zn-containing alloy is improved more than an order of magnitude.Strengthening is attributed to a substantial refinement of and more spherical Ag3Snprecipitates in the solidification microstructure. In this case, Zn is incorporated inthe more corrosion-resistant Ag3Sn precipitates.These precipitates suppress the for-mation of Sn dendrites and leave the Sn-rich matrix primarily free of Zn in solidform.28,29

The addition of a small amount (<1 percent) of Cu in 95Sn-3.5Ag-1Zn-0.5Cualloy refines the effective grain size while retaining uniform distribution of Ag3Snprecipitates in the solidification microstructure, thus dramatically improving ductil-ity in the 95.5Sn-3.5Ag-1Zn alloy. The quaternary 95Sn-3.5Ag-1Zn-0.5Cu alloy hasbetter mechanical properties than 96.5Sn-3.5Ag because it has a uniform fine dis-persion of precipitates and small effective grain size. However, addition of Cu or Zn> 1 percent is not desirable as it causes precipitates of additional intermetallic com-pound phases that deplete the finely dispersed precipitates in the surroundingmatrix and induce nonuniformities in the microstructure that consequently deterio-rate the mechanical properties.30

AT&T reported that small alloying additions of Ag dramatically improve themechanical properties of 87Sn-8Zn-5In alloy (melting point 188°C) due to elimina-tion of the coarse and nonuniform distribution of platelike dendrites and refiningeffective grain size in the solidified microstructure.25

However, care should be taken in the selection of elements because some ele-ments may cause grain coarsening. For instance, addition of 0.001 percent Co toeutectic Sn-Bi inhibits dissolution of Cu and coarsens the microstructure relative tothe solidified structure of the alloy in pure form or with small additions of As, Fe, orCu.21–23

12.4.2.5 Impurity Tolerance. Some impurity elements have significant adverseimpact on soldering performance and physical properties. The sensitivity of soldertoward impurities is a function of the impurity element.15 Table 12.6 shows the detri-mental effect of some impurities on the properties of 60Sn-40Pb.

The tolerance of solder alloys toward impurities may be increased by the addi-tion of some other elements. For instance, Mei et al. studied the effect of Pb con-tamination on Sn-Bi eutectic and found that formation of an Sn-Pb-Bi ternaryeutectic phase resulted in drastic failure of the solder. The Pb dissolves into moltenBi-Sn during the soldering process, resulting in the formation of a 52Bi-30Pb-18Sn(melting point 96°C) ternary eutectic structure in the solidified solder joint. The sol-der joints became mechanically weak when subjected to thermal cycling at tem-peratures exceeding 96°C because the low-melting-point ternary eutectic phase



TABLE 12.6 Lowest Impurity Levels Producing Detrimental Effect on a 60Sn-40Pb Solder

Impurity Impurity,element % Effect

Ag 2 Increases spread and strength of solder; grittiness in excess of solubility.

Ag3Sn intermetallic compound is soft, ductile, and nonembrittling.

Al 0.0005 Oxide-promoting element; causes a lack of adhesion, grittiness,and dull solder surface.

No dewetting on Cu or brass; 0.001% showed onset of dewetting on steel and nickel.

Sb eliminates Al by promoting rapid drossing out of AlSb compound.

As 0.2 25% decrease in area of spread.

0.005 Dewetting and grittiness on brass, probably due to formation of As-Zn intermetallic compound.

Au 0.1 Gritty joints and surfaces. Weakens solder dramatically at 4%.

Bi 0.5 Discoloration and oxidation of solder coating. Very slightly reduces the area of spread. Increases the rate of spread.

Cd 0.15 25% decrease in area of spread. Dull surface due to oxide film.

Cu 0.29 Grittiness due to Cu-Sn intermetallic compound. Excessive solder increases the liquidus temperature of the

solder, making it more viscous or sluggish. Negligible effect on wetting.

Fe 0.02 Grittiness of solder coating.

Ni 0.05 Grittiness at over 0.02%.

P 0.01 Deoxidant. Dewetting at 0.012% on Cu and steel. Grittiness at 0.1% on Cu.

S 0.0015 Additions up to 0.25% produce no dewetting effects, but give asevere gritty appearance of the solder coating due to the presence of discrete intermetallic compound particles of SnS and PbS.

Powerful grain refiner.

Sb 1 Area of spread decreases slightly with increase in Sb content. Prevents transformation of beta Sn to alpha Sn at subzero

temperatures. Drosses out An, Al, and Cd from solder.

Zn 0.003 Oxide-forming element. Dewetting at 0.001%. Loss of solder brightness at 0.005%.

accelerated grain growth and phase agglomeration. The addition of small amountsof indium (2 to 3 percent) into 58 Bi-42Sn solder may eliminate the formation of theternary eutectic phase, as indicated by the disappearance of the ternary phase peakin differential scanning calorimeter measurements.31

12.5 LEAD-FREE ALLOYS INVESTIGATED

The Pb-free solder alloys investigated are summarized in Table 12.7.32 Also listed arethe two controls, 63Sn-37Pb and 62Sn-36Pb-2Ag. The nomenclature of each alloycategory is based on the elemental composition percentage, with the compositionwith a higher percentage listed first.

12.6 FAVORITE Pb-FREE ALLOYS

The favorite choice of Pb-free alloys differs from region to region, as discussed in thefollowing text.

12.6.1 JAPAN

As discussed in Chap. 11, Japan is leading in terms of implementing Pb-free solder-ing processes. Although the selection of alloys in Japan is not standardized yet, theJapanese Electronic Industry Development Association (JEIDA) does provide rec-ommendations about alloys based on applications, as shown in Table 12.8.106 TheJEIDA recommendations can also be presented according to the melting tempera-ture for mid–melting range applications, as shown in Fig. 12.9.105 Figure 12.10 showsthe survey results on lead-free soldering implementation status in Japan conductedby Senju in April 2001.10 It is interesting to note the high overlap between JEIDArecommendations and industry implementation status, suggesting an active partici-pation of Japanese industrial Pb-free soldering manufacturers in the JEIDA pro-gram.

Due to the wide range of products in the electronics industry and the highlydiversified coverage for major electronic manufacturers, the selection of Pb-freealloys may be a multiple choice. This phenomenon is well exemplified by Fig. 12.11,which shows the road map of Panasonic on Pb-free alloy development.11

12.6.2 EUROPE

The European consortium, BRITE-EURAM, recommends that 95.5Sn-3.8Ag-0.7Cu be considered as an all-purpose alloy. Other alloys with potential are 99.3Sn-0.7Cu, 96.5Sn-3.5Ag, and Sn-Ag-Bi. In the UK, the Department of Trade andIndustry (DTI) provided its perception on Pb-free alloy choices as shown in the fol-lowing list.

High professional group (automotive, military): Sn-Ag-Cu(Sb) Medium professional group (industrial, telecommunications): Sn-Ag-Cu, Sn-Ag General consumer and low professional group (TV, audio-video, office equip-

ment): Sn-Ag-Cu(Sb), Sn-Ag, Sn-Cu, Sn-Ag-Bi


TABLE 12.7 Pb-Free Solders Investigated

Solidus/liquidus Manufacturer or

Alloy category Composition (°C) Advantages Disadvantages investigator

Sn-Pb 63Sn-37Pb 183 Overall good properties, low cost. Structural coarsening; NCMS (control)1

UTS 4442 psi. YS 3950 psi. prone to creep.Elongation 48%, YM 15.7 GPa.33 σ

464 dyn/cm,34 380 dyn/cm.35 UTS 27MPa,36 SS 39 MPa.37

Sn-Pb-Ag 62Sn-36Pb-2Ag 179/180 UTS 6904 psi, elongation 31%, YS NCMS (control)1

6287 psi, YM 18.0 GPa.33

Au-Sn 80Au-20Sn 280 Creep and corrosion resistant. Hard and brittle; meltingpoint too high; expensive.

Bi-Cd 60Bi-40Cd 144 Toxic. Indium

Bi-In 67Bi-33In 109 Poor wetting on Cu. Indium

Bi-In-Sn 57Bi-26In-17Sn 79 Melting point too low.

Bi-Sn 50Bi-50Sn 138/152 YS 8263 psi, UTS 8965 psi, Wide pasty range. NCMS33

elongation 21% and 53%.33

52Bi-48Sn 138/151 YS 6414 psi, UTS 8834 psi, Wide pasty range. NCMS33

elongation 57%.33

57Bi-43Sn YS 7972 psi, UTS 8540 psi, NCMSelongation 77%.33

58Bi-42Sn 138 Good fluidity. UTS 8766 psi.1 Strain rate sensitivity;Elongation 46%.33 Low poor wetting. Concerns:

σ, 349 dyn/cm,34 300 (1) eutectic 52Bi-32Pb-dyn/cm.35 16Sn (96°C); (2) Bi is by-

product of Pb mining.YM 11.9 GPa.33

95Bi-5Sn 134/251 Indium

Bi-Sn-Ag 56Bi-43.5Sn-0.5Ag Ternary NCMS33

eutectic

12.1

4

57Bi-42.9Sn-0.1Ag 138/140 NCMS33

57Bi-42Sn-1Ag BGA bend strength using HP39

95.8Sn-3.5Ag-0.7Cu balland 57Bi-452Sn-1Agpaste is 65% of that of Sn63 paste.38

57Bi-41Sn-2Ag 140/14733 YS 9487 psi, UTS 10,390 psi.33 Elongation 31%.33 NCMSThermal fatigue life > Sn63 >

58Bi-42Sn.40

Bi-Sn-Ag-Sb 56Bi-40.5Sn-2Ag- 137/14533 YS 9063 psi, UTS 9946 psi.33 Low elongation, 27%.33 NCMS1.5Sb

55.5Bi-40Sn-3Ag- 137/14733 YS 8665 psi, UTS 9379 psi.33 Elongation 45%.33 Wide NCMS1.5Sb pasty range.

55Bi-40Sn-3Ag-2Sb 138/15033 YS 8984 psi, UTS 9807 psi.33 Elongation 44%.33 Wide NCMSpasty range.

54Bi-39Sn-3Ag-2Sb 138/15433 Low elongation, 3.7%.33 NCMSWide pasty range.

Bi-Sn-Ag-Sb-In 54Bi-39Sn-3Ag-2Sb- 99/13833 YS 5055 psi, UTS 11,640 psi.33 Low elongation, 13%.33 NCMS2In Very wide pasty range.

Bi-Sn-Ag-Sb-Cu 54Bi-39Sn-3Ag-2Sb- YS 11,440 psi UTS 12,280 psi.33 Low elongation, 4%.33 NCMS2Cu

Bi-Sn-Cu 55Bi-42Sn-3Cu >400 High Cu, wide pasty NCMSrange, high liquidus, lowelongation.33

55Bi-43Sn-2Cu 138/14033 YS 8985 psi, UTS 9478 psi.33 Elongation 41%.33 NCMS48Bi-48Sn-4Cu >400 High Cu, wide pasty NCMS

range, high liquidus, lowelongation.33

Bi-Sn-Fe 54.5Bi-43Sn-2.5Fe 137 Creep and fatigue resistance. Developmental stage. AT&T

Bi-Sn-In 56Bi-42Sn-2In 126/14033 YS 7224 psi, UTS 8429 psi, Quenched alloy shows IBM, NCMSelongation 116%.33 ternary melting (99°C),

116% total elongation.33

12.1

5

(Continues)

TABLE 12.7 Pb-Free Solders Investigated (Continued)



Bi-Sn-In (cont.) 57Bi-42Sn-1In 132/13833 Poor wetting.33 IBM

57Bi-41Sn-2In 127/14033 YS 7304 psi, UTS 8436 psi, NCMSelongation 72%.33

(37-57)Bi-(37-53)Sn- Ford41

(6-10)In

Bi-Sn-In-Cu 56.7Bi-42Sn-1In- 132/13833 YS 8359 psi, UTS 8985 psi.33 Low elongation, 38%.33 NCMS, IBM0.3Cu

Bi-Sn-Sb 57Bi-41Sn-2Sb 141/15033 YS 8521 psi, UTS 9586 psi.33 Elongation 47%.33 Wide NCMSpasty range.

57Bi-42Sn-1Sb 138/14933 YS 8285 psi, UTS 8944 psi, NCMSelongation 60%.33

Bi-Sn-Zn 55Bi-43Sn-2Zn Ternary NCMS33

eutectic

Bi-Sb 95Bi-5Sb ∼275/ Ford∼308

In-Ag 97In-3Ag 143 Poor wetting; expensive. Indium

90In-10Ag 141/237 Indium

In-Bi-Sn 48.8In-31.6Bi-19.6Sn 59

51.0In-32.5Bi-16.5Sn 60 Indium

In-Sn 60In-40Sn 118/127

52In-48Sn 118 Au soldering. Melting point too low; Indiumpoor fatigue andmechanical properties;expensive.

50In-50Sn 118/125 Indium

12.1

6

Sn 100Sn 232 Wetting. UTS 21 MPa vs. 17.5 MPa Whisker and tin pest Indiumfor Pb,36 SS 26 MPa vs. 13 MPa growth.for Pb.37

Sn-Ag 95Sn-5Ag 221/24542 No coarsening. UTS 10,100 psi, SS Welco Castings42

8,400 psi.42

96.5Sn-3.5Ag 221 Good strength; creep resistance, Poor isothermal fatigue at IndiumFatigue life 1.1 times that of Sn63,13 low strain; melting point

better than 95.5Sn-3.8Ag-0.7Cu, slightly too high. Padcomparable with 99Sn-1Cu.43 Shear trace may crack due tostrength not affected by baking time high rigidity of solder.44

and better than for Sn62.44 YM 26.2 UTS 3873 psi, YS 3256GPa,33 56 GPa.45 UTS 55 MPa vs. psi, elongation 24%,33

31–46 MPa for Sn6345 and 21 MPa 35% vs. 35–176% forfor Sn.36 UTS 8900 psi, SS 4600 Sn63.45 σ 493 dyn/cm.34

psi.42 SS 61.2 MPa46 vs. 26 MPa for Sn.37

98Sn-2Ag 221/226 NCMS33

Sn-Ag-Au Balance Sn-(1- Motorola47

2.2)Ag-(1-2.2)Au

Sn-Ag-Bi 93.5Sn-3.5Ag-3Bi 200/217 SMT defect rate <50% of that for Matsushita Nihonor Sn63.11 Handa

208/21733

95.5Sn-3.5Ag-1Bi 219/220 Fatigue much better than for Sn63. H-Technol13

94Sn-3Ag-3Bi 213

95Sn-3Ag-2Bi 216/22033 YS 5463 psi, UTS 7930 psi.1,33 Low elongation, 30%.33 NCMS

95.5Sn-2.5Ag-2Bi 215/22133 YS 6592 psi, UTS 7564 psi.33 Low elongation, 26%.33 NCMS

Sn-Ag-Bi-Cu 92.6Sn-3.3Ag-3Bi- 204/216 Ref. 48

1.1Cu

Sn-Ag-Bi-Cu-Ge 92.7Sn-3.2Ag-3Bi- Tensile strength 1.8 times that of Impact strength 16% that Japanese Solder1.1Cu-1Ge Sn63.49 of Sn63.49

12.1

7

(Continues)




Sn-Ag-Bi-In Balance Sn-(1-4)Ag- Mitsui50

(Ag+1.23Bi+0.52In)>5

Sn-Ag-Bi-In-Zn Balance Sn-(1-6)Ag- Lucent51

(0.2-0.6)Bi-(0.2-0.6)In-(0.2-0.6)Zn

Sn-Ag-Bi-Sb 94Sn-2.5Ag-2Bi- 219/22633 YS 7070 psi, UTS 8117 psi.33 Low elongation, 21%.33 NCMS1.5Sb

93Sn-3Ag-2Bi-2Sb 219/22633 YS 6918 psi, UTS 9212 psi.33 Low elongation, 36%.33 NCMS

(90.3-99.2)Sn-(0.5- AIM52

3.5)Ag-(0.1-2.8)Cu-(0.2-2)Sb

Sn-Ag-Bi-Zn-Cu (93.5-94)Sn-(2.5- IBM53

3)Ag-(1-20bi-(1-2)Zn-1Cu

Sn-Ag-Cd-Sb 95Sn-3.5Ag-1Cd- 221/223 YS 7545 psi.33 Low elongation, 15%.33 NCMS,33 Alpha0.5Sb Low usage, contains Cd.

(89.4-95.1)Sn-(3- IBM54

3.8)Ag-(0.7-1.3)Cd-(0.2-0.5)Sb

Sn-Ag-Cu 93.6Sn-4.7Ag-1.7Cu 217/24433 Sandia, IowaState University

95Sn-4Ag-1Cu 217/220

95.5Sn-4Ag-0.5Cu 217/22533 Published 50 years ago. Creep slower Heraeusor than for Sn63.56

221/23055

95.5Sn-3.9Ag-0.6Cu 217 Recommended by NEMI. NEMI

12.1

8

95.5Sn-3.8Ag-0.7Cu 217/220 Nokia and Multicore—yield and Nokia andreliability equal or better than for Multicore,Sn63. BRITE-EURAM project BRITE-reports better reliability and EURAMsolderability than for SnAg andSnCu, recommends this alloy forgeneral-purpose use.

95.4Sn-3.6Ag-1Cu 217/ SS 67 MPa.46

217.946

95.2Sn-3.5Ag-1.3Cu NIST alloy,NCMS33

95.6Sn-3.5Ag-0.9Cu 217 Eutectic. NIST

95.75Sn-3.5Ag- 218 Creep rate much lower than for Sn63.57 Tensile strength 0.9 times Senju,58 Hitachi0.75Cu Impact strength 2.5 times that of that of Sn63.49

Sn63.49

96.1Sn-3.2Ag-0.7Cu NIST alloy,NCMS59

96.3Sn-3.2Ag-0.5Cu 217/218 Alpha

95.4Sn-3.1Ag-1.5Cu 216/217 Low cycle fatigue life 2.4 times that H-Technolof Sn63.13

96.5Sn-3Ag-0.5Cu 220 Harris60

97.25Sn-2Ag-0.75Cu 217/21933 NEC

Sn-Ag-Cu-Bi-Zn Balance Sn-(3.5- Iowa State7.7)Ag-(1-4)Cu- University,(0-10)Bi-(0-1)Zn- Sandia61

(Si, Sb, Mg, Ca,rare earth,miscellaneousmetal <1)

12.1

9

(Continues)




Sn-Ag-Cu-Co 95.27Sn-3.59Ag- 217/ SS 65.5 MPa.46

0.99Cu-0.15Co 218.546

95.12Sn-3.59Ag- 217/ SS 56.1 MPa.46

0.99Cu-0.30Co 219.346

94.98Sn-3.58Ag- 217/ SS 65.7 MPa.46

0.99Cu-0.45Co 218.946

Sn-Ag-Cu-Ni (91.5-96.5)Sn-(2- Ford62

5)Ag-(0-2.9)Cu-(0.1-3)Ni

Sn-Ag-Cu-P-Ni Balance Sn-(<10)Ag- Fukuda63

(<3)Cu-(0.05-1.5)P-(0.05-1.5)Ni

Sn-Ag-Cu-Sb 93Sn-3Ag-2Cu-2Sb 221/22433 YS 6684 psi, UTS 7655 psi.33 Elongation 32%.33 NCMS

96.2Sn-2.5Ag-0.8Cu- 211/22633 Low cycle fatigue life 2.4 times that Slightly high melting AIM (CASTIN)0.5Sb of Sn63.13 Elongation 50% vs. 53% point. UTS 3749 psi.1 YS

for Sn63; UTS 5730 psi vs. 4920 psi 3311 psi.33 Lowfor Sn63; YS 4860 psi vs. 4380 psi elongation, 9%.33 High σ,for Sn63, YM 7420 ksi vs. 4870 ksi 510 dyn/cm.35

for Sn63.64

93Sn-3Ag-2Cu-2Sb NCMS

95Sn-3Ag-1.5Cu- 216/217 Good fatigue life, high modulus and H-Technol65

0.5Sb strain. UTS equals that of Sn63.65

(93-98)Sn-(1.5- AIM66

3.5)Ag-(0.2-2)Cu-(0.2-2)Sb

Balance Sn-(3-5)Ag- Senju58

(0.5-3)Cu-(0-5)Sb

12.2

0

Sn-Ag-Cu-Ti-V- (35-95)Sn-(0.5- GTE30

Zr-Ni-Cr 70)Ag-(0.5-20)Cu-(0.1-4)Ti, V, Zr-(0-5)Ni-(0-2)Cr

Sn-Ag-In 95Sn-3.5Ag-1.5In 214/22033 YS 4616 psi, UTS 4987 psi.33 Low elongation, 26%.33 NCMS, Alpha

Sn-Ag-In-Cu 95.3Sn-3Ag-1In- UTS comparable with that of Sn63.49

0.7Cu Impact strength 2.4 times that ofSn63.49

Sn-Ag-In-Bi 93Sn-3.5Ag-3In- UTS 1.1 times that of Sn63.49 Impact Harima, Mitsui0.5Bi strength 1.5 times that of Sn63.49 Metals

Sn-Ag-Sb 65Sn-25Ag-10Sb 233 High strength. Expensive; melting point Motorolatoo high. Old Motoroladie attach solder. Verybrittle.

95Sn-3Ag-2Sb 225/22833 YS 5749 psi, UTS 6124 psi.33 Low elongation, 25%.33 NCMS

(61-69)Sn-(23- Motorola67

28)Ag-(8-11)Sb

Sn-Ag-Sb-Bi-Cu 93.5Sn-3Ag-1.5Sb- 220/22433 YS 8361 psi, UTS 9256 psi.33 Low elongation, 21%,33 NCMS1Bi-1Cu complex system, although

meets NCMS criteria.

Sn-Ag-Sb-Bi-In Balance Sn-(0.8- Kabushiki68

5)Ag-(0.1-10)Sb-(>0.1)Bi-(>0.1)In

Sn-Ag-Sb-Cd 95Sn-3.5Ag-0.5Sb- NCMS1Cd

Sn-Ag-Zn 95.5Sn-3.5Ag-1.0Zn 217 Good mechanical strength. Slightly high melting AT&Tpoint.

Sn-Ag-Zn-Cu 95Sn-3.5Ag-1.0Zn- 219/22133 Good ductility. Slightly high melting AT&T alloy,0.5Cu point. NCMS

Sn-Au-Bi-Ag Sn86.85Sn-5Au-5Bi- Sandia69

3.15Ag

12.2

1

(Continues)




Sn-Bi 55Sn-45Bi 138/16433 26°C pasty range. NCMS

Sn-Bi-Ag 90.5Sn-7.5Bi-2Ag 190/21633 Tamura Kaken

92Sn-5Bi-3Ag 210

91.8Sn-4.8Bi-3.4Ag 200/216, Considered one of most promising by Small peak at 137°C from Sandia,70 NCMS201/205, the NMCS. 0–100°C temperature quench.33 High σ, 420

or cycling much better than for Sn63.11 dyn/cm.35

211/21633 UTS 10,349 psi.1

(91-96)Sn-(>3)Bi- Sandia70

(3.2-4.83)Ag

91.7Sn-4.8Bi-3.5Ag 211/215 Low cycle fatigue comparable to that Low cycle fatigue life 0.9 H-Technol13

of Sn63. YS 6,712 psi, UTS 10,349 times that of Sn63.13 Lowpsi.33 elongation, 16%.33

96.5Sn-3Bi-0.5Ag 223/226 Low cycle fatigue better than Sn63. H-Technol13

90.5Sn-6Bi-3.5Ag 220 Matsushita

90.8Sn-6.1Bi-3.1Ag 137/21533 Low solidus, very wide NCMSpasty range.

86.5Sn-10Bi-3.5Ag 137/20833 Very wide pasty range. Matsushita,NCMS

81.7Sn-15Bi-3.3Ag 137/20033 Very wide pasty range. Matsushita,NCMS

78Sn-19.5Bi-2.5Ag 138/19633 YS 12,070 psi, UTS 13,450 psi.33 Elongation 17%, wide NCMS, Kesterpasty range.33

63.2Sn-30Bi-6.8Ag 137/28233 Very wide pasty range. NCMS

56Sn-41Bi-3Ag 138/16633 YS 9287 psi, UTS 10,130 psi.33 Elongation 39%, wide NCMS, IBM,pasty range.33 Endicott

(40-60)Sn-(>40)Bi- Lucent71

(0.05-1)Ag

12.2

2

Sn-Bi-Ag-Cu 86.6Sn-10Bi-2.8Ag- Ono0.6Cu

90Sn-7.5Bi-2Ag- 193/21333 YS 12,370 psi, UTS 13,440 psi.33 Low elongation, 12%.33 Alloy H, Alpha0.5Cu Metals,

developed atITRI; NCMS33

90.8Sn-5Bi-3.5Ag- 198/213 NCMS33

0.7Cu

91.0Sn-4.5Bi-3.5Ag- 210 Senju1.0Cu

93.3Sn-3.1Bi-3.1Ag- 209/212 Low cycle fatigue life 1.8 times that H-Technol0.5Cu of Sn63.13

94.25Sn-3Bi-2Ag- 205/21733 NCMS0.75Cu

Sn-Bi-Ag-Cu-Ge 93.4Sn-4Bi-2Ag- 202/217 NCMS33

0.5Cu-0.1Ge

Sn-Bi-Ag-In Balance Sn-(6-14)Bi- Samsung72

(3-4)Ag-(2-5)In

Sn-Bi-Ag-In-Cu (83-92)Sn-(5-18)Bi- Matsushita59

(2.5-4)Ag-(0-1.5)In-(0-0.7)Cu

Sn-Bi-Au Balance Sn-(30- Motorola73

70)Bi-Au bump

Sn-Bi-Cu 50Sn-48Bi-2Cu 138/15333 YS 8899 psi, UTS 9495 psi.33 Low elongation, 19%,33 NCMSwide pasty range.

Sn-Bi-Cu-Ag 48Sn-46Bi-4Cu-2Ag 137/146 YS 9806 psi, UTS 10,070 psi.33 Low elongation, 3%.33 IBM, NCMSPoor ductility.

90Sn-7.5Bi-2Ag- NCMS0.5Cu

12.2

3

(Continues)




Sn-Bi-Cu-Ag-P (0.08-20)Bi-(0.02- Cookson74

1.5)Cu-(0.01-1.5)Ag-(0-0.10)P-(0-0.2) rareearth mixture—balance Sn

Sn-Bi-In-Ag 80Sn-11.2Bi-5.5In- 170/22133 Wide pasty range. NCMS3.3Ag

80.8Sn-11.2Bi-5.5In- 169/20033 Wide pasty range. NCMS2.5Ag

Sn-Bi-Zn 65.5Sn-31.5Bi-3Zn 133/17133 Elongation 53%, UTS 11,210 psi, YS Wide pasty range. NCMS, Alpha10,500 psi.33

Sn-Bi-Zn-Sb-Mg- Balance Sn-(5-15)Bi- Korea Institute ofAl-Te (0.01-3)Zn-(0.01- Machinery and

3)Sb-(0.01-3)Mg- Metals39

(0.01-3)Al-(0.01-3)Te

Sn-Cd 67.8Sn-32.2Cd 177 Toxic. Indium

Sn-Cu 97Sn-3Cu 227/33542 UTS 6420 psi. Ford, WelcoCastings42

99Sn-1Cu 227

99.3Sn-0.7Cu 227 SS 29.8 MPa.46 σ 461 dyn/cm.34 Fatigue resistance 0.3times that of Sn63.13 UTS0.5 times that of Sn63.75

Sn-Cu-Ag 95.5Sn-4Cu-0.5Ag 218/22633 Melting range too wide Engelhardand too high. YS 3724 (Silvabrite 100),psi, UTS 4312 psi. Low NCMSelongation, 27%.33

12.2

4

93Sn-4Cu-3Ag 221/>300 YS 6276 psi, UTS 7006 psi.33 Low elongation, 22%. NCMS95°C pasty range, liquidus>300°C.33

(92-99)Sn-(0.7-6)Cu- Engelhard76

(0.05-3)Ag

Sn-Cu-Ag-Bi-Se (79-97)Sn-(3015)Cu- Touchstone77

(0-4)Ag-(0-1)Bi-(0-1)Se

Sn-Cu-Ag-Ni (92.5-96.9)Sn-(3- Harris78

5)Cu-(0-5)Ag-(0.1-2)Ni

Sn-Cu-Bi-Ag (88-99.35)Sn-(0.5- Oaley79

6)Cu-(0.1-3)Bi-(0.05-3)Ag

Sn-Cu-In-Ag (80-81)Sn-(10- IBM80

12)Cu-(5-6)In-(2-4)Ag

Sn-Cu-Se-Te Balance Sn-(3-6)Cu- Tanacorp81

(0.1-1)Se-(0.1-1)Te

Sn-Cu-Sb-Ag 95.5Sn-3Cu-1Sb- 256 Melting point too high. Motorola0.5Ag

97Sn-2Cu-0.8Sb- 219/23033 YS 3758 psi, UTS 4323 Kester SAF-0.2Ag psi. Low elongation, A-LLOY,

27%.33 NCMS

Sn-Cu-Zn-Ag-Ni 95.68Sn-(2.8-3.5)Cu- Kale Sadashiv S82

(0.2-0.5)Zn-(0.08-

0.16)Ag-(0.08-

0.16)Ni

Sn-In 58Sn-42In 118/145 Wide pasty range, high In. Indium

70Sn-30In 120/∼175 Poor creep.

12.2

5

(Continues)




Sn-In-Ag 77.2Sn-20.0In-2.8Ag 175/186 Creep resistant; virtually drop-in Slightly expensive. 114°C Indium, NCMSreplacement. small peak due to eutectic

Sn-In. σ 390 dyn/cm.35

73.2Sn-20In-6.8Ag 113/24233 Low solidus, very wide NCMSpasty range.

82Sn-15In-3Ag NCMS

80Sn-14.4In-5.6Ag 189/19933 High In content. NCMS33

96.9Sn-10In-3.1Ag 204/205 No Sn-In eutectic problem, potential Indiumuse for flip chip applications.83

86.4Sn-8.6In-5Ag NCMS

(70-92)Sn-(4-35)In- Indium83

(1-6)Ag

(71.5-91.9)Sn-(4.8- Indium83

25.9)In-(2.6-3.3)Ag

(70.5-73.5)Sn- IBM84

balance In-(6.5-7.5)Ag

Sn-In-Ag-Bi 91.5Sn-4In-3.5Ag- 208/213 Low cycle fatigue life 3.3 times that H-Technol1Bi of Sn63.13

Sn-In-Ag-Cu 85.9Sn-10In-3.1Ag- Joints may deform due to Delphi Delco85,86

1Cu phase change attemperature cycling.

88.5Sn-8In-3Ag- 196/202 Low cycle fatigue life 5.3 times that Low yield strength. H-Technol87

0.5Cu of Sn63.13

Sn-In-Ag-Sb 85.7Sn-10.9In-3Ag- 201/ Qualitek, NCMS0.4Sb 217.6

88.5Sn-10.0In- 211 Qualitek1.0Ag-0.5Sb

12.2

6

86.4Sn-8.6In-5Ag- 200/20533 Meets NCMS acceptance NCMS33

2Sb criteria.

Sn-In-Bi 70Sn-20In-10Bi UTS 6938 psi.33 Low elongation, 4%. NCMSHigh In content.33

82Sn-15In-3Bi 113 NIST33

80Sn-10In-10Bi 153/199 Wide pasty range. IBM, NCMSor

170/20033

85Sn-10In-5Bi NCMS33

90Sn-8In-2Bi 206/21533 High strength. YS 7160 psi, UTS Melting point too high. IBM,88 NCMS7970 psi.33 Low elongation, 25%;

high In content.33

(70-90)Sn-(8-20)In- IBM89

(2-10)Bi

Sn-In-Bi-Ag 80Sn-10In-9.5Bi- 179/201 Creep and fatigue resistant. Slightly expensive. Ford0.5Ag

82Sn-10In-5Bi-3Ag NCMS33

78.4Sn-9.8In-9.8Bi- 163/19533 YS 14,560 psi, UTS 15,380 psi.33 Low elongation, 7%.33 IBM,90 NCMS2Ag 32°C pasty range, high

In content.

80Sn-(5-14.5)In-(4.5- Ford91

14.5)Bi-0.5Ag

Sn-In-Bi-Ag-Cu (86-97)Sn-(0-9.3)In- U.S. Army92

(0-4.8)Bi-(0.3-4.5)Ag-(0-0.5)Cu-intermetallic filler

Sn-In-Cu 93.3Sn-6In-0.7Cu 213/217 Low cycle fatigue life 2.1 times that H-Technol75

of Sn63. YM 1.5 times that of Sn63.UTS 1.2 times that of Sn63.75

94.3Sn-5In-0.7Cu 213/217 Low cycle fatigue life 1.4 times that H-Technol75

of Sn63. YM 2.1 times that of Sn63.UTS 1.1 times that of Sn63.75

12.2

7

(Continues)




Sn-In-Cu-Ga 92.5Sn-6In-1Cu- 213/217 YM 1.7 times that of Sn63. UTS 1.2 Low cycle fatigue life 0.8 H-Technol75

0.5Ga times that of Sn63.75 times that of Sn63.

92.8Sn-6In-0.7Cu- 210/215 Low cycle fatigue life 3 times that of H-Technol13,75

0.5Ga Sn63. YM 1.7 times that of Sn63.UTS 1.2 times that of Sn63.75

93Sn-6In-0.5Cu- 209/214 Low cycle fatigue life 1.7 times that H-Technol75

0.5Ga of Sn63. YM 1.6 times that of Sn63.UTS 1.3 times that of Sn63.75

94.5Sn-4In-1Cu- 215/218 Low cycle fatigue life 1.2 times that UTS 0.9 times that of H-Technol75

0.5Ga of Sn63. YM 1.6 times that of Sn63. Sn63.75

94.8Sn-4In-0.7Cu- 215/218 Low cycle fatigue life 1.8 times that UTS 0.85 times that of H-Technol75

0.5Ga of Sn63. YM 1.4 times that of Sn63. Sn63.75

95Sn-4In-0.5Cu- 215/219 Low cycle fatigue life comparable UTS 0.96 times that of H-Technol75

0.5Ga with that of Sn63. YM 1.4 times Sn63.75

that of Sn63.

Sn-In-Zn 77.2Sn-20In-2.8Zn 106/18033 YS 5095 psi, UTS 5381 psi.33 Low elongation, 31%.33 NCMSWide pasty range.

83.6Sn-8.8In-7.6Zn 178/19533 YS 6033 psi, UTS 6445 psi.33 Low elongation, 14%.33 NCMS

Sn-Sb 95Sn-5Sb 232/240 Creep resistant; good high- Melting point too high, Motorolatemperature shear; mechanically poor wetting. YS 3720strong. UTS 5110 psi, YM 44.5 psi. Low elongation,GPa.33 UTS 5900 psi, SS 6200 psi.42 22%.33

97Sn-3Sb 232/238 Indium

99Sn-1Sb 232/235 Indium

Sn-Sb-Ag-Bi 90Sn-7.5Sb-2Ag- 229/23833 YS 8230 psi, UTS 8773 PSI.33 Low elongation, 19%.33 Alpha

0.5Bi

12.2

8

Sn-Sb-Ag-Cu 88.9Sn-5Sb-4.5Ag- Sandia alloy,

1.6Cu NCMS33

Sn-Sb-Bi-Ag (90-95)Sn-(3-5)Sb- Willard(1-4.5)Bi-(0.1-0.5)Ag Industries93

95Sn-3.9Sb-1Bi- Plumbing solder. NCMS33

0.1Ag

95Sn-3Sb-1.5Bi- Plumbing solder. NCMS33

0.5Ag

Sn-Sb-Bi-Cu 93.5Sn-3Sb-2Bi- 225/23133 YS 7343 psi, UTS 9350 psi.33 Low elongation, 28%.33 NCMS1.5Cu

(93-94)Sn-(2.5- IBM94

3.5)Sb-(1.5-2.5)Bi-(1-2)Cu

Sn-Sb-Cu 95Sn-3Sb-2Cu 227/234 Low cycle fatigue life 1.9 times that H-Technolof Sn63.13

Sn-Sb-Cu-Ag 94.5Sn-3Sb-2Cu- 218/232 Low cycle fatigue life 2 times that of H-Technol13

0.5Ag Sn63.

Sn-Sb-Cu-Ag-Ni (87-92.9)Sn-(4-6)Sb- Harris78

(3-5)Cu-(0-0.5)Ag-(0-2)Ni

Balance Sn-(0.75- Johnson95

2)Sb-(0.05-0.6)Ag-(0.05-0.6)Cu-(0.05-0.6)Ni

Sn-Sb-Zn-Ag (90-98.5)Sn-(0.5- Harris96

4)Sb-(0.5-4)Zn-(0.5-2)Ag

Sn-Sb-Zn-Ag-Cu (86.8-98.8)Sn-(0.5- Harris60

4)Sb-(0.5-4)Zn-(0.1-3)Ag-(0.1-2)Cu

12.2

9

(Continues)




Sn-Zn 91Sn-9Zn 199 Good strength; abundant. YS 7478 Poor corrosion resistance Indium, NCMSpsi, UTS 7708 psi.33 and wetting; high

drossing. Low elongation,27%.33 σ 487 dyn/cm.34

Balance Sn-(4-12)Zn Motorola47,97

Sn-Zn-Ag (59-82)Sn-(16-30)Zn- Lucent98

(2-11)Ag

Sn-Zn-Bi 89Sn-8Zn-3Bi 192/19733 Matsushita,Senju, ShowaDenko99

88Sn-7Zn-5Bi 185/194 Zn drossing. NCMS, Alpha

Sn-Zn-Bi-Cu Balance Sn-(7-9)Zn- Mitsui100

(<3)Bi-(0.1-0.5)Cu

Sn-Zn-Ge (50-70)Sn-(25-40)Zn- Kronberg101

(0.1-10)Ge

Sn-Zn-In 90Sn-9Zn-1In

87Sn-8Zn-5In 175/188 Poor wetting; eutectic AT&T52In-46Sn-2Zn (106°C) aconcern.

(72.8-89.4)Sn-(6.7- Indium102

19.2)Zn-(2.7-16.4)In

12.3

0

Sn-Zn-In-Ag 87Sn-8Zn-5In-0.1Ag AT&T

Sn-Zn-In-Bi 86.5Sn-5.5Zn-4.5In- 181/189 Zn causes high dross and Indium103

3.5Bi corrosion concerns.103

(82-90)Sn-(4.5-6)Zn- Indium103

(3.5-6)In-(1-5)Bi

Sn-Zn-In-Bi-Ag- (>70)Sn-(6-10)Zn-(3- AT&T104

Cu-Sb-Au 10)In-(<10)Bi-(<5)Ag-(<5)Cu-(<5)Sb-(<5)Au

Sn-Zn-In-Cu 87Sn-8Zn-5In-0.1Cu AT&T

Zn-Sn-Ag-Al (15-98)Zn-(2-85)Sn- Asahi Glass105

(0-5)Ag-(0.01-0.5)Al

SS, shear strength; UTS, ultimate tensile strength; YM, Young’s modulus; YS, yield strength; σ, surface tension.

12.3

1

TABLE 12.8 List of Lead-Free Solder Alloy Candidates Recommended by JEIDA

Composition preferredAlloys used for practical from point of view of cost

Process applications and performance Notes

Sn-3.5Ag

Sn-(2–4)Ag-(0.5–1)Cu Sn-3Ag-0.5Cu Sn-Pb plating on

Wave Sn-0.7Cu with a very smallcomponents might cause

amount of other elements (Ag,fillet lifting and damage to

Au, Ni, Ge, In, etc.) addedboards.

Sn-3.5Ag Needs temperature control

Sn-(2–4)Ag-(0.5–1)Cu Sn-3Ag-0.5Cu for reflow at higher temperatures.

Sn-(2–4)Ag-(1–6)Bi, including Incompatibility with Sn-Pb–Medium and those with 1–2% In plated components when ithigh temperatures contains some percentage of Bi.

Reflow Handle Sn-Zn carefully inSn-8Zn-(0–3)Bi Sn-8Zn-3Bi corrosive environment.

Ni-Au finishes preferred forCu electrode at high temperatures.

Incompatibility with Sn-Pb–Low temperatures Sn-(57–58)Bi Sn-57Bi-1Ag plated components

Sn-3.5Ag

Sn-(2–4)Ag-(0.5–1)Cu Sn-3Ag-0.5Cu Incompatibility withManual/robot (thread solder) Sn-0.7Cu with a very small different solder alloys in

amount of other elements (Ag, reworking.Au, Ni, Ge, In, etc.) added

Sn-3.5Ag-0.5Cu is JEIDA’s primary recommendation.

12.3

2

12.6.3 NORTH AMERICA

In the U.S., the National Electronics Manufacturing Initiative (NEMI) recommends99.3Sn-0.7Cu for wave soldering and 96.5Sn-3.5Ag and 95.5Sn-3.9Ag-0.6Cu forreflow soldering. The NCMS recommends 96.5Sn-3.5Ag, 91.7Sn-3.5Ag-4.8Bi, and58Bi-42Sn.

12.6.4 COMPARISON OF REGIONAL PREFERENCES

Eutectic or near-eutectic Sn-based alloys containing Ag and/or Cu are the mostcommonly selected families, regardless of region. However, beyond that, discrepan-cies show up, as discussed in the following text.

12.6.4.1 Bi. In general, Bi is avoided in Europe and North America, mainly dueto the general perception that Bi is a by-product of Pb and not a good choice envi-ronmentally.107 Bi-containing alloys generally are more rigid, and may pose concernsabout impact resistance. For example, Richard D. Parker, staff engineering supervi-sor, advanced substrate assembly, advanced engineering at Delphi Automotive, says“Automotive components are subjected to intense vibration. Sn-Ag-Cu system sol-der is superior to Sn-Ag-Cu-Bi system solder in terms of mechanical strength, andoffers better reliability.”108

Fillet lifting or lift-off is another reason Bi additives are not favored. This defectis common when using Bi-containing alloys such as Sn-Ag-Cu-Bi system solders, andoccurs when a through-hole component is flow-soldered and the solder separatesfrom the printed circuit board land. When Bi levels are high, the fillet lifting occur-rence rate increases. On top of that, difficulty in separating Bi from Cu by smelteralso presents challenges in recycling. At present, no manufacturers plan to use Sn-Ag-Cu-Bi system solder.


FIGURE 12.9 JEIDA road map of possible midrange melting tempera-ture lead-free alloys.

In Japan, the Bi-containing alloy group represents the second most popularchoice, and composes 20 to 30 percent of Pb-free solders used. This is attributed toits low melting temperature and superior wetting performance.109 The former reasonmay not be significant, since the addition of Bi allows for only a minor reduction inmelting temperature. For instance, the melting point of the Sn-2.5Ag-0.5Cu-1Bi sol-der used by Sony is 222°C, about the same as that of normal Sn-Ag-Cu system sol-


FIGURE 12.10 Senju survey results on lead-free soldering implemen-tation status in Japan.

(b)

(a)

der. If the Bi content were increased further, the melting point could be dropped fur-ther, but with the consequence that the solder would become hard and brittle. Sony,Mitsubishi, and Hitachi all use Sn-Ag-Cu-Bi system solder, and Sharp is said to beconsidering using the same type in the future. NEC uses an Sn-Zn-Bi solder, andMatsushita uses an Sn-Ag-Bi-In blend. The Bi additive in Sn-Zn system solder isused to improve wetability and to reduce the sensitivity of Zn toward corrosion.Themelting temperature remains almost unchanged from the 199°C of Sn-Zn alloy with-out Bi.

The sensitivity toward Pb contamination presumably is addressed by limiting theapplications to consumer electronics and by suppressing the sensitivity via the intro-duction of other elements such as In.31 Separating Bi from Cu was also noted bySuga as an issue resolved in the Japanese smelting industry.110

12.6.4.2 Zn. Zn-containing alloys are not considered in Europe and NorthAmerica due to the high reactivity of Zn.This high reactivity poses problems such as(1) excessive oxidation of Zn during soldering, (2) poor wetting of Zn alloys,109 (3)finding a flux that can be compatible with Zn alloys,109 and (4) potential vulnerabil-ity of solder joints to corrosion. In Japan, Zn-containing alloys are used by NEC forlaptops, presumably due to the low melting temperature of these alloys.To cope withthis need, flux technology has been developed for reflow applications that is com-patible with Zn systems.99,111

12.6.4.3 Sb. Toxicity of Sb appears to be more of a concern in Japan than inEurope or North America. In the UK, the DTI considers Sb-containing alloys one ofprimary choices. In North America, during the straw poll survey of preferred selec-tions conducted by NEMI on November 16, 1999, Sb-containing alloys were numberthree in preference as Pb-free solder alternatives for both reflow and wave-soldering


FIGURE 12.11 Panasonic lead-free alloy development: alloy groupings.

applications. However, in Japan, Sb in general is dropped from consideration due toits toxicity.

12.6.4.4 In. At low In levels, In-containing alloys have received great interestworldwide. In Japan, Sn-Ag-Bi-In has already been adopted as the third most popu-lar solder group for reflow applications. In North America, resistance against In-containing alloys due to cost and availability concerns is gradually giving way tointerest in their reliability.13,73,87,108,112–114

12.7 PATENT ISSUES

There are three lead-free solder patents that cover the prevailing Pb-free soldersfavored by the industry, as shown in Table 12.9.5 Of the three patents, the Japanesepatent to Senju and Matsushita and the U.S. patent to the Iowa State UniversityResearch Foundation (ISURF) covering the Sn-Ag-Cu family are the most important,due to the fact that Sn-Ag-Cu alloys are the number one choice worldwide. The cov-erage of both patents on Sn-Ag-Cu composition is illustrated in Fig. 12.12.Also shownin Fig. 12.12 are the three Sn-Ag-Cu compositions recommended by JEIDA, BRITE-EURAM, and NEMI. The numerical ranges for these three popular Sn-Ag-Cu com-positions fall right in and only in the claim range of the Senju patent, suggesting thatpatent is the only valid patent affecting the global Pb-free solder implementation.However, the ISURF patent covers both joints and solder material. Since most elec-tronic soldering involves the use of copper pad or leads, dissolution of Cu into soldermay bring the joint composition into the ISURF patent range. In addition, the U.S.patent law’s “doctrine of equivalents” may allow the patent’s coverage to extendbeyond the literal range of its claims and outlaw the compositions that differ some-what from those that are claimed literally in the patent.115 Since Senju and Matsu-shita were granted the patent in Japan, while the Ames Laboratory held patentrights in the U.S., neither side could sell in the other country. In view of these con-siderations, a prudent business approach for a multinational company may beacquiring licensing agreement from both patent holders. However, this implies pay-ing double royalty fee and thus incurring a higher manufacturing cost.

Fortunately, this Pb-free solder patent problem is being resolved gradually. Senjuand Nihon Superior have resolved their patent dispute concerning Sn-Ag-Cu–basedPb-free solder. Rights held by Senju and Matsushita, and other rights held by NihonSuperior and the U.S. Department of Energy’s Ames Laboratory at Ohio State Uni-versity, have been unified into a set for licensing to other solder manufacturers. Themove makes it possible for Japanese equipment manufacturers to purchase Sn-Ag-Cusolder from Senju, Nihon Superior, or any vendors licensed by both companies,


TABLE 12.9 Patents Critical for Pb-Free Solder Implementation

RoyaltyAlloy Patent holder Coverage Patent territory requirement

Sn-Ag-Cu family Ames/Iowa State Solder and joints U.S. YesUniversity

Senju Metals Solder Japan Yes

Sn-Ag-Bi-Cu Oaley Solder North America Yes

and to sell equipment products in both Japan and America without fear of infringe-ment.116 With Sn-Ag-Cu solder being the most promising Pb-free solder, the decisionto unify rights is expected to accelerate its adoption by manufacturers as the stan-dard lead-free material.

12.8 CONCLUSION

Numerous Pb-free solder systems, including preexisting alloys and newly modifiedalloys, have been developed and studied. The modification is often accomplished byaddition of a small amount of additional elements to preexisting alloys with thegoals of improving wetability, bond strength, oxidation resistance, and impurity tol-erance level; reducing the melting temperature; refining the grain structure; and soon.The Sn-Ag-Cu family is the favorite choice. Other popular choices include Sn-Ag,Sn-Cu, Sn-Ag-Cu-Bi, Sn-Ag-Bi-In, and Sn-Zn-Bi systems. Patent overlap issues arebeing resolved through unifying rights.

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CHAPTER 13PREVAILING LEAD-FREE ALLOYS

Among the numerous lead-free solder options available, the following families areof particular interest and are the prevailing choices of industry: eutectic Sn-Ag,eutectic Sn-Cu, Sn-Ag-Bi, Sn-Ag-Bi-In, Sn-Ag-Cu, Sn-Ag-Cu-Bi, Sn-Ag-Cu-In, Sn-Ag-Cu-Sb, Sn-Zn, and Sn-Zn-Bi. Their characteristics and potential performance inelectronic applications follow.

13.1 EUTECTIC Sn-Ag

13.1.1 PHYSICAL PROPERTIES

Eutectic Sn-Ag, 96.5Sn-3.5Ag, has been used in hybrid applications for many years.Some physical properties of this alloy, as well as 99.3Sn-0.7Cu and 63Sn-37Pb, areshown in Table 13.1.1–3

The melting temperature of 96.5Sn-3.5Ag is 38°C higher than eutectic Sn-Pb,suggesting that a considerable thermal stress can be introduced into the devicesbeing assembled during soldering. The high surface tension of the eutectic Sn-Agsystem predicts a higher contact angle during soldering, hence a greater difficulty insolder spreading.The lower density and lower electrical resistivity of eutectic Sn-Agversus eutectic Sn-Pb promises a 12 percent reduction in the weight of solder mate-rials to be used and about 30 percent improvement in signal quality dictated by thesolder joint electrical conductivity. However, the lower thermal conductivity ofeutectic Sn-Ag may hamper the heat dissipation of devices. The difference in thecoefficient of thermal expansion (CTE) between eutectic Sn-Ag (30) and copper(16.6) is greater than the difference between eutectic Sn-Pb (25) and copper, sug-gesting a possibly greater local stress for Sn-Ag at the solder-copper interface. Thehigher hardness of eutectic Sn-Ag may have multiple implications, as will be dis-cussed later.

13.1.2 MECHANICAL PROPERTIES

Some tensile and shear properties of 96.5Sn-3.5Ag, as well as of 99.3Sn-0.7Cu and63Sn-37Pb, are shown in Table 13.2. Some data on alloy compositions close to theeutectic binary systems are also listed. Wide data scattering is observed from sourceto source and may be attributed to the variation in test conditions and sample prepa-ration. To allow a better comparison of alloy systems, data from the same source arelisted side by side. In general, the ultimate tensile strength and yield strength ofeutectic Sn-Ag are about the same as eutectic Sn-Pb. Eutectic Sn-Ag also exhibits ahigher Young’s modulus but a lower elongation than eutectic Sn-Pb, presumably dueto the stiff nature of eutectic Sn-Ag, as indicated by its higher hardness shown inTable 13.1. The shear strength of eutectic Sn-Ag is comparable with or higher thaneutectic Sn-Pb.

13.1


Compilation and comparison of room-temperature elongation of 96.5Sn-3.5Agand eutectic Sn-Pb as a function of strain rate and loading geometry have also beendone by Glazer,3 as shown in Fig. 13.1. Data measured in shear were compared byGlazer with data measured in tension by using equivalent strain, e = r/(3)1/2 if truestrains in tension and shear are known. Again, a wide data scattering is observed,although in most cases the elongation of eutectic Sn-Ag for both tension and sheartests appears to be lower than that of eutectic Sn-Pb. Glazer reported that eutecticSn-Ag has comparable elongation to eutectic Sn-Pb at moderate strain rates at roomtemperature, but is probably less strain rate sensitive (i.e., its elongation does notrise as rapidly at slow strain rates).22 At a strain rate of 6.2 × 10−4 s−1, strain hardeningand softening rates were by far the slowest for 96.5Sn-3.5Ag and the fastest for63Sn-37Pb, with eutectic Sn-Bi in between.22

Figure 13.2 shows the tensile stress-strain behavior of several alloys measured at6.56 × 10−4/s and 300 K, as reported by Hwang.4 In this case, the eutectic Sn-Ag dis-plays a lower tensile strength but a higher elongation than eutectic Sn-Pb.

The creep rate of eutectic Sn-Ag is comparable with Sn-Ag-Cu and Sn-Ag-Bi,but is much lower than that of 60Sn-40Pb at low stress, suggesting a higher creepstrength associated with those Pb-free solders. However, the difference rapidlydiminishes with increasing stress, as indicated by Fig. 13.3.23 The National Institute ofStandards and Technology (NIST) also reported that the creep strength of 96.5Sn-3.5Ag is comparable with Sn-Ag-Cu and is higher than eutectic Sn-Cu or Sn, asshown in Table 13.3.8

The high creep strength of 96.5Sn-3.5Ag is also reflected in Grusd’s creep-rupture study.24 At a given applied stress, time to failure increases in the followingorder: 60Sn-40Pb < 99.3Sn-0.7Cu < 96.5Sn-3.5Ag < Sn-Ag-Cu at 25°C, as shown inFig. 13.4.24 At 100°C, 96.5Sn-3.5Ag exhibits the highest creep resistance, followed indecreasing order by Sn-Ag-Cu, 99.3Sn-0.7Cu, and 60Sn-40Pb, as shown in Fig. 13.5.24

This is consistent with the data reported earlier, where stress rupture life at roomtemperature of eutectic Sn-Ag, Sn-Bi, Sn-In, and Sn-Pb solders as a function ofapplied stress decreases in the following order: Sn-Ag > Sn-Bi > Sn-Pb > Sn-In.22

13.2 CHAPTER THIRTEEN

TABLE 13.1 Some Physical Properties of 96.5Sn-3.5Ag, 99.3Sn-0.7Cu, and 63Sn-37Pb

Property 96.5Sn-3.5Ag 99.3Sn-0.7Cu 63Sn-37Pb

Melting temperature (°C) 221 227 183

Surface tension (dyne/cm) 460 at 260°C; 431 at 491 at 277°C (air); 380 at 260°C; 417at 260°C 271°C (air); 493 at 461 at 277°C at 233°C (air); 464

271°C (nitrogen) (nitrogen) at 233°C (nitrogen)

Density (gm/cm3) 7.36 7.31 8.36

Electrical resistivity (µΩ-cm) 10.8 10–15 15.0

Thermal conductivity 0.33 at 85°C — 0.509 at 30°C;(W/cm·°C) 0.50 at 85°C

Coefficient of thermal expansion 30 — 25(CTE) at 20°C (ppm/K)

Hardness 16.5 [Vickers hardness — 12.8 (HV); 17(VH)]; 40 (Brinell) (Brinell)

PREVAILING LEAD-FREE ALLOYS 13.3

TABLE 13.2 Tensile and Shear Properties of 96.5Sn-3.5Ag, 99.3Sn-0.7Cu, and 63Sn-37Pb

99.3Sn-Property 96.5Sn-3.5Ag 0.7Cu 63Sn-37Pb Notes/References

Ultimate tensile 35 23 46 Ref. 4strength, — — 46 Ref. 5in MPa 61.4 — 45.1 Ref. 6

52 (as drawn) — — Ref. 754.6 (annealed) — — Ref. 7

58 — — Ref. 861.4 — 49.2 Ref. 9

69.6 (Sn-5Ag) — — Ref. 931.7 (Sn-5Ag) — — Ref. 10

— — 40.7 Ref. 11— — 33.92 Ref. 11

55 — 31–46 Ref. 1226.7 — 30.6 Ref. 13— — 26.7 Ref. 2

20–56 — 19–56 Ref. 3

Yield strength, 49 37 37 Ref. 14in MPa 24.8 (Sn-5Ag) — — Ref. 10

— — 28.1 Ref. 11— — 30.2 Ref. 11

22.5 — 27.2 Ref. 13

Young’s — — 38.1 (−70°C) Ref. 15modulus, — — 30.2 (20°C) Ref. 15in GPa — — 19.7 (140°C) Ref. 15

— — 32 Ref. 16— — 33.58 Ref. 11

56 — 35 Ref. 1726.2 — 15.7 Ref. 13— — 31.03 Ref. 2

Elongation, 39 45 31 Ref. 4in % 38.9 — 35.5 Ref. 6

— — 43.66 Ref. 11— — 52.87 Ref. 11

35 — 35–176 Ref. 1224 — 48 Ref. 13— — 35 Ref. 2

Shear strength, 27 20–23 23 At 0.1 mm/min, 20°C; Ref. 8in MPa 17 16–21 14 At 0.1 mm/min, 100°C; Ref. 8

At 0.1 mm/min; gap61.2 29.8 36.5 thickness: 76.2 µm;

(Sn-40Pb) cooling rate = 10°/s,tested at 22°C; Ref. 18

At 0.1 mm/min; gap20.5 10.1 4.5 thickness: 76.2 µm;

(Sn-40Pb) cooling rate = 10°/s;tested at 170°C; Ref. 18

39 28.5 34.5 (Sn-1Cu) (Sn-40Pb) At 1 mm/min at reflow

temperature (RT); Ref. 19

Stress (MN m−2) to rupture in 1000 h as a function of temperature for eutectic Sn-Agand Sn-Pb, Sn-Pb-Ag is as follows:22

96.5Sn-3.5Ag. 14 (at 20°C), 4 to 5.5 (at 100°C), 3.9 (at 125°C), and 2.3 (at 150°C)60Sn-40Pb. 3.5 to 4 (at 20°C) and 1 to 1.1 (at 100°C)62Sn-36Pb-2Ag. 5 (at 20°C) and 1 (at 100°C)

Hwang et al.25 reported that the creep resistance, in descending order, was 62Sn-36Pb-2Ag > 96.5Sn-3.5Ag > 63Sn-37Pb > 58Bi-42Sn > 60Sn-40Pb > 70Sn-30In >60In-40Sn at room temperature.

96.5Sn-3.5Ag absorbed considerably more strain before failure than 63Sn-37Pb.The acceleration factor in a thermal cycling test versus field service will be greaterfor 96.5Sn-3.5Ag than for 63Sn-37Pb.22 96.5Sn-3.5Ag has far superior room-temperature isothermal fatigue behavior to 63Sn-37Pb at high shear strain ampli-tudes, due to the resistance of 96.5Sn-3.5Ag to fatigue crack initiation, but is farinferior to 63Sn-37Pb at low strain amplitudes.22,26 The creep activation energy of the96.5Sn-3.5Ag is higher for equivalent stresses and temperatures than the value ofthe eutectic tin-lead alloy, as reported by Villain.27


TABLE 13.2 Tensile and Shear Properties of 96.5Sn-3.5Ag, 99.3Sn-0.7Cu, and 63Sn-37Pb (Continued)

99.3Sn-Property 96.5Sn-3.5Ag 0.7Cu 63Sn-37Pb Notes/References

Shear strength, 21.2 21.6 At 1 mm/min at 100°C; Ref. 19in MPa 23.5 (Sn-1Cu) (Sn-40Pb)

54.95 — 40.27 By ring-and-plug test;Refs. 2, 6, 12, 20, 21

31.7; 57.9 (Sn-5Ag) — 41.832.1 — 28.4

— — 23.837.8 — 48.4

FIGURE 13.1 Comparison of elongation of 96.5Sn-3.5Ag and 63Sn-37Pb as functions of strain rate at room temperature.


FIGURE 13.2 Tensile test results of several solder alloys (96.5Sn-3.5Ag, 99.3Sn-0.7Cu,93.3Sn-3.1Ag-3.1Bi-0.5Cu, and 63Sn-37Cu). The test condition is 6.56 × 10−4/s and 300 K.

FIGURE 13.3 Creep behavior of several eutectic or near-eutectic solder alloys.

13.1.3 Wetting Properties

The wetting times determined with the use of wetting balance for several solderalloys are shown in Fig. 13.6, using an unactivated flux and copper coupons.28 Thewetting ability descends in the following order: eutectic SnPb > SnAgCu > SnAg >SnCu under both air and nitrogen when tested at the same temperature, with nitro-gen atmosphere always yielding a shorter wetting time than air atmosphere.28 On theother hand, Glazer reported that the wetting ability in three out of four casesdecreases in the following order: 60Sn-40Pb > 100Sn > 95.5Sn-4Ag-0.5Cu > 95Sn-5Sb > 96.5Sn-3.5Ag when tested at 260 to 280°C, as shown in Fig. 13.7 in a contactangle study and in Fig. 13.8 in a wetting time study.22,28

The difference in wetting time as a function of superheat temperature due to avariation in alloys or atmosphere diminishes if an activated flux is used on coppersheet, as shown in Fig. 13.9.28 However, it should be noted that while this may be trueunder certain conditions for wetting balance test results, considerable differenceamong those alloys is reported for solder paste reflow performance by Huang andLee.29 On a scale of 0.0 to 10 for full spreading, the wettability of alloys at reflowincreases in the following order: 89Sn-8Zn-3Bi (0.5) < 96.5Sn-3.5Ag (4.6) < 95Sn-5Sb (4.7) < 99.3Sn-0.7Cu (5.2), 96.2Sn-2.5Ag-0.8Cu-0.5Sb (5.2) < 93.6Sn-4.7Ag-1.7Cu (5.3) < 95.5Sn-3.8Ag-0.7Cu (5.4) < 58Bi-42Sn (6.0) < 91.7Sn-3.5Ag-4.8Bi(6.8) < 90.5Sn-7.5Bi-2Ag (7.0) < 63Sn-37Pb (9.8).

The wetting behavior of alloys is affected by surface finishes as well.Therefore, foreasily wettable surfaces, such as Sn-Pb surface finish on a small-outline integrated cir-cuit (SOIC), both eutectic Sn-Ag and eutectic Sn-Pb exhibit virtually identical wet-


FIGURE 13.4 Creep-rupture data for several candidate lead-free alloys com-pared to 60Sn-40Pb at 25°C.

TABLE 13.3 Creep Strength* of Several Lead-Free Solders

95.8Sn-3.5Ag- 95.5Sn-3.8Ag-Temperature Sn 96.5Sn-3.5Ag 99.3Sn-0.7Cu 0.7Cu 0.7Cu

20°C 3.3 13.7 8.6 13 13

100°C 1 5 2.1 5 5

* Creep strength (N/mm2) determined at 0.1 mm/min.

ting time under air at the same superheat using an activated flux. However, if the sur-face is less wettable, such as a Pd-Ni surface finish, the wetting time for eutectic Sn-Ag solder becomes considerably longer even if an activated flux is used, while that ofSn-Pb solder still remains about the same as that of the Sn-Pb surface finish.28

The wetting time of eutectic Sn-Ag may also be shorter than eutectic Sn-Pb,depending on the test condition, as shown in Fig. 13.10.30 At 260°C, the wetting timedescends in the following order: 96Sn-2.5Ag-1Bi-0.5Cu > 96.2Sn-2.5Ag-0.5Sb-0.8Cu > 63Sn-37Pb > 99.3Sn-0.7Cu > 96.5Sn-3.5Ag > 95.5Sn-4Ag-0.5Cu. Melton alsoreported that 96.5Sn-3.5Ag wets better than 63Sn-37Pb and is less sensitive toreflow atmosphere than 63Sn-37Pb.31


FIGURE 13.5 Creep-rupture data for several candidate lead-free alloys com-pared to 60Sn-40Pb at 100°C.

FIGURE 13.6 Wetting times as a function of temperature using copper coupons with a range of sol-der alloys and unactivated flux. (a) Air and (b) nitrogen.28


FIGURE 13.7 Contact angle of 60Sn-40Pb, 100Sn, 95Sn-5Sb, 96.5Sn-3.5Ag, and95.5Sn-4Ag-0.5Cu at 260 to 280°C using four different fluxes.

FIGURE 13.8 Wetting time of 60Sn-40Pb, 100Sn, 95Sn-5Sb, 96.5Sn-3.5Ag, and95.5Sn-4Ag-0.5Cu at 260 to 280°C using four different fluxes.

Loomans et al. studied the contact angle of multicomponent lead-free soldersand reported that for binary eutectic solders, the contact angles using rosin-IPA fluxare: Sn-Bi, 40° (166°C); Sn-Zn, 60° (225°C); Sn-Ag, 45° (250°C).32 Vianco et al.reported the following contact angle values: 96.5Sn-3.5Ag, 60 to 75°; 95Sn-5Sb and95.5Sn-4Cu-0.5Ag, 35 to 55°; 60Sn-40Pb, 20 to 35°. The high contact angle of 96.5Sn-3.5Ag is probably related to the high surface tension of Ag,33 as well as the inabilityof flux to significantly lower the solder-flux interfacial tension.34 Here, 96.5Sn-3.5Agwas also noted to have a slower wetting than the rest of the alloys.

The poor wetting of 96.5Sn-3.5Ag is consistent with the observation of Melton etal.35 In that study, compared with 63Sn-37Pb, the wetting of Sn on Cu is superior,95.5Sn-4Cu-0.5Ag and eutectic Sn-Bi is acceptable, while 96.5Sn-3.5Ag is quitepoor. Wetting of 96.5Sn-3.5Ag did not improve significantly in inert atmosphere,perhaps because Ag is not readily oxidized. On a Ni-Au-plated substrate, 96.5Sn-3.5Ag and 58Bi-42Sn are acceptable on wetting, but poorer than 63Sn-37Pb.22 Onthick film Au76-Pt21-Pd3 over Al2O3, eutectic Sn-Ag also wets more poorly thaneutectic Sn-Pb.36


FIGURE 13.9 Wetting times as a function of superheat using copper coupons with a range of sol-der alloys and 0.5 percent activated flux. (a) Air and (b) nitrogen.

FIGURE 13.10 Effect of solder alloy and solder temperature on wetting time on oxi-dized copper.

(a) (b)

Suganuma reported the wetting area decreases in the following order: 63Sn-37Pb > 96.5Sn-3.5Ag > 75Sn-25Bi > 100Sn > 91Sn-9Zn. Wetting area of Sn increaseswith increasing doping level (up to 4 percent) of Ag, but decreases with increasingdoping level (up to 9 percent) of Zn.37 The relative wetting performance of 96.5Sn-3.5Ag and 100Sn contradicts the observation of Melton,35 as described earlier.

The relative poorer wetting performance of eutectic Sn-Ag versus Sn-Pb is alsoreflected in the capillary flow test, as reported by Vianco et al.38 Here the capillaryrise of 96.5Sn-3.5Ag (2.0 cm) is lower than that of 60Sn-40Pb (2.8 cm) at the 0.025-cm gap, and is 1.8 cm versus infinity at the 0.008-cm gap. The rise rate (dyne/s) is 29versus 32 and is consistent with the rise data. However, the void area (percent) of96.5Sn-3.5Ag appears to be equal or smaller than 60Sn-40Pb, as indicated by 3.7 ver-sus 3.8 at the 0.025-cm gap, and 11.9 versus 14.9 at the 0.008-cm gap.

For 96.5Sn-3.5Ag and 63Sn-37Pb, at a lower soldering temperature Cu6Sn5 for-mation dominates, while at a higher temperature the Cu3Sn layer is much thicker.Activation energy for Cu3Sn growth is 58 kJ mol−1, and for the total compound layerit is 21 kJ mol−1.22 Au dissolves more rapidly into eutectic Sn-Ag and Sn than intoeutectic Sn-Pb solder for the same amount of superheating.22,39–42

13.1.4 RELIABILITY

Eutectic Sn-Ag is more tolerant of Au than eutectic Sn-Pb. 96.5Sn-3.5Ag containing5 percent Au is ductile and elongation deteriorated only very little due to muchsmaller AuSn4 intermetallic compound (IMC) grain size. 63Sn-37Pb containing 5%Au is brittle and the elongation decreased dramatically.22,39,40

96.5Sn-3.5Ag on Cu with thin IMC layers fractured at or near the solder-Cu6Sn5

layer. For joints with thicker IMC layers, fracture occurred at the Cu6Sn5-Cu3Sninterface.43 Glazer reported that for 95Sn-5Ag, no microstructural coarseningoccurred and only the IMC layer thickness increased. Cracks propagated at thesolder-intermetallic interface and through the solder. However, no complete failureswere observed after 70 cycles.22 The fatigue test results indicate that the fatigueresistance of alloys can be ranked in increasing order: 63Sn-37Pb < 64Sn-36In <58Bi-42Sn < 50Sn-50In < 99.25Sn-0.75Cu < 100Sn < 96Sn-4Ag.22

The microstructures of various Pb-free solder-Cu interfaces has been examinedprimarily by Suganuma.37 Most Sn alloys, including pure Sn, Sn-Ag, Sn-Bi, or theirternary alloys, form two IMCs at the interfaces with Cu [i.e., Cu6Sn5 (15 µm) andCu3Sn (5 µm)].The former is much thicker than the latter, and the interface integrityis strongly influenced by the presence of the Cu6Sn5 layer.37

A study of Siow et al. showed that 63Sn-37Pb solder joint has a higher toughnessthan that of 96.5Sn-3.5Ag.This trend could be attributed to the sharp Ag-rich phasespresent in the latter. Both types of solder joints failed by fracture through the solderinstead of yielding, though there are signs of local plasticity. The failure mode wasprimarily microvoid nucleation and coalescence. Equiaxed dimples were observedin the fracture surface of the mode I loaded sample while elongated dimples wereobserved in the fracture surface of the mixed-mode loaded sample. Both solderspreferred to fail in the shear mode.44

The temperature cycling performance of Pb-free solder joints has been reported ina number of works.Table 13.4 shows the brief summary on results of those work.

Therefore, depending on the applications, the reliability of eutectic Sn-Ag mayrange from the best to the worst when compared with other alloys studied in thoseworks. The performance of eutectic Sn-Ag appears to be comparable with eutecticSn-Pb, with three cases being better46,48 and four cases being poorer14,50–52 than Sn-Pb.


TABLE 13.4 Temperature Cycling Performance of Sn-Ag-Bi, Sn-Ag-Bi-In, 96.5Sn-3.5Ag, 99.3Sn-0.7Cu, and SnPb—The Performance May Also BeRanked in Descending Order for Some Works, with 1st Being the Best

Applica- Notes/Test condition tion SnPb 96.5Sn-3.5Ag 99.3Sn-0.7Cu Sn-Ag-Bi Sn-Ag-Bi-In References

Pull strength, QFP 1st 2nd (Sn-Ag-3Bi), ∼1st; Ref. 45−40/+85°C with 3rd (Sn-Ag-6Bi, or

90Sn10 -10Bi, or -15Bi)Pb leadfinish

Pull strength QFP 2nd; 3.1/0 1st (93.5Sn-3.5Ag- Ref. 45(Kgf), cycle, 3Bi) 2/0 cycle,−40/+85°C 2.1/200 2.1/200 cycles,

cycle, 2.2/500 cycles on1.9/500 Pd-Nicycle

−40/+80°C, SMD 3rd 2nd Ref. 46crack

0/+100°C SMD 2nd 1st (91.84Sn-3.33Ag- Ref. 4710,000 cycles, 4.83Bi), no electricalshear strength failure, shear strength

higher than freshSn63

−50/+150°C PBGA 1× >2× Sn-Pb (but Ref. 48high moduluscauses pad tracefracture)

0/100°C, 30-min CBGA Poorest 96.5Sn-3.5Ag-3Bi ∼ Ref. 49cycle 95.5Sn-3.8Ag-0.7Cu

> 63Sn-37Pb

13.1

1

TABLE 13.4 Temperature Cycling Performance of Sn-Ag-Bi, Sn-Ag-Bi-In, 96.5Sn-3.5Ag, 99.3Sn-0.7Cu, and SnPb—The Performance May Also BeRanked in Descending Order for Some Works, with 1st Being the Best (Continued)

Applica- Notes/Test condition tion SnPb 96.5Sn-3.5Ag 99.3Sn-0.7Cu Sn-Ag-Bi Sn-Ag-Bi-In References

0/100°C, 60-min CBGA Poorest 96.5Sn-3.5Ag-3Bi ∼ Ref. 49cycle 95.5Sn-3.8Ag-0.7Cu

> 63Sn-37Pb

0/100°C, 240- CBGA Poorest 96.5Sn-3.5Ag-3Bi > Ref. 49min cycle 95.5Sn-3.8Ag-0.7Cu

≥ 63Sn-37Pb

−40/+125°C, 42- CBGA 1× 96.5Sn-3.5Ag-3Bi ≥ Ref. 49min cycle, 95.5Sn-3.8Ag-0.7Cufatigue life ≥ 63Sn-37Pb

−40/+125°C, CBGA 1×, poorer 96.5Sn-3.5Ag-3Bi Ref. 49240-min cycle, than 95.5Sn- slightly poorer fatigue life 3.8Ag- than 63Sn-37Pb

0.7Cu

−40/+60°C, 30- CBGA Poorest 95.5Sn-3.8Ag-0.7Cu Ref. 49min cycle, ≥ 96.5Sn-3.5Ag-3Bifatigue life > 63Sn-37Pb

−50/+150°C, PBGA Sn-Ag ball/Sn-Pb Ref. 48fatigue life paste > Sn-Ag

ball/SAC paste Sn62 ball/SACpaste > Sn62ball/Sn63 paste

−40/+125°C, PBGA Sn-Ag ball/Sn63 Ref. 48fatigue life paste > Sn-Ag

ball/SAC paste Sn62 ball/SACpaste, Sn62ball/Sn63 paste

13.1

2

−40/+125°C, FlexBGA 6th–7th 8th Ref. 50*fatigue life

Flip chip 2nd Best on Ref. 14cracking

Fatigue life Flip 30–60% of Sn63 Ref. 51chip,unfilled

Fatigue life Flip chip 2nd 3rd 1st Ref. 52

0/+100°C General 2nd (on 3rd 1st Ref. 14NiP-Au)

−40/+85°C, General 1st 2nd (Sn-Ag-3Bi), ∼1st Ref. 45tensile test

−40/+85°C, General 2nd ∼1st 1st (Sn-Ag-3Bi) Ref. 45shear test

Fatigue cycle General 1st 2nd, close Ref. 53life to 1st

Fatigue cycle General 3rd 4th (3Bi), 7th (4.8Bi), 2nd Refs. 54 and life 8th (7.5Bi) (2.5In- 55, with

2.5Bi), Sn-Ag-Cu6th (3In- best in0.5Bi) cycle life

* CBGA, ceramic ball grid array; PBGA, plastic ball grid array; QFP, quad flat pack; SAC, Sn-Ag-Cu; SMD, surface-mount device.† Fatigue life: 1st (3.5Ag-1.5In), 2nd (Sn-2.5Ag-0.8Cu-0.5Sb), 3rd (Sn-4Ag-1Cu); 4th (Sn-4.6Ag-1.6Cu-1Sb-1Bi), 5th (Sn-4Ag-0.5Cu), 6th (Sn-3.4Ag-1Cu-3.3Bi) ∼ 7th.

13.1

3

Plastic ball grid arrays (PBGAs) with eutectic Sn-Ag spheres performed a mini-mum of two times better than Sn-Pb-Ag spheres in two automotive thermal cyclingconditions, −50 to 150°C and −40 to 125°C,48 as shown in Figs. 13.11 and 13.12. Thehigher modulus of the Sn-Ag solder balls appeared to put more stress on the tracesconnected to non-solder-mask-defined (NSMD) motherboard pads resulting in frac-tures and opens, but only in the most severe cycling condition of −50 to 150°C. ForPb-free spheres, the Sn-Ni IMC appeared to grow at the same rate in both 125 and150°C. whereas the growth rate in the Pb-containing PBGAs baked at 150°C was 2.6times of that at 125°C.

Shangguan reported that 96.5Sn-3.5Ag eutectic solder has superior overall prop-erties and is suitable for solder interconnects in thick-film automotive electronicspackages when used with a mixed bonded Ag conductor.56 For 0.4-mm-pitchsurface-mount technology (SMT) assembly applications,Artaki et al. concluded thateutectic Sn-Ag, together with eutectic Sn-Bi and 91.8Sn-4.8Bi-3.4Ag, 77.2Sn-20In-2.8Ag, and 96.2Sn-2.5Ag-0.8Cu-0.5Sb, are all feasible, although with narrower pro-cessing windows.57 This narrow processing window is concurred by Yang et al.,58 whostudied the effect of processing conditions on joint quality, and concluded that lowsoldering temperatures, fast cooling rates, and short renew times are suggested forproducing joints with the best shear strength, ductility, and creep resistance.

13.2 EUTECTIC Sn-Cu

13.2.1 Physical Properties

Some physical properties of 99.3Sn-0.7Cu can be found in Table 13.1. The meltingtemperature of eutectic Sn-Cu is the highest among the prevailing Pb-free solders,suggesting a greater difficulty in adopting this alloy. Its surface tension, electricalresistivity, and density are comparable with eutectic Sn-Ag, presumably due to thehigh content of Sn in both alloys.


The tensile and shear properties of 99.3Sn-0.7Cu are shown in Table 13.2 and Fig.13.2. Eutectic Sn-Cu is lower in tensile strength but higher in elongation than botheutectic Sn-Ag and Sn-Pb, reflecting the softness and ductility of Sn-Cu. On theother hand, shear strength of Sn-Cu appears to be comparable with Sn-Pb, but lowerthan Sn-Ag.

The creep strength of eutectic Sn-Cu is higher than 100Sn, but lower than eutec-tic Sn-Ag and Sn-Ag-Cu at both 20 and 100°C, as shown in Table 13.3. The data areconsistent with the creep-rupture data shown in Figs. 13.4 and 13.5, where the timeto rupture increases in the following order: eutectic Sn-Ag, Sn-Ag-Cu < eutectic Sn-Cu < 60Sn-40Pb at 25 and 100°C.

13.2.3 WETTING PROPERTIES

The wetting properties of eutectic Sn-Cu and eutectic Sn-Ag are considered by Vin-cent et al. as having a great potential as replacements for Sn-Pb in wave and reflowprocesses.59 Wetting balance test results by Hunt et al., as shown in Fig. 13.6, indicate


FIGURE 13.11 Two-parameter Weibull plot as of 5619 thermal cycles of −50 to 150°C. No solder joint failures had been recorded on the configuration with Sn-Ag solder balls assembled with Sn-Pb paste.

13.1

5

FIGURE 13.12 Two-parameter Weibull plot as of 9187 thermal cycles of −40 to 125°C. Only one failure had been recorded on the configuration with Sn-Agsolder balls assembled with Sn-Pb paste, and this occurred at 7555 cycles.

13.1

6

that the wetting ability decreases in the following order: eutectic Sn-Pb > Sn-Ag-Cu > Sn-Ag > Sn-Cu when an unactivated flux is used.28 The difference in wettingvanishes when an activated flux is used and when the wetting time is plotted againstsuperheating, as shown in Fig. 13.9.28 However, depending on the test conditions ofthe wetting balance test, wetting time of eutectic Sn-Cu may also be shorter thaneutectic Sn-Pb, as demonstrated in Fig. 13.10.30 In the Prismark report, Nortel foundsoldering quality equal to eutectic Sn-Pb in Meridian desktop telephone manufac-turing. However, in air reflow the wettability was reduced, the fillet exhibited arough and textured appearance, and the flux residue was dark brown.60 Preferablythe use of eutectic Sn-Cu should be confined to wave soldering because low soldercost and inerting of waves is not costly.

At reflow, on a full scale of 0 to 10, the reflow spreading of eutectic Sn-Cu (5.2/10)is better than eutectic Sn-Ag (4.6/10), but it is considerably poorer than eutectic Sn-Pb (9.8/10), as reported by Huang and Lee.29 Toyoda also studied spreading per-formance of several alloys, and observed the following spreading behavior indecreasing order: 63Sn-37Pb > Sn-Ag-Cu-4.5Bi, Sn-Ag-Cu-7.5Bi > Sn-3.5Ag-0.75Cu > 99.25Sn-0.75Cu > 89Sn-8Zn-3Bi, as shown in Fig. 13.13.46

13.2.4 RELIABILITY

Although the tensile strength of eutectic Sn-Cu is fairly poor, the fatigue resistanceis fairly good. In Glazer’s study, the fatigue resistance increases in the followingorder: 63Sn-37Pb < 64Sn-36In < 58Bi-42Sn < 50Sn-50In < 99.25Sn-0.75Cu < 100Sn <96Sn-4Cu.22 However, the low-cycle isothermal fatigue (strain 0.2 percent, 0.1 Hz,R = 0.8, 300 K) performance shows a different trend, as shown in Table 13.5.4 Herethe number of cycles to failure for eutectic Sn-Cu is less than one-third of that foreutectic Sn-Pb.

Table 13.4 shows that for the two cases involving comparison of Sn-Cu with Sn-Pb,the former is consistently better than the latter.14,52 In addition, Syed studied the thermal cycling reliability of Pb-free solder joints for several package assemblies.50


FIGURE 13.13 Spreading performance of several Pb-free solders and eutec-tic Sn-Pb.

For a 27-mm, 256-PBGA assembly, at 0 to 100°C cycling, no failures in any alloyswere observed after 9730 cycles.At −55 to 125°C cycling, after 6830 cycles, more than50 percent failure rate occurred in eutectic Sn-Pb, Sn-Ag, and Sn-Cu. Eight failuresoccurred in Sn-3.4Ag-0.7Cu (30 percent higher life performance than Sn-Pb), andone failure at 6288 cycles occurred in Sn-4Ag-0.5Cu. At −40 to 125°C cycling, after5080 cycles, Sn-Pb and Sn-Cu just started to fail, while no failures were observed inSn-Ag, Sn-4Ag-0.5Cu, and Sn-3.4Ag-0.7Cu.

For a 12-mm, 144-flexible ball grid array (fleXBGA) assembly, at 0 to 100°Ccycling, eutectic Sn-Cu is 1.4 times better in performance than Sn-Pb, as shown inFig. 13.14.50 Sn-4Ag-0.5Cu and Sn-3.4Ag-0.7Cu are similar in performance, and bothare 1.6 to 1.7 times better in performance than eutectic Sn-Pb. Sn-Ag is the best, withno failure recorded after 10,740 cycles.At −55 to 125°C cycling, eutectic Sn-Cu is bet-ter in performance than Sn-Pb, as shown in Fig. 13.15.50 Sn-4Ag-0.5Cu and Sn-3.4Ag-0.7Cu are comparable, both being 20 percent better in performance than Sn-Pb.Sn-Ag again is the best, with only three failures observed, and is 1.4 times better inperformance than Sn-Pb. At −40 to 125°C cycling, Sn-Cu is similar to Sn-Pb in per-formance, as shown in Fig. 13.16.50 Not much improvement is observed for Sn-Ag-Cuover Sn-Pb, with Sn-4Ag-0.5Cu being slightly better than Sn-3.4Ag-0.7Cu. EutecticSn-Ag again comes to the top and is 1.4 times better in performance than Sn-Pb.

For ball grid array (BGA) assembly, eutectic Sn-Cu may seem to be inferior toeutectic Sn-Ag in temperature cycling performance; however, the opposite trend isobserved for flip chip assembly, as shown in Table 13.4. In Maestrelli’s study, eutec-tic Sn-Cu is observed to be the best in temperature cycling (0 to 100°C) perform-ance, with Sn-4Ag-0.5Cu and eutectic Sn-Pb being the next, while eutectic Sn-Agturns out to be the poorest in performance, as shown in Fig. 13.17.14 The higher reli-


TABLE 13.5 Relative Performance in Fatigue Resistance of Lead-Free Solders in Low-Cycle Isothermal Fatigue Test*

Alloy Melting temperature (°C) Nf†

88.5Sn-3Ag-0.5Cu-8In 195–201 19,501

91.5Sn-3.5Ag-1Bi-4In 208–213 12,172

92.8Sn-0.7Cu-0.5Ga-6In 210–215 10,800

95.4Sn-3.1Ag-1.5Cu 216–217 8,936

96.2Sn-2.5Ag-0.8Cu-0.5Sb 216–219 8,751

95.5Sn-3.5Ag-1Bi 219–220 8,129

94.5Sn-0.5Ag-2Cu-3Sb 218–232 7,120

95Sn-2Cu-3Sb 227–234 6,821

93.3Sn-3.1Ag-3.1Bi-0.5Cu 209–212 6,522

96.5Sn-0.5Ag-3Bi 223–226 4,283

96.5Sn-3.5Ag 221 4,186

92Sn-3.3Ag-4.7Bi 210–215 3,850

63Sn-37Pb 183 3,650

91.7Sn-3.5Ag-4.8Bi 211–215 3,179

99.3Sn-0.7Cu 227 1,125

* Strain, 0.2 percent; 0.1 Hz; R = 0.8; 300 K.† Nf: number of cycles to failure at 300 K (50 percent load drop, 0.2 percent strain range).


FIGURE 13.14 Temperature cycling (0 to 100°C) performance for 12-mm 144-fleXBGA assembly.


FIGURE 13.15 Temperature cycling (−55 to 125°C) performance for 12-mm 144-fleXBGAassembly.


FIGURE 13.16 Temperature cycling (−40 to 125°C) performance for 12-mm 144-fleXBGAassembly.

ability of eutectic Sn-Cu in flip chip applications is attributed to its compliant nature.Figure 13.18 shows the cross-section of flip chip solder joints after the same numberof thermal cycles.14 Both Sn-Pb and Sn-Ag-Cu display a fracture without solder jointdeformation. Sn-Cu solder joints, on the other hand, exhibits a deformed solderinterconnect.This deformation of solder material due to the compliant nature of sol-


FIGURE 13.17 Plot of fatigue life as a function of thermalstrain for a variety of solders over a temperature range of 0to 100°C.

FIGURE 13.18 Flip chip solder interconnects after the same number of thermal cycles for eutecticSn-Pb, Sn-Cu, and Sn-Ag-Cu.

der is considered helpful in offsetting the impact of mismatch in the coefficient ofthermal expansion (CTE), and hence is the main reason for Sn-Cu to be superior infatigue life performance under an application with a large mismatch in CTE. Frearet al. also reported that for flip chip assembly, the thermal fatigue life descends in thefollowing order: eutectic Sn-Cu > Sn-3.8Ag-0.7Cu, eutectic Sn-Pb > eutectic Sn-Ag.52

13.3 Sn-Ag-Bi AND Sn-Ag-Bi-In

As discussed in an earlier chapter, Sn-Ag-Bi is favored by Brite Euram, Departmentof Trade Industry of UK, NEMI of USA, and JEIDA. Sn-Ag-Bi-In is not investigatedas extensively as Sn-Ag-Bi system. However, it is recommended by JEIDA, and is oneof the major Pb-free alloys used for reflow soldering in Japan, as shown in Fig. 12.10.

13.3.1 PHYSICAL AND MECHANICAL PROPERTIES

The equilibrium phase diagram of the Sn-Ag-Bi system is shown in Fig. 13.19.61 Theternary eutectic point exhibits a melting temperature around 138°C, which is fairlycomparable with the binary eutectic Sn-Bi alloy. For the ternary Sn-Ag-Bi composi-tion with a narrow pasty range and a melting temperature close to 63Sn-37Pb, the


FIGURE 13.19 Equilibrium phase diagram for Sn-Ag-Bi system.

desirable composition can be prescribed by the shaded area in the lower left corner.Unlike the ternary eutectic point where the composition can be narrowed down eas-ily, the desirable Sn-Ag-Bi composition for the higher melting temperature rangecannot be identified easily. The most favorable composition appears to contain 1 to5 at % Bi and 1 to 4 at% Ag, with the balance as Sn, as shown in Table 13.6. Alsoshown in Table 13.6 are the physical and mechanical properties of Sn-Ag-Bi and Sn-Ag-Bi-In alloys, with eutectic Sn-Pb included for comparison. In most instances, thedata listed from the same source were determined under the same test conditions.

Most of the Sn-Ag-Bi alloys exhibit a pasty range from 210 to 220°C. Few alloyshave a lower solidus temperature, but a wider pasty range, such as 90.5Sn-2Ag-7.5Bi.Additional Sn-Ag-Bi melting temperature information can be found in Fig. 12.6. In allinstances except for 96.5Sn-0.5Ag-3Bi, the melting temperature is higher than that of63Sn-37Pb, but lower than eutectic Sn-Cu (227°C) or Sn-Ag (221°C), suggesting aslight advantage on soldering temperature. On the other hand, the surface tension ofSn-3.5Ag-4.8Bi is higher than 63Sn-37Pb, implying a disadvantage in solder spreading.

The density of Sn-3.4Ag-4.8Bi (7.53 g/cm3) is close to that of pure Sn (7.3gm/cm3), due to the high content of Sn. The lower density of Sn-Ag-Bi system thaneutectic Sn-Pb (8.4 g/cm3) indicates that a reduced solder weight of the joints can beexpected. The hardness of Sn-Ag-Bi, as demonstrated by Sn-3Ag-5Bi (29.9 HV), isconsiderably higher than both eutectic Sn-Pb (12.9 HV) and Sn-Ag (16.5 HV) (seeTable 13.1), and may pose a greater resistance against solder deformation duringtemperature cycling of assemblies.

Tensile strength, yield strength, and shear strength of Sn-Ag-Bi systems are signif-icantly higher than eutectic Sn-Pb, while the elongation or plasticity is much lowerthan Sn-Pb. This is consistent with the high hardness of Sn-Ag-Bi, mentioned earlier.Baggio studied the Sn-Ag-Bi system for Panasonic Mini Disk Player applications.45

By replacing some Sn of 96.5Sn-3.5Ag with Bi, the pull strength decreases linearlywith increasing content of Bi, while the melting point decreases rapidly first, thendecreases at a slower linear rate at Bi content above 6 percent, as shown in Fig. 12.6.

Figs. 13.20 and 13.21 show examples of tensile stress-strain behavior for Sn-Ag-Biand Sn-Ag-Bi-In systems, respectively, with 63Sn-37Pb and 96.5Sn-3.5Ag includedfor comparison.4 Addition of In to Sn-Ag-Bi enhances either the strength or theplasticity and is considered the optimal lead-free solder by Hwang4 in the absence ofCu, if aiming for a melting temperature of less than 215°C. In the study by Tanaka et al. on lead-free solders for mobile equipment, the impact resistance of soldersdescends in the following order: Sn-3.5Ag-0.75Cu > Sn-3Ag-1In-0.7Cu > Sn-3.5Ag-0.5Bi-3In > 63Sn-37Pb > Sn-3.2Ag-3Bi-1.1Cu-Ge, Sn-Ag-X, as shown in Fig. 13.22.5

The superior impact resistance of Sn-Ag-Bi-In is attributed to the high plasticity ofsolder due to the presence of In.

The creep rate of eutectic Sn-Ag-Bi is slightly lower than that of Sn-Ag-Cu and ofSn-Ag, and it is much lower than that of 60Sn-40Pb and eutectic Sn-Bi at low stress,presumably due to the high hardness and high strength of the Sn-Ag-Bi system, asindicated by Fig. 13.3. In the creep rupture tests conducted by Tanaka et al., the creeprupture performance of Sn-3.5Ag-0.5Bi-3In, as well as Sn-3.5Ag-0.75Cu, Sn-3Ag-1In-0.7Cu, Sn-3.2Ag-3Bi-1.1Cu-Ge, and Sn-Ag-X, are all better than 63Sn-37Pb.5


Presence of Bi significantly improves the solder spreading properties of lead-freesolders. In the reflow soldering compatibility study done by Huang and Lee, the sol-der spreading performance at solder paste reflow increases in the following order:89Sn-8Zn-3Bi (0.5) < 96.5Sn-3.5Ag (4.6) < 95Sn-5Sb (4.7) < 99.3Sn-0.7Cu (5.2),



TABLE 13.6 Physical and Mechanical Properties of Sn-Ag-Bi, Sn-Ag-Bi-In, and 63Sn-37Pb Systems

Notes/Property Sn-Ag-Bi Sn-Ag-Bi-In 63Sn-37Pb References

Melting temperature 208–217 (93.5Sn- — 183 Ref. 13(°C) 3.5Ag-3Bi)

219–220 (95.5Sn- 208–213 (91.5Sn- — Ref. 43.5Ag-1Bi) 3.5Ag-1Bi-4In)—

216–220 (95Sn- — — Ref. 133Ag-2Bi)

215–221 (95.5Sn- — — Ref. 132.5Ag-2Bi)

223–226 (96.5Sn- — — Ref. 40.5Ag-3Bi)

219–220 (95.5Sn- — — Ref. 43.5Ag-1Bi)

210 (92Sn-3Ag- — — —5Bi)

202–215 (91.8Sn- — — Ref. 293.5Ag-4.8Bi)

191–215 (90.5Sn- — — Ref. 292Ag-7.5Bi)

213 (94Sn-3Ag- — — —3Bi)

Surface tension 420 (Sn-3.5Ag- — 380 (260°C) Ref. 62(dyne/cm) 4.8Bi)

Density (g/cm3) 7.53 (Sn-3.4Ag- — 8.4 Ref. 294.8Bi)

Electrical resistivity 11.6 (Sn-3Ag- 12.1 (Sn-3Ag-5Bi- 17 Ref. 63(µΩ-cm) 5Bi) 5In)

Hardness [Vickers 29.9 (Sn-3Ag- — 12.9 Ref. 63hardness (HV), 5Bi)kg/mm2]

Ultimate tensile 82.5 (92Sn-3.3Ag- — 46 6.56 × 10−4/s,strength (MPa) 4.7Bi) 300 K, Ref. 4

43 (95.5Sn-3.5Ag- — — 6.56 × 10−4/s,1Bi) 300 K, Ref. 4

54.7 (Sn-3Ag-2Bi) 106.0 (Sn-2Ag- 30.6 Ref. 139.8Bi-9.8In)

52.2 (Sn-2.5Ag- — — Ref. 132Bi)

92.7 (Sn-2.5Ag- — — Ref. 1319.5Bi)

71.4 (Sn-3.4Ag- — — Ref. 134.8Bi)

Yield strength, 0.2% 37.7 (Sn-3Ag-2Bi) 100.4 (Sn-2Ag- 27.2 Ref. 13(MPa) 9.8Bi-9.8In)

45.5 (Sn-2.5Ag- — — Ref. 132Bi)

83.2 (Sn-2.5Ag- — — Ref. 1319.5Bi)

46.3 (Sn-3.4Ag- — — Ref. 134.8Bi)

96.2Sn-2.5Ag-0.8Cu-0.5Sb (5.2) < 93.6Sn-4.7Ag-1.7Cu (5.3) < 95.5Sn-3.8Ag-0.7Cu(5.4) < 58Bi-42Sn (6.0) < 91.7Sn-3.5Ag-4.8Bi (6.8) < 90.5Sn-7.5Bi-2Ag (7.0) < 63Sn-37Pb (9.8).29 The figures in parentheses represent the scores of spreading, with ascore of 10 representing full spreading. 63Sn-37Pb exhibits the best spreading per-formance. Among the Pb-free alloys, except for Zn-containing solder, all Bi-containing solders, including 58Bi-42Sn, 91.7Sn-3.5Ag-4.8Bi, and 90.5Sn-7.5Bi-2Ag,display a better wetting than non-Bi-containing alloys. Among the three Bi-containing alloys, eutectic Sn-Bi is too low in melting temperature as a drop-inreplacement for 63Sn-37Pb, and Sn-Ag-Bi systems are the most attractive due totheir higher melting temperature and superior wettability.

13.3.3 RELIABILITY

Bi-containing Pb-free alloys are sensitive to the presence of Pb, due to the formationof a 52Bi-30Pb-18Sn ternary eutectic structure with a melting temperature of 96°Cin the solidified solder joint. The solder joints become weak in mechanical strengthwhen subjected to thermal cycling with the temperature exceeding 96°C because thelow-melting ternary eutectic phase accelerates grain growth and phase agglomera-tion.64 Bi-containing solders also tend to have a fillet-lifting phenomenon, particu-larly at the wave-soldering stage, therefore posing reliability concerns.65 Both failuremechanisms will be discussed in detail in Chap. 16.

Hwang studied the isothermal low-cycle fatigue performance of a series of Pb-freesolders, as shown in Fig. 13.4.4 Among the alloys investigated, Sn-Ag-Bi systems arecomparable or better than 96.5Sn-3.5Ag and 63Sn-37Pb, and are considerably betterthan 99.3Cu-0.7Sn. The low-cycle fatigue life performance (numbers in parentheses)can be ranked as follows: 95.5Sn-3.5Ag-1Bi (8129) > 96.5Sn-0.5Ag-3Bi (4283) >96.5Sn-3.5Ag (4186) > 92Sn-3.3Ag-4.7Bi (3850) > 63Sn-37Pb (3650) > 91.7Sn-3.5Ag-4.8Bi (3179) > 99.3Sn-0.7Cu (1125).


TABLE 13.6 Physical and Mechanical Properties of Sn-Ag-Bi, Sn-Ag-Bi-In, and 63Sn-37Pb Systems(Continued)

Notes/Property Sn-Ag-Bi Sn-Ag-Bi-In 63Sn-37Pb References

Elongation (%) 10 (92Sn-3.3Ag- 13 (90Sn-3.3Ag- 31 6.56 × 10−4/s,4.7Bi) 3Bi-3.7In) 300 K, Ref. 4

31 (95.5Sn-3.5Ag- 30 (91.5Sn-3.5Ag- — 6.56 × 10−4/s,1Bi) 1Bi-4In) 300 K, Ref. 4

30 (Sn-3Ag-2Bi) 7 (Sn-2Ag-9.8Bi- 48 Ref. 139.8In)

26 (Sn-2.5Ag-2Bi) — — Ref. 1317 (Sn-2.5Ag- — — Ref. 13

19.5Bi)16 (Sn-3.4Ag- — — Ref. 13

4.8Bi)

Shear strength 81.36 (Sn-3.33Ag- — 40.27 Ref. 36(MPa) 4.83Bi)

Impact strength — 48 (Sn-3.5Ag- 31 Ref. 5(J/cm2) 0.5Bi-3In)


FIGURE 13.20 Tensile stress-strain properties of 92Sn-3.3Ag-4.7Bi, 95.5Sn-3.5Ag-1Bi, 96.5Sn-3.5Ag, and 63Sn-37Pb at 300 K and 6.56 × 10−4/s.

FIGURE 13.21 Tensile stress-strain properties of 90Sn-3.3Ag-3Bi-3.7In, 91.5Sn-3.5Ag-1Bi-4In, 96.5Sn-3.5Ag, and 63Sn-37Pb at 300 K and 6.56 × 10−4/s.

It is interesting to note that all three In-containing alloys, including Sn-Ag-Bi-In,are ranked at the top of the list, with Sn-Ag-Cu or Sn-Ag-Cu-X being equal or betterthan Sn-Ag-Bi systems. Presumably, the superior low-cycle fatigue performance ofIn-containing alloys might be attributed to the enhanced plasticity introduced by In.

The temperature cycling fatigue performance of Sn-Ag-Bi and Sn-Ag-Bi-In sys-tems are shown in Table 13.4. Again, the performance is highly dependent on appli-cations. Therefore, in four cases,45,49,54,55 Sn-Ag-Bi systems are inferior to Sn-Pb or62Sn-36Pb-2Ag, while in eight other cases, the trend is opposite.45,47,49

Bradley studied the temperature cycling reliability of Sn-Ag-X systems, withresults shown in Fig. 13.23.54,55 In his results, all three Sn-Ag-Bi alloys are more inferiorthan 62Sn-36Pb-2Ag, and the performance descends with increasing Bi content: Sn-Ag-3Bi > Sn-Ag-4.8Bi > Sn-Ag-7.5Bi.The favorable performance of lower Bi contentis also observed by Baggio, as shown in Fig. 13.24.45 Here the pull strength of solderjoints is plotted as a function of number of cycles. Sn-Ag-Bi systems with 3 percent Biexhibit the best performance, and are comparable with Sn-Pb. For Sn-Ag-Bi alloyswith 6 to 15 percent Bi content, the pull strength dropped considerably, although itremains stable with an increasing cycle number.

The work of Vianco et al. indicates that Sn-Ag-Bi is far more superior than eutec-tic Sn-Pb in a thermal fatigue test.47 In their study, an evaluation was performed that


FIGURE 13.22 Charpy impact test results for several lead-free alloys and 63Sn-37Pb.

FIGURE 13.23 Reliability performance of Sn-Ag-X alternatives.

examined the reliability of surface-mount solder joints made with a 91.84Sn-3.33Ag-4.83Bi alloy. A 1206-chip capacitor, an SOIC gull-wing, and plastic leaded chip car-rier (PLCC) J-lead solder joints were thermally cycled between 0 and 100°C for1,000, 2,500, 5,000, or 10,000 cycles. Continuous dc signal monitoring did not revealany open events through the 10,000-cycle mark, and there was no evidence of solderdegradation for any of the evaluated solder joint geometries through 2,500 cycles.Thermal cycles of 5,000 and 10,000 caused some surface cracks and isolated through-cracks in the thinner reaches of the solder fillets, as shown in Fig. 13.25.47 However,it should be pointed out that regardless of the development of cracks, the Sn-Ag-Bistill exhibits a shear strength greater than the as-fabricated 63Sn-37Pb solder jointsafter 10,000 thermal cycles, as shown in Fig. 13.26.47

Bartelo reported that at the 0/100°C temperature cycling condition with cycletimes from 30 to 240 min, the joint reliability of 96.5Sn-3.5Ag-3Bi is comparable orbetter than 95.5Sn-3.8Ag-0.7Cu, which in turn is better than 63Sn-37Pb when assem-bling 625 I/O CBGA with 32 × 32 × 0.8 mm dimension.49 For temperature range −40/+125°C thermal cycling test, 96.5Sn-3.5Ag-3Bi was better than 63Sn-37Pb at 42min cycle time, but slightly poorer than Sn-Pb at a 240-min cycle time.49 For −40/60°Ctemperature cycling condition with 30-min cycle time, Sn-Ag-Cu is slightly betterthan Sn-Ag-Bi, and both are better than 63Sn-37Pb. In Bartelo’s study, the alloysused for solder ball and solder paste are the same.

The superior thermal cycling fatigue performance of the Sn-Ag-Bi system per-haps can be partly attributed to its high rigidity and thus low creep rate. This highrigidity is mainly caused by two solder-strengthening mechanisms. The first is solu-tion strengthening, achieved by dissolving 4 to 5 percent Bi into Sn. The second isprecipitation strengthening, achieved by Bi and Ag3Sn particles, as shown in Fig.13.27.47 The high concentration of Ag3Sn and Bi precipitates is also responsible forthe rough and grainy solder joint appearance, as shown in Fig. 13.28.47

Addition of In to the Sn-Ag-Bi systems significantly improves the fatigue resist-ance, as shown in Table 13.5 for isothermal low-cycle fatigue and in Fig. 13.21 fortemperature cycling performance. In the former case, the Sn-Ag-Bi-In is far supe-rior than eutectic Sn-Pb, while in the latter case, both systems are comparable inperformance.


FIGURE 13.24 Reliability of Sn-Ag-Bi and Sn-Pb sys-tems.


FIGURE 13.25 Maximum shear load (N) versus fillet-top cracking of the 1206-chip capacitor91.84Sn-3.33Ag-4.83Bi solder joints as a function of number of thermal cycles at condition 0/+100°C.Note that the cracks developed with an increasing number of cycles.

FIGURE 13.26 Maximum shear load (N) versus fillet-top cracking of the 1206-chip capacitor91.84Sn-3.33Ag-4.83Bi solder solder joints as a function of number of thermal cycles at condition0/+100°C. Note that the shear load of Sn-Ag-Bi is higher than or equal to as-fabricated 63Sn-37Pbjoints, even up to 10,000 thermal cycles.

13.4 Sn-Ag-Cu AND Sn-Ag-Cu-X

Sn-Ag-Cu is the most prevailing choice of Pb-free alternative globalwise. Besidesthat, the Sn-Ag-Cu-X family (including Sn-Ag-Cu-In, Sn-Ag-Cu-Bi, Sn-Ag-Cu-Sb—particularly the latter two) also received quite a lot of attention and will be intro-duced in more detail later.


The physical properties of Sn-Ag-Cu, Sn-Ag-Cu-In, Sn-Ag-Cu-Bi, Sn-Ag-Cu-Sb,and 63Sn-37Pb are shown in Table 13.7.

Figure 13.29 shows the Sn-Ag-Cu phase diagram, as reported by Handwerker.61

NIST experimental work showed that the ternary eutectic composition is approxi-


FIGURE 13.27 Microstructure of 91.84Sn-3.33Ag-4.83Bi. (a) Scanning electron microscope (SEM)photograph; (b) thermal expansion mismatch (TEM) photograph. The solder is strengthened by (1)solution strengthening (Sn with 4 to 5 percent Bi) and (2) precipitation strengthening (Bi and Ag3Snparticles).

FIGURE 13.28 The as-fabricated solder joints made with the 91.84Sn-3.33Ag-4.83Bi solder. (a)SEM micrograph; (b) optical micrograph.

(a) (b)

(a) (b)


TABLE 13.7 Physical Properties of Sn-Ag-Cu, Sn-Ag-Cu-X, and 63Sn-37Pb

63Sn- Sn-Ag- Sn-Ag- Sn-Ag- Notes/Properties 37Pb Sn-Ag-Cu Cu-In Cu-Bi Cu-Sb References

Melting 183 217 207–216 Ref. 66temperature (95.5Sn- (Sn-3.3(°C) 3.8Ag- Ag-3Bi-

0.7Cu) 1.1Cu)

216–217 195–201 209–212 216–219 Ref. 4(95.4Sn- (88.5Sn- (93.3Sn- (Sn-2.5Ag-3.1Ag- 3Ag- 3.1Ag- 0.8Cu-1.5Cu) 0.5Cu- 3.1Bi- 0.5Sb)

8In) 0.5Cu)

218–232 Ref. 4(94.5Sn-0.5Ag-2Cu-3Sb)

217–244 Ref. 67(93.6Sn-4.7Ag-1.7Cu)

217 Ref. 61(95.6Sn-3.5Ag-0.9Cu)

217–225 Ref. 68(95.5Sn-4Ag-0.5Cu)

218 Ref. 69(95.75Sn-3.5Ag-0.75Cu)

220 Ref. 70(96.5Sn-3Ag-0.5Cu)

Density 8.36 7.44 (Sn- 7.56 (Sn- 7.39 (Sn- Ref. 71(g/cm3) 4Ag- 2Ag- 2.5Ag-

0.5Cu) 0.5Cu- 0.8Cu-7.5Bi) 0.5Sb)

7.39 (Sn- Ref. 724Ag-0.5Cu)

8.4 7.5 (Sn- Ref. 83.8Ag-0.7Cu)

Electrical 17 10.6 (Sn- 12.1 (Sn- Ref. 63resistivity 3Ag-3Cu- 2.5Ag-(µΩ-cm) 2Bi) 0.8Cu-

0.5Sb)


TABLE 13.7 Physical Properties of Sn-Ag-Cu, Sn-Ag-Cu-X, and 63Sn-37Pb (Continued)

63Sn- Sn-Ag- Sn-Ag- Sn-Ag- Notes/Properties 37Pb Sn-Ag-Cu Cu-In Cu-Bi Cu-Sb References

14.5 10–15 (Sn- Ref. 34Cu-0.5Ag)

13 (Sn- Ref. 83.8Ag-0.7Cu)

Hardness 12.8 [Vickers hardness (HV), kg/mm2],Ref. 6

10.25 (as (HV), kg/mm2),drawn), Ref. 712.45(annealed)

Sn-4.7Ag-1.7Cu

12.9 34.5 (Sn- 28.6 (Sn- (HV, kg/mm2),3Ag-3Cu- 3Ag-2Cu- Ref. 632Bi) 2Sb)

10.08 18.28 (Sn- Rockwell2.5Ag- hardness, 15-W0.8Cu- scale hardness,0.5Sb) Ref. 11

12.2 13.5 (Sn- Rockwell2.5Ag- hardness, 16-W0.8Cu- scale hardness,0.5Sb) Ref. 11

15 (Sn- Brinell hardness,3.8Ag- Ref. 80.7Cu)

Surface 380 at 510 (Sn- Ref. 62tension 260°C 2.5Ag-(mN m−1) 0.8Cu-

0.5Sb)

417 (air), Ref. 3464 (N2)at 233°C

CTE (ppm) 18.74 14.83 (Sn- Ref. 83Ag-4Cu)

25 Ref. 2

21 Ref. 16

24(−70°C, Ref. 1520°C,140°C)

mately 95.6Sn-3.5Ag-0.9Cu (±0.1 percent), with a melting point of 217°C. Aroundthe ternary eutectic point, as shown by the shaded area in Fig. 13.29, there are a num-ber of Sn-Ag-Cu compositions developed, with melting temperatures all within 10°Cof the ternary eutectic melting temperature. Figure 13.30 shows several examples ofdifferential scanning calorimeter (DSC) thermograms of such alloys.73 Although theexact ternary eutectic composition is to be determined, generally it is consideredthat the physical, mechanical, and soldering properties, and the reliability of Sn-Ag-Cu compositions near this ternary eutectic point should be fairly comparable.

The density and electrical resistivity of Sn-Ag-Cu and Sn-Ag-Cu-X are all com-parable with each other, and are similar to other high-Sn alloys (see also Tables 13.1and 13.6), due to the dominant presence of Sn. The hardness, however, does differfrom system to system. Therefore, Sn-Ag-Cu is comparable with Sn-Pb, while Bi-containing alloys exhibit a considerably higher hardness than Sn-Pb. The high hard-ness can be explained by the precipitation and Bi-dissolution strengtheningmechanisms, discussed in Sec. 13.3.3.


The mechanical properties of Sn-Ag-Cu, Sn-Ag-Cu-In, Sn-Ag-Cu-Bi, Sn-Ag-Cu-Sb,and 63Sn-37Pb are shown in Table 13.8. Examples of tensile stress-strain relation at300 K and 6.56 × 10−4/s for Sn-Ag-Cu, Sn-Ag-Cu-In, and Sn-Ag-Cu-Bi alloys,together with 63Sn-37Pb for comparison, are shown in Figs. 13.31, 13.32, and 13.33,respectively.4 Some additional information on the time to break in creep tests forseveral other alloys is shown in Fig. 13.34.46

The tensile strength of Sn-Ag-Cu is slightly higher [e.g., Sn-3.8Ag-0.7Cu or Sn-3.5Ag-0.7Cu (Ref. 8)] or higher [e.g., Sn-4.7Ag-1.7Cu (Ref. 4)] than eutectic Sn-Pb.In the case of composition near the ternary eutectic point, such as Sn-3.8Ag-0.7Cu,


FIGURE 13.29 Sn-Ag-Cu phase diagram. Alloys inshaded area have a freezing range of less than 10°C.

its yield strength,14 shear strength,8,18 impact strength,5 and creep resistance8 are allhigher than Sn-Pb. Figure 13.34 shows that Sn-3.5Ag-0.75Cu exhibits the longesttime to break in creep tests among all of the alloys tested, including both Pb-free andSn-Pb alloys. For Sn-Ag-Cu alloys further away from ternary eutectic composition,such as 93.6Sn-4.7Ag-1.7Cu, not only does the melting temperature (217 to 244°C67)increase, but also the tensile4,7 and shear18 strengths increase, at the expense ofreduction in elongation.4

The data on the mechanical properties of Sn-Ag-Cu-In system is fairly scarce.Hwang studied the yield strength σy, tensile strength σT, and plastic strain εp at frac-ture versus Ag contents for the Sn-Ag-Cu-In system at both 0.5Cu-8In and 0.5Cu-4In, with results shown in Fig. 13.35.4 Results indicate that alloys with 8 percent Inexhibit a higher yield strength and tensile strength but a lower plastic strain at frac-ture than alloys with 4 percent In. For an 8 percent In system, the optimal Ag con-tent for tensile properties is 3 percent within the range of 0 to 4.1 percent Ag. Thedifference between 3 and 4.1 percent Ag alloys is fairly small, and all exhibit a highertensile strength but a lower elongation than 63Sn-37Pb, as shown in Fig. 13.32. For a4 percent In system, increasing Ag content from 3 to 4.1 percent results in a linearincrease in tensile strength and yield strength, but a decrease in plastic strain.Tanakaet al.5 reported that Sn-3Ag-0.7Cu-1In exhibits a tensile strength comparable withthat of 63Sn-37Pb. However, the former displays a considerably higher impactstrength and an exceptionally lower creep rate than eutectic Sn-Pb (see Table 13.8and Fig. 13.34).


FIGURE 13.30 Differential scanning calorimeter thermograms of fourSn-Ag-Cu solder alloys: (a) Sn-4Ag-0.5Cu, (b) Sn-3.6Ag-1Cu, (c) Sn-3.7Ag-0.9Cu, and (d) Sn-3Ag-0.5Cu.


TABLE 13.8 Mechanical Properties of Sn-Ag-Cu, Sn-Ag-Cu-In, Sn-Ag-Cu-Bi, Sn-Ag-Cu-Sb,and 63Sn-37Pb

Properties 63Sn-37Pb Sn-Ag-Cu Sn-Ag-Cu-In Sn-Ag-Cu-Bi Sn-Ag-Cu-Sb Notes/References

Ultimate 46 48.5 64 78 (93.3Sn- 36.5 (Sn- 6.56 × 10−4/s,tensile (95.4Sn- (87.4Sn- 3.1Ag-3.1Bi- 2.5Ag- 300 K, Ref. 4strength 3.1Ag- 4.1Ag- 0.5Cu) 0.8Cu-(MPa) 1.5Cu); 0.5Cu-8In); 0.5Sb)

75 63(93.6Sn- (88.5Sn-4.7Ag- 3Ag-1.7Cu) 0.5Cu-8In)

46 42 (Sn- 43 (Sn- Ref. 53.5Ag- 3Ag-1In-0.75Cu) 0.7Cu)

45.1 Ref. 6

44 (as Ref. 7drawn), 53(annealed)

93.6Sn-4.7Ag-1.7Cu

48 (for Ref. 8both Sn-3.8Ag-0.7Cu andSn-3.5Ag-0.7Cu)

49.2 Ref. 9

40.7 38.3 (Sn- Ref. 112.5Ag-0.8Cu-0.5Sb)

33.92 39.5 (Sn- Ref. 112.5Ag-0.8Cu-0.5Sb)

31–46 Ref. 12

30.6 48.3 (Sn- 92.7 (Sn-2Ag- 52.8 (Sn- Ref. 133Ag-4Cu); 7.5Bi-0.5Cu) 3Ag-2Cu-29.7 (Sn- 2Sb); 25.80.5Ag- (Sn-2.6Ag-4Cu) 0.8Cu-

0.5Sb);29.8 (Sn-0.2Ag-2Cu-0.8Sb)

26.7 Ref. 2

Yield 37 45 (Sn- Ref. 14strength 3.8Ag-(MPa) 0.7Cu)

28.1, 30.2 27.8 and 33.5 Ref. 11(Sn-2.5Ag-0.8Cu-0.5Sb)


TABLE 13.8 Mechanical Properties of Sn-Ag-Cu, Sn-Ag-Cu-In, Sn-Ag-Cu-Bi, Sn-Ag-Cu-Sb, and63Sn-37Pb (Continued)


27.2 43.3 (Sn- 85.3 (Sn-2Ag- 46.1 (Sn- Ref. 133Ag-4Cu); 7.5Bi-0.5Cu) 3Ag-2Cu-25.7 (Sn- 2Sb); 22.80.5Ag- (Sn-2.6Ag-4Cu) 0.8Cu-

0.5Sb);25.9 (Sn-0.2Ag-2Cu-0.8Sb)

Shear 23 27 (Sn- At 0.1 mm/min,strength 3.8Ag- 20°C, Ref. 8(MPa) 0.7Cu)

14 17 (Sn- At 0.1 mm/min,3.8Ag- 100°C, Ref. 80.7Cu)

36.5 (Sn- 67 (Sn- 64.1 (Sn- At 0.1 mm/min;40Pb) 3.6Ag- 3.8Ag- gap thickness:

1.0Cu); 58 0.7Cu- 76.2 µm;(Sn-4.7Ag- 0.5Sb) cooling1.7Cu); rate = 10°/s,63.8 (Sn- tested at 22°C,3.8Ag- Ref. 180.7Cu)

4.5 (Sn- 24.4 (Sn- 28.9 (Sn- At 0.140Pb) 3.6Ag- 3.8Ag- mm/min; gap

1.0Cu); 0.7Cu- thickness: 76.221.6 (Sn- 0.5Sb) µm; cooling4.7Ag- rate = 10°/s;1.7Cu); tested at 170°C,25.1 (Sn- Ref. 183.8Ag-0.7Cu)

34.5 (Sn- At 1 mm/min at 40Pb) reflow temper-

ature, Ref. 19

21.6 (Sn- At 1 mm/min at40Pb) 100°C, Ref. 19

40.27 Ring-and-plugtest, Ref. 20

41.8 Ref. 21

28.4 Ref. 12

23.8 Ref. 2

48.4 Ref. 6

Elongation 31 36.5 21.5 19 (93.3Sn- 38.5 (Sn- 6.56 × 10−4/s,(%) (95.4Sn- (87.4Sn- 3.1Ag-3.1Bi- 2.5Ag- 300 K, Ref. 4

3.1Ag- 4.1Ag- 0.5Cu) 0.8Cu-1.5Cu); 20 0.5Cu-8In); 0.5Sb)(93.6Sn- 22.54.7Ag- (88.5Sn-1.7Cu) 3Ag-

0.5Cu-8In)


TABLE 13.8 Mechanical Properties of Sn-Ag-Cu, Sn-Ag-Cu-In, Sn-Ag-Cu-Bi, Sn-Ag-Cu-Sb, and63Sn-37Pb (Continued)


35.5 Ref. 6

43.66; 52.87 50 (Sn- Ref. 112.5Ag-0.8Cu-0.5Sb)

35–176 Ref. 12

48 22 (Sn- 12 (Sn-2Ag- 32 (Sn- Ref. 683Ag-4Cu); 7.5Bi-0.5Cu) 3Ag-2Cu-27 (Sn- 2Sb); 90.5Ag- (Sn-2.6Ag-4Cu) 0.8Cu-

0.5Sb); 27(Sn-0.2Ag-2Cu-0.8Sb)

35 Ref. 2

Impact 31 77 (Sn- 75 (Sn- Ref. 5strength 3.5Ag- 3Ag-1In-(J/cm2) 0.75Cu) 0.7Cu)

Young’s 33.58 51.16 (Sn- Ref. 11modulus 2.5Ag-(Gpa) 0.8Cu-

0.5Sb)

Creep 6 27 (Sn- 25°C, 100 h to(60Sn- 4Ag- failure, MPa,40Pb) 0.5Cu) Ref. 24

2.8 7.5 (Sn- 25°C, 1000 h(60Sn- 4Ag- to failure, MPa,40Pb) 0.5Cu) Ref. 24

323 (Sn- 1007 (Sn- 218 (Sn-Ag-Cu- Time to break1Ag- 3Ag- 7.5Bi); 1747 (h), Ref. 460.5Cu); 0.7Cu- (Sn-Ag-Cu-3849 (Sn- 1In) 4.5Bi); 22033.5Ag- (Sn-Ag-Cu-2Bi)0.75Cu)

13 for both Creep strength,Sn-3.5Ag- N/mm2 at 0.10.7Cu and mm/min, 20°C,Sn-3.8Ag- Ref. 80.7Cu

5.0 for Creep strength,both Sn- N/mm2 at 0.13.5Ag- mm/min, 100°C,0.7Cu and Ref. 8Sn-3.8Ag-0.7Cu


FIGURE 13.31 Tensile stress-strain relationships of 93.6Sn-4.7Ag-1.7Cu, 95.4Sn-3.1Ag-1.5Cu, andeutectic Sn-Ag, Sn-Cu, and Sn-Pb alloys at 6.56 × 10−4/s, 300 K.

FIGURE 13.32 Tensile stress-strain relationships at 300 K and 6.56 × 10−4/s for 88.5Sn-3Ag-0.5Cu-8In, 87.4Sn-4.1Ag-0.5Cu-8In, 95.4Sn-3.1Ag-1.5Cu, and 63Sn-37Pb.


FIGURE 13.33 Tensile stress-strain relationships at 300 K and 6.56 × 10−4/s for 93.3Sn-3.1Ag-3.1Bi-0.5Cu, 96.5Sn-3.5Ag, 99.3Sn-0.7Cu, and 63Sn-37Pb.

FIGURE 13.34 Time to break for solder joints in creep tests for several solder alloys.

Sn-Ag-Cu-Bi alloys exhibit a higher tensile strength4,13 and yield strength,13 alower elongation,4,68 and a slower creep rate46 than eutectic Sn-Pb. Figure 13.33exemplifies the tensile stress-strain behavior of the Sn-Ag-Cu-Bi alloy versus eutec-tic Sn-Pb, Sn-Ag, and Sn-Cu systems. Toyoda observed that for the Sn-Ag-Cu sys-tem, the creep resistance, or time to break in creep test, increases considerably withaddition of Bi, as shown in Fig. 13.34.46 Addition of 2 percent Bi results in a consid-erable increase in time to break (2203 h). However, further increase in Bi contentresults in a rapid drop in benefit in creep resistance, and the time to break is reduceddown to 1747 h for 4.5 percent Bi and 218 h for 7.5 percent Bi, compared with 1 h foreutectic Sn-Pb. Figure 13.36 shows that the tensile strength of the Sn-Ag-Cu-Bi sys-tem increases with increasing Bi content, then levels off at around 10 percent Bi.46

On the other hand, the elongation of this system drops rapidly with increasing Bicontent until it reaches the 3 percent level, then it decreases slowly and later levelsoff with further increase in Bi content.

The Sn-Ag-Cu-Sb system exhibits a more diverse variation in mechanical prop-erties as a function of composition. The most widely studied composition is 96.2Sn-2.5Ag-0.8Cu-0.5Sb (CASTIN). CASTIN was reported to be comparable11 orlower4,13 in tensile strength, comparable11 or lower13 in yield strength, lower,68 com-parable,4 or higher11 in elongation than eutectic SnPb. Other compositions of inter-est that were studied include Sn-3Ag-2Cu-2Sb,13 Sn-3.8Ag-0.7Cu-0.5Sb,18 andSn-0.2Ag-2Cu-0.8Sb (SAF-A-LLOY).13 Sn-3Ag-2Cu-2Sb is higher in tensilestrength and yield strength, but lower in elongation than eutectic Sn-Pb. Sn-3.8Ag-0.7Cu-0.5Sb is higher in shear strength, while Sn-0.2Ag-2Cu-0.8Sb is comparable intensile strength and yield strength, but lower in elongation than 63Sn-37Pb.


FIGURE 13.35 Yield strength σy, tensile strength σT, and plastic strain εp at fracture versusAg contents for Sn-Ag-Cu-In system at both 0.5Cu-8In and 0.5Cu-4In.


The wetting properties of Sn-Ag-Cu and Sn-Ag-Cu-X alloys are shown in Table 13.9.The contact angle results of Table 15.9, together with Fig. 13.7, indicate that alloywetting decreases in the following order: 60Sn-40Pb > Sn-Ag-Cu > Sn-Ag-Cu-Sb.18,22

Spread test results of Baggio show 62Sn-36Pb-2Ag > Sn-Ag-Cu-Bi > Sn-Ag-Cuwhen tested with a profile with a peak temperature of 240°C, a dwell time above liq-uidus of 60 s for Pb-free alloys, and a peak temperature of 215°C, 60-s dwell for Sn-Pb-Ag. Both forced-air convection, air reflow atmosphere, and vapor phase reflowwere used in his study.45 The spread performance is rated with a scale of 1 to 5, with5 being the best. Toyoda also studied spreading performance of several alloys, andobserved the following spreading behavior in decreasing order: 63Sn-37Pb > Sn-Ag-Cu-4.5Bi, Sn-Ag-Cu-7.5Bi > Sn-3.5Ag-0.75Cu > 99.25Sn-0.75Cu > 89Sn-8Zn-3Bi, asshown in Fig. 13.13.46

A wetting time study done by Baggio showed that there was no significant dif-ference between 62Sn-36Pb-2Ag (at 235°C), Sn-3.8Ag-0.7Cu, and Sn-3.3Ag-3Bi-1.1Cu (at 260°C) for metallized PCB surface finishes, including immersion Pd,immersion Sn, immersion Ag, and Ni/Au.45 However, 62Sn-36Pb-2Ag wettedslightly faster on organic solderability preservative (OSP) than Sn-Ag-Cu and Sn-Ag-Cu-Bi.45 Lotosky’s results show that at 260°C when tested on oxidized copper,the wetting time increases in the following order: Sn-4Ag-0.5Cu (1.1 s) < 63Sn-37Pb(1.85 s) < Sn-2.5Ag-0.8Cu-0.5Sb (2.05 s) < 96.5Sn-2.5Ag-1Bi-0.5Cu (2.7 s).30 Toyodastudied the wetting time of meniscograph for Sn-Pb, Sn-Cu, Sn-Ag-Cu, and Sn-Ag-Cu-Bi alloys, and observed that the wetting time increases in the following order:63Sn-37Pb < Sn-Ag-Cu-2Bi ∼ Sn-Ag-Cu-1Bi < Sn-3.5Ag-0.75Cu < Sn-1Ag-0.5Cu <Sn-0.7Cu-0.3Ag < Sn-0.75Cu. The wetting time decreases with increasing tempera-ture at a slightly different rate, as shown in Fig. 13.37.45 In Toyoda’s work, the Bicontent between 1 and 2 percent seems to have no effect on the wetting perform-ance, including spreading and wetting time, of Sn-Ag-Cu-Bi system. Both Sn-Ag-Cu-1Bi and Sn-Ag-Cu-2Bi display a wetting behavior that is fairly comparable with63Sn-37Pb.


FIGURE 13.36 Effect of Bi content on the tensile strength and elonga-tion of Sn-Ag-Cu-Bi and Sn-Zn-Bi alloys.


TABLE 13.9 Wetting Properties of 63Sn-37Pb, Sn-Ag-Cu, Sn-Ag-Cu-In, Sn-Ag-Cu-Bi, and Sn-Ag-Cu-Sb

63Sn- Sn-Ag- Sn-Ag- Sn-Ag-Properties 37Pb Sn-Ag-Cu Cu-In Cu-Bi Cu-Sb Notes/References

Contact 17 (Sn- 21 (Sn- 44 (Sn- Ref. 18angle 40Pb) 4.70Ag- 3.8Ag-(degree) 1.70Cu) 0.7Cu-

0.5Sb)22 47 Flux A611, 260–

280°C, Ref. 2232 45 Flux A260HF,

260–280°C, Ref. 2231 35 Flux B2508,

260–280°C, Ref. 22

Wetting 0.28 s 0.24 s (Sn-time (s) 0.36 s (Sn-3.8Ag- 3.3Ag-3Bi-

at 235°C 0.7Cu) at 1.1Cu) at Immersion Pd(Sn62) 260°C 260°C PCB, s, Ref. 45

0.23 s 0.26 s (Sn-0.27 s (Sn-3.8Ag- 3.3Ag-3Bi-

at 235°C 0.7Cu) at 1.1Cu) at Immersion Sn(Sn62) 260°C 260°C PCB, s, Ref. 45

0.25 s 0.19 s (Sn-0.20 s (Sn-3.8Ag- 3.3Ag-3Bi-

at 235°C 0.7Cu) at 1.1Cu) at Immersion Ag,(Sn62) 260°C 260°C PCB, s, Ref. 45

0.42 s 0.44 s (Sn-0.32 s (Sn-3.8Ag- 3.3Ag-3Bi-

at 235°C 0.7Cu) at 1.1Cu) at NiAu, PCB, s,(Sn62) 260°C 260°C Ref. 45

0.26 s 0.26 s (Sn-0.20 s (Sn-3.8Ag- 3.3Ag-3Bi-

at 235°C 0.7Cu) at 1.1Cu) at(Sn62) 260°C 260°C OSP 1, s, Ref. 45

0.23 s 0.25 s (Sn-0.21 s (Sn-3.8Ag- 3.3Ag-3Bi-


0.27 s 0.27 s (Sn-0.24 s (Sn-3.8Ag- 3.3Ag-3Bi-


2.75 s 2.05 s (Sn-1.1 s (Sn- (96Sn- 2.5Ag-

4Ag- 2.5Ag-1Bi- 0.8Cu- At 260°C, RF 800,1.85 s 0.5Cu) 0.5Cu) 0.5Sb) oxidized Cu, Ref. 30

Spread 4.2 (Sn- 4 (Sn-4.55 3.8Ag- 3.3Ag-3Bi-

(Sn62) 0.7Cu) 1.1Cu) OSP 3*, Ref. 454.55 (Sn- 4.6 (Sn-

4.7 3.8Ag- 3.3Ag-3Bi- Immersion Ag*, Ref. 45(Sn62) 0.7Cu) 1.1Cu)

3.9 (Sn- 4.4 (Sn-4.4 3.8Ag- 3.3Ag-3Bi- Immersion Pd*, Ref. 45

(Sn62) 0.7Cu) 1.1Cu)

For Sn-Ag-Cu-In, no wetting information is available. For the Sn-Ag-Cu-Sb sys-tem, Seelig et al.11 studied the effect of Ag content on wetting performance whenusing rosin-based, mildly activated (RMA), no-clean, and organic acid (OA) fluxes.Results indicate that at around 2.5 percent Ag content, the Sn-Ag-Cu-Sb systemexhibits the shortest wetting time for two out of three fluxes, as shown in Fig. 13.38.11


TABLE 13.9 Wetting Properties of 63Sn-37Pb, Sn-Ag-Cu, Sn-Ag-Cu-In, Sn-Ag-Cu-Bi, and Sn-Ag-Cu-Sb(Continued)

63Sn- Sn-Ag- Sn-Ag- Sn-Ag-Properties 37Pb Sn-Ag-Cu Cu-In Cu-Bi Cu-Sb Notes/References

4.4 (Sn- 4.7 (Sn-3.8Ag- 3.3Ag-3Bi-

5 (Sn62) 0.7Cu) 1.1Cu) NiAu*, Ref. 454.35 (Sn- 4.45 (Sn-

3.8Ag- 3.3Ag-3Bi-5 (Sn62) 0.7Cu) 1.1Cu) OSP 3,† Ref. 45

4.8 (Sn- 4.95 (Sn-4.7 3.8Ag- 3.3Ag-3Bi- Immersion Ag,† Ref. 45

(Sn62) 0.7Cu) 1.1Cu)3.9 (Sn- 4.65 (Sn-

4.7 3.8Ag- 3.3Ag-3Bi- Immersion Pd,† Ref. 45(Sn62) 0.7Cu) 1.1Cu)

5 (Sn- 5 (Sn-3.8Ag- 3.3Ag-3Bi-

5 (Sn62) 0.7Cu) 1.1Cu) NiAu,† Ref. 45

* Peak 240°C, dwell 60 s for Pb-free, 215°C, 60-s dwell for Sn-Pb-Ag, scale 1 to 5 (best), forced-air convection, air.† Peak 240°C, dwell 60 s for Pb-free, 215°C, 60-s dwell for Sn-Pb-Ag, scale 1 to 5 (best), 230°C bp VPR.

FIGURE 13.37 Wetting time of meniscograph for Sn-Pb, Sn-Cu, Sn-Ag-Cu, and Sn-Ag-Cu-Bi alloys.

13.4.4 RELIABILITY

The isothermal low-cycle fatigue (strain 0.2 percent, 0.1 Hz, R = 0.8, 300 K) perform-ance of several Sn-Ag-Cu and Sn-Ag-Cu-X alloys was studied by Hwang,4 with resultsshown in Table 13.5. The number of cycles to failure at 300 K (50 percent load drop,0.2 percent strain range) decreases in the following order: 88.5Sn-3Ag-0.5Cu-8In(19,501) > 95.4Sn-3.1Ag-1.5Cu (8,936) > 96.2Sn-2.5Ag-0.8Cu-0.5Sb (8,751) > 94.5Sn-0.5Ag-2Cu-3Sb (7,120) > 93.3Sn-3.1Ag-3.1Bi-0.5Cu (6,522) > 63Sn-37Pb (3,650). Thedata above suggest that Sn-Ag-Cu-X is a very viable family as lead-free alternatives.

Some temperature cycling and heat treatment reliability data for Sn-Ag-Cu, Sn-Ag-Cu-In, Sn-Ag-Cu-Bi, Sn-Ag-Cu-Sb, and 63Sn-37Pb or 62Sn-36Pb-2Ag areshown in Table 13.10.

Since all of the viable Pb-free alternatives comprise a high Sn content, the possi-bility of a high IMC formation rate and its impact on reliability are a concern. Grusdreported that the Cu dissolution rate during soldering increases in the followingorder: 60Sn-40Pb < Sn-Ag-1Cu < Sn-2.5Ag-0.8Cu-0.5Sb < Sn-Ag-Cu-Bi < Sn-Ag-0.5Cu, and thus validates this concern.24 Figure 13.39 shows examples of intermetallicformations between Sn-Ag-Cu and copper substrate.73 Although the intermetallicsthickness may be thicker than the Sn-Pb system, it does not adversely affect the sol-der joint reliability. Feldmann and Reichenberger studied the effect of 160°C storageaging time on the shear strength of chip resistor 1206.66 Results indicate that while62Sn-36Pb-2Ag decreased in shear strength for about 30 percent after 1000 h of agingat 160°C, the Pb-free alternatives Sn-3.8Ag-0.7Cu and Sn-3.3Ag-3Bi-1.1Cu reducedonly 12 and 4 percent, respectively, under the same test condition.

In all of the results reported in Table 13.10, the Sn-Ag-Cu system is always equalor better than 63Sn-37Pb. This widely acceptable reliability performance of Sn-Ag-Cu system, regardless of applications, strongly validates the acceptability of this sys-tem as a substitute for eutectic Sn-Pb. Figures 13.14 through 13.17 show the Weibullplots for Sn-Ag-Cu versus Sn-Pb in the BGA applications over several temperaturecycling test conditions, thus demonstrating the superiority of the Sn-Ag-Cu system.


FIGURE 13.38 Effect of Ag content on the wetting time of Sn-Ag-0.8Cu-0.5Sb systemusing RMA, no-clean, and OA fluxes.

TABLE 13.10 Accelerated Life Testing Performance of Sn-Ag-Cu, Sn-Ag-Cu-In, Sn-Ag-Cu-Bi, Sn-Ag-Cu-Sb, and Sn-Pb—The Performance MayAlso Be Ranked in Descending Order for Some Works, with 1st Being the Best

Test Sn-Pb Notes/condition Applications (or Sn62) Sn-Ag-Cu Sn-Ag-Cu-In Sn-Ag-Cu-Bi Sn-Ag-Cu-Sb References

0–100°C General Good Comparable Ref. 14(on with Sn63NiP/Au) (Sn-4Ag-

0.5Cu onTiW/CuUBM)

−40°C, 15 TQFP64, Good Comparable Comparable Ref. 66min/+125°C, CR1206 with Sn62 with Sn6215 min,shearstrengthafter 2000cycles

160°C heat CR1206, 68/55/48 68/62/60 80/83/77 Ref. 66treatment, 100Sn (Sn62) (Sn-3.8Ag- (Sn-3.3Ag-shear component 0.7Cu) 3Bi-1.1Cu)strength finish to(N): OSP board0/500/1000 finishh

Single-lead TQFP64, 7/6.9 8.8/8 (Sn- 6.5/10.3 Ref. 66shear Sn-Pb vs. (Sn62) 3.8Ag- (Sn-3.3Ag-strength Ni-Pd 0.7Cu) 3Bi-1.1Cu)(N) component

finish toOSP boardfinish

−55/+125°C Cylindrical 1st (Sn- 3rd Ref. 46resistor, 3.5Ag- (Sn-Ag-Cu-crack vs. 0.75Cu) 7.5Bi)cycles

13.4

6

−40/80°C, 3rd 1st (Sn- 1st 1st Ref. 46crack 3.5Ag- (Sn-Ag-Cu- (Sn-Ag-Cu-

0.75Cu) 1In) 7.5Bi)

−40/125°C, 625 I/O 3rd, 2nd (Sn- Ref. 4942-min CBGA, close to 3.8Ag-cycle time 32 × 32 × 0.8 2nd 0.7Cu),

mm, same close toalloy for Sn-Ag-Biball and paste

−40/125°C, 625 I/O 2nd 1st (Sn- Ref. 49240-min CBGA, 3.8Ag-cycle time 32 × 32 × 0.8 0.7Cu)

mm, samealloy forball and paste

0/100°C, 30- 625 I/O 2nd 1st (Sn- Ref. 49min cycle CBGA, 3.8Ag-time 32 × 32 × 0.8 0.7Cu),

mm, same same asalloy for Sn-Ag-Bi,ball and paste better than

Sn63

0/100°C, 60- 625 I/O 2nd 1st (Sn- Ref. 49min cycle CBGA, 3.8Ag-time 32 × 32 × 0.8 0.7Cu),

mm, same same asalloy for Sn-Ag-Bi,ball and paste better than

Sn63

0/100°C, 625 I/O 2nd 1st (Sn- Ref. 49240-min CBGA, 3.8Ag-cycle time 32 × 32 × 0.8 0.7Cu),

mm, same better thanalloy for Sn63ball and paste

13.4

7

TABLE 13.10 Accelerated Life Testing Performance of Sn-Ag-Cu, Sn-Ag-Cu-In, Sn-Ag-Cu-Bi, Sn-Ag-Cu-Sb, and Sn-Pb—The Performance MayAlso Be Ranked in Descending Order for Some Works, with 1st Being the Best (Continued)

Test Sn-Pb Notes/condition Applications (or Sn62) Sn-Ag-Cu Sn-Ag-Cu-In Sn-Ag-Cu-Bi Sn-Ag-Cu-Sb References

−40/60°C, 625 I/O 2nd, 1st Ref. 4930-min CBGA, close tocycle 32 × 32 × 0.8 Sn-Ag-Cu

mm, samealloy forball and paste

−40/+125°C fleXBGA 7th ∼ 3rd (Sn- 6th (Sn- 2nd (Sn- Ref. 516th 4Ag-1Cu); 3.4Ag-1Cu- 2.5Ag-

5th (Sn- 3.3Bi) ∼ 7th 0.8Cu-4Ag-0.5Cu) 0.5Sb)

Flip chip Good Comparable Ref. 14with Sn63

Flip chip of 1st 2nd, Ref. 51nonunderfilled 96.2Sn-systems 2.5Ag-

0.8Cu-1Sbis about80% ofSn63-Pb37

Flip chip Good Comparable Ref. 52with Sn63(Sn-3.8Ag-0.7Cu)

General Good (Sn- Good (Sn- Ref. 53.5Ag- 3Ag-1In-0.75Cu) 0.7Cu)

General Best Refs. 54, 55

13.4

8

FIGURE 13.39 SEM migrographs (2500×) using backscatteredelectron imaging to show the intermetallic layer between copperand Sn-3.6Ag-1Cu (top), Sn-3Ag-0.5Cu (middle), and Sn-4Ag-0.5Cu (bottom).

13.49

Among the viable Sn-Ag-Cu compositions, Sn-3.8Ag-0.7Cu and Sn-4Ag-0.5Cuappear to be more commonly accepted.

Sn-Ag-Cu-In is much less studied than Sn-Ag-Cu. Tanaka et al. studied Pb-freetechnology solder for mobile equipment at NEC, and concluded that Sn-3.1Ag-1In-0.7Cu, which is similar to Sn-3.5Ag-0.75Cu and Sn-3.5Ag-0.5Bi-3In, is equal to 63Sn-37Pb in the fatigue test, but is inferior to Sn-Ag-Bi-1.1Cu-Ge and Sn-Ag-X.5

However, the latter two alloys are unacceptable for the Charpy Impact Test. All fivePb-free alloys are excellent in the creep rupture test when compared with 63Sn-37Pb. Overall,Tanaka et al. concluded that Sn-3Ag-1In-0.7Cu is equal to eutectic Sn-Pb, is poorer than both Sn-3.5Ag-0.75Cu and Sn-3.5Ag-0.5Bi-3In, but is better thanSn-Ag-Bi-1.1Cu-Ge and Sn-Ag-X.

Toyoda investigated the temperature cycling performance of several Pb-free sol-ders.46 The cycling condition is −40 to +80°C, with a 30-min temperature cycle, usinga wave-soldered nylon connector in a paper phenol substrate. The failure is moni-tored by observing externally the crack development. Results indicate that thefatigue resistance increases in the following order: 63Sn-37Pb < Sn-3.5Ag < Sn-Ag-Cu-7.5Bi, Sn-Ag-Cu-1In, Sn-3.5Ag-0.75Cu up to 1000 cycles, as shown in Fig. 13.40.46

Hwang also studied the isothermal low-cycle fatigue behavior of Pb-free alloys at300 K, 0.2 percent strain, 0.1 Hz, and R = 0.8.4 For the Sn-Ag-Cu-In system, shereported that the optimum In content for low-cycle fatigue life performance is 8 per-cent for both Sn-In-4.1Ag-0.5Cu and Sn-In-3.1Ag-0.5Cu systems, as shown in Fig.13.41.4 Hwang also studied the optimum Ag content for Sn-Ag-In-0.5Cu,with In main-tained at 4 and 8 percent. Results shown in Fig. 13.42 indicate that the Ag content maybe better to be higher than 3 percent.4 In the fatigue life test, 88.5Sn-3Ag-0.5Cu-8In(19,501 in Nf value) is the best among all the rest alloys, as shown in Table 13.5.

Sn-Ag-Cu-Bi is outstanding in creep resistance and wetting, as indicated in Figs.13.34 and 13.37. However, the presence of the Bi ingredient brings up concerns on


FIGURE 13.40 Temperature cycling performance of 63Sn-37Pb, Sn-Ag-Cu-7.5Bi, Sn-Ag-Cu-1In,Sn-3.5Ag-0.75Cu, and Sn-3.5Ag (test conditions: −40°/+80°C at 30-min cycle time).


FIGURE 13.41 Fatigue life Nf versus In content for the Sn-Ag-Cu-In system at bothSn-In-4.1Ag-0.5Cu and Sn-In-3.1Ag-0.5Cu.

FIGURE 13.42 Fatigue life Nf as a function of Ag content for Sn-Ag-Cu-In system at both Sn-Ag-0.5Cu-8In and Sn-Ag-0.5Cu-4In.

the sensitivity toward lead contamination. Feldmann and Reichenberger evaluatedthe solder joint shear strength of TQFP64 with Sn-Pb or Ni-Pd component finishessoldered onto PCB with OSP board finish using 62Sn-36Pb-2Ag, Sn-3.8Ag-0.7Cu,and Sn-3.3Ag-3Bi-0.7Cu, with the results shown in Fig. 13.43.66 The joint strength of62Sn-36Pb-2Ag and Sn-3.8Ag-0.7Cu is insensitive to the component finish type.However, in the case of Sn-3.3Ag-3Bi-1.1Cu, solder joints with the Sn-Pb compo-nent lead surface finish are much lower than joints with Ni-Pd finish due to the for-mation of a ternary eutectic low melting phase, 52Bi-30Pb-18Sn, as will be discussedin Chap. 16.

The temperature cycling performance of Sn-Ag-Cu-Bi was studied by Toyoda.46

At test conditions of −40°/+80°C at a 30-min cycle time, Sn-Ag-Cu-7.5Bi outper-forms 63Sn-37Pb and Sn-3.5Ag and, similar to Sn-Ag-Cu-1In and Sn-3.5Ag-0.75Cu,exhibits no failure up to at least 1000 cycles, as shown in Fig. 13.40. However, at testconditions of −55/+125°C and a cycle time of 30 min, the temperature cycling per-formance of the solder joints of a cylindrical chip resistor using Sn-Ag-Cu-7.5Bi isslightly poorer than 62Sn-36Pb-2Ag and is considerably poorer than Sn-3.5Ag-0.75Cu, as shown in Fig. 13.44.46

In Syed’s work as part of the National Center for Manufacturing Sciences(NCMS) project, fatigue life of the fleXBGA package [144 input/output (I/O), 18-mil ball size, 0.8-mm ball pitch) was determined at the following conditions:−40/+125°C, 15-min ramp and dwell, 1 cycle/h, single-zone cycling chamber for aseries of Pb-free solders.The results are shown in Table 13.11, indicating that Sn-Ag-Cu-Bi is comparable with or slightly better than control eutectic Sn-Pb solder,poorer than Sn-Ag-Cu, Sn-Ag-Cu-Sb, Sn-Ag-Cu-Sb-Bi, Sn-Ag-In, but better thanSn-Ag.50 The results are roughly consistent with the findings of Toyoda46 and Feld-mann and Reichenberger.66

Sn-Ag-Cu-Sb is one of the alloy systems that received a relatively good evalua-tion in the early stage of Pb-free soldering development. In Syed’s results, Sn-2.5Ag-0.8Cu-0.5Sb is leading in fatigue life evaluation (see Table 13.11). In Elenius andYeh’s work, 96.2Sn-2.5Ag-0.8Cu-1Sb is about 80 percent of Sn63-Pb37 for unfilled


FIGURE 13.43 Single lead shear strength of TQFP64 with Sn-Pb or Ni-Pd surface finishes.The component is soldered onto PCB with an OSP sur-face finish with 62Sn-36Pb-2Ag, Sn-3.8Ag-0.7Cu, and Sn-3.3Ag-3Bi-0.7Cu.

flip chip assembly.51 Seelig and Suraski reported that the fatigue life of 96.1Sn-2.6Ag-0.8Cu-0.5Sb is comparable with or better than 96.5Sn-3.5Ag, as shown inTable 13.12,11 according to ASTME 606, 1-Hz triangular waveform oscillatedbetween 0.15 and −0.15 percent strain. The passing mark was set at 10,000 cycles.

The reliability potential of Sn-Ag-Cu-Sb was also investigated by determiningthe intermetallics growth rate at 125°C on several commonly used base metals,including copper, brass, nickel, and alloy 42. Results indicate that 96.1Sn-2.6Ag-0.8Cu-0.5Sb (CASTIN) is most stable when compared with 60Sn-40Pb, 96.5Sn-3.5Ag, and 99.3Sn-0.7Cu, as shown in Fig. 13.45.11 The advantage in slowintermetallics growth rate of Sn-Ag-Cu-Sb is particularly profound on copper.


FIGURE 13.44 Temperature cycling performance of 62Sn-36Pb-2Ag, Sn-Ag-Cu-7.5Bi, and Sn3.5-Ag0.75-Cu (test conditions: −55°C/+125°C and cycle time 30 min).

TABLE 13.11 Relative Comparison of Fatigue Life of Pb-Free Alloys*

Relative fatigue life

Alloy By first failure By mean life

A1 (Sn-Ag) 0.69 0.94

A11 (Sn-Ag-Cu) 1.27 1.28

A14 (Sn-Ag-Cu) 1.14 1.26

A21 (Sn-Ag-Cu-Sb) 1.29 1.33

A32 (Sn-Ag-Cu-Sb-Bi) 1.17 1.31

A62 (Sn-Ag-Cu-Bi) 1.01 1.12

A66 (Sn-Ag-In) 1.29 1.25

B63 (Sn-Pb) 1.00 1.00

* At the following conditions: −40/+125°C, 15-min ramp and dwell, 1 cycle/h, single-zone cycling chamber,fleXBGA package, 144 I/O, 18-mil ball size, 0.8-mm ball pitch.


TABLE 13.12 Fatigue Life of 96.1Sn-2.6Ag-0.8Cu-0.5Sb and 96.5Sn-3.5Ag*

96.1Sn-2.6Ag-0.8Cu-0.5Sb 96.5Sn-3.5Ag

11,194 10,003

26,921 6,267

24,527 11,329

* Test conditions: ASTME 606, 1-Hz triangular waveformoscillated between 0.15 and −0.15 percent strain. The pass-ing mark was set at 10,000 cycles.

13.5 Sn-Zn AND Sn-Zn-Bi


91Sn-9Zn is attractive mainly due to its relatively low melting temperature. How-ever, the high activity of Zn results in a great challenge in solder paste reflow appli-cations. Bi is added accordingly in order to reduce the corrosivity of Zn underhumid conditions and to reduce the melting temperature further.Table 13.13 showsthe physical properties of 91Sn-9Zn, 89Sn-8Zn-3Bi, and eutectic Sn-Pb, althoughinformation on Sn-Zn-Bi is very scarce. In general, the physical properties of theSn-Zn alloy are dictated by the property of Sn due to high Sn content. Sn-Znexhibits a higher surface tension than eutectic Sn-Pb, as predicted by Fig. 12.2,hence inferring a poorer wetting behavior. Addition of low-surface-energy Bi isexpected to lower the surface tension of the Sn-Zn system (see Fig. 12.2), thuspromising a better wetting.

FIGURE 13.45 Rate of intermetallic growth of 60Sn-40Pb, 96.5Sn-3.5Ag, 99.3Sn-0.7Cu, and96.1Sn-2.6Ag-0.8Cu-0.5Sb (CASTIN) at 125°C on copper, brass, nickel, and alloy 42.


The mechanical properties of 63Sn-37Pb, 91Sn-9Zn, and 89Sn-8Zn-3Bi are shown inTable 13.14. In general, the Sn-Zn alloy is higher in yield strength, equal or higher intensile strength, equal in shear strength, and equal or lower in elongation than eutec-tic Sn-Pb. On the other hand, Sn-Zn-Bi is higher in creep resistance than Sn-Pb.

The effect of Bi addition on tensile properties of Sn-Zn results in embrittlementof the alloy, and is fairly similar to that of Sn-Ag-Cu. Figure 13.36 shows that at 8percent Bi content, the tensile strength of the Sn-Zn-Bi system is at the maximumand is about twice that of Sn-Zn, while the elongation is at the minimum and is aboutone-sixth of that of the Sn-Zn.


Zn-containing alloys are notorious in terms of wetting performance, mostly due to thesusceptibility of Zn to oxidation and the high surface energy of the systems.The latterresults in difficulty in wave soldering bath maintenance, as reported by Baggio.45 Toy-oda studied the solder spread of Pb-free alloys, and concluded that 89Sn-8Zn-3Bi isthe lowest in spread when compared with Sn-Pb, Sn-Cu, Sn-Ag-Cu, and Sn-Ag-Cu-Bi,


TABLE 13.13 Physical and Mechanical Properties of 91Sn-9Zn and 89Sn-8An-3Bi

Notes/Property 63Sn-37Pb 91Sn-9Zn 89Sn-8Zn-3Bi References

Melting temperature 183 199 187–197 Ref. 6(°C)

Density (g/cm3) 8.4 7.27 Ref. 1

Electrical resistivity 17 Ref. 63(µΩ-cm) 15 Ref. 1

14.5 10–15 Ref. 314.6 Ref. 2

Thermal 50.9 Ref. 2conductivity 61 Ref. 1[W/(m·K)]

Specific heat 0.167 0.239 Ref. 1[J/(g·K)]

CTE (ppm/K) 16.8 23.9 Ref. 624 (−70°C, 20°C, Ref. 15

140°C)21 Ref. 1625 Ref. 218.74 Ref. 8

Hardness 12.8 21.3 HV;kg/mm2,Ref. 6

17 21.5 Brinel hardness,Ref. 1

Surface tension 417 (air), 464 (N2) 518 (air), 487 (N2) Ref. 3(mN m−1) at 233°C at 249°C

as shown in Fig. 13.13.46 In his results, it is also interesting to note that, opposite to thebehavior of other alloys, the spread of Sn-Zn-Bi decreases with increasing tempera-ture. Presumably this can be attributed to the oxidation of Zn during testing.

Huang and Lee studied the reflow spreading performance of Pb-free solders, withresults shown in Fig. 13.46.29 In this work, each alloy was evaluated in solder pasteform using 10 different flux chemistries, with the average performance expressed as awetting index, where 10 and 0 represent full wetting and no wetting, respectively.Thereflow was conducted at two different reflow profiles, one with a peak temperature of15°C above the liquidus, and another one with a peak temperature of 30°C above liq-uidus temperature. The spreading of Sn-Zn-Bi was extremely poor, with the solderpastes barely reflowed in many cases. The poor wetting performance of Sn-Zn-Bi ispresumably attributable to the highly oxidative nature of Zn.

Loomans et al. studied the contact angle of binary eutectic alloys using rosin-IPAflux, and reported the following results: Sn-Bi, 40° (166°C); Sn-Zn, 60° (225°C); andSn-Ag, 45° (250°C).32 These angles were little affected by a number of 1 percent ter-nary additions to the solders. The significantly larger contact angle of Sn-Zn thanthat of Sn-Bi and Sn-Ag reflects the greater difficulty in wetting, and is consistentwith results discussed earlier.

Suganuma studied the microstructure of lead-free solders and of their interfaceswith copper. He reported that the Sn-Zn alloys only form different Cu-Zn IMCs(beta-Cu-Zn and gamma-Cu5Zn8) at the interface. Even with the small amount ofZn added to Sn, Zn segregates to the interface to form the Cu-Zn IMC with Sn.However, most other Sn alloys, including pure Sn, Sn-Ag, Sn-Bi, or their ternaryalloys, form two IMCs at the interfaces with Cu [i.e., Cu6Sn5 (15 µm) and Cu3Sn (5µm).The former is much thicker than the latter and the interface integrity is stronglyinfluenced by the presence of the Cu6Sn5 layer.37

Sn-Zn-(Bi) is not stable with most fluxes when used as a solder paste, mainly dueto the high reactivity of Zn. However, few solder paste products have been devel-oped, including no-clean applications, for this alloy system.6

13.5.4 RELIABILITY

The reliability of Sn-Zn-Bi system was studied by determining the shear strength ofsolder joints as a function of storage time at 125°C, as shown in Fig. 13.47.6 The sur-


TABLE 13.14 Mechanical Properties of 63Sn-37Pb, 91Sn-9Zn, and 89Sn-8Zn-3Bi

Properties 63Sn-37Pb 91Sn-9Zn 89Sn-8Zn-3Bi Notes/References

Ultimate 45.1 45.4 Ref. 6tensile 30.6 53.1 Ref. 13strength (MPa) 51.7 54.7 Ref. 1

Yield strength, A 27.2 51.6 Ref. 13(0.2%, MPa)

Shear strength 48.4 48.8 Ref. 6(MPa)

Elongation (%) 35.5 40 Ref. 648 27 Ref. 1335 Ref. 2

33 Ref. 1

Creep (time to 1 94 Ref. 46break, h)

face finishes for PCBs tested included Au-Ni and preflux. Results indicate that bothSn-Zn-Bi and Sn-Pb maintained a steady shear strength with storage time up to atleast 3000 h. Ni-Au resulted in a slightly higher shear strength than preflux, and Sn-Zn-Bi joints appear to be slightly stronger than those of Sn-Pb. However, inanother test where the solder joints were pretreated with thermal cycling, the shearstrength started to decline after 2000 cycles for Sn-Zn-Bi, and from the very begin-ning of the Sn-Pb system, as shown in Fig. 13.48.6 In the case of Sn-Zn-Bi, no differ-ence in strength and reliability can be discerned between Ni-Au and prefluxfinishes.

While Showa Denko’s results indicate a lack of sensitivity of Sn-Zn-Bi towardheat treatment, Suganuma reported that the tensile strength (MPa) of Cu/solder/Cujoints for Sn-Zn did deteriorate with heat treatment. Thus, the tensile strength foras-soldered/after-exposure at 150°C for 100 h can be shown as follows: Sn-8Zn(Au-Pd-Ni), 73/54; Sn-8Zn, 53/15; Sn-3.5Ag, 52/37; Sn-7.5Bi-2Ag-0.5Cu, 44/35; andSn-37Pb, 28/18.37 The deterioration extent for Sn-Zn on Cu without Au-Pd-Nisurface finish is greater than the rest systems studied, suggesting a change in Sn-Znsolder joint microstructure may have occurred in the absence of Au-Pd-Ni. Indeed,this may very well be the case. In the absence of a Ni diffusion barrier for copper,Zn and Cu form Cu-Zn IMCs (beta-Cu-Zn and gamma-Cu5Zn8) at the interface, asreported by Suganuma for the Sn-Zn system.37 A similar Cu-Zn IMC is also ob-served in the Sn-Zn-Bi system on Cu, as shown in Fig. 13.49.6 The development ofCu-Zn IMC can be prevented by predepositing a Ni barrier on top of Cu, as shownin Fig. 13.50.6


FIGURE 13.46 Solder paste wetting (spreading) performance for Pb-free and 63Sn-37Pb solder pastes when reflowed at a peak temperature 15 and 30°C above the liq-uidus temperature of alloys. An index value of 10 represents full wetting, and 0represents no wetting.

FIGURE 13.47 Shear strength of 89Sn-8Zn-3Bi and Sn-Pb soldered ontoAu-Ni or preflux-coated copper at 125°C storage temperature.

FIGURE 13.48 Shear strength of 89Sn-8Zn-3Bi and Sn-Pb soldered ontoAu-Ni or preflux-coated copper as a function of thermal cycle number.

FIGURE 13.49 Effect of thermal cycle test on microstructure of89Sn-8Zn-3Bi (electron probe microanalysis view).

13.58

13.6 SUMMARY

Sn-Ag-Cu is the most attractive system, mainly due to overall superior reliabilityand acceptable soldering property and cost. Addition of In, Bi, or Sb introducesimproved performance with a trade-off. Sn-Ag-Bi-X is outstanding on soldering, butsensitive to Pb-contamination and fillet lift. Reliability of Sn-Ag can range frombeing poor to good, and is highly dependent on applications. Its soldering perform-ance is marginally acceptable. Sn-Cu is poor in mechanical strength, and its reliabil-ity can range from poor to good, too, depending on the applications. Sn-Zn-Bi isattractive mainly due to the low melting temperature. The high activity of Zn pro-hibits this system to be used in high-end applications.

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FIGURE 13.50 Effect of thermal cycle test on microstruc-ture of 89Sn-8Zn-3Bi (electron probe microanalysis view) inthe presence of a Ni barrier layer.

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CHAPTER 14LEAD-FREE SURFACE FINISHES

14.1 INTRODUCTION

To accomplish a lead-free soldering environment, not only the solders used for form-ing solder joints but also the surface finishes of pads on printed circuit boards(PCBs) and leads of components have to be lead-free.This is based on the consider-ations of both environmental safety and joint reliability issues. At this stage, an Sn-Pb surface finish is widely used in the electronics industry. Figure 14.1 shows thePCB surface finishes technology adoption status in the United States, published byIPC Technology Marketing Research Council.1 Although Sn-Pb hot-air solder level-ing (HASL) still remains the dominant choice since 1998, its usage has definitelybeen declining with time even before the heat on lead-free soldering was felt by theindustry.

In Fig. 14.1, several lead-free surface finishes are cited, including organic solder-ability preservative (OSP), Ni-Au, and Pd. Because the pressure on lead-free solderingis increasing at a tremendous rate, many new lead-free surface finish technologieshave emerged since the late 1990s. In this chapter, the options on lead-free surfacefinishes will be introduced, with the chemistry, process, and performance of the mostpromising choices discussed. Since the same surface finish may be used on productsvarying considerably in design and application, the considerations for selecting asurface finish need to address a wide range of possible applications, including sol-derability, compatibility with solder alloys, solder joint reliability, wire bondability,connector abrasion resistance, electrical contact resistance, shelf life, and contrast inautomated optical inspection or registration system.

14.2 OPTIONS FOR PCB LEAD-FREE

SURFACE FINISHES

Table 14.1 lists the options of lead-free surface finishes for PCBs. The system is cat-egorized by the key element used. Each category is further classified by the type ofprocess and chemistry. Examples are given for certain groups.

14.3 OSP

Organic solderability preservative (OSP) refers to organic coating that is applied toPCB pads as a preservative for solderability, and is also referred to as antitarnish. Itincludes rosins, resins, and azole chemicals.2–17 Organic solderability preservative iswidely used in Japan. Depending on the PCB type, single-sided PCBs use OSP as vir-tually the only means for preserving solderability. For double-sided, multilayerboards, 40 percent of the surface finishes are OSP types, as shown in Fig. 14.2.18

14.1


Within the azole group, benzotriazole, imidazoles, and substituted benzimidazolesare the most popular group, and will be discussed individually. Regarding the rosinor resin, it is often referred to as preflux, and will be discussed briefly. In general, theprimary benefits of OSP surface finishes are low-cost and flat-surface.

14.3.1 BENZOTRIAZOLE

Use of benzotriazole (BTA) can be dated back to at least the early 1960s. Instead ofrelying on a physical layer that is mechanically applied onto the top of base-metalcopper to provide protection against corrosion and oxidation, benzotriazole chemi-cally reacts with cuprous oxide and forms polymeric copper salt, as shown in Fig.14.3.19 These polymeric salt molecules align with each other and form a protectivefilm composed of a semipermeable, colorless, three-dimensional polymeric layer onthe surface of copper. Cuprous oxide is much faster in reacting with BTA than cupricoxides. Although this film thickness was considered a nominal 50 to 100 angstroms(Å), with an average of about 80 Å, the polymer thickness may continue to grow(within limits) if the metal is continually exposed to oxidant and triazole, especiallyat low (acidic) pH. This multilayer can be up to 5000 Å thick, depending on manyfactors, such as temperature, time, pH, corrodent, oxidation reduction environment,and so on.20 This continual growth can also repair defects in an existing film by react-ing with entrapped BTA molecules.21

Additional work that was later done by Tornkvist et al. on the surface orientationof BTA suggests a mean orientation of the first BTA layer on a cuprous oxide sur-face.As shown in Fig. 14.4, the upper nitrogen allows the adsorbed BTA molecule tocoordinate to another Cu(I) ion and thereby form a multilayer protective film, con-sisting of [-Cu(I)-BTA-]n chains.22 Some other surface absorption mechanism hasalso been postulated.23 However, the latter mechanism appears to allow monolayerBTA protection only.

14.2 CHAPTER FOURTEEN

FIGURE 14.1 Surface finish technology market study.(Source: IPC Technology Marketing Research Council, 1999.)

LEAD-FREE SURFACE FINISHES 14.3

TABLE 14.1 List of Lead-Free Surface Finishes*

Surfacefinishsystem Finish process and chemistry Example

OSP Benzotriazole COBRATEC, ENTEK CU-56

Imidazole AT&T, Protecto 5630 (Kester)

Benzimidazole (substituted) ENTEK PLUS CU-106A (Enthone-OMI), Glicoat (Shikoku)

Preflux (rosin/resin) Sealbrite, Solderite RT-05R

Ni-Au Electrolytic Ni-Au, or EG

Electroless Ni/electroless (immersion) Au, or ENIG

Electroless Ni/electroless (autocatalytic) Au

Electroless Ni/electroless (substrate-catalyzed) Au

Ag Electroless (immersion, or galvanic) Ag Alpha Level (Alpha Metals); SterlingSilver (MacDermid)

Bi Electroless (immersion) Bi

Electrolytic Pd or Pd alloys

Pd Electroless (autocatalytic) Pd

Electroless (autocatalytic) Pd/electroless (immersion) Au

Ni-Pd Electroless Ni/electroless (immersion) Pd

Electroless Ni/electroless (autocatalytic) Pd

Electroless Ni/electroless (autocatalytic) Pd/electroless (immersion) Au

Ni-Pd (X) Electrolytic Ni/PdCo/Au flash

(Electroless) Ni/(electroless) Pd-Ni/electroless(immersion) Au

Sn Electrolytic Sn Matte—Lucent (large, polygonized)

Electrolytic Sn Florida CirTech

Electrolytic Sn

Electroless (immersion) Sn White (new)

Electroless (immersion) Sn Gray (old)

Electroless (modified immersion +autocatalytic) Sn Flat Solderable Tin (FST)—Dexter

Ni-Sn Electrolytic Ni/electrolytic Sn Satin bright Sn—Lucent ECS

Sn-Ag Electrolytic Sn-Ag 96.5Sn-3.5Ag

Electrolytic Sn-Bi 90Sn-10Bi

Sn-Bi Electroless (immersion) Sn-Bi Motorola

Sn-Cu Electrolytic Sn-Cu 99Sn-1Cu

Sn-Ni Electrolytic Sn-Ni

* For multilayer designs, the sequence of materials starts from the bottom layer.

Substitution in the triazole ring (positions 1 and 2) results in a considerabledecrease in inhibiting efficiency versus BTA. A methyl group in the benzene ring(positions 4 and 5) gives, on the other hand, a value of the inhibiting efficiency, whichis even higher than that obtained for BTA. These results are closely related to theability of the molecules to chemisorb and form stable film on the surface.Therefore,the inhibiting efficiency is 4Me- (30 Å) and 5Me-BTA (70 Å) > BTA (<100 Å) >1Me- and 2Me-BTA.22

Fabrication Process. The application process of BTA is detailed in Fig. 14.5.20

After the initial acid clean, the copper surface is microetched to produce a subtleroughness in order to enhance subsequent solder bonding, as well as possible in-


FIGURE 14.2 Organic solderability preservative in Japan.(a) Surface finishes for double-sided, multilayer boards, and(b) surface finishes for single-sided PCBs.

(b)

(a)


FIGURE 14.3 Scheme of triazole inhibitor mechanism and chemistry.

FIGURE 14.4 Surface orientation of BTA on copper.

circuit test probe contact. The etchant is then removed by deionized (DI) rinse, fol-lowed by acid rinse, DI rinse, then BTA coating, and completed with DI rinsing anddrying. This OSP application process is slightly simpler than the HASL applicationprocess: (1) acid clean, (2) water rinse, (3) etch, (4) water rinse, (5) flux application,(6) preheat to 105 to 150°C, (7) solder coat (2 to 10 s/250 to 260°C), (8) excessblowoff, (9) water rinse, and (10) dry.23 Following is a list that compares the applica-tion processes for HASL and OSP:

HASL OSP1. Acid clean 1. Acid clean2. Water rinse 2. Water rinse3. Etch 3. Microetch4. Water rinse 4. Water rinse5. Flux application 5. Acid rinse6. Preheat 105–150°C 6. Water rinse7. Solder coat (2–10 s/250–260°C) 7. OSP coat8. Excess blowoff 8. DIW rinse9. Water rinse 9. Dry

10. Dry

For BTA coating process, 2.5 percent concentration is satisfactory. The shelf lifeof BTA-coated PCB is about 2 years at normal storage condition. Generally, the Cu-BTA complex thickness is 40 to 140 Å.24 However, under high humidity, the shelf lifemay be as short as 3 to 6 months only.23


FIGURE 14.5 Deposition process for BTA on copper.

FIGURE 14.6 Effect of temperature on the oxidation of BTA-treated and -nontreated copper.

Performance. Walker studied the effect of temperature on the oxidation extent ofBTA-treated copper and nontreated copper by monitoring the weight gain of cop-per after thermal aging. Results indicate that BTA is much more effective in reduc-ing oxidation at lower temperature than higher temperature, as shown in Fig. 14.6.25

Therefore, aging at 200°C for 400 min, the treated copper had no weight gain,whereas the nontreated sample exhibited a weight gain of about 2 mg/80 cm2. Thedifference between treated and nontreated copper diminished with increasing tem-perature. After aging at 400°C for 400 min, the BTA-treated copper only oxidizedslightly less than the plain copper, and both exhibited a weight gain of around 50mg/80 cm2.

Obviously, increasing oxidation will deteriorate the solderability. Thwaites inves-tigated the effect of aging on solder spread and wetting time of copper foil with var-ious treatments, with results shown in Fig. 14.7.26 Benzotriazole, tin-lead coating, andpreflux (resin lacquer) all preserved solderability very well in terms of area ofspread after 6 months of normal storage. The uncoated copper showed no spread atall after the same period of storage. However, the difference between various treat-ments becomes more pronounced in terms of wetting time, with tin-lead being themost effective in retaining the wetting time, followed in order by resin lacquer, BTA,and uncoated copper. After 6 months’ storage, tin-lead coating increased onlyslightly in wetting time, while BTA increased from about 0.2 to around 5 s. Foruncoated copper, no wetting can be observed at all.

14.3.2 IMIDAZOLES

The chemical structures and oxidation inhibition mechanism of alkylimidazoles isshown in Fig. 14.8. Similar to BTA, alkylimidazole reacts with copper oxide andforms polymeric alkylimidazolium copper film on the surface of copper. The


FIGURE 14.7 Solderability for various treatments of copper foil.


FIGURE 14.8 Scheme of chemical structure andinhibition mechanism of alkylimidazole.

thickness of this compound is approximately 100 Å.27 The reaction takes placealmost immediately, on the order of seconds. Once the surface reaction has beencompleted, extended dwells in the bath do not advance the reaction; the processpassivates. Consequently, the process is easily controlled. The chemistry is aque-ous based and can be stored for long periods of time. The most critical part of the process is rinsing between the conditioner and OSP components of theprocess.27

The efficiency of oxidation inhibition of imidazole is temperature-sensitive.Ray et al. studied the effect of heat treatment on oxide thickness for imidazole-coated copper, with results shown in Fig. 14.9.5 At a low temperature (e.g., 100°C),the oxidation inhibition efficiency is very high, and no oxidation can be discernedafter 4 h. The efficiency declines with increasing temperature. At 200°C, the oxidethickness increases about six times after 4 h of aging. The sensitivity toward agingtemperature can result in poor wetting due to storage and due to multiple sol-dering processes, as demonstrated by Fig. 14.10.12 In reflow applications, poorwetting can cause opens for joints with the use of a zipper stencil aperturedesign.

14.3.3 BENZIMIDAZOLES

Use of benzimidazoles as an OSP for the electronics industry evolved from imida-zole chemistry. In the 1970s, Sanwa of Shikoku Chemical Co. patented alkylimida-zole and benzylimidazole coating. These were adopted in the United States in 1985.The coating thickness is typically around 0.3 µm, and may range from 0.2 to 0.5 µm.Product suppliers include MacDermid (M-Coat), Chemcut (Schercoat), Kester(Protecto), and Enthone-OMI (Entek).23,28 The protection mechanism is similar tothat of imidazole, with a polymeric imidazolium copper film, as shown in Fig. 14.11.20


FIGURE 14.9 Effect of heat treatment on oxide thickness forimidazole-coated Cu.

FIGURE 14.10 Examples of good wetting and poor wetting for PCBs with imidazole coating. (a)Incomplete wetting of pads, (b) open caused by a zipper stencil aperture pattern, (c) completely filledplated-through-hole (PTH) via, and (d) partially filled PTH via.

In 1997, Sirtori et al. from IBM-Semea studied the stability of a 2-butyl-5-chloro-benzimidazole film (BCB) before and after a thermal treatment, which simulatedthe annealing of a typical soldering process.The chemical bonds inside the BCB filmand between this film and the copper substrate were investigated by x-ray photo-electron spectroscopy (XPS). The thickness of that film was measured by opticalellipsometry. Protection of the film versus oxidation and its physical modificationduring the thermal treatment was also investigated. The XPS valence data showedthat a part of the chlorine was linked to the benzene ring, and the other part formedeither copper chloride or another chloride linked to the nitrogen of the BCB mole-cule.The passivation film hardened and became an electric insulator during the ther-mal treatment, so that any successive electrical test was impaired.29

Introduction of substituted benzimidazoles (BAs) results in an improvement insoldering defect rate. In a study conducted by Siemens Information and Communi-cation Networks, Inc., the board quality was the leading cause of wave-solderingdefects in their experiment. When Entek 106A, a substituted BA, is used as an OSP,the bridging defect rate was slightly higher than HASL, and increased slightly withincreasing residual oxygen level in the soldering atmosphere, as shown in Fig.14.12.30 The solder mask quality is a more dominant factor than the oxygen level,with an improperly cured solder mask causing much more crossing and bridgingthan the oxygen does.

Fabrication Process. The fabrication process of Entek 106A is shown in Fig.14.13.20 The process is similar to that of other azole coating processes. Typically,BA chemistry deposits 0.1 to 0.4 µm and relies on ionic copper in the working solu-tion to develop the protective coating. However, there is one limitation about thisprocess: the mixed-metal surfaces can become stained or discolored when pro-cessed through the OSP. The discoloration is due to nonuniform deposition of theOSP on the different metals due to the presence of the copper ion.31 This problemcan be corrected by utilizing a copper-free formula. By pretreating the copper sur-face with an alkaline precoat solution, the copper on the printed wiring board(PWB) becomes available to build a consistent OSP coating. The precoat is mildlyalkaline and will not discolor active metal surfaces such as gold or aluminum, evenif contaminated with copper. The ionic copper can be eliminated from the OSPsolution since the copper on the PWB can react with OSP to form the desired coat-


FIGURE 14.11 Inhibition mechanism of substituted benzimidazole on copper surface.

ing.20 This new process produces a slightly thinner OSP layer, about 0.2 to 0.25 µmthinner than the standard process. However, the resistance against being burnedout by heat treatment under reflow atmosphere is still comparable with the stan-dard OSP, as shown in Fig. 14.14.

The new process produces a very thin, stain-free OSP coating on an active metalsurface. The thickness is less than 0.05 µm and is virtually absent on the metal sur-face, as shown in Fig. 14.15 when tested on a Ni-Au or matte Ni surface.20 Like theoriginal, it provides similar protection against incidental handling contact, 12months’ shelf life, and similar solderability even with mixed technology in a no-cleanair assembly environment. Because of this new mechanism, a thinner but equallyrobust OSP coating is possible while being selective to copper only.

The OSP fabricated with a new process is more heat-resistant than the standardOSP, as shown in Table 14.2.32 Among the OSPs tested, BTA is the least heat-resistant.

Azoles react with metallic as well as with cuprous or cupric oxides. Ray et al.reported that films grown on the native oxide are more compact and polymerized toprovide greater corrosion protection. Films grown from acidic media are thicker, butmore permeable to oxygen.


FIGURE 14.12 Effect of nitrogen purity on the wave-soldering defectrate. Most of the defects are bridging.

FIGURE 14.13 Typical substituted BA application process.

Imidazole film thickness is typically 50 Å. Benzimidazole or alkylated BAs couldrange from 100 to 10,000 Å in film thickness, depending on solution concentration,immersion time, and other factors. The copper-BA film is polymerized and posessesa high degree of submicroscopic physical porosity. The alkylated BA is more non-porous and less polymerized. Unheated thick films may or may not be solderable,depending on azole chemistry.

The thin films (<100 Å) decompose into volatile products at temperatures above100°C and leave the copper substrate susceptible to oxidation. Films 50 to 100 timesthicker than the monolayer azole films do not suffer a significant change in thicknessduring thermal treatment, but are still ineffective for solderability retention. Thesefilms undergo dealkylization/oxidation reactions during thermal treatment, whichrender them unsolderable.

The chemical structure of substituted BAs has a great impact on the solderability.Figure 14.16 shows the effect of OSP coating thickness on wetting tension, calcu-lated from wetting force.5 A short alkyl group on benzene ring C5 of the BA resultsin a higher wetting tension, whereas a long alkyl group on imidazole carbon providesthe highest wetting tension and is independent of coating thickness.

In summary, the benefits of substituted BAs include the following:

Simple conveyorizable process. Ability to withstand the rigors of multiple heat cycles. At least three reflow cycles

(even more in an inert environment). However, this capability degrades if theboards are washed due to misprint or mishandling prior to reflow.


FIGURE 14.14 Organic solderability preservative coating thickness on copper using the standardprocess and the new process.

FIGURE 14.15 Impact of the new OSP coating process on the OSP thickness of Au and Ni surfaces.

BA OSP thickness:−0.2 to 0.5 µm (0.3 µm is typical)

Excellent coplanarity is offered. Microwave applications: best. Shelf life: 2 months. Cost of surface finish is 0.2 to 0.3 times that of HASL. Corrosion resistance of OSP-coated Cu pads may be a concern during the product

service life.


FIGURE 14.16 Effect of the coating thickness of various OSPs on solderability.

TABLE 14.2 Thickness Reduction Due to Reflow Atmosphere

Reflow Std OSP New OSP BTA Imidazole

0× 0.32 µm 0.16 µm ∼80 ang. 218.4 ang.

1 × air 0.30 µm 0.16 µm 0* 219.5 ang.

1 × N2 0.14 µm 0.12 µm 0* 205.0 ang.

Reduction air 6.25% 0.0% 100% 0.0%

Reduction N2 56.25% 25% 100% 6.1%

* Based on presence test.

14.3.4 PREFLUX

The preflux (rosin/resin-based coatings) have been used extensively as solderabilitypreservatives. Adequate formulations could retain solderability of copper after sev-eral thermal excursions experienced by mixed technology. However, due to thenature of rosin/resin, the PCBs often remain somewhat tacky and, accordingly, canintroduce contamination through manual handling or entrapment of particulates inthe air. In addition, these preflux coatings are sensitive to flux chemistry that is utilized and are virtually compatible only with rosin-based solder pastes or fluxes.

Prefluxes are not as robust as modified azole A in terms of heat and moisture resist-ance, as shown in Fig. 14.17.33 However, they are better than other azole chemistry.

14.4 Ni/Au

Au overcoat on top of Ni has been used as an alternative to HASL for many years,due to the advantages of a flat surface, stability against environment, good shelf lifeand solderability, and reduced bridging at assembly. It is one of the favorite surfacefinishes for fine-pitch surface-mount technology (SMT) and ball grid array (BGA)packages. Ni serves as a solderable diffusion barrier and prevents copper migrationinto solder. Upon soldering and aging, Ni forms a Ni3Sn4 intermetallic layer withSn-Pb or Sn-Ag alloys, although other intermetallics may also be formed, such as(Cu1-p-qAupNiq)6Sn5 on top of (Ni1-yCuy)3Sn4 intermetallics when soldering with Sn-Ag-Cu alloys.34 Due to the lower coefficient of thermal expansion (CTE) of Nithan Cu (12.96 versus 16.56 ppm/°C), Ni can also stabilize plated-through-holes


FIGURE 14.17 Effect of various surface treatments on solderability after multiple reflows or afterhumidity treatment.

during thermal excursion.35 Although Ni forms a more stable solder joint interfacethan does Cu, it is prone to oxidation, and the Au overcoat serves as an oxidationbarrier for the Ni underlayer.

If the Au layer is thin, it will quickly dissolve into solder and be removed from theinterface, thus eliminating the concern about Au embrittlement. If the Au layer is notthin, Au embrittlement may be a concern. Au dissolves more rapidly into Sn thaninto eutectic Sn-Pb solder for the same amount of superheating.36,37 The same is truefor eutectic Sn-Ag, although Sn-Ag is more tolerant of Au than eutectic Sn-Pb.38–40

The two most commonly used Ni/Au finishes are electrolytic Ni/Au and electrolessNi/immersion Au.

14.4.1 ELECTROLYTIC Ni/Au

Electrolytic Ni/Au comprises an inner layer of electrolytic Ni plus an outer layer ofelectrolytic Au (often expressed as EG). It is the traditional surface finish used forwire-bonded applications. This process requires the bars to make an electrical con-nection from the edge of the panel to the area to be plated. The tie bars complicatesubstrate layout and compromise the routing density of PCB traces. This becomes acritical issue when the circuit connection density increases and is the main reasonthat EG is less common than immersion gold, to be discussed later.11 The typicalthickness of nickel in EG is 150 to 200 µin (3.75 to 5 µm), although 200 to 250 µin(5 to 6.25 µm) is desired in order to prevent slivering during etching and subsequentprocessing.35

For an EG surface finish, the thickness and hardness of Au are highly depend-ent on the applications. In general, 3 to 15 µin (0.075 to 0.375 µm) of hard Au isneeded for soldering, with greater than 30 µin (0.75 µm) of hard Au for the con-nector and greater than 30 µin (0.75 µm) of soft Au for wire bonding.35 How-ever, these amounts should only be considered as a guideline. This is particularlytrue when a surface finish is used for mixed applications. For instance, Nakajimaet al., from Flextronics, employed a metal finish of 20 µin (0.5 µm) of soft Au and200 µin (5 µm) of Ni as a wire-bondable and solderable surface finish in Per-sonal Computer Memory Card International Association (PCMCIA) card as-sembly involving chip-scale packages (CSPs) and chips on board (COBs). Non-solder-mask-defined (NSMD) pad designs were applied on this double-sidedboard.41

Fabrication Process. The steps of the fabrication process of EG itself can bedescribed as follows35:

1. Use cleaner to clean the copper surface.2. Etch copper surface.3. Plate nickel at 52 to 57°C.4. Plate gold at 29 to 38°C.

The PCB fabrication process is affected by the type of surface finish selected. Inthe case of an EN surface finish, the steps of the PCB fabrication process are asfollows35:

1. Inner-layer (IL) manufacture.2. Layup and lamination.


3. Drill.4. Desmear and electroless copper plating.5. Outer-layer (OL) imaging.6. Plate copper.7. Plate nickel.8. Plate gold.9. Strip and etch.

10. Surface preparation.11. Solder mask.12. Fabrication.13. Electrical test.14. Final inspection.

For connector applications using an EG surface finish, the steps of the PCBprocess are as follows35:

1. IL manufacture.2. Layup and lamination.3. Drill.4. Desmear and electroless copper plating.5. OL imaging.6. Plate copper.7. Plate Ni/Au.8. Acid clean.9. Flat mask.

10. Plate gold tips.11. Strip and etch.12. Surface preparation.13. Solder mask.14. Fabrication.15. Electrical test.16. Final inspection.

Performance. For Au-Au thermosonic wire bonding, the bondability decreaseswith increasing hardness of the Au surface. At Au layer thickness of 0.15, 0.10,∼0.05, and 0 µm, the Knoop hardness (HK) is 235, 554, 862, and 1007, respectively.A minimum Au layer of 0.1-µm thickness is considered to be necessary for goodbondability by Lai and Liu.42 In general, the hardness of the Au surface decreaseswith increasing Au thickness, while the Au thickness increases with increasing Pcontent in the Ni layer, and decreasing pH value of the nickel bath. For instance, atpH levels of 4 to 5, 8 to 9, and 9 to 10, the P level (wt%) in the Ni layer is 10 to 12,5 to 8, and 2 to 5, respectively. Thus, at a P content (wt% in Ni) of 10.3, 9.6, 8.4, and6.5, the Au layer thickness is 0.15, 0.10, 0.08, and 0.02 µm, respectively. For a goodAu wire (25-µm diameter) bonding, the pull strength is 7.8 gf, and loops broke


right above the ball. For a poor bond (pull strength of 2.7 gf), the bond ball liftedoff the pad.42

For soldering applications, gold embrittlement is an issue if the gold content insolder joints exceeds 3 percent by weight.43–45 This is equivalent to about 30 µin(0.75 µm) of Au on the pad.Therefore, if the gold thickness is specified at 3 to 15 µin(0.075 to 0.375 µm), the gold embrittlement can be well prevented. Figure 14.18shows the histogram of gold layer thickness in the EG process.35 The gold thicknessranges from 3 to 9 µin and holds a process capability (Cp) value better than 1.0.

Electrolytic gold is less porous than immersion gold; therefore, it is more effec-tive in preserving the solderability of the pad.35 Cinque and Morris, Jr., from


FIGURE 14.18 Histogram of EG gold thickness by x-ray fluorescence. Total of 297 measurements,mean = 5.1, Cp = 1.6.


Lawrence Berkeley Lab, investigated the possibility of flux soldering on an EGfinish.46 The results suggest that electrodeposition of a thin gold plate (0.14 µm)and the concurrent reduction of nickel oxide produce a gold-nickel system thatwill wet without flux. Nickel oxidation was observed to occur via nickel out-diffusion and by direct exposure of the substrate through pinhole plating defects.Auger chemical analysis indicates that pinholes do not produce oxidation of thesurrounding substrate area.

In the work of Ludwig et al., the soldering performance of EG was comparedwith OSP and two new lead-free surface finishes: satin-bright Ni/Sn and Ni/PdCo/Au.47 EG is found to be good in wettability and voiding, and medium in lap shearstrength. It is not sensitive to aging, flux chemistry, reflow atmosphere; it is slightlysensitive to alloy type and profile length. Figures 14.19, 14.20, and 14.21 show thewetting, voiding, and lap shear bond strength performance of EG versus three othersurface finishes, respectively.47 The data shown are the overall average performanceof systems including five different fluxes and four lead-free solders, 42Sn-58Bi,91.8Sn-4.8Bi-3.4Ag, 95.5Sn-3.8Ag-0.7Cu, 96.5Sn-3.5Ag, together with eutectic63Sn-37Pb.

The EG process does have a limited ability to throw nickel; therefore, it cannotbe used without extreme measures in high–aspect ratio boards. Consequently, theprocess is not the finish of choice for thick boards with small holes. An aspect ratioof less than 8:1 is required.35 Other shortcomings experienced by the EG finishinclude: cost, solder joint embrittlement, slivers, contamination from solder maskfumes.43,48 The EG process is adequate for boards with connectors, PCMCIAs, mem-ory, or technologies that do not require a high aspect ratio.

14.4.2 ELECTROLESS Ni/IMMERSION Au

Electroless nickel (100 to 200 µin, or 2.5 to 5.0 µm) with an immersion gold flash (6to 10 µin, or 0.15 to 0.25 µm) (ENIG) is another major type of Ni/Au surface finish.

Fabrication Process. The fabrication process of EG itself can be described as fol-lows35:

1. Use cleaner to clean the copper surface.2. Etch copper surface.3. Pre-dip.4. Catalyst.5. EN at 79 to 85°C.6. Neutralization.7. Immersion gold at 85 to 91°C.

The PCB fabrication process is affected by the type of surface finish selected. Inthe case of an ENIG surface finish, the PCB fabrication process is as follows35:

1. IL manufacture.2. Layup and lamination.3. Drill.4. Desmear and electroless copper plating.

14.19

FIGURE 14.19 Effect of surface finish on wetting.

FIGURE 14.20 Effect of surface finish on voiding.

FIGURE 14.21 Effect of surface finish on lap shear bondstrength.

5. OL imaging.6. Plate copper.7. Plate nickel.8. Plate gold.9. Strip and etch.

10. Surface preparation.11. Solder mask.12. Fabrication.13. Electrical test.14. Final inspection.

For connector applications using an ENIG surface finish, the PCB process can beshown as follows35:

1. IL manufacture.2. Layup and lamination.3. Drill.4. Desmear and electroless copper plating.5. OL imaging.6. Plate copper.7. Plate Ni/Au.8. Acid clean.9. Flat mask.

10. Plate gold tips.11. Strip and etch.12. Surface preparation.13. Solder mask.14. Fabrication.15. Electrical test.16. Final inspection.

Chemistry of the ENIG Process. The first step in the ENIG process is to cat-alyze the copper surface for Ni deposition. Proper catalysis of the copper fornickel deposition has been demonstrated to affect the highest-priority failuremodes. The electromotive potentials of nickel and several other elements areshown in Table 14.3.

Since Ni is less noble than Cu, activation of the copper surface for EN plating canbe achieved in several ways. Normally, it is done by seeding a noble metal such aspalladium from an acidic solution of palladium sulfate/chloride.49 Recently, a gal-vanic displacement reaction is used to deposit a catalytic seed layer of ruthenium(Ru) metal onto the Cu. The use of ruthenium catalysis is a more selective and con-trollable process for catalyzing copper. Ruthenium is favored since it is readily avail-able, cost-effective, resistant to colloid formation, and inspectable during deposition.The ruthenium-nickel-boron system reduces the occurrence and increases the detec-tion of failure modes associated with EN deposition.50


The Ni is then deposited from the solution onto the catalyzed copper surface.Table 14.4 shows the formulations, including components, functions, and examples ofEN plating bath.51

The Ni deposition is aided by a hypophosphite (H2PO2−) reducing agent that

decomposes during this reaction and results in the codeposition of phosphorous inthe EN layer.49

3ΝaΗ2PΟ2 + 3Η2Ο + ΝiSΟ4 → 3ΝaΗ2PΟ3 + Η2SΟ4 + 2Η2 + Νi0

or alternatively

Νi2+ + Η2PΟ2− + Η2Ο → Νi0 + Η2PΟ3

− + 2Η+


TABLE 14.3 Electromotive Potential ofSeveral Selected Elements

Element Electromotive potential

Au +1.40 V

Pd +0.83 V

Ag +0.80 V

Ru +0.45 V

Cu +0.34 V

H 0.00 V

Ni −0.25 V

TABLE 14.4 Formulations Including Components, Functions,and Examples of EN Plating Bath

Components Functions Examples

Nickel ions Source of nickel ions Nickel chloride, nickel sulfate,nickel acetate

Hypophosphite Ions Reducing agent Sodium hypophosphite, sodiumborohydride, hydrazine

Complexants Form nickel complexes, prevent Monocarboxylic acids, dicarboxylicexcess free-Ni ion concen- acids, hydroxycarboxylic acidstration, so stabilizing the Nibath.

Accelerators Activate hypophosphite ions Sulfur compounds anions of someand accelerate deposition. monodicarboxylic acids, fluoridesMode of action opposesstabilizers.

Stabilizers Prevent solution breakdown by Sulfur compounds, transition metals,shielding catalytically active etc.nuclei.

Buffers pH stabilizers, for long-term pH Sodium salt of certain complexantscontrol

pH regulators For subsequent pH control Sulfuric acid/caustic soda ammonia

The nickel cation obtains two electrons from the hypophosphite and is reduced tonickel metal. However, release of electrons from hypophosphite occurs only at thesite of a catalyst. The catalyst is a physical site where the reducing agent is adsorbedand rearranges to an intermediate product, donating its electrons. Figure 14.22shows a scanning electron microscope (SEM) photo of ruthenium-catalyzed nickeldeposition.50

Phosphorus is also codeposited with nickel as shown in the following:

2H2PO2− + Hads → H2PO3

− + H2O + OH− + P

3H2PO2− → H2PO3

− + H2O + 2OH− + 2P

Table 14.5 shows characteristics of EN.50

Another EN plating chemistry is boron based, as shown in the following52:

2R2ΝΗ ⋅ ΒΗ3 + 3Νi++ + 2Η2Ο → ΝiΒ + 2Νi + 2R2ΝΗ + ΗΒΟ2 + 6Η+ + Η2

Mei et al. stated that EN becomes more corrosion-resistant with increasing phospho-rus content.51 The phosphorus content in electroless Ni should not be too high, becausethe wetting on nickel-phosphorus degrades with the phosphorus content.51 The phos-phorus content increases with decreasing pH value. Typically, the nickel-phosphorus

32


FIGURE 14.22 Scanning electron microscope image of ruthenium-catalyzed nickel deposition.

chemistry is categorized as low-phosphorus (1 to 5 percent), midphosphorus (6 to 10percent), and high-phosphorus (>10 percent) content. Midphosphorus is the mostcommonly used for microelectronics applications.52 Atotech investigations haveshown that the phosphorus content in the upper range of 8 to 10 percent gives manyof the desired properties that are needed for the ENIG finish.The phosphorus contentin an EN deposit normally increases 1 to 2 percent as the nickel metal turnovers(MTOs) increase, as shown in Fig. 14.23.49

Cullen noted that, in theory, 1 µm of electroless NiP will prevent any migrationunder soldering conditions. In addition to the migration of copper through nickel,the solder will also dissolve the nickel during reflow.The dissolution also occurs to amuch lesser extent after soldering. Based on the dissolution rate of nickel into solderto form the Ni3Sn4 intermetallic, 0.5 µm is considered more than enough nickelthickness to perform this function at typical soldering conditions.50 As mentionedearlier, the commonly employed EN thickness is 2.5 to 5 µm.

Immersion Gold Process. An immersion reaction is an oxidation/reduction sys-tem in which a metal ion in solution is reduced to the metal at the expense of the sur-face metal, which is oxidized to an ion. The exchange occurs in one direction onlyand is determined by the relative positions of the interacting metals in the electro-motive force series. In principle, any metal ions in solution higher in the electromo-


TABLE 14.5 Physical Characteristics of EN

Property Measured value Test method, comments

Hardness 500–700 HV100 Vickers

500–700 HK100 Knoop

45–55 RC Rockwell

Density 7.9–8.3 g/cm3 Pure Ni = 8.90

Melting point 890°C 7 < P < 10%

Thermal expansion 12–15 µm/m°C 0–100°C

Wear resistance 14–18 TWI (Taber Wear Index) Weight loss, 1000 rev, 10 N

Electrical resistivity 55–90 µΩ⋅cm

Ductility 1–2.5% elongation Instron pull test

Internal stress 0–5 Kpsi tensile From 10–6% P, 1 mil

Thermal conductivity 0.01 cal/° cm s−1

Phosphorous 6–10% ICP AA

Grain size 0.001–0.01 µm (10–100 Å) X-ray diffraction

Tensile strength 500–750 N/mm2 Instron pull test

FIGURE 14.23 Phosphorus content of EN as a func-tion of nickel MTO.

tive series will displace any base metal below it in the series. Since the base metal isoxidized, it is also considered a corrosion process for the base metal. In immersionplating of gold over nickel, gold ions from solution are reduced to gold metal. Theelectrons needed for this reduction are supplied by the nickel substrate itself.Immersion deposition or displacement plating will cease as soon as the substrate iscompletely covered by the immersion coating52:

Νi0 + 2Αu+ → Νi2+ + 2Αu0

or

Νi + 2Αu(CΝ)2− → Νi2+ + 2Αu + 4CΝ−

The typical thickness of gold flash layer is less than 5.0 µin, or 0.125 µm.53

The immersion gold thickness used in the electronics industry ranges from 2 to 8µin (0.05 to 0.2 µm). The studies carried out by Atotech indicated that an ideal goldthickness is 3 to 4 µin, with an operating window of 2.5 to 4.5 µin.Too low gold thick-ness will result in oxidation of the nickel and consequently poor wetting of the sol-der during assembly.Too high gold thickness will result in high levels of attack on thenickel surface, resulting in the possibility of interfacial fracture leading to poor sol-derability.49 At 150 µin (3.75 µm) of nickel and 3 to 5 µin (0.075 to 0.125 µm) of gold,the copper circuits are completely encapsulated. The solderability of the nickel ispreserved by the gold to at least a 1-year shelf life.

Performance. Similar to EG, ENIG offers advantages such as coplanarity, Al-wirebondability, and the ability to survive multiple soldering cycles (up to three reflows).Again, the nickel layer allows multiple hand reworks without copper dissolutionbeing a factor. As described in the EG section, the nickel also acts like a rivet toimprove through-hole thermal integrity.49 The gold finish on the nickel has goodreflectivity. This allows this finish to be suitable for automated optical inspection(AOI). The difference in color between the gold and the solder after assemblymakes for ease of visual inspection at that stage. The electrical conductivity of thefinish does not interfere with electrical testing before or after assembly.53

Electroless nickel/immersion gold is the more common Ni/Au surface finish speci-fication. However, if not properly fabricated, its thin or porous gold can allow nickel tomigrate to the surface and oxidize, causing a nonsolderable surface mount pad. Thenickel thickness and phosphorus content in the nickel also play an important role inobtaining reliable solder connections.11 Other problems encountered by ENIG includeblack pad, skip plating, extraneous plating, and embrittlement of the solder mask.

Black pad is a symptom that is associated with some weak solder joints formedon ENIG surface finishes.After the weak joint is ruptured, the exposed nickel pad isblack. The black-pad defect was found to be due to a hyperactive corrosive immer-sion gold (IG) process that changes the near-surface microstructure of P-Ni into onewith a marginal to total nonwetting state. Figure 14.24 shows a high-magnificationcross-sectional view of a solder joint formed on black pad.54

The black-pad defect shall be classified in terms of hyperactive corrosive activ-ity.55 Charge buildup near the module boundaries triggers hypercorrosion. Longtraces (with several ohms of resistance) with differential pad plating surface areamay induce the condition of charge buildup by the galvanic reaction. The minor dif-ference in electrical potential among the packages and leads caused different typesor different degrees of chemical reactions between the ENIG bath, resulting in thepreferential occurrence of black pads. Johal reported that a rapid buildup of animmersion gold layer encourages a higher attack on the E-Ni. This rapid attackoccurs along the E-Ni grain boundaries, resulting in a possible black-pad defect.49


Skip plating is seen on some boards as areas with missing nickel on copper pads, asshown in Fig. 14.25.49 Extraneous plating is an excessive buildup of nickel on copper,as shown in Fig. 14.26.49 Both symptoms can be attributed to an activation process ofthe copper surface. As discussed earlier, activation of the copper surface for EN plat-ing can be achieved in several ways, and is normally done by seeding a noble metalsuch as palladium from an acidic solution of palladium sulfate/chloride. However,there is a balance of the amount of palladium seeding in combination to the activity ofthe nickel bath that must be maintained; otherwise, the skip plating or extraneous plat-ing will occur.49 Besides the unbalanced activation process as a cause of skip plating, itwas also hypothesized that a static is created in the solder mask operation on certaincapacitive areas of the circuitry and that this static attracts volatiles during the maskcure operation and is the underlying cause of skip plating.56

Embrittlement of the solder mask is mainly caused by the EN bath. This bath, aswell as the immersion gold bath, is operated at around 82°C with a 20-min soak timefor nickel and 10 min for gold. Besides being affected by the elevated temperature,the strong reducing agent may also be absorbed into the soft porous mask, making itbrittle and, consequently, may lead to peeling at the mask surface junction. Bakingthe boards after the ENIG process may minimize this effect.56

14.4.3 ELECTROLESS Ni/ELECTROLESS (AUTOCATALYTIC) Au

Most wire-bonded COB assemblies include SMT components on the same sub-strate. Thick gold is necessary for the wire-bonded devices, but is unacceptable forthe SMT devices. The electroless gold process can be selectively plated over the


FIGURE 14.24 A 2000× cross-sectional view of a gullwing solder joint formed on a black pad.Severe corrosion can be noted at the side of the pad surface.

immersion gold. Following the immersion gold, a plating mask is applied to the panelexposing only the area requiring the thicker gold. The electroless gold is thendeposited onto the unmasked areas. The finished gold thickness is 20 to 60 µin overnickel. The plating chemistry of electroless gold (cyanide-based) can be expressedby the following:

R2ΝΗ ⋅ ΒΗ3 + 4ΟΗ− + 3Αu(CΝ)2− → R2ΝΗ + Η2 + ΒΟ2

− + 2Η2Ο + 3Αu + 6CΝ−

14.5 IMMERSION Ag

Immersion silver is another lead-free surface finish formed by galvanic reaction. Sil-ver is selected due to the following four considerations: (1) electromotive potentialof Ag (+0.80 V) relative to copper (+0.344 V) allows use of immersion depositionprocess; (2) high electrical conductivity of Ag is compatible with touchpad applica-tions, in-circuit probe test processes, and signal transmission requirement; (3) Ag is anoble metal and thus promises good stability; (4) Ag dissolves into solder quicklyand thus promises good solderability.1

Fabrication Process. The immersion silver coating process consists of four baths, aprecleaner, a microetch followed by the conditioner, and finally the plating bath.Table 14.6 shows the detailed process conditions for organic modified silver (Ster-ling™ Silver).1 The Ag coating is applied by an immersion process that exchanges

32


FIGURE 14.25 Example of skip plating in the ENIG process.

copper from the base metal for silver in the silver nitrate bath, as illustrated by Fig.14.27.1 As discussed in the section on the immersion Au process, a particular benefitof immersion processes is the self-terminating feature; that is, the process terminateswhen the coating completely covers the base material. As a result, the plating thick-ness is consistent and easily controlled.54 The process can be horizontal or vertical,with 8 min of cycle time for a conveyorized process. Typically, 120 panels/h can betreated in 8-m equipment at 50°C process temperature. Figure 14.28 shows anexample of PCB with an immersion Ag surface finish.1

Silver finish is applied after the solder mask, with the silver deposited on theexposed copper surface. Since silver surfaces are readily tarnished, an organicinhibitor is included in the plating bath to protect the surface. The thickness of


FIGURE 14.26 Example of extraneous plating in the ENIG process caused by overactivation ofcopper.

TABLE 14.6 Immersion Process Conditions for Sterling™ Silver

Process Temperature (°C) Conveyorized Immersion

Sterling™ acid cleaner 50 30 s 5 min

Sterling™ surface prep 40 60 s 60 s

Sterling™ predip 30 30 s 30 s

Sterling™ Silver 50 60 s 60 s

organic modified silver is typically 0.2 to0.3 µm, although other thickness ranges(e.g., 0.08 to 0.16 µm) have also beenused. Obviously, the coplanarity will beexcellent for such a thin coating. Figure14.29 shows the topology of the organicmodified silver.1 Inclusion of an organicinhibitor is fairly evenly distributed inthe Ag layer down to a depth of about 3µin (0.075 µm), as indicated by theAuger depth profiling for organic modi-fied silver finish (see Fig. 14.30).1

For Alpha-Level immersion silver, the coating consists of a layer of silver approx-imately 4 to 5 µin (0.1 to 0.125 µm) thick with a thin inhibitor layer superimposed, asshown in Fig. 14.31.57 The inhibitor layer is essentially an OSP, which is approxi-mately 5 Å thick.58

Microetch. The immersion Ag finish can be produced as a shiny or a matte finish.This shiny or matte surface characteristics are due to the surface roughness, and arecontrolled by the etching rate of copper. This microetch step provides a secondarycleaning of the copper surface. In addition, it also microroughens the surface andincreases the surface area. As a result, after the immersion Ag coating process, the


FIGURE 14.27 Copper displaced by silver inthe immersion silver process.

FIGURE 14.28 Picture of a PCB with an immersion Ag surface finish.


FIGURE 14.29 Topology of organic modified silver coating.

FIGURE 14.30 Auger depth profiling for organic modified silver finish. The silverdeposit contains 2 to 4 percent carbon (by wt%) or approximately 30 percent (by at%).(Source: Auger analysis data provided by Arch Chemicals, Inc.)

surface with increased surface roughness or greater surface area appears as a mattefinish, and the surface with minimal surface roughness appears bright and shiny.

Plating Chemistry. The reaction of the immersion Ag process can be expressed asfollows:

Cu + 2Αg+ → Cu2+ + 2Αg

The chemicals used in the plating bath are listed in Table 14.7. Ag coating thicknessis affected by time, temperature, pH, and Ag ion concentration of silver nitratebath.54 Figure 14.32 shows the effect of immersion time on Ag coating thickness.59

Performance. Perhaps the first concern about immersion Ag is the potential for Agmigration. Cullen investigated the Bellcore TR-78 electromigration performance ofimmersion silver (Sterling™ Silver) together with copper and HASL.1 Results indi-cate that, at 85°C/85%RH and 10 VDC bias condition, immersion silver exhibits aresistance value higher than that of copper, and HASL finishes at both 96 and 596 h,as shown in Fig. 14.33, thus eliminating the concern on Ag migration. Figure 14.34shows the posttest coupon from the electromigration test, displaying no sign of den-drite formation. Results on a surface insulation resistance (SIR) test also show a safepass for Bellcore TR-78 specifications. Chada et al., from Motorola, also reportedthat the immersion Ag surface finish performs adequately in the SIR and electromi-gration (EM) tests and is not readily prone to dendritic growth in the presence ofhigh humidity.60 However, ENIG and OSP are superior in the water droplet condi-tions simulating condensation and are less likely to electromigrate under those cir-cumstances.


FIGURE 14.31 Schematic of alpha-level immersion Ag.

TABLE 14.7 Chemistry of Immersion Ag Plating Bath

Chemicals Functions

Ag Metal source, 0.46 V relative to Cu

HNO3 Produce Ag anion, and accelerate reaction

Cu complexation Prevent the copper in solution to affect reaction

Inhibitors Prevent bath sensitivity to light, and assure deposituniformity

Surfactants Prevent electromigration and inhibit tarnish

Buffers Control pH of bath

The solderability of immersion Ag finish has been tested by Beigle by evaluatingthe hole-filling performance at wave soldering. Results indicate that immersion Agis almost as good as HASL Sn-Pb, and is considerably better than OSP for bothsteam-aged and unaged samples, as shown in Table 14.8.59 Beigle also studied thewetting force of immersion Ag, HASL, two OSPs, and electrolytic Ni-Au surface fin-ishes in meniscograph test with five different aging treatments: (1) fresh, (2) threereflows, (3) 40°C/93% RH for 96 h, (4) 40°C/93% RH for 96 h, followed by threereflows, and (5) 150°C for 2 h. Results indicate that the wetting force for all systemsis comparable for fresh samples.Aging treatment results in a declining wetting forcefor systems other than HASL. Immersion Ag is less sensitive to aging treatment thanboth electrolytic Ni-Au and OSPs.

The wetting defect rate of reflow soldering is expected to be closely correlatedwith wetting force performance. Cullen studied the wetting defect rate (IPC-J-STD003 Test F) of a convection reflow soldering system for OSP, ENIG, and immersionAg with three different thicknesses. Results indicate that only the OSP finishexhibits wetting defects, as shown in Fig. 14.35.

Gordon et al. also reported that the solderability of immersion silver is relativelyinsensitive to storage at 85°C/85% RH conditions. Depending on the type and thick-


FIGURE 14.32 Effect of immersion time on Ag coating thickness.

FIGURE 14.33 Electromigration data of immersion Ag (Sterling), copper, and HASL at96 and 596 h, per Bellcore TR-78 specifications: 85°C/85% RH, 10 VDC bias.

ness, however, the immersion silver is sensitive to assembly processes. Immersion sil-ver has demonstrated superior moisture and insulation resistance behavior comparedwith HASL, and modules manufactured with an immersion silver finish have passedall appropriate module validation testing for automotive electronic applications.61

Chada and Bradley reported sensitivity of immersion Ag toward corrosive atmo-sphere storage condition.60 In their work, wetting and spreading of both Pb-containing and Pb-free solder pastes over the immersion Ag surface are adequateeven when the surface is mildly corroded. However, ENIG and OSP exhibit greatersolder spread than immersion Ag finish for all testing conditions studied, althoughthe silver was more consistent. Also, if subjected to a corrosive environment forextended periods of time (>96 h flowing mixed gas), wetting deteriorates drastically


FIGURE 14.34 Posttest coupon from electromigration test, indicating no sign of dendrite forma-tion from the silver-finished traces.

TABLE 14.8 Effect of Surface Finish Type andAging Treatment on the Hole-Filling Performance

Percentage of hole fill

Coating No aging Steam-aged

Silver-plated 99.7 99.9

HASL 100 99.97

Organic A 93.57 92.62

for immersion Ag and ENIG finishes. This sensitivity of immersion Ag toward cor-rosive storage environment is consistent with the findings of Reed, who reportedthat immersion Ag is sensitive to corrosive environments62: the solderability ofimmersion silver is severely impacted by exposure to Cl2, SO2, and NO2 gases in thepresence of water vapor. It is recommended by Reed that boards be packaged inpolyethylene bags no matter what the storage environment is. If the raw boards arestored in dry air, a 1-year shelf life can be expected.

Reed also noted that exposed silver after assembly will corrode, although theproducts of this chemical attack appear to be benign for SIR and dendritic growth.Regardless, it may cause difficulties for field repairs. As to the unprotected immer-sion silver, the test points and etched-on symbols will tarnish, and the solder mask isconsidered an effective protectant for silver in corrosive environments in service.Since the amount of silver on the board termination is estimated to contribute lessthan 0.1 percent in a typical 20-mil pitch solder joint, no effect is expected on the sol-der joint life.62

Immersion Ag is also good for ultrasonic Al wire-bonding applications. Figure14.36 shows comparison of clad Al pad and immersion Ag (Sterling Silver) finish onultrasonic 10-mil Al wire bonding.1 Results indicate that immersion Ag is slightlybetter than clad Al, and thus it is adequate for wire-bonding applications.

Cullen also reported the test results on touchpad applications. In this work, thecontact resistance of four different surface finishes following 100,000 touchpad acti-vations was compared, as shown in Table 14.9.1 Immersion Ag, electroless Pd, andENIG remain very good electrical contact, while the resistance of conductive carbonincreases to 0.28 mΩ.

In-circuit test is another important criterion to be met by any PCB surface fin-ishes. Gordon et al. studied the in-circuit test performance of immersion Ag.61 Intheir work, the test pad was probed with blade probes made of heat-treated steel,coated with gold on top of a hard-nickel finish. For each probe, a spring force of 8.1oz was provided. Results indicate that percent of reseats and percent of failures


FIGURE 14.35 Effect of PCB surface finish types on solderability defect rateper IPC-J-STD-003 Type F in convection reflow soldering process.

decrease with increasing Ag thickness and increasing surface roughness (or etchingrate).61 This increase in surface roughness appears to be crucial for enhancing thecontact between pad and probe, at least in the case of blade probes.

Chada et al. observed that immersion Ag/BGA solder joints appear to havelower load levels at failure than OSP and ENIG finishes in the as-reflowed condi-tion.The effects of silver thickness and peak reflow temperature are insignificant onthe reliability. However, solid-state aging and multiple reflows lead to a lowering offailure load.60 In Parker’s study at Viasystems, the rupture strength of the jointsformed on immersion Ag finish consistently exceeds 1.0 lb, which is within the rangegenerally attributed to a joint formed on an HASL surface. More important, how-ever, is the fact that the rupture always occurs at the lead interface and never at thepad/board interface. This indicates that the bond at the pad is superior to thatformed at the lead.54

Chase et al., at Raytheon and Nokia, compared the effect of surface finishes,including immersion Ag, Ni/Au (ENIG), and solder HASL on the second-level reli-ability of fine-pitch area array assemblies.63 A temperature cycling test with a tem-perature range of −40 to +125°C was used, with dwell times of at least 20 min at hightemperature and 15 min at low temperature. Chamber temperature ramp rates were8 to 10°C/min.The average time for 1 cycle was 75 min. Results indicate that, for 144I/O and 0.8-mm-pitch BGAs, the reliability increases in the following order: Ni-Au <immersion Ag < HASL, as shown in Fig. 14.37.63 However, it should be noted that


TABLE 14.9 Contact Resistance of Touchpads After 100,000 Activations

Surface finishes Contact resistance (mΩ) Note

Conductive carbon 0.2773 Heavy gloss on one pad, slight glosson all others. Circuit board between one pad is worn to shine.

Immersion Ag (Sterling) 0 Pads slightly dulled.

Electroless Pd 0 Green circuit between is turningbrown.

ENIG 0 Some nicks and cuts in gold pads.

FIGURE 14.36 Effect of surface finish types on ultrasonic 10-mil Al wire-bonding per-formance.

14.3

5

FIGURE 14.37 Comparison of 144 I/O BGA test results by surface finish. The temperature cycling range is −40 to +125°C.

some premature failure points of the immersion Ag system started from 254 cycleswere considered to fall in another failure mechanism and were not plotted. Thesepremature failures were attributed to the presence of postassembly hairline crackson the board side; the cause of formation of those hairline cracks is not understoodyet. For 156 I/O with 1.0-mm-pitch BGA system, immersion Ag is comparable withHASL and is better than ENIG.

14.6 IMMERSION Bi

Immersion Bi was introduced in 1996 as a nonprecious metal surface finish intendedto address the coplanarity problem experienced by HASL in fine-pitch applica-tions.59

Fabrication Process. The immersion bismuth process features three steps:

1. An acidic cleaner that removes surface oils and solder mask residue.2. Microetch step that prepares the copper substrate topography for the deposit of

immersion bismuth. For the bismuth to deposit on copper, the bath is highlyacidic.

3. Bi plating step.

The immersion bismuth bath is operated at 50°C, and a typical contact time of 1to 2 min to produce a deposit of pure metallic bismuth onto the copper surface. Sim-ilar to other immersion processes, the reaction is complete when the copper can nolonger be released and the surface is completely covered.

Performance. The plated Bi metal is uniform dark gray, and easily distinguishedfrom the substrate copper. During aging or thermal excursions, the Bi finishbecomes more copperlike in appearance. This is believed to be caused by diffusionof the bismuth into the underlying copper. Beigle considered this to be a cosmeticeffect and has no major impact on solderability under normal conditions.59

The solderability of immersion Bi was found to be better than Pd in a through-hole filling test. For as-plated finishes, Bi exhibited 99 percent hole fill, while Pd onlydisplayed 60 percent hole fill.59 In another experiment comparing Bi with OSP, PCBswith through-holes varying in hole size from 0.062 to 0.022 in were used to examinethe hole fill with 11 different no-clean fluxes. A highly activated organic acid–basedflux was used as a control. All boards were preconditioned with two air-atmosphereheating cycles with a profile having a peak temperature of 220°C and a dwell time of40 s. At wave soldering, each board was applied with 600 to 800 µg/in2 of flux. Thesoldering performance was evaluated using complete topside pad coverage as theacceptance criteria. Results indicate that immersion Bi is less sensitive than OSP toflux selection, as shown in Fig. 14.38.59

The solderability of immersion Bi for reflow applications was also reported byBeigle.59 The spread performance of solder paste was determined on 20-mil-pitchQFP pads with 70-mil length, using the following procedure:

1. Steam-age half of the test boards.2. Precondition test boards—one infrared reflow pass in air.3. Screen solder paste.


4. Reflow solder in the air.5. Inspect and measure solder wetting distance.

Results indicate that the reflow spread performance decreases in the followingorder: HASL > immersion Ag > OSP > immersion Bi, as shown in Table 14.10.59 Allsurface finishes are considered acceptable.

The temperature cycling reliability of solder joints on immersion Bi has beenstudied by Beigle and Guy with the use of LCCC68 component.59,64 Both works indi-cate that immersion Bi provides comparable temperature cycling performance toHASL. For instance, Beigle used thermal cycle condition from −55 to 125°C with a0.5-h ramp and an additional 0.5 h at temperature for 1000 cycles. The continuity ofeach component was monitored during each cycle, and resistance over 600 Ω wasconsidered open and as a component failure. Results indicate that within statisticalsignificance, immersion Bi is comparable with HASL in reliability, as shown in Fig.14.39.59

The wire bondability of immersion Bi is very poor. No adhesion can be registeredin a pull test of gold wire-bondingattempt. One of the concerns aboutimmersion Bi finish is sensitivity towardPb. When used with Sn-Pb solder alloys,the ternary eutectic alloy 8Sn-52Pb-40Bi(melting point 95°C) formed may causeearly failure and extensive porosity dur-ing temperature cycling or service. Thefailure mechanism induced by the forma-tion of ternary eutectic alloy 8Sn-52Pb-40Bi was elucidated by Mei et al. oneutectic Sn-Bi system.65


FIGURE 14.38 Effect of flux selection on the hole filling yield of immersion Bi andOSP finishes.

TABLE 14.10 Reflow SpreadPerformance of Various Surface Finishes

Mean solder wetting distance

Coating Not aged Steam-aged

Bismuth 65.85 64.72

Silver 68.46 68.24

HASL 69.28 68.39

OSP 67.86 66.19

14.7 Pd

Palladium (Pd) is an attractive, cost-effective alternative finish for printed wiringboards (PWBs) that is both solderable and wire bondable. Similar to gold, Pd is anoble metal; therefore, it is stable against many chemical reactions, such as oxida-tion. In addition, Pd exhibits several advantages over Au as a potential surface finishconstituent:

1. Pd is cheaper than Au.2. Pd exhibits a density 38 percent lower than Au (12.02 versus 19.32 g/cm3), thus

further reduces the cost of Pd needed for surface finish applications.3. Pd displays a tensile strength about 35 percent higher than Au.4. Pd exhibits a hardness at 250 to 290 Vickers, about twice that of copper and three

times that of gold, thus making it more suitable for contact purposes.5. Pd dissolves in molten 60Sn-40Pb at a much lower rate than Au (about 0.01 ver-

sus 5 µm/s), thus it is less prone to contaminate the solder pot.66,67

14.7.1 ELECTROLYTIC Pd WITH OR WITHOUT IMMERSION Au

Electrolytic Pd or electrolytic Pd with Au flash provides a thin deposit on top of cop-per. The Pd is less than 0.5 µm, and is typically 0.25 µm in thickness. The Au flash is0.025 µm in thickness.The fabrication process is described in the following subsection.

Fabrication Process. The electrolytic Pd plating process is integrated with thePCB patterning process as follows:

1. Cu plate2. Pd plate3. Resist strip4. Cu etch5. Solder mask application


FIGURE 14.39 Solder joint reliability in temper-ature cycling of −55 to 125°C test using LCCC68component soldered on various surface finishes.

During the plating cycle, Pd is applied immediately after acid copper plating. Thedwell time in the Pd is 1 to 2 min.After Pd plating, the photoresist is stripped and theboards are processed through the copper etcher, and finally the solder mask isapplied. The Pd etch resist can then be activated for further processing if required.For instance, electroless Pd and/or immersion Au can be selectively applied, depend-ing on the specific solderability and bonding requirements.68

Performance. The major advantage of a Pd finish is its use as an etch resist replace-ment for Sn-Pb. It reduces the manufacturing steps and the cycle time by reducingthe plating time, and it eliminates the Sn-Pb stripping step. Other advantages pro-vided include:

It is wire bondable. It is solderable. It has uniform thickness and excellent coplanarity, thus making it suitable for high-

density interconnect applications.

The solderability stability of a Pd finish against storage conditions is evaluatedwith steam and thermal aging. Table 14.11 shows the solderability test results of Pdagainst Ni/Au and Ni/Ag finishes.68 Apparently, Pd exhibits a superior solderabilityand stability. This is attributed to the low porosity of Pd versus that of Ni/Au orNi/Ag.Applying a layer of Au flash on top of the Pd layer further improves the stor-age stability. Kakija et al. also reported that Pd and Pd-alloy electrodeposits pre-serve the integrity of the surface finish and provide good solderability by limitingporosity, inhibiting thermal diffusion, and increasing wetting speeds.69

However, it also has been reported that Pd may not always be a good diffusionbarrier.Wang and Tu, at UCLA, noted that an intermetallic compound (IMC), whichgrows at a rate greater than 1 µm/s, has been observed in the liquid/solid reaction at250°C between molten eutectic Sn-Pb solder and solid Pd. The intermetallic PdSn4

that is formed does not serve as a diffusion barrier between the reactants. Instead, itgrows as lamellae into the molten solder, with the growth direction being normal tothe liquid/solid interface. The molten solder between the lamellae serves as fast dif-fusion channels during the reaction. On the other hand, molten Sn reacts with Pd ata rate that is slower by one order of magnitude than Sn-Pb. The IMCs formed heregrow as a diffusion barrier between the Sn and Pd.70

The bond strength of solder joints on a Pd finish is considerably lower than sev-eral other finishes. Ray et al. examined the pull strength of 50-mil-pitch, 20 I/O gull-


TABLE 14.11 Solderability of Pd Versus Ni/Au and Ni/Ag After Steam Aging and Thermal Aging*

After Cu etch After Cu etch, After Cu etch,85°C/85% SA,† 150°C TA,†

16 h 16 h

0.25 µm Pd/Cu laminate 99 96 96

0.025 µm Au/0.25 µm Pd/Cu laminate 99 99 98

1.25 µm Ag/1.25 µm Ni/Cu laminate 96 55 70

0.375 µm Au/1.25 µm Ni/Cu laminate 99 60 85

* All values are expressed as percentages.† SA: steam aging; TA: thermal aging.

wing-leaded small-outline integrated circuits (SOICs) before and after thermalcycling (0 to 100°C, 3 cycles/h) using a Pb-free alloy, 96.2 Sn-2.5Ag-0.8Cu-0.5Sb(CASTIN).71 Four surface finishes were compared: (1) ENIG (150 µin Ni/5 to 10 µinAu), (2) EN (150 µin)/electroless Pd (5 to 10 µin), (3) Pd (20 µin, or 0.5 µm) on cop-per, and (4) imidazole. Results indicate that the bond strength of the Pd surface fin-ish is the lowest one among the four surface finishes, as shown in Fig. 14.40.71 This canbe attributed to the formation of a large quantity of PdSn4 intermetallic. Similar toAu, the volume of intermetallics formed between Pd and Sn is significantly higherthan that formed between Sn and other metals such as Cu or Ag. This is because theamount of Sn consumed for formation of intermetallics is much higher for Au(AuSn4) and Pd (PdSn4) than for Cu (Cu6Sn5 or Cu3Sn) and Ag (Ag3Sn). Here the Pd


FIGURE 14.40 Mechanical pull test data for 50-mil-pitchSOICs before and after 5000 thermal cycling (0 to 100°C, 3cycles/h) using alloy 96.2Sn-2.5Ag-0.8Cu-0.5Sb (CASTIN).The component is 20 I/O gullwing-leaded SOIC.

TABLE 14.12 Pull Test Results of 1-mil Au Wire on Au-FlashedElectrolytic Pd Before and After Aging at 150°C for 64 h

Ball bond Ball bondminimum setting maximum setting

Force (g) 50 50

Power (mW) 3.0 9.9

Temp (°C) 120 120

Time (µs) 10 10

Pull force, 5.35 6.53

as plated (g)

Std. deviation 1.69 0.21

Pull force, 6.38 6.29

64 h, 150°C (g)

Std. deviation 1.24 0.53

thickness (20 µin) in the Pd finish is thicker than the Au thickness (5 to 10 µin) inENIG or the Pd thickness (5 to 10 µin) in Ni/Pd, thus causing a greater deteriorationin bond strength. Additional cycling treatment does not cause further deteriorationin bond strength for the Pd finish, suggesting that the Pd finish may be a viableoption as a surface finish. Similar tests conducted using 256 I/O 0.4-mm-pitch plasticquad flat pack (PQFP) also shows that the Pd finish exhibits the lowest pull strength.However, treatment with 2500 thermal cycles results in a slight decrease in bondstrength for both Pd and Ni/Pd systems, as shown in Fig. 14.41.71 Presence of a largequantity of PdSn4 intermetallics can be seen easily.

The wire bondability of Pd with Au flash was studied with the use of a 1-mil Auwire-and-ball-bonding process. Results indicate that the wire bondability is main-tained after thermal aging at 150°C for 64 h, as shown in Table 14.12.68


FIGURE 14.41 Mechanical pull test data for 256 I/O, 4-mm-pitch gullwing-leaded PQFPs beforeand after thermal cycling using alloy 96.2Sn-2.5Ag-0.8Cu-).5Sb (CASTIN). The thermal cycling con-dition is 0 to 100°C, 3 cycles/h.

14.7.2 ELECTROLESS (AUTOCATALYTIC) Pd WITH OR WITHOUT

IMMERSION Au

An electroless Pd coating process is essentially an autocatalytic process with or with-out an immersion Au flash (<1 µin, or 0.025 µm). The thickness of Pd is dependenton the applications. For soldering purposes, a Pd thickness of 3 to 9 µin (0.075 to0.225 µm), often at 4 to 6 µin (0.1 to 0.15 µm), is employed. For wire-bonding appli-cations, the Pd thickness is about 0.6 µm.

Fabrication Process. Milad and Roberts, from Atotech USA, Inc., reported thatthe fabrication process for PD-Tech PC, a trade name for their electroless Pd finishproduct, consists of the following four steps67:

1. Acid clean2. Microetch3. Activation4. Electroless Pd

The acid clean and microetch steps are required to provide a properly treatedcopper surface. The activator is formulated to be selective to copper only. The acti-vation step is an immersion process and produces a thin layer of Pd with a thicknessless than 1 µin via an exchange reaction with the copper. This immersion process isself-limiting and takes 3 to 5 min at 40°C for complete coverage in a bath bufferedin the pH range of 1.1 to 1.4. The electroless Pd deposition step is linear over time,regardless of the dwell period.The deposition of a pure Pd is assured through the useof an exclusive reducing agent, with the Pd concentration of the solution maintainedat around 1.0 g/L. The bath temperature is typically maintained between 60 and70°C, while the pH of the process is maintained within a range of 5.3 to 5.7.The dep-osition rate of Pd is affected by bath temperature and is typically in the range of 1.0to 2.5 µin/min.

The Pd in the electroless bath will only deposit on a Pd metal surface formed inthe activation step via the immersion Pd process. In the diffusion layer, adsorbedhydrogen on the Pd surface reduces the complexed Pd2+ in the electroless bath to themetallic state, which deposits and subsequently adsorbs additional hydrogen gener-ated by the reducing agent. As a result, the reaction becomes autocatalytic and con-tinues linearly. The process is maintained by control of the Pd concentration in theactivation bath and the pH of the electroless Pd bath. Replenishment of the Pd inthe electroless bath is determined by the concentration in the activation stage.

Performance. Pd finish serves as a sacrificial layer. Pure Pd at a thickness of 4 to 6µin provides a highly solderable finish, comparable with that of HASL. During waveor reflow soldering, the Pd layer is dissolved into the solder and is held in suspen-sion. At the solder–base metal interface, the intermetallic that is formed is a copper-tin intermetallic.67 The solderability performance of Pd is comparable with Au-Ni,but the advantage is primarily shelf life. Pd finishes perform better after acceleratedaging tests than do those with Au/Ni. This is because Pd acts as a thermal/migra-tion/diffusion barrier, whereas Au or Ag allows migration of Ni or Cu through to thesurface. Since Cu will not readily diffuse through Pd, Pd can be applied directly overCu and protect it from oxidation. Seto et al. studied the use of an electroless Pd sur-face finish for soft-touch switches and high-density SMT assemblies in a joint pro-gram between Chrysler Huntsville Electronics and Photocircuits Corporation.72 ThePd thickness employed ranges from 3 to 9 µin (0.075 to 0.225 µm). Surface insulation


resistance on Pd-finished PWBs was reported to be much lower than for an HASLproduct. Some other test results are shown in Table 14.13.72 Results indicate that therequirements of Chrysler are met for each test listed.

Since April 1996, a running total of more than 2.5 million assemblies have beenbuilt at Chrysler, with an average production rate of 7500 assemblies per day.The in-process PWB defect rate at the assembly level is reported to be less than 30 ppm. Pdhas been qualified for Chrysler reflow processes for new SMT products employingdouble-sided, two-pass convection reflow.

Electroless Pd with a thickness of 0.6 µm (24 µin) with a Au flash (<0.025 µm)was compared with Ni/Pd for wire-bonding performance. In the second finish, ENwas deposited to a thickness of 5.0 µm (200 µin) followed by an electroless Pddeposit of 0.2 µm (8 µin) also with a Au flash. Both finishes were then subjected toAu (thermosonic) and Al (ultrasonic) wire bonding. Based on the test results,Milad and Roberts concluded that Pd over Cu with a Au flash or a Ni/Pd/Au flashboth offer excellent Au wire-bonding properties. For Al bonding, similar resultswere only achieved for Ni/Pd/Au flash. Al wire bonding to Pd/Au flash, althoughpossible, is limited due to a very narrow operating range of optimal bondingparameters.67

14.8 ELECTROLESS Ni/Pd/(Au FLASH)

Electroless Ni/Pd with or without an overcoat of immersion Au is a cheaper alterna-tive to ENIG (Ni/Au).A thin layer of Pd on copper is sufficient for delivering a goodsolderable finish. However, in cases where Au wire bonding is required, an under-coat of Ni will be required for best performance. For soldering and Al wire-bondingapplications, the thickness of an EN underlayer is typically 100 to 200 µin (2.5 to 5.0µm). On top of that is the electroless Pd layer, with a thickness of 5 to 10 µin (0.125to 0.25 µm), and more commonly 6 to 8 µin (0.15 to 0.2 µm).The overcoat immersionAu flash typically is less than 1 µin (0.025 µm). If the Pd layer is increased to 15.0 to20.0 µin (0.375 to 0.50 µm), the surface finish is then solderable and gold-wire-bondable.

Fabrication Process. The EN process is the same as that described in Sec. 14.4.2.After the EN process, the surface to be Pd plated is catalyzed with an immersiondeposit of Pd before electroless Pd plating can proceed. This catalyzation restrictsthe deposit to the metallic substrate without extraneous plating on the mask or lam-inate.


TABLE 14.13 Results of Tests Conducted at Chrysler on Pd-Finished PWB Assemblies

Test performed Results Comments

Key contact resistance Passed After 22,000 cycles

Life test Passed Underhood environment; assembled by reflow

Thermal cycle Passed −40–105°C

Process sensitivity Good PWBs with Pd thickness extremes processed at high, nominal, and low reflow temperature profiles

Electroless Pd is an autocatalytic process that involves using a chemical reducingagent to plate palladium metal out of solution. The reducing agent varies and mayincorporate other elements in the deposit, such as phosphorus if sodium hypophos-phite is the reducing agent. The thickness of the autocatalytic deposit is not self-limiting and will increase continuously as long as the bath chemistry balance andoperating parameter are maintained properly.The Pd bath operates under relativelymild conditions, with a pH in the range of 4.0 to 6.0 and the temperature not exceed-ing 65°C.67

Performance. If properly fabricated, Ni/Pd preserves solderability better thanNi/Au, since Pd is inherently less porous than Au. This makes Ni migration lesslikely, thus giving a pure Pd surface on which to solder. Stacy et al. studied the sol-derability of EN/Pd with and without Au flash before and after steam aging (SA)with the use of three commercial fluxes. Results indicate that, for both finishes, nodegradation in solderability can be discerned after 8 h of SA, as shown in Table14.14.68 However, Toben and Kanzler reported that the use of a Ni underlayer com-promises solderability somewhat for applications where both soldering and wirebonding are specified on the same board.73

The wire bondability of EN/Pd/Au flash is well preserved against storage condi-tion. Table 14.15 shows the pull test results of 1-mil Au wire on EN/Pd/Au flash. Nodeterioration of the pull force can be discerned after 8 h of treatment at 85°C/85%RH.68

However, Pd dissolves much less readily into tin alloy solder than does Au. Thisrequires that the Pd layer be extremely thin to prevent the formation of a weakinterface layer between the nickel and solder. Melton and Fuerhaupter suggested aPd thickness of 1 to 2 µin (0.025 to 0.05 µm).This thin layer is susceptible to mechan-ical damage from scratches and electrical test probes and can cause the Ni under-layer to be exposed, which is detrimental to the solderability of the Pd surface.11

Currently, the electroless Pd layer is about 6 to 8 µin (0.15 to 0.2 µm) in thickness.The mechanical pull test data for 50-mil-pitch SOICs on EN/Pd before and after5000 thermal cycling (0 to 100°C, 3 cycles/h) indicate that the formation of a weak Pdinterface layer between solder and nickel may not be an issue, as shown in Fig. 14.40.Here the alloy used is 96.2 Sn-2.5Ag-0.8Cu-0.5Sb (CASTIN), and the component is20 I/O gullwing-leaded SOIC.71


TABLE 14.14 Solderability of EN/Pd with and Without Au FlashBefore and After SA*

K-135Rosin No. K-951 K-1515

Flux type Act. MA Act. Rosin

As-plated 95 98 98electroless Pd/EN

As-plated 97 99 97GF/electroless Pd/EN

8 h SA 95 95 97electroless Pd/EN

8 h SA 97 98 98GF/electroless Pd/EN

* All values are expressed as percentages.

Pd tends to react with organic molecules in the atmosphere to form an insulative,nonsolderable organic film over long periods of storage through catalytic reaction.11

Other typical problems encountered with Pd plating include:

Inability to visually discern Pd from Ni if plating over Ni, which may lead to ship-ping parts that have no Pd on them.

Poor bath maintenance practice can lead to Pd deposits that won’t solder or wire-bond.

Poor activation of surface to be plated with Pd, hence delamination of Pd depositoccurs.

Unclean rinses and poor handling after Pd plating causes solderability and wire-bonding failures.

14.9 Ni/Pd(X)

14.9.1 ELECTROLYTIC Ni/PdCo/Au FLASH

This surface finish, developed by Lucent Technology, is composed of a 100-µin nickelbottom layer, a 10-µin 80Pd-20Co (w/w) middle layer, and a 2- to 3-µin Au top layer.All three layers are electrolytically plated. Ludwig et al. studied the solder pastereflow soldering-related performance, and the results are compared with OSP,Ni/Au, and Ni/Sn. Four lead-free solders [Sn42-Bi58 (Sn-Bi), Sn91.8-Bi4.8-Ag3.4(Sn-Bi-Ag), Sn95.5-Ag3.8-Cu0.7 (Sn-Ag-Cu), and Sn96.5-Ag3.5 (Sn-Ag)] wereused, with eutectic Sn63-Pb37 (Sn-Pb) also included as a reference. Also included inthe test matrix are five fluxes, three reflow profiles, two reflow atmospheres, and twoaging treatments.

The study concluded that, overall, the Ni/PdCo/Au is poor in wettability (see Fig.14.42), fairly low in lap shear strength (see Fig. 14.43), and high in voiding (see Fig.14.44). However, it is fairly stable, and its soldering performance is not sensitive toprofile length, reflow atmosphere, aging treatment, and flux chemistry. It does seemto be sensitive to Bi-containing alloy in terms of voiding and lap shear strength.74

14.9.2 ELECTROLESS Ni/PdNi/Au FLASH

Electroless PdNi alloy with a Au flash over EN is the preferred contact metallurgy,reported by Yeung and Nakamura from Hitachi Micro Systems.75 It is said to provide


TABLE 14.15 Pull Test Results of 1-mil AuWire on EN/Pd/Au Flash Before and After 8 hof Treatment at 85°C/85% RH

Mean load DeviationFinish (g) (g)

GF eP2-EN,* 5.21 1.09as plated

GF eP2-EN, 6.45 0.938 h SA

* eP2: electroless Pd on immersion Pd; GF: Au flash.

cost savings, extend contact life, and requires no organic lubricants. It also enhancescorrosion resistance. Using this surface finish on contact pads enables the design ofhigh-speed, high-density, small form-factor memory modules/cards with improvedreliability, superior electrical characteristics, and enhanced resistance to corrosioncompared with traditional electrolytic plating.75

14.10 Sn

Tin (Sn) is a very attractive option as a lead-free surface finish. Merits of Sn include:

1. It’s lead free.2. It has good solderability.


FIGURE 14.42 Effect of surface finish on wetting.

FIGURE 14.43 Effect of surface finish on bond strength.

3. It has good corrosion resistivity.4. It has reasonably good electrical conductivity.5. It has good mechanical strength.6. It is nontoxic.7. It is abundant in supply and low in cost.8. It is compatible with virtually all solders, particularly Sn-bearing alloys.9. It has a long history, with reliability data available.

10. It has the option of being a metal etch resist.11. It is fusible, if needed.12. It provides a flat surface contour.

However, the following three concerns have to be addressed before full acceptanceis possible:

1. Tin whisker2. Tin pest3. Sliver

14.10.1 ELECTROLYTIC Sn

Several decades ago, electroplated tin was used as a final surface finish. It was aban-doned later as final finish due to problems with current distribution thickness anddeposit stability, besides the whisker and tin pest problems.76 Today, improved ver-sions of electrolytic tin processes are reintroduced as an option for lead-free surfacefinishes, mainly driven by the global lead-free soldering move. The thickness of a tinlayer may vary from 3 µm for the SnTech Sn Satin Bright (SB) process to 7.5 µm toas much as 8 to 15 µm, depending on the process and chemistry.76–78


FIGURE 14.44 Effect of surface finish on voiding.

Fabrication Process. There is a wide variety of plating chemistry for electroplatingtin in the electronics industry. Baths using pyrophosphate, potassium stannate, sodiumstannate, and phenol sulfonic acid have been used in the past. However, they may notbe compatible with the aqueous developing photo resists used for pattern plating.Themost commonly used bath chemistries are the stannous sulfate, stannous fluoroborate,and methanesulfonic acid plating baths.79 Newer plating solutions (e.g., the neutralstannous sulfate/gluconate plating) may also be used. The stannous sulfate bath is thelowest in cost and it is cheaper than the tin-lead baths that are presently used, althoughthe cost of the tin anodes is considerably more than the cost of 60Sn-40Pb anodes.78

The stannous sulfate plating solution is susceptible to reaction with the atmo-sphere to form stannic oxide.The stannic oxide codeposits with the tin and will cause

grainy-appearing solder joints and dewetting atsoldering. Stannic oxide is a very fine-grainedwhite crystal and extremely difficult to filter fromthe plating solution, unless a pertinent coagulantis used for filtering the stannic oxide and clarify-ing the plating bath. Additives for grain refiningand leveling often are available. Examples of aplating bath producing a fine-grained pure-tinplate are given in Table 14.16. Figure 14.45 showsthe topology of the electroplated pure tinobtained accordingly.80


TABLE 14.16 Plating BathProducing Fine-Grained Tin Plate

Item Condition

Sn content 45 g/L

Acid content 200 mL/L

Additive 105 mL/L

Temperature 40°C

Current density 20 Å/dm2

FIGURE 14.45 Topology of electroplated pure tin obtained with the use of a plating bath, as shownin Table 14.16.

Zhang noted that a tin whisker can be depressed by tin coatings with large, well-polygonized grains and a very low inclusion of organics, and reported a tin-platingchemistry that produces such, as shown in Table 14.17.77,81 Figure 14.46 shows thetopology of the electroplated tin with large polygonized grains.77

For most operations the plating is semimatte to bright finish.The bright and semi-bright finishes are harder than the matte finishes, offer good storage life, and areresistant to fingerprinting. Unfused tin is preferable, if a flat surface is required forsurface mount attachment. Figure 14.47 shows the roughness of a surface finish with


TABLE 14.17 Tin Plating Bath Produces Large Polygonized Grains

Feature Typical Range

Sn as metal, g/L 40 10–100

70% methane sulfonic acid (MSA), 200 150–250mL/L

Surfactant, mL/L 40 20–60

Grain refiner 10 8–15

Current density, ASF 10–250 10–1000

Agitation Low to medium Low to vigorous

Temperature, °C 55 50–60

Anode/cathode ratio 1:1 1:1–3:1

FIGURE 14.46 Optical micrograph of electroplated tin with large polygonized grains obtainedwith the use of a plating bath, as shown in Table 14.17.

various treatments. Note that the reflow process deteriorates flatness only when aflux is used.77

Hinton estimated that the cost of tin plating on copper conductors is about equiva-lent to the hot-air-solder-level process, and it is slightly cheaper than the tin-lead pat-tern plating process, if the tin sulfate bath is used instead of the fluoroborate solutions.78

Performance. The tin deposit is highly pliable and ductile.76 The growth rate of thetin-copper IMCs on tin-coated copper is reported to be very nearly the same as60Sn-40Pb solder-coated copper, therefore should not be a major issue for elec-trolytic tin finish.82,83 This minor effect of 100Sn on Cu-Sn intermetallics formationrate is further confirmed by Hunt.The data reported by Hunt indicate that althoughhigher tin content results in a greater intermetallics formation rate, the differencebetween 100Sn and 60Sn-40Pb is less than 20 percent at 155°C.83 Zhang reportedthat, for electroplated pure tin, the deposit consistency in terms of reflectance, melt-ing enthalpy, surface morphology, solderability, and SA resistance are very good upto at least 2.5 bath turnover.77 For large-grained coating, SA (95°C, 95% RH) wasnot sufficient to induce whiskers.

A severe bending test (ASTM standard B489-85, bend coatings around a 0.5-mm-diameter mandrel) did induce whiskers, but whiskers were only observed attensile stressed areas. Four years later, the nonstressed area is still free fromwhiskers.

14.10.2 IMMERSION Sn

Immersion Sn has been in existence for many years, with only limited use. It is oftenused for low-cost products and has a process advantage since it is applied after theetching process of a PCB is complete. The old processes have a short storage life,from a few days to a few months. New processes, such as immersion white tin or flatsolderable tin (FST), made considerable improvement in shelf life.76,84 The typicalthickness is approximately 1 µm. Immersion white tin was defined by IBM in the1980s, referring to an immersion tin coating that would have long-term solderable


FIGURE 14.47 Surface roughness of electroplated tin with largepolygonized grains versus various treatments.

finish applications.This immersion white tin structure contains other properties thatmake it suitable for other production uses in fabricating PWBs. On the other hand,IBM used the term immersion gray tin to define an immersion tin coating that didnot meet solderability requirements.76

Fabrication Process. The FST fabrication process is as follows84,85:

1. Acid cleaner, 49°C, 4 min2. Cascade rinse, ambient3. Persulfate microetch, 0.75 to 1.0 µm Cu, 27°C4. Cascade rinse, ambient5. Sulfuric dip, ambient6. Tin module, 66°C, 8 min7. Warm rinse, 43°C, 1 min8. Cascade rinse9. Hot-air drying, ambient

10. Finish

The tin is applied as a stannous sulfate or chloride solution, and a displacementreaction occurs with the copper. Since copper (+0.342 V) is more positive in electro-chemical potential than Sn (−0.138 V), thiourea is used to drive the reverse potentialneeded for Sn to replace Cu as a deposit.The basic reaction is Sn2+ + Cu → Cu2+ + Sn.The bath usually contains a stannous halide and thiourea, and the tin is deposited onthe surface of the copper. The reaction is self-limiting and stops as soon as the tincoating prevents any further transfer of copper into the solution.As a result, the fin-ishes are thin and are on the order of 0.1 to 1.5 µm, depending on concentration,temperature, and porosity of the deposit. The immersion tin formulations often aremodified to provide autocatalytic deposition of the metal to supplement the immer-sion deposit thickness.The immersion tin process will also codeposit copper with theimmersion tin coating as the bath is used and as the copper concentration increases.As the copper concentration in the bath increases, the solderability of the boarddecreases and the tin coating thickness decreases.78 The modified immersion tinprocess incorporates an organometallic complex, which suppresses Cu-Sn inter-metallics, surface oxidation, and whiskering.

The immersion white tin process is similar to the FST process, as shown in the fol-lowing76:

1. PC5009 cleaner. 10 percent, 2 to 4 min at 29 to 43°C for copper cleaning2. Water rinse. 1 to 2 min3. Acidic surface conditioner CirEtch 100 microetch. 1 lb/gal, 30 to 60 s, 24 to 29°C4. Water rinse. 1 to 2 min5. OV-4 predip. 25 percent, 1 to 2 min, 16 to 32°C6. OA-8 immersion tin solution. 100 percent, 6 to 12 min, 61 to 71°C7. Water rinse (warm). 2 to 4 min8. Water rinse. 1 to 2 min

Total operating cost should be comparable if not less than hot-air-solder-levelingcosts. Immersion white tin is also used as an ammoniacal etch resist.


Performance. Earlier-immersion tin is more prone to oxidation. Ray et al., atAT&T, studied the influence of temperature and humidity on the wettability ofimmersion tin-coated PWBs.86 Exposure to temperature and humidity was variedfrom near ambient (35°C/85% RH) to harsh (SA). A minimum thickness of about1.5 µm was reported to be critical for assembly operations involving multiple ther-mal excursions. Formation of Cu-Sn IMCs at the copper-tin interface in the immer-sion tin finish does not adversely affect the soldering performance, as long as theIMC phase is protected by a tin surface layer. Immersion tin finishes are relativelystable to thermal exposure, but oxidized readily under humid conditions andresulted in solderability degradation. An electroless copper substrate caused signifi-cantly more intermetallic formation, and consequently resulted in poor solderabilityeven under moderate temperature and humidity conditions.

Immersion white tin provided considerable improvement against oxidationresistance. Edgar studied the effect of aging conditions on the oxide thickness


TABLE 14.18 Effect of Aging Condition on the Oxide Thickness Formed on Immersion Tin and theRemaining Tin Layer Thickness

Aging Surface White Immersion Immersion Immersion treatment analysis tin tin 1 tin 2 tin 3

Nonaged Average thickness 0.99 1.01 0.98 1.05of Sn coating (µm)

Stannous oxide (Å) 30 32 28 32

Stannic oxide (Å) 3 4 7 5

6 months’ Average thickness 0.95 Not tested 0.79 0.88aging of Sn coating (µm)

Stannous oxide (Å) 28 Not tested 30 35

Stannic oxide (Å) 3 Not tested 5 9

155°C for Average thickness 0.49 ND 0.05 ND4 h of Sn coating (µm)

Stannous oxide (Å) 13 Oxidized IMC 17 Oxidized IMC onon surface surface

Stannic oxide (Å) 4 Oxidized IMC 7 Oxidized IMC onon surface surface

Steam aging Average thickness 0.74 0.36 0.59 0.558 h of Sn coating (µm)



One-pass air Average thickness 0.63 0.13 0.54 0.28reflow of Sn coating (µm)



Three-pass air Average thickness 0.34 ND 0.05 NDreflow of Sn coating (µm)



formed on immersion tin and the remaining tin layer thickness with the use ofsequential electrochemical reaction analysis (SERA). White tin was evaluatedagainst three conventional immersion gray tin finishes, with results shown in Table14.18.76 Aging consumes Sn and results in a decrease in the Sn layer. Immersion graytin finishes show a more rapid decrease in tin thickness than immersion white tin. Intesting various aging methods, it was noted that the tin surface stability was notdetermined by the original thickness of the coating. Deposit characteristics of tininfluenced the stability of the coating.

The wettability of the surface of aged coatings was tested by stenciling solderpaste with a 1⁄4-in-diameter circle pattern. The solder paste spread on the coatingswas measured and percentage comparison was noted. Results indicate that immer-sion white tin outperformed all three immersion gray tin finishes and is quite com-parable with HASL for aged surface finishes, as shown in Fig. 14.48.76

The SERA can identify the coating’s thickness, but it does not identify the poros-ity of the coating. When the immersion tin coatings are aged at 155°C for 4 h andcompared using SEM, the surface characteristic observed helps to explain the sol-derability results. The immersion gray tin has a honeycomb pattern that is not seenwith an immersion white tin. Oxygen and moisture penetrate the surface and oxidizethe copper. It is this porosity that allows for the failure of the coatings under agingconditions.An immersion white tin structure is completely different from immersiongray tin and allows for long-term stability and solderability.

In Fig. 14.48, the wettability of immersion tin 1 and immersion tin 3 is extremelypoor for samples conditioned at 155°C and 4 h. In Table 14.18 it can be seen that forsamples treated with the same conditions, both finishes exhibit no detectable tinlayer. Furthermore, oxidized intermetallics are present at the surface.This close cor-relation strongly indicates that oxidized intermetallics are not wettable. Obviously,the effect of oxidized intermetallics on wettability is much greater than that of sol-der oxide formed on the top of finishes.

Ormerod studied the oxidation rate of modified immersion Sn (100Sn) com-pared with 80Sn-20Pb and 60Sn-40Pb. Results indicate that the oxidation rate is notsensitive to the Sn content of the surface finishes, as shown in Fig. 14.49.84

It is interesting to note that the effect of oxide thickness on wettability is fairlymoderate. For instance, the wetting force of a 60Sn-40Pb surface finish decreasesrapidly at first with increasing oxide thickness, then slowly at an oxide thickness thatis greater than 4 nm.84


FIGURE 14.48 Effect of surface finish types and aging conditionson the solder paste spread percentage of surface finishes.

The effect of temperature on the intermetallics formation rate of immersion tin(100Sn) on Cu is shown in Fig. 14.50.84 Besides the intermetallic thickness increasingwith increasing temperature, the type of intermetallics may also alter with tempera-ture. At higher temperatures another intermetallic species, Cu3Sn (ε phase), isformed below the Cu6Sn5 (η phase) material. These intermetallics consume theamount of fusible tin, and multiple heat cycles quickly degrade any remaining sol-derability. Since the immersion tin layer is typically 1 µm, care should be taken toensure that the tin layer is not depleted by the formation of Cu-Sn intermetallicsprior to assembly.


FIGURE 14.49 Effect of temperature and alloy composition of sur-face finishes on the oxidation rate.

FIGURE 14.50 The effect of temperature on the intermetallic formation rate of immersion tin(100Sn) on Cu.

For FST finishes, the effect of the aging treatment on solderability is similar tothat of immersion white tin, although the morphology of FST distinctly differs fromthat of white tin.The former exhibits a large round-grained texture influenced by theorganics, while the latter exhibits a fine-grained texture. Raising the temperature toincrease the deposit thickness will also increase the grain size. As a comparison, theconventional tin shows a characteristic angular crystal structure.

Ormerod also reported that FST displays a very low ionics level after processing,as shown in Fig. 14.51.84 Here the FST ionics reading is comparable with Ni/Au, andis one order of magnitude lower than HASL.The IPC-TM 650# 2.6.14 electromigra-tion performance of FST is comparable with bare copper, OSP, Ni/Au, and is consid-erably higher than HASL.

Applications of the Immersion Sn. Applications of immersion Sn include the fol-lowing: As a low-cost solderable metallic finish for non-wire-bonded applications As a replacement for OSPs as a more robust finish As a planar replacement for HASL As a lead eliminator As a solderable substrate for solid solder deposit technology.

14.11 ELECTROLYTIC Ni/Sn

Electrolytic tin with an electrolytic nickel underlayer is a low-cost alternative forprecious-metal finishes, such as Ni-Au or Ni-Pd. Tin has an excellent solderability,probably only next to Sn-Pb finishes. Nickel is often used as a migration barrier


FIGURE 14.51 Flat solderable tin ionic residue after processing compared withbare Cu, OSP, HASL, and Ni/Au. The readings are expressed as chloride equivalentsbased on conductivity measurement.

under the tin to prevent copper from forming a thick, brittle layer of intermetallicswith solder. The cost of plated tin is approximately the same as HASL. The cost ofthe tin plate over nickel is more than the HASL process, especially if the copper,nickel, and tin-plating processes are not in the same continuous plating system.When comparing it with any of the noble metal finishes, tin over nickel will be theleast expensive finish.

The thickness of the Ni/Sn finish is plating chemistry and process dependent. Forinstance, Hinton, at Hinton PWB Engineering, reported a semimatte to bright tinfinish with 8 to 15 µm thickness on top of the nickel underlayer (1 to 2 µm).78 On theother hand, Ludwig et al., at Lucent, reported a satin-bright tin (3 µm) electrolyti-cally plated on a 2.5-µm nickel layer, which is electrolytically plated on the basemetal copper.47 Melton and Fuerhaupter considered the typical electrolytic tin layerto be 7.5 to 10 µm in thickness.11

Fabrication Process. The fabrication process of EN itself can be described as fol-lows, and it is the same as that in Sec. 14.4.135:

1. Use the cleaner to clean the copper surface.2. Etch the copper surface.3. Plate the nickel at 52 to 57°C.

The typical electrolytic Sn plating process has been reviewed by Price.87 Some exam-ples can be found in Sec. 14.10.1.

Copper contamination in the plating predips, which precede the tin-plating bath,may cause depositions on the nickel. This copper deposit may also cause dewettingof the plated surface when soldered. All processes between the nickel bath and thetin bath (e.g., transport, rinse, and preplate clean dwell times) should be as short aspossible to prevent the nickel from being passivated and, as a result, being dewettedat soldering.The nickel underplate is often plated from a low-stress nickel sulfamatechemistry.As discussed earlier, a 1- to 2-µm thickness of nickel is commonly appliedin order to act as a barrier plate and etch resist.

When a tin finish is required on only the solderable lands, a second layer of pho-toresist is applied after the nickel pattern plating operation. The second imageallows for tin plate only on the lands that are to be soldered and is known as theSanta Clara process.Assembled printed boards made by the Santa Clara process willhave all of the high-purity electrodeposited tin plating alloyed with lead, antimony,or other tin whisker and α-tin-inhibiting metals during the soldering process, thusreducing the chances of having those problems.

Performance. The tin layer in the satin-bright tin process comprises large grains,with organic inclusion less than 0.004 w/w %, and exhibits the lowest propensity fortin whisker development.47 The performance in reflow wetting, lap shear strength,and voiding is shown in Figs. 14.42 through 14.44, respectively. In general, this elec-trolytic satin-bright tin Ni-Sn by Lucent, although being sensitive to aging (see Fig.14.52), reflow atmosphere (see Fig. 14.53), solder alloy type (see Fig. 14.54), andvariation in flux chemistry (see Fig. 14.55), it is the highest in wettability, one of thehighest in lap shear strength, and the lowest in voiding.47 It performs better underlong profile.The high sensitivity may be attributed to the relatively high reactivity oftin. Under most instances, the soldering performance is comparable with or betterthan OSP and Ni-Au.



FIGURE 14.52 Effect of aging on wetting of satin-bright tinon nickel. Aging was regulated by sending the substratesthrough a Btu furnace 0 (aging 0) and 1 (aging 2) time using amedium-reflow atmosphere, 245°C peak temperature, and airreflow atmosphere. Aging 2 subjected the coupon to an85°C/85% RH environment for 24 h prior to soldering.

FIGURE 14.53 Effect of reflow atmosphere on wetting of var-ious surface finishes.A ratio value greater than 1 indicates a pos-itive effect of nitrogen. Use of nitrogen provides a better spreadfor the Ni-Sn system, but a negligible effect for the remainingthree systems.


FIGURE 14.54 Effect of solder alloy on wetting. Sn-Pb soldergenerally wets better than the Pb-free alloys. Most of the Pb-free alloys are comparable in wetting, except that Sn-Bi displaysan exceptionally high spread for the Ni-Sn system. Overall, Ni-Sn is more sensitive to alloy type than other surface finishes.

FIGURE 14.55 Effect of flux chemistry on wetting for a sys-tem with Sn-Pb alloy, medium profile, air reflow, and no agingtreatment. Both Ni-Sn and OSP are sensitive to the variation influx type. The sensitivity toward flux type can be ranked as fol-lows: Ni-Sn > OSP > Ni-Au, Ni-PdCo-Au.

14.12 Sn-Bi

Sn-Bi alloy is attractive as a surface finish because it is lead-free, has good solder-ability, is low-cost, and has stability against environment. It is one of the four lead-free alloys—Bi-Sn, In-Sn, Ag-Sn, and Ag-Sb—recommended by the OccupationalSafety and Health Administration as a replacement for Sn-Pb, based on their rela-tive safety in the manufacturing environment. It is also one of three solder alloys—Sn-Ag, Sn-Bi, and Sn-Ag-Bi—recommended by the National Center forManufacturing Sciences as a lead-free solder alloy option. The tin-bismuth alloy hasalready been used in PCB manufacturing as an etch resist.88 However, Bi-containingfinish is sensitive to lead contamination due to the formation of a low-melting(95°C) ternary eutectic alloy 52Bi-30Pb-18Sn, thus compromising the reliability ofsolder joints.65 Another concern is the potential of having fillet lifting. Both phe-nomena will be discussed in detail in Chap. 16.

14.12.1 IMMERSION Sn-Bi ALLOY

Motorola has developed an immersion plating process to deposit approximately a1.0-µm thickness of 70Sn-30Bi alloy onto copper surface mount pads as a PCB sur-face finish.11,89,90

Fabrication Process. The immersion tin-bismuth process is relatively simple toperform. A mild etch should be given to the copper surfaces prior to plating withtin-bismuth, and a rinse should follow the actual tin-bismuth plating process. Theplating reaction can be performed by dipping in a stationary tank, or spraying/flooding in a horizontal conveyorized system. The chemistry is based on salts ofmethane sulfonic acid. The immersion plating process deposits approximately 1.0µm of a 70/30 Sn-Bi alloy in 1 min at 30°C. As other immersion plating processes,this is self-limiting, and the plating reaction stops once the maximum thickness isreached.

Performance. The finish has a matte gray appearance, and thus it can be easily dis-tinguished from the rest of the PCB by automated optical assembly and inspectionequipment. Melton, at Motorola, conducted various tests on PCBs coated with theimmersion tin-bismuth surface finish. Overall, this surface finish passed all of the testswith similar results as tin-lead. Capital costs are expected to be low, since the processis immersion plating. Material costs are low due to the thin thickness to be formed.The cost per square foot of deposited Sn-Bi alloy is estimated to be slightly more thanthat charged for OSP surface finishes.

14.12.2 ELECTROLYTIC Sn-Bi ALLOY

The tin-bismuth alloy has already been used in PCB manufacturing as an etchresist.91 With the addition of Bi, the chance of tin whiskering is highly reduced.One of the features of this electroplated tin alloy is the prevention of tin whiskers.Therefore, M&T marketed one tin electroplating chemical as a whisker-free tinformulation, which contains a small amount of bismuth that is codeposited withthe tin.92


14.13 Sn-Cu (HASL)

99Sn-1Cu is attractive as a board surface finish due to promising solder alloy per-formance. Table 14.19 shows comparison between eutectic Sn-Pb and eutectic Sn-Cu.Through their 1991 to 1994 study, Notel concluded that Sn-Ag and Sn-Cu are themost promising solder alloys among 200 alloy candidates. In the 1994–1995 lab study,Sn-Cu HASL was selected as the most promising lead-free board finish. In 1997, ver-tical Sn-Cu HASL was tried, with encouraging results. In 1998, further developmentof the Sn-Cu HASL board finish was conducted, including a horizontal HASLprocess.93

Fabrication Process. The process condition for manufacturing Sn-Cu HASL isshown in Table 14.20.94 Also shown is the process for Sn-Pb HASL as a comparison.

Performance. The performance of a 99Sn-1Cu HASL surface finish was evaluatedby comparing the pull strengths of solder joints made on various surface finishes, asshown in Table 14.21.93 The components used were plastic leaded chip carrier withSn-Pb surface finish. Results indicate that horizontal Sn-Cu HASL is comparablewith horizontal Sn-Pb HASL, Entek 106, and immersion Ag (Alpha-Level), and isbetter than Ni-Au and vertical Sn-Pb HASL, thus is a very viable option as a boardsurface finish. It should be noted that the HASL process is pertinent for conven-tional SMT assembly, but inadequate for fine-pitch SMT applications, due to uneven


TABLE 14.19 Properties of 63Sn-37Pb and 99Sn-1Cu95

Property 63Sn-37Pb 99Sn-1Cu

Melting point (°C) 183 227

Density (g/ml) 8.4 7.31

Thermal conductivity (W/m⋅K) 56.61 65.73

Electrical conductivity (M mho/cm) 8.73 9.52

Cost per kg (bar) $0.85 $1.03

Cost per kg (paste) $140.8 $163.2

TABLE 14.20 Comparison of Process Conditions Between 63Sn-37Pb and 99Sn-1Cu in HASL Process95

Parameter 63Sn-37Pb 99Sn-1Cu

Bath temperature (°C) 250 280

Air knife temperature (°C) 250 280

Oil temperature (°C) 230 255

Air heat exchanger 250 300

Air pressure Lower Higher

PCB preheat (°C) 150 200

thickness in solder coating for large and small pads. This constraint holds true,regardless of the alloys selected for pad coating.

14.14 ELECTROLYTIC Sn-Ni

Sn-Ni alloy electroplate has been used as a final finish for PCB applications.At a tin-nickel ratio of 65 wt % tin and 35 wt % nickel, the alloy forms a metastable phase,Ni-Sn. Thickness of the plating for the higher process temperature environments isusually 5 µm. For the more benign, lower process temperature environments, one-half of that thickness may be sufficient.78

Fabrication Process. Tin-nickel is typically plated from a chloride-fluoride platingsolution containing ammonium ion.94 It can also be plated with the nickel chlo-ride/tin chloride solution complexed with potassium pyrophosphate, mainly used inAsia.95 Organic additives are used with all of the solutions for grain refinement andleveling. The cost of tin-nickel electroplating is about the same as solder mask overbare copper, with one of the major costs being the plating anodes, which may be tinand nickel or tin-nickel.

Performance. The solderability of the deposit, although not as good as a pure tinor solder coating, is good enough for surface mount land finish applications. It has aflat surface, good storage life, and is compatible with a variety of solders. The Sn-Nifinished board can be etched in normal etching chemistries and may have a numberof other finishes (e.g., Au, Sn-Pb, or Sn) overplated on the Sn-Ni land. Being non-melting in nature, the Sn-Ni finish accepts a solder mask easily and can be used toproduce solder coating on the lands only by the HASL process. Sn-Ni final finish ismost often used when a higher process temperature is required.

Sn-Ni electroplate is much harder than copper, Sn, and 63Sn-37Pb, with hardnessbeing 750, 30, 100, and 12.8 HV, respectively. It maintains a low electrical resistanceand is very corrosion-resistant, thus it can be used for edge connector applications,although the contact resistance is not as low as a gold overplate. The Sn-Ni depositis free from problems such as tin whiskers or tin pest. When aged for a long time at150°C, it will form copper-tin intermetallics with the underlying copper plating in theholes and surface.78


TABLE 14.21 Comparison of Pull Strengths of Joints on VariousSurface Finishes Using 63Sn-37Pb and 99Sn-1Cu for Solder Joints95

Pull strength (N)

PCB finish 63Sn-37Pb 99Sn-1Cu

Ni/Au 19.3 18.8

Alpha-Level 25.6 20.2

Entek 106 23.6 22.7

Vertical Sn-Pb HASL 15.4 17.9

Horizontal Sn-Pb HASL 21.2 23.5

Horizontal Sn-Cu HASL 22.5 21.5

14.15 SOLID SOLDER DEPOSITION (SSD)

Solid solder depositions (SSDs) are a method of depositing solid solder onto landsof PCB.96,97 This solid solder not only serves as the surface finish for board lands,but also provides solder needed for forming solder joints, thus eliminating theneed for the solder paste stencil printing process. Since the solder paste printingstep often contributes approximately two-thirds of the defect rate of the PCBassembly process, SSD represents a potential of improving the yields. This is par-ticularly true for fine-pitch applications with certain SSD processes. AlthoughSSDs are not confined to lead-free solders only, they definitely provide options forlead-free surface finishes to PCB. The types of SSD may include HASL, Optipad,Sipad, PPT, solder cladding, solder jetting, and Super Solder, and are discussedbriefly.

14.15.1 HASL

Hot-air solder level (HASL) techniques were developed in the early 1980s in a ver-tical mode. In the mid-1980s, horizontal machines were designed and graduallybecame the choice of larger PWB manufacturers. As of today, about two-thirds ofthe PWBs are with HASL finish, and the majority of HASL finishes are processedin the horizontal mode.1,98

Fabrication Process. The typical fabrication process for HASL is as follows:

1. Acid clean2. Water rinse3. Etch4. Water rinse5. Flux application6. Preheat (105 to 150°C)7. Molten solder coat (2 to 10 s at 250 to 260°C)8. Excess solder blowoff9. Water rinse

10. Dry

In a vertical HASL process, after the solder mask is applied, the panels are dippedvertically into a molten solder bath, then drawn vertically out of the bath and pasthigh-pressure hot-air knives, which drive the molten solder through the plated-through-holes, and provide some leveling of the solder along the exposed coppersurface. In a horizontal HASL process, after the solder mask is applied, the orien-tation of the panel is changed to the horizontal plane. The panel is then pulledthrough the molten solder bath and past high-pressure hot-air knives. The hori-zontal orientation of the panels allows a conveyorized process, reducing the effectsof gravitational pull on the molten solder and thus minimizing the tendency forpuddling. The angle at which a PWB is presented to the air knives is important; thebest results are achieved on quad flat packs (QFPs) at 45° to the air knife. ThePWB is usually in contact with solder for about 2 s, so the copper tin intermetallic


formed in that time is typically 0.15 to 0.30 µm, although the thickness of IMC canbe as high as 1.9 µm through the HASL process.98,99 Horizontal HASL is capableof providing a solder finish that meets assembly requirements. For 0.010- to 0.020-in pitch, a mean solder coating thickness of 12.5 µm, with an LCL of 1.75 µm and aUCL of 25 µm, is achieved with no solderability problems on a large pad.Typically,the smaller the pad, the thicker the solder coating is.99,100 Table 14.22 shows therange of finished solder coating thicknesses of all pads for horizontal and verticalHASL processes. However, a coating thickness of up to 75 µm at the center of a viahole has been noted.101 Improper cleaning was a leading cause of exposed copperand dewetting in HASL process, and high viscosity with minimal thickness isdesired.102

Performance. The advantages of the HASL process include:

Excellent solderability Long shelf life (12 months) Universal acceptance Multiple heat cycle capability Easy visual inspection Good mask integrity Fair electrical contact Fair microwave applications Compatible with solder mask on bare copper applications No solder reflow under solder mask

The disadvantages of the HASL process include:

Difficult process Boards being thermally stressed Poor surface contrast between solder and pad Not compatible with wire-bonding process Inconsistent solder volume deposition from pad to pad Poor surface coplanarity Hole compensation needed (50 to 75 µm) due to the nonuniform HASL solder

deposit


TABLE 14.22 Finished Solder Coating Thickness for Vertical and Horizontal Sn-Pb HASL Processes

Process Hole wall Board surface Note

Vertical HASL 12.5–25 µm 0.125–25 µm Not recommended for 20 mil

pitch or less

Horizontal HASL 12.5–25 µm 0.25–15 µm Recommended for fine pitch

14.15.2 Optipad

Optipad is a process that delivers flattened solder deposits on pads serving as boththe surface finish of pads and solder source for joint formation. In this process, atemporary dry film (Optimask) is applied to the board, printed, and developed,forming photodefined wells.A liquid solder is forced into the wells.The board is keptflat as it moves through the machine and cools. The temporary dry film is thenstripped from the board.The solder thickness is typically 50 to 200 µm, which is con-trolled by the temporary solder mask.96,97

Fabrication Process. Detailed fabrication process steps for Optipad follow andare schematically shown in Fig. 14.56.96

1. Apply the temporary solder mask.2. Apply the molten solder.3. Flatten the solder.4. Remove the temporary solder mask.5. Apply the sticky flux.6. Place the components.7. Reflow the package.


FIGURE 14.56 Schematic of Optipad process. (a) Bare board, (b) applicationof temporary mask, (c) liquid solder application, (d) stripping of temporarymask.

Performance. The primary benefit in the Optipad process is the consistent soldervolume attained and the flat deposit. The negative aspects include

The cost associated with the application and removal of the temporary soldermask

The initial cost of the equipment The inability of maintaining the planarity of the second side

14.15.3 Sipad

Similar to Optipad, Sipad also forms flattened solder deposit on pads serving asboth surface finish and solder source for joint formation. Instead of using a tempo-rary solder mask and molten solder for deposition, Sipad employs a permanent sol-der mask for solder volume control and solder paste as a solder source fordeposition. The average thickness of solder deposits formed is 50 µm and can be ashigh as 130 µm.

Fabrication Process. The detailed fabrication process for Sipad can be describedas follows96,97,103:

1. Analyze and alter the computer-aided design (CAD) as necessary.2. Apply a photoimagable dry-film solder mask (Simask) with a thickness of no

less than 100 µm.3. Laminate and develop the Simask to form a well around each pad.4. Print solder paste into a well via the standard solder paste stencil printing

process.5. Reflow solder paste via the standard process for form bumps with meniscus

above the plane of the solder mask.6. Wash the board to remove flux residue and solder balls.7. Place the PCB into a flattening system.8. Reheat solder to the melting point.9. Flatten the pads between the platens of a cold press to freeze the solder deposit

into a planar SSD flush with the surface of the solder mask.10. Print adhesive no-clean flux onto the planar SSD surface.11. Dry the flux to a tacky finish.12. Cover the tacky finish with a release paper.13. Remove the release paper, and place the component.14. Reflow to form solder joints.

Performance. The benefits of Sipad include

The very flat deposit The ability to alter the volume of solder Excellent solderability Good compatibility with all other surface finishes (e.g., OSP or Ni/Au) for solder

paste deposition process


The solder volume is accomplished by regulating the opening of the solder maskaround the surface mount pad and filling the entire well with solder paste. The con-cerns associated with the Sipad process include

The potential for foil formation The potential for meniscus formation The cost associated with the use of a thick dry film mask The inability of maintaining the planarity of the second side

14.15.4 PPT

Precision pad technology (PPT) forms a flattened solder deposit with the use of aregular solder mask and solder paste. A vibrating stainless-steel mesh is seated ontop of the PCB printed with paste.The paste is then reflowed with a traveling hot-airknife, and solidified afterward with a pass of a cool-air knife.The thickness of solderis 50 to 200 µm.

Fabrication Process. The fabrication process of PPT can be described as follows:

1. Prepare the PCB with a solder mask no less than 25 µm in thickness.2. Stencil-print the solder paste onto the pads within the aperture of the solder

mask.3. Load the board into the reflow/formation system.4. Place a vibrating tensioned stainless-steel screen onto the surface of the board.5. Reflow the paste with a hot-air knife traveling over the board.6. Solidify the solder with a pass of a cool-air knife.7. Board-exit the system.8. Clean the board, if necessary.

The dwell time above liquidus temperature isnormally less than 20 s. Table 14.23 shows a typicalPPT reflow profile for a 63Sn-37Pb system.104 Thescreen serves as a mold to flatten, shape, andremove excess solder during reflow. Excess solderwicks above the mesh during reflow in the form ofsolder balls. The reflowed solder deposits aremacroplanar with an embossed surface topographythat facilitates retaining tack flux at subsequentassembly. Shorts and solder balls are eliminated atassembly, and the copper lands are encapsulated ina thick solder deposit, which increases bare-boardshelf life.

Performance. The advantages of PPT are simplicity and the low cost associatedwith it. Furthermore, the mesh impression left on the surface of the board providesa flat surface for component leads to rest on and an area for tacky flux to pool priorto reflow at assembly. It offers the planarity and uniformity that are needed for fine-pitch and BGA assembly by reassigning solder paste printing responsibilities to thesupplier. The limitation is the inability to maintain the planarity of the second side.


TABLE 14.23 Typical ReflowProfile for PPT Process Using63Sn-37Pb105

Peak temperature 214.5°C

Time over 150°C 57.5 s




14.15.5 SOLDER CLADDING

Solder cladding forms solid solder deposits by reflowing solder paste, a method inuse since the 1960s.97

Fabrication Process. The fabrication process of solder cladding is described as fol-lows:

1. Print the solder paste.2. Reflow the solder paste.3. Apply the tacky flux.4. Place the components.5. Reflow the package.

Performance. Simplicity and cost are the two advantages to solder cladding. Themain drawback is the formation of the rounded meniscus, thus causing potential dif-ficulty in placing fine-pitch components.

14.15.6 SOLDER JETTING

The solder jetting process deposits liquid solder droplets of a controlled size to theland areas of the circuit board. Pressure and vibration via a piezoelectric mechanismare applied to a liquid metal reservoir, which forces the solder through an orifice toform a liquid droplet that flies through a charge electrode, electrostatic deflectionplate, and a catcher, and then onto the surface mount pads. The size of the balls is inthe range of 0.004 to 0.012 in.97

Fabrication Process. The fabrication process of solder jetting is described as fol-lows:1. Apply the jetted solder droplet onto the fine-pitch pads.2. Apply the tacky flux to the fine-pitch pads.3. Place components onto the fine-pitch sites.4. Reflow via a hot bar.5. Print the paste onto coarse-pitch sites.6. Place the coarse-pitch components.7. Reflow the coarse-pitch components.

Performance. Ideally, this method allows the deposition of solder onto fine-pitchpads automatically through programming. Unfortunately, this is at the expense ofbeing able to process the entire board in one step. Furthermore, uniformity, copla-narity, jetting landing precision, and throughput are issues that still need to beresolved before this process becomes widely acceptable.

14.15.7 SUPER SOLDER

Super Solder is a chemical deposition process. Applied in paste form, materialmainly contains (RCOO)2Pb (a lead salt of an organic acid) and tin powder. Uponheating, the two components react, with lead being reduced to metal and tin being


oxidized to tin salt. The lead metal forms alloys with tin powder to create parti-cles; the particles then settle, forming a solder deposit. A minimum pitch of 0.1 to0.15 mm can be sustained.106 The thickness of the solder deposit is between 40 and70 µm.

Fabrication Process. The fabrication process of Super Solder is described as fol-lows:

1. Preclean the panels via the microetch process.2. Apply Super Solder, a proprietary organic lead and tin powder paste.3. Heat to bond Super Solder to fine-pitch sites to form solder bumps.4. Clean the panels.5. Apply tacky flux to the fine-pitch sites.6. Place the fine-pitch components.7. Reflow the fine-pitch sites via a hot bar.8. Paste-print the coarse-pitch sites.9. Place the components.

10. Reflow the coarse-pitch sites.

Performance. This process is well suited for applications such as tape automatedbonding, where the insertion of solder mask dams may be difficult but is notdesigned to predeposit solder over all of the component sites on a board.97 SuperSolder is used in Toshiba’s notebook and Panasonic’s notebook.105 The solderdeposit is dome-shaped. The cost of the complete system may make Super Solderprohibitive for some applications. Most important, the exchange chemistry deposi-tion approach, adequate for a Sn-Pb alloy, is highly questionable to be applicable forPb-free solder systems.

14.16 SUMMARY OF PCB SURFACE FINISHES

As discussed previously, there is a wide range of lead-free coating options availablefor PCB surface finish applications. However, due to the complicated functionalrequirement of various electronic products, it is virtually unlikely to identify a singlesurface finish that will satisfy all of the requirements. Following are the pros andcons of finish options for specific applications.

Solderability. Similar to Sn-Pb HASL, Pb-free HASL is also considered to besuperior in solderability, particularly for a reflow temperature higher than themelting temperature of HASL materials. However, the primary limitation ofHASL is the inability to provide even finish thickness and quality for fine-pitchapplications. This is true whether the solder is Sn-Pb or Pb-free; this excludesHASL as a major candidate for PCB finish applications. Plated metallization sur-face finishes often exhibit better solderability, if the metal can be dissolved intosolder rapidly during soldering.Therefore, options such as Au,Ag, and Sn are oftenbetter in solderability than others, including OSPs.

Aging resistance. Metal surface finishes often exhibit better resistance than OSPagainst aging (e.g., thermal or steam). This is particularly true for noble metal fin-


ishes. However, under corrosive gas storage conditions, OSPs turn out to be out-standing in retaining their solderability.

Tolerance against Pb contamination. Bi-containing systems are sensitive to Pb-contamination, mainly due to the formation of low-melting ternary eutectic 52Bi-30Pb-18Sn alloy.

Thick-board through-holes. Due to the throw-power limitation, electrolytic plat-ings typically fail to provide an even coating for thick-board through-holes. As aresult, the immersion process becomes a favorite choice.

Wire bonding. Only nonmelting metallic surface finishes can be considered forwire-bonding applications. This quickly rules out OSP, Sn-, or Bi-containing sys-tems. Presence of nickel underlayer improves the wire bondability, although thesolderability may be compromised, as demonstrated by Pd systems.

In-circuit probe testing. For in-circuit probe testing purposes, finishes with eithertarnish or too high a hardness will have greater difficulty in probe penetration. Ingeneral, OSPs are not the most promising in this regard. Metallic surface finisheswith a matte appearance often are more promising, due to a greater contact areaprovided by the rough surface topology.

Tolerance against cleaning. Organic solderability preservatives have a very lowtolerance against PCB cleaning prior to paste reflow, because OSPs tend to beremoved by the cleaners as well. Metallic surface finishes are all fairly robustagainst board cleaning.

IMC formation. Both Au and Pd suffer excessive intermetallics formation issueswhen the thickness of those metals is greater than approximately 0.25 µm. This isparticularly an issue for Au when the same finish is to be used for wire bonding aswell.

Connector applications. Usually surface finishes with high electrical conductiv-ity, high abrasion resistance, high oxidation resistance, and high hardness aredesired for connector applications. The OSPs are ruled out immediately due topoor electrical conductivity. Noble metallic finishes often shine in this category.

Finish consistency/inspectability. Finishes with challenges in both quality consis-tency and inspection capability are the trickiest manufacturing issues. For instance,ENIG tends to have black-pad problems from time to time. And, unfortunately,the symptom is not easily inspectable prior to assembly, and it is not inspectablewithout destructive testing after assembly.

Tin whisker/tin pest. All tin-based metals or alloys suffer from potential prob-lems with tin whisker and tin pest.Although both symptoms can be prevented to acertain extent, such as nickel underlying or alloying tin with secondary elements,100 percent symptom-proof is still unassured. These potential problems highlyreduced the options of finding a low-cost lead-free surface finish, as will be dis-cussed later.

Cost. The OSP is probably the lowest in cost. Next to that may be nonprecious-metal plating finishes, especially the immersion processes. Additional savingspromised by the electroless process include elimination of tie bars from circuitboard design, hence the additional size reduction that is achievable.

Overall, it can be seen that none of the surface finishes can be considered an idealsolution as the answer for all PCB finish requirements.Therefore, the finish selectionhas to be determined by the requirements of specific applications involved in theproduct design.


14.17 OPTIONS FOR COMPONENT

SURFACE FINISHES

The requirements for component surface finishes are similar to those for PCB sur-face finishes. However, some of the features may be more important to componentsthan to PCBs, such as ductility, due to the lead trimming and forming requirements.Shipley uses the following criteria as their plating process requirements80:

Must be compatible with existing high-speed plating equipment Must have a minimum deposition rate of 7.5 µm/min Must have familiar chemistry, preferably an MSA electrolyte Must have all products in the process that are fully analyzable Must have deposits that possess good solderability (same as or better than Sn-Pb) Must have deposits that possess good ductility (same as or better than Sn-Pb)

Table 14.24 lists the options of lead-free surface finishes for components.The sys-tem is categorized by the key element that is used. Each category is further classifiedby the type of process and chemistry. Examples are given for certain groups.

14.18 Ni/Au (ENIG)

Fabrication Process. The characteristics and manufacturing process of ENIG arethe same as those discussed in Sec. 14.4.2.

Performance. Popelar et al., at IC Interconnect, have investigated using ENIG forflip chip under-bump metallurgy (UBM) applications.106 In their process, theexposed aluminum I/O pads are plated with an EN cap with a typical thickness of 5


TABLE 14.24 List of Lead-Free Surface Finishes

Surfacefinishsystem Finish process and chemistry Example

Ni/Au Electroless Ni/electroless (immersion) Au, or ENIG

Pd Electrolytic Pd or Pd alloys

Ni/Pd Electroless Ni/electroless (autocatalytic) Pd

Electroless Ni/electroless (autocatalytic) Pd/electroless(immersion) Au

Pd-Ni Electrolytic Pd-Ni

Sn Electrolytic Sn

Sn-Ag Electrolytic Sn-Ag 96.5Sn-3.5Ag

Sn-Bi Electrolytic Sn-Bi 90Sn-10Bi

Sn-Cu Electrolytic Sn-Cu 99Sn-1Cu

For multilayer design, the sequence of materials starts from the bottom layer.

µm, followed by a flash of immersion Au. The appropriate solder paste alloy isdeposited via stencil printing, and subsequently reflowed and cleaned. No degrada-tion in shear load or failure mode occurred among the three alloys (63Sn-37Pb,90Pb-10Sn, and 95.5Sn-3.8Ag-0.7Cu) that were tested, indicating no critical UBMconsumption (i.e., no excessive intermetallics growth) during reflow. Additionaltests were performed comparing nickel UBM thicknesses of 1, 2, and 5 µm.Again, allbumps exhibit comparable shear strength, indicating no critical UBM consumption.These data suggest that ENIG can be a viable candidate as UBM for flip chip solderpaste bumping applications.

14.19 ELECTROLYTIC Pd

Fabrication Process. See Sec. 14.7.1.

Performance. Fan et al., from Lucent Technologies, studied electrolytic Pd or elec-trolytic Pd with Au flash as a potential leadframe surface finish.107 The postetch sol-derability comparison was made on 34 µm of copper laminate of 0.25 µm ofpalladium, 0.025 µm of gold flash over 0.25 µm of palladium, 1.25 µm of silver over1.25 µm of nickel, and 0.38 µm soft gold over 1.25 µm of nickel. Both gold-flashedpalladium and palladium alone passed the steam and thermal aging with only aslight degradation. The gold-flashed palladium was better than all of the samplestested, probably due to the gold on the palladium performing two functions: (1)inhibiting the diffusion of copper into the palladium, and (2) acting as an oxidationbarrier.The failure of both silver and gold over nickel after thermal and steam agingwere probably due to migration of nickel through the silver or gold layer and form-ing an oxide on the surface. Palladium, as an excellent migration/diffusion barrier,unlike gold and silver, produces a more stable final finish, which does not require anactive flux to be solderable.The results conclude that thin deposits (<0.5 µm in thick-ness) of palladium and gold-flashed palladium provide good solderability and goodwire and die bondability.

14.20 ELECTROLESS Ni/Pd

Fabrication Process. See Sec. 14.8.

Performance. Fan et al. studied the potential of EN/Pd as a surface finish for lead-frame, with emphasis on the solderability.107 Samples consisted of 0.015 gm/cm2 (0.5oz/ft2) of copper laminate plated with 2.5 µm 6 percent phosphorous EN and then0.5 µm of electroless palladium with and without a gold flash. Samples were solder-ability-tested in the as-plated condition and again after an 85% RH/85°C 8-h steamage.Three different fluxes were used.All deposits exhibited 95 percent or higher sol-der coverage, even with a rosin nonactive flux. The electroless palladium with thegold flash performed slightly better than the palladium without gold, presumablydue to the oxidation barrier function of gold flash. Results indicate that EN/electro-less Pd is promising as a leadframe surface finish.

The preceding findings are supported by the work at UCLA.108 In the solderingreaction at 200°C between eutectic Sn-Pb and plated Ni/Pd on Cu leadframes, two


thicknesses of Pd—760 and 2500 Å—were used. Even very thin Pd can lead to con-tinuous spreading of the solder on the leadframe, as evidenced by the lack of a sta-ble wetting angle. Anisotropic spreading and spike formation were found to becaused by the mechanical effect of rolling of the leadframe. This phenomenon ismore pronounced in the thinner Pd samples. In the interfacial reaction, formation ofa ternary Pd-Ni-Sn compound and Ni3Sn4 were observed. While spalling of the ter-nary compound occurs, no spalling of the Ni3Sn4 was found.The latter forms a ratheruniform layer consisting of small scallop-type grains. The growth rate of Ni3Sn4 isabout one order of magnitude slower than that of Cu6Sn5 in the reaction between Cuand eutectic Sn-Pb.108

For a component leadframe surface finish, either a two-layer plate [consisting ofPd flash (0.075 to 0.25 µm) over Ni preplating] or a four-layer plate (consisting of Niflash–Pd flash–Ni plate–Pd flash) is utilized. Plating cracks occur on all outer-bendradii of the lead. Solderability is poorer than Sn-Pb plated leadframes. For connec-tors or contacts, the thickness of Pd used is around 0.5 to 0.75 µm.

14.21 ELECTROLYTIC Pd/Ni

Fabrication Process. See Sec. 14.9.2.

Performance. An electrolytic Pd-Ni alloy with 80/20 (w/w) composition on copperwas tested for its potential as a leadframe surface finish.107 The coating thickness is0.12 µm.A Pd-Ni alloy exhibits good solderability for an as-plated sample. However,the solderability degraded after thermal aging when using a nonactivated rosin flux.The nickel in the palladium/nickel alloy is believed to oxidize at 125°C, and using amildly active or fully active flux is necessary to remove the oxide before soldering.

14.22 Sn

Fabrication Process. See Sec. 14.10.

Performance. Hunt, at the National Physical Laboratory, investigated the solder-ability of tin on leadframes made of Cu, Ni, and Alloy 42. He concluded that oxidegrowth degrades wetting, but it is intermetallic growth at the surface that has beenshown to be the critical factor influencing solderability. Good-quality coatingsshould be a minimum of 5 µm to protect components, and 10 µm is recommended asideal for Sn-Pb coating. A similar thickness requirement may also be applicable toSn coating. An intermetallic oxide is impervious to flux. Hence, for thin or inferiortin coatings, it is the presence of intermetallic oxide at the surface that spells the endof a component’s solderability.83

14.23 ELECTROLYTIC Sn-Ag

Eutectic Sn-Ag alloy (96.5Sn-3.5Ag) exhibits a low electrical resistivity (12.31 µΩ-cm) and high elongation (73 percent). The metal supply is sufficient at a reasonably


low cost.Although Ag extract in the groundwater may still be a concern, overall it isstill a promising option as lead-free surface finish.

Fabrication Process. Due to the electrochemical potential shown, Ag tends todeposit much more readily than does Sn. Consequently, it is very difficult to codeposittin-rich alloys of Sn-Ag:

Ag+ + 1e− → Ag0 0.7996 VSn2+ + 2e− → Sn0 −0.1375 VSn4+ + 4e− → Sn0 −0.9450 V

Codeposition of near-eutectic Sn-Agalloys requires use of complexing agents withexotic chemistries.This inevitably complicatesthe waste treatment. Table 14.25 shows theplating parameter for an Sn-Ag surface fin-ish.80 Figure 14.57 shows the topology of theelectrolytic Sn-Ag surface finish.80

Performance. The Sn-Ag deposition com-position is sensitive to the plating currentdensity. Figure 14.58 shows the relationbetween current density and Ag content in

Sn-Ag deposition.80 The Ag content decreases rapidly initially, then slowly withincreasing current density. Unlike eutectic Sn-Pb, the melting temperature of which


TABLE 14.25 Electroplating Para-meters for Sn-Ag Finish

Parameter Setting

Sn content (g/L) 40

Ag content (g/L) 7

Additive (mL/L) 20

Temperature (°C) 45

Current density (Å/dm2) 0.2

FIGURE 14.57 Topology of electroplated Sn-Ag surface finish.

is not sensitive to composition variation, variation in Ag content causes a significantchange in melting temperature of Sn-Ag binary alloys. For instance, the liquidustemperatures of 96.5Sn-3.5Ag and 90Sn-10Ag are 221 and 300°C, respectively. As aresult, a very tight composition control is required. Overall, the electrolytic Sn-Agfinish exhibits good mechanical properties, good solderability, and good compatibil-ity with solder alloys and fluxes. However, it suffers low deposition rates, compli-cated plating chemistry, and requires the use of complexing agents; therefore, it isnot easy to get implemented.

14.24 ELECTROLYTIC Sn-Bi

The electrolytic Sn-Bi used as a component surface finish has been reported bySchetty, from Shipley Ronal, with a composition of 90Sn-10Bi (w/w).80

Fabrication Process. The electroplating proc-ess parameters for a 90Sn-10Bi finish aredescribed in Table 14.26.80

Performance. The topology of a 90Sn-10Bisurface finish is shown in Fig. 14.59.80 Overall,Sn-Bi finishes have good mechanical proper-ties and low toxicity. Due to abundant globalsupply, the cost is also low, as discussed in theimmersion Sn-Bi section. With the use ofmethane sulfonic acid chemistry, the deposi-tion rate is high. However, Bi immersion onto


FIGURE 14.58 Effect of current density (Å/dm2, or ASD) on the Ag content in electroplatedSn-Ag.

TABLE 14.26 Process Parametersfor Sn-Bi Electroplating

Parameter Setting

Sn content (g/L) 60

Bi content (g/L) 8.5

Acid content (mL/L) 200

Additive (mL/L) 105


Current density (Å/dm2) 15

anodes forms gray deposit. The solderability is very good. Prasad et al. conductedplating chemical evaluations and reliability of Pb-free leadframe packages.109 Whencompared with pure Sn and Sn-Cu, the Sn-Bi system exhibits the minimum whiskers,although no plating system is totally whisker-free. Whisker growth is found to berelated to substrate materials, and alloy 42 leadframe does not seem to have whiskerproblems for the Pb-free plating chemical systems within the time frame studied.The main disadvantage is incompatibility with Sn-Pb solder due to formation of ter-nary eutectic Sn-Pb-Bi solder, as discussed earlier. As a result, Sn-Pb solder jointscontaminated with Bi are prone to fracture. Therefore, a Sn-Bi finish is only recom-mended where total control of the assembly parts and process is maintained.

14.25 Sn-Cu

Shipley has developed electrolytic Sn-Cu alloy plating chemistry for component sur-face finish.80 Sn-Cu alloy with a composition of 99.3Sn-0.7Cu exhibits the followingattractive features as a component surface finish:

Low electrical resistivity (11.67 µΩ-cm) High elongation (>30 percent) Low toxicity Abundant world reserve Low cost


FIGURE 14.59 Scanning electron microscopic image of a 90Sn-10Bi surface finish.

Fabrication Process. The reportedly high-speed, methane sulfonic acid–based elec-trolyte chemistry developed by Shipley canbe described in Table 14.27.80

Performance. The topology (2000×) of Sn-Cu,plated at 20 Å/dm2 current density, is shownin Fig. 14.60.80 The Sn-Cu deposit compositionis fairly insensitive to variation in temperatureand current density, as shown in Fig. 14.61.80

To evaluate the potential of tin whiskergrowth, two Sn-Cu samples deposited with 25and 30 Å/dm2 current density, respectively,

were subjected to 60°C/95% RH for 500 h.As a control, pure tin after 500 h of agingshowed tin whisker formation, while Sn-Cu finishes exhibit no whiskers at all forboth samples. The solderability of Sn-Cu was studied by determining the zero-forcecross time (ZCT) in the wetting balance test.Also evaluated were 90Sn-10Pb, 100Sn,90Sn-10Bi, and 97Sn-3Ag. Results shown in Table 14.28 indicate that the solderabil-ity decreases in the following order: 90Sn-10Pb > 90Sn-10Bi > 97Sn-3Ag > 100Sn and99Sn-1Cu.80 All finishes are considered acceptable in solderability.

14.26 SUMMARY OF COMPONENT

SURFACE FINISHES

Schetty has compared the pros and cons of several viable component surface fin-ishes, with results summarized in Table 14.29.80 Ni/Pd is too expensive and marginal


TABLE 14.27 Plating Parameters forSn-Cu Alloy Electrolyte

Parameter Setting

Sn content (g/L) 60

Cu content (g/L) 1.1

Acid content (mL/L) 200

Additive (mL/L) 105


Current density (Å/dm2) 25

TABLE 14.28 Solderability Test Results for Various Component Surface Finishes

Component finish Aging condition ZCT (s) Coverage (%)

Sn-Pb 90-10 As plated 0.29 >95

Sn-Pb 90-10 Steam aged 0.46 >95

Sn-Pb 90-10 Heat aged 0.32 >95

Sn As plated 0.58 >95

Sn Steam aged 0.83 >95

Sn Heat aged 0.68 >95

Sn-Bi 90-10 As plated 0.33 >95

Sn-Bi 90-10 Steam aged 0.41 >95

Sn-Bi 90-10 Heat aged 0.37 >95

Sn-Ag 97-3 As plated 0.42 >95

Sn-Ag 97-3 Steam aged 0.54 >95

Sn-Ag 97-3 Heat aged 0.53 >95

Sn-Cu 99-1 As plated 0.39 >95

Sn-Cu 99-1 Steam aged 0.83 >95

Sn-Cu 99-1 Heat aged 0.79 >95


FIGURE 14.60 Topology (2000×) of a Sn-Cu surface finish plated at 20 Å/dm2 current density.

FIGURE 14.61 Effect of current density and temperature on Sn-Cu deposit composition.

on soldering and mechanical properties. Sn still suffers concerns on tin whiskeringand tin pest problems. Sn-Bi is sensitive to Pb contamination. Sn-Ag is very difficultto plate high Sn deposit because of unfavorable electrochemical potentials of Snversus Ag. Overall, the most promising component finish appears to be Sn-Cu, dueto the lack of any obvious weakness.80

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TABLE 14.29 Pb-Free Electronic Finish Comparison Summary

Pb-free Solderability Mechanical Melting Whiskering Compatibility Plating Relativefinish properties point feasibility cost

NiPd ∗ ∗ G G G G X

Sn G G ∗ X G G G

SnBi G G ∗ G X G G

SnAg G G G G G X ∗SnCu G G G G G G G

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CHAPTER 15IMPLEMENTATION OF

LEAD-FREE SOLDERING

15.1 COMPATIBILITY OF LEAD-FREE SOLDERS

WITH SMT REFLOW PROCESS

Due to the toxicity of lead, there is a tremendous amount of effort to eliminate leadfrom the solders used in the electronics industry. The move toward lead-free solderalternatives in North America and Europe accelerated significantly1 since the Jap-anese industry announced its aggressive lead-free road map.2 For instance, Toshiba,Matsushita, and Hitachi have announced plans to eliminate all lead interconnectsin their products by 2001, 2004, and 2004, respectively. However, the preferredsolution for lead-free alternatives varies from region to region, and there are anumber of alloys considered promising. The most favorable Pb-free solder systemsidentified by the industry3,4 comprise primarily alloys of Sn with Ag, Bi, Cu, Sb,or Zn, such as 99.3Sn0.7Cu, 96.5Sn3.5Ag, 95.5Sn3.8Ag0.7Cu, 93.6Sn4.7Ag1.7Cu,96.2Sn2.5Ag0.8Cu0.5Sb, 91.7Sn3.5Ag4.8Bi, 90.5Sn7.5Bi2Ag, 89Sn8Zn3Bi, 95Sn5Sb,and 58Bi42Sn. Unfortunately, although some reliability data have been generated inthe past,3 most of the promising alloys were evaluated under a single flux system.The compatibility between flux and alloy often dictates the performance of reflowsoldering, such as solder balling, wetting, processing window, and stability. Since theflux chemistry varies from supplier to supplier, and since the use of more than onesupplier is considered crucial for ensuring a steady process, an alloy compatible witha wider range of flux systems obviously will have greater prospects of being acceptedby the surface-mount technology (SMT) industry. In this study, a group of the mostpromising Pb-free alloys reported are tested against a broad range of commonlyused flux chemistries, such as water wash, no-clean, halide-containing, halide-free,nitrogen reflow systems, and air reflow systems, in the form of solder paste.The han-dling and reflow soldering performance of these pastes is evaluated and ranked inorder to assess the prospect of the alloys being widely used for reflow solderingapplications by the industry.

15.1.1 EXPERIMENTAL DESIGN FOR COMPATIBILITY EVALUATION

15.1.1.1 MaterialsAlloys. Among the most promising lead-free alloys, 10 representative alloys were

chosen, including 99.3Sn0.7Cu, 96.5Sn3.5Ag, 95.5Sn3.8Ag0.7Cu, 93.6Sn4.7Ag1.7Cu,96.2Sn2.5Ag0.8Cu0.5Sb, 91.7Sn3.5Ag4.8Bi, 90.5Sn7.5Bi2Ag, 89Sn8Zn3Bi, 95Sn5Sb,and 58Bi42Sn. The eutectic tin-lead 63Sn37Pb was used as a control.

Fluxes and Solder Pastes. Ten fluxes varying widely in chemistry, as shown inTable 15.1, were used to make solder pastes in order to evaluate the compatibility ofalloys with reflow soldering applications. The solder paste samples were made bymixing each flux with solder powder (−325/+500 mesh, 25 to 45 µm) for each alloy.

15.1


The metal content of solder paste for each alloy system is shown in Table 15.2, and isset to provide approximately the same solder volume as that of eutectic SnPb solderpastes with 90 percent metal content when the flux density is 1 g/ml. For the purposeof calculation, densities of F1 to F9 can be approximated as 1.0 g/ml, while a densityof F10 is 1.25 g/ml.The density of the alloys shown in Table 15.2 was determined witha pycnometer.

15.1.1.2 Tests. As stated earlier, the scope of this work involves assessing thecompatibility of lead-free alloys with reflow soldering, with emphasis on the han-dling and soldering performance of solder pastes. In the case of handling, incompat-ibility of an alloy with a certain flux chemistry often results in excessive chemical

15.2 CHAPTER FIFTEEN

TABLE 15.1 Fluxes Used for Lead-Free Solder Pastes

Flux Description

F1 No-clean, halide-free, air reflow, probe-testable

F2 No-clean, halide-free, air reflow, probe-testable

F3 No-clean, halide-containing, air reflow

F4 No-clean, halide-containing, air reflow

F5 Rosin-based, mildly activated type, halide-containing, air reflow

F6 No-clean, halide-free, medium residue, nitrogen reflow

F7 No-clean, halide-free, low residue, nitrogen reflow

F8 No-clean, halide-free, ultra-low residue, nitrogen reflow

F9 Water-washable, halide-free, medium-temperature process, air reflow

F10 Water-washable, halide-containing, high-temperature process, air reflow

TABLE 15.2 Metal Content of Solder Paste Samples for Each Alloy System

Metal Metalcontent for content for

Density F1–F9 system F10 systemAlloy (g/ml) (wt/wt %) (wt/wt %)

63Sn37Pb 8.40 90.0 88.0

96.5Sn3.5Ag 7.36 89.0 86.5

99.3Sn0.7Cu 7.34 89.0 86.7

95.5Sn3.8Ag0.7Cu 7.38 89.0 86.5

93.6Sn4.7Ag1.7Cu 7.43 89.0 86.6

96.2Sn2.5Ag0.8Cu0.5Sb 7.47 89.0 86.5

91.7Sn3.5Ag4.8Bi 7.57 89.0 86.8

90.5Sn7.5Bi2Ag 7.58 89.0 86.8

58Bi42Sn 8.56 90.0 88.2

95Sn5Sb 7.25 89.0 86.3

89Sn8Zn3Bi 7.39 89.0 86.5

reaction between the alloy and flux either at storage temperature or on exposure tothe ambient atmosphere. This in turn results in a thickened or crusted paste, andaccordingly a poor shelf life and poor tack time.

On the other hand, for certain alloys, the solder oxide may not be readily remov-able by certain flux chemistries. This would result in poor solder balling, poor wet-ting, and often a poor solder surface appearance. In certain other cases, some solderalloys may react with base metal very slowly and thus would exhibit fairly poor wet-ting when compared with eutectic SnPb systems. For situations like those, conven-tional flux systems may be inadequate, and a more aggressive flux may be needed inorder to achieve wetting comparable with that of SnPb systems.

In view of the aforementioned situations, it becomes clear that the tests requiredin order to assess the compatibility of solder alloys with reflow soldering shouldinclude (1) shelf life, (2) tack time, (3) solder balling, (4) wetting ability, and (5) sol-der joint surface appearance. However, before embarking on these performanceevaluations, some other information may need to be generated, such as the meltingtemperature. Knowledge of melting behavior—particularly liquidus temperature—is required for setting up a reflow profile.

Melting Temperature. For each alloy system, the melting temperature (see Table15.3) was determined on a Seiko differential scanning calorimeter. The sample waspreconditioned at 300°C, followed by cooling to 0°C at a cooling rate of 5°C/min,then reheated to 300°C at a heating rate of 5°C/min. The onset of the meltingendotherm was recorded as solidus temperature, and the peak of the endotherm wasrecorded as liquidus temperature. The liquidus temperature data in Table 15.3 havenot been corrected for thermal lag effect, which is 0.9°C.

Wetting Ability. The wetting ability of solder pastes was tested by printing sol-der paste onto copper pads coated with organic solderability preservative (OSP) ona printed circuit board (PCB) and followed by reflow.The stencil thickness was 6 mil(150 µm), and the ratio of aperture opening versus pad dimension was 1 to 1. Theregistration of paste to pad was set 70 percent off so that only 30 percent of the pastewas printed onto the pads and 70 percent was printed onto the solder mask. Uponreflow, the solder paste coalesced and pulled away from the solder mask and wettedto the pad side (see Fig. 15.1). The wetting ability of solders was determined by

IMPLEMENTATION OF LEAD-FREE SOLDERING 15.3

TABLE 15.3 Melting Temperatures for Solder Alloys

Alloy Solidus (°C) Liquidus (°C) Note

63Sn37Pb 182.1 183.0 Eutectic

96.5Sn3.5Ag 219.7 220.8 Eutectic

99.3Sn0.7Cu 225.7 227.0 Eutectic

95.5Sn3.8Ag0.7Cu 216.3 217.5 Multicore

93.6Sn4.7Ag1.7Cu 215.9 217.1 Ames

96.2Sn2.5Ag0.8Cu0.5Sb 216.9 218.2 AIM

91.7Sn3.5Ag4.8Bi 202.1 215.1 Sandia

90.5Sn7.5Bi2Ag 190.6 214.7 Tamura

58Bi42Sn 136.3 138.5 Eutectic

95Sn5Sb 238.3 240.3 Indium

89Sn8Zn3Bi 190.6 195.4 Senju

examining the extent of solder spreading on the pads, and the average of 10 pads wasexpressed as wetting index (WI), which is defined in Table 15.4. A higher WI valuerepresents a better wetting ability.

The reflow process was conducted with the use of a BTU VIP70 forced-air con-vection oven. Two tent-shaped reflow profiles were used for each alloy, with thepeak temperature being a function of the liquidus temperature. The first profile(cool profile) exhibits a peak temperature 15°C above the liquidus temperature,with the second profile (warm profile) being 30°C above the liquidus. The ramp-uprate was about 0.7 to 0.8°C/s. For flux systems F6, F7, and F8, a nitrogen atmospherewas used for reflow, while for the remaining systems an air reflow atmosphere wasused.The use of two profiles provided insights on (1) minimal temperature requiredand (2) potential for improving soldering performance with the use of a higher tem-perature.


FIGURE 15.1 Schematic of wetting test.

TABLE 15.4 Definition of WI

Spread areaWI (% of pad)

0 0

1 10

2 20

3 30

4 40

5 50

6 60

7 70

8 80

9 90

10 100

Solder Balling. Solder balling performance was evaluated by examining undera 20× optical microscope the average number of solder balls per pad for the reflowresults just described. The average performance of 10 pads is expressed as solderballing index (SBI), as defined in Table 15.5. A higher SBI value represents a bettersolder balling performance.


TABLE 15.5 Definition of SBI

SBI Number of solder balls

0 No reflow

1 >501, with some reflow

2 401–500

3 301–400

4 201–300

5 151–200

6 101–150

7 51–100

8 21–50

9 11–20

10 0–10

Tack Time. The tack time of a solder paste was determined using the followingprocedure: (1) Print solder paste onto ceramic coupons, as prescribed by J-STD-006procedure. (2) Condition the specimen under 76 percent relative humidity. (3) Mea-sure the tack value, per J-STD-006 procedure, of the conditioned specimen at fresh,8 h, 24 h, 48 h, and 72 h. The specimen was discarded after each tack measurement.The tack data are expressed as tack time index (TTI), which is defined in Table 15.6.A higher TTI value represents a longer tack time.

Shelf Life. The shelf life of solder pastes was determined by monitoring theirviscosity stability at 25°C over a period of 1 month. A changing viscosity, typicallyincreasing with time, was considered undesirable. For each solder paste sample, theviscosity was determined at 1 day, 7 days, and 30 days after paste manufacturing.The

TABLE 15.6 Definition of TTI

TTI Description

0 Decreasing tack curve reaching a value at <10 g on the 3rd day.

2 Decreasing curve reaching 10–20 g on the 3rd day.

4 Decreasing curve reaching 25–20 g on the 3rd day.

6 Tack initially increases, reaching maximum, and continuously decreases.

8 Continuously increasing curve.

10 Constant over 3 days.

percentage change in viscosity for each sample was calculated for the period from 1day to 7 days (change rate A) and for the period from 7 days to 30 days (change rateB). The overall instability was calculated with the following equation:

Overall instability = 0.3 × (change rate A) + 0.7 × (change rate B)

The shelf life is expressed as shelf life index (SLI), and is defined in Table 15.7. Ahigher SLI value represents a longer shelf life.


TABLE 15.7 Definition of SLI

SLI Description

0 Overall instability > 25%

2 Overall instability = 20–25%





Solder Surface Appearance. The solder bump surface was examined under anoptical microscope, and the appearance is expressed as solder appearance index(SAI), as defined in Table 15.8. A higher SAI value suggests a more desirable solderjoint quality.

Compatibility. The compatibility of an alloy with reflow soldering was deter-mined by adding up the performance of all five categories. However, much moreweight was assigned to solder balling and wetting performance. The solder appear-ance received a slightly higher weight than shelf life and tack time. Hence, the com-patibility C was calculated according to the following formula:

C = 1 × SBI + 1 × WI + 0.3 × SLI + 0.3 × TTI + 0.4 × SAI

TABLE 15.8 Definition of SAI

SAI Description

0 Not reflowed, with “powder” appearance

1 Partially reflowed with some melted area

2 No less than one large cavities

3 Numerous small pinholes

4 Solder dewetted to form a few bumps

5 Rough area 70–100%




9 Smooth and dull, or shiny with slight roughness (rough area <10%)

10 Shiny and smooth

The higher the compatibility value, the more compatible the alloy is with reflow sol-dering. A value of 30 represents 100 percent compatibility.

Cross Section. In order to understand the relationship between surface appear-ance and solder interior integrity, some solder bumps were cross-sectioned andexamined under optical as well as electron microscopy.

15.1.2 RESULTS OF COMPATIBILITY STUDY

The results of studies on wetting, solder balling, shelf life, tack time, and solder sur-face appearance for each combination of fluxes and alloys are shown in Table 15.9.For each alloy, the average performance of paste systems using a variety of fluxesis summarized in Table 15.10. On the other hand, for each flux, the average per-formance of paste systems using a variety of alloys is summarized in Table 15.11.Although the data are presented for all conditions including cool profile, warmprofile, and the average value of cool profile and warm profile, the primary com-parison of alloys or fluxes is based on the warm profile data. This is due to the con-sideration that the warm profile simulates the current industrial practice better.Figure 15.2 shows the schematic ranking of alloys in terms of compatibility forreflow soldering.

15.1.2.1 Compatibility of Alloys63Sn37Pb. The control 63Sn37Pb exhibits the highest compatibility with reflow

soldering, as shown in Table 15.10. This should not be a surprise, since most of thefluxes were developed for eutectic or near-eutectic tin-lead systems. The primaryfactor distinguishing 63Sn37Pb from the rest of the alloys is the soldering perform-ance, particularly the wetting and solder appearance. As to solder balling, although63Sn37Pb is also the best, it is fairly close to the best lead-free systems. In materialhandling performance, including shelf life and tack time, 63Sn37Pb is merelymediocre and is outperformed by several lead-free systems.

SnAgBi Systems. Both SnAgBi alloys studied here, 91.7Sn3.5Ag4.8Bi and90.5Sn7.5Bi2Ag, turned out to be among the top lead-free systems. This is mainlyattributed to the better wetting and solder balling performance. Shelf life and tacktime of the SnAgBi systems are also fairly good, while the solder appearance is atbest considered average.

SnCu, SnAgCu, SnAgCuSb, SnBi, and SnSb Systems. 99.3Sn0.7Cu,95.5Sn3.8Ag0.7Cu, 93.6Sn4.7Ag1.7Cu, 96.2Sn2.5Ag0.8Cu0.5Sb, 58Bi42Sn, and95Sn5Sb show fairly comparable performance, with compatibility ranging from 19.3to 20.3 (with full compatibility being 30) for warm profile. In general, the wholegroup displays a quite noticeably poorer wetting than SnAgBi systems. 58Bi42Snexhibits a fairly poor solder balling performance, but an outstanding solder appear-ance among lead-free systems. 96.2Sn2.5Ag0.8Cu0.5Sb shows a relatively poor per-formance in both wetting and solder appearance among these six alloys.

SnAg Systems. 96.5Sn3.5Ag is ranked below the alloys just described in com-patibility, mainly due to poor performance in solder balling and particularly poorwetting.

SnZnBi Systems. 89Sn8Zn3Bi falls far short in every category when comparedwith all other alloy systems. Obviously, this is attributable to the very reactive natureof zinc, which results in excessive oxidation of metal and excessive reaction withfluxes, and consequently a definitely unacceptable performance for solder pasteapplications.


TABLE 15.9 Compatibility Between Fluxes and Solder Alloys

Cool profile Warm profile

Flux Alloy SBI WI SLI TTI SAI C SBI WI SLI TTI SAI C Average C

F1 63Sn37Pb 9.0 10.0 8.0 10.0 10.0 28.4 10.0 8.9 8.0 10.0 10.0 28.3 28.4

96.5Sn3.5Ag 6.0 4.0 6.0 10.0 5.0 16.8 4.0 3.0 6.0 10.0 6.0 14.2 15.5

99.3Sn0.7Cu 8.0 5.8 8.0 4.0 5.0 19.4 9.0 5.0 8.0 4.0 8.0 20.8 20.1

95.5Sn3.8Ag0.7Cu 8.0 5.2 10.0 8.0 8.0 21.8 8.0 4.1 10.0 8.0 8.0 20.7 21.3

93.6Sn4.7Ag1.7Cu 7.0 3.5 10.0 6.0 6.0 17.7 8.0 5.8 10.0 6.0 7.0 21.4 19.6

96.2Sn2.5Ag0.8Cu0.5Sb 8.0 5.7 10.0 10.0 5.0 21.7 8.0 3.3 10.0 10.0 6.0 19.7 20.7

91.7Sn3.5Ag4.8Bi 10.0 5.5 10.0 8.0 5.0 22.9 10.0 6.1 10.0 8.0 5.0 23.5 23.2

90.5Sn7.5Bi2Ag 9.0 6.8 10.0 8.0 5.0 23.2 10.0 6.5 10.0 8.0 5.0 23.9 23.6

58Bi42Sn 6.0 8.0 10.0 10.0 9.0 23.6 6.0 6.3 10.0 10.0 9.0 21.9 22.8

95Sn5Sb 10.0 4.5 10.0 8.0 5.0 21.9 9.0 5.7 10.0 8.0 9.0 23.7 22.8

89Sn8Zn3Bi 0.0 0.0 0.0 2.0 0.0 0.6 0.0 0.0 0.0 2.0 0.0 0.6 0.6

Average 7.4 5.4 8.4 7.6 5.7 19.8 7.5 5.0 8.4 7.6 6.6 19.9 19.9

F2 63Sn37Pb 10.0 10.0 10.0 8.0 10.0 29.4 10.0 10.0 10.0 8.0 10.0 29.4 29.4

96.5Sn3.5Ag 4.0 4.5 8.0 8.0 7.0 16.1 4.0 5.5 8.0 8.0 6.0 16.7 16.4

99.3Sn0.7Cu 7.0 6.1 10.0 6.0 7.0 20.7 8.0 7.0 10.0 6.0 9.0 23.4 22.1

95.5Sn3.8Ag0.7Cu 7.0 5.4 10.0 6.0 7.0 20.0 9.0 3.1 10.0 6.0 8.0 20.1 20.1

93.6Sn4.7Ag1.7Cu 7.0 3.5 8.0 8.0 6.0 17.7 8.0 5.8 8.0 8.0 2.0 19.4 18.6

96.2Sn2.5Ag0.8Cu0.5Sb 7.0 5.0 10.0 6.0 5.0 18.8 8.0 7.0 10.0 6.0 6.0 22.2 20.5

91.7Sn3.5Ag4.8Bi 10.0 6.1 6.0 8.0 6.0 22.7 10.0 5.3 6.0 8.0 6.0 21.9 22.3

90.5Sn7.5Bi2Ag 10.0 6.8 10.0 6.0 6.0 24.0 10.0 5.7 10.0 6.0 6.0 22.9 23.5

58Bi42Sn 5.0 7.3 8.0 8.0 9.0 20.7 6.0 7.5 8.0 8.0 9.0 21.9 21.3

95Sn5Sb 9.0 4.3 4.0 8.0 6.0 19.3 8.0 5.5 4.0 8.0 9.0 20.7 20.0

89Sn8Zn3Bi 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Average 6.9 5.4 7.6 6.5 6.3 19.0 7.4 5.7 7.6 6.5 6.5 19.9 19.5

15.8

F3 63Sn37Pb 9.0 10.0 8.0 8.0 10.0 27.8 9.0 10.0 8.0 8.0 10.0 27.8 27.8

96.5Sn3.5Ag 6.0 3.1 2.0 8.0 7.0 14.9 7.0 6.3 2.0 8.0 8.0 19.5 17.2

99.3Sn0.7Cu 8.0 6.3 8.0 8.0 8.0 22.3 8.0 7.5 8.0 8.0 8.0 23.5 22.9

95.5Sn3.8Ag0.7Cu 8.0 6.7 10.0 8.0 8.0 23.3 8.0 7.3 10.0 8.0 8.0 23.9 23.6

93.6Sn4.7Ag1.7Cu 8.0 6.1 4.0 8.0 7.0 20.5 8.0 7.3 4.0 8.0 7.0 21.7 21.1

96.2Sn2.5Ag0.8Cu0.5Sb 8.0 5.3 10.0 8.0 6.0 21.1 9.0 6.5 10.0 8.0 7.0 23.7 22.4

91.7Sn3.5Ag4.8Bi 8.0 7.0 10.0 8.0 8.0 23.6 9.0 6.5 10.0 8.0 8.0 24.1 23.9

90.5Sn7.5Bi2Ag 7.0 6.1 8.0 8.0 8.0 21.1 8.0 6.6 8.0 8.0 8.0 22.6 21.9

58Bi42Sn 6.0 5.1 6.0 8.0 9.0 18.9 6.0 5.8 6.0 8.0 9.0 19.6 19.3

95Sn5Sb 8.0 5.1 10.0 8.0 8.0 21.7 8.0 6.8 10.0 8.0 8.0 23.4 22.6

89Sn8Zn3Bi 0.0 0.0 0.0 4.0 0.0 1.2 0.0 0.0 0.0 4.0 0.0 1.2 12

Average 6.9 5.5 6.9 7.6 7.2 19.7 7.3 6.4 6.9 7.6 7.4 21.0 20.4

F4 63Sn37Pb 10 10 8 8 10 28.8 10 9.5 8 8 10 28.3 28.6

96.5Sn3.5Ag 9 5.5 2 8 7 20.3 9 6 2 8 7 20.8 20.3

99.3Sn0.7Cu 8 5.2 8 8 8 21.2 9 6.5 8 8 8 23.5 22.4

95.5Sn3.8Ag0.7Cu 8 5.3 10 8 8 21.9 8 6.5 10 8 8 23.1 22.5

93.6Sn4.7Ag1.7Cu 8 5.3 4 8 6 19.3 8 6.7 4 8 7 21.1 20.2

96.2Sn2.5Ag0.8Cu0.5Sb 8 5 10 8 5 20.4 8 6.7 10 8 7 22.9 21.7

91.7Sn3.5Ag4.8Bi 8 6.3 10 8 6 22.1 9 6.2 10 8 7 23.4 22.8

90.5Sn7.5Bi2Ag 8 4.6 8 8 7 20.2 9 6.5 8 8 8 23.5 21.9

58Bi42Sn 6 4.6 6 8 9 18.4 5 4.8 6 8 9 17.6 18.0

95Sn5Sb 8 2.9 10 8 6 18.7 8 5.5 10 8 8 22.1 20.4

89Sn8Zn3Bi 0 0 0 4 0 1.2 0 0 0 4 0 1.2 1.2

Average 7.4 5.0 6.9 7.6 6.5 19.3 7.5 5.9 6.9 7.6 7.2 20.7 20.0

(Continues)

15.9

TABLE 15.9 Compatibility Between Fluxes and Solder Alloys (Continued)



F5 63Sn37Pb 9.0 10.0 6.0 2.0 10.0 25.4 9.0 10.0 6.0 2.0 10.0 25.4 25.4

96.5Sn3.5Ag 8.0 8.0 2.0 2.0 2.0 18.0 7.0 7.5 2.0 2.0 8.0 18.9 18.5

99.3Sn0.7Cu 8.0 5.5 4.0 2.0 6.0 17.7 8.0 6.6 4.0 2.0 8.0 19.6 18.7

95.5Sn3.8Ag0.7Cu 7.0 7.0 10.0 0.0 8.0 20.2 7.0 7.3 10.0 0.0 8.0 20.5 20.4

93.6Sn4.7Ag1.7Cu 9.0 6.1 4.0 2.0 2.0 17.7 9.0 6.3 4.0 2.0 7.0 19.9 18.8

96.2Sn2.5Ag0.8Cu0.5Sb 8.0 7.3 8.0 0.0 6.0 20.1 8.0 6.0 8.0 0.0 7.0 19.2 19.7

91.7Sn3.5Ag4.8Bi 10.0 7.8 8.0 0.0 5.0 22.2 10.0 6.7 8.0 0.0 2.0 19.9 21.1

90.5Sn7.5Bi2Ag 10.0 8.0 6.0 0.0 7.0 22.6 8.0 8.8 6.0 0.0 7.0 21.4 22.0

58Bi42Sn 6.0 5.0 4.0 2.0 9.0 16.4 7.0 5.2 4.0 2.0 9.0 17.6 17.0

95Sn5Sb 9.0 6.0 6.0 2.0 7.0 20.2 8.0 6.0 6.0 2.0 2.0 17.2 18.7

89Sn8Zn3Bi 1.0 0.0 0.0 0.0 0.0 1.0 1.0 0.0 0.0 0.0 0.0 1.0 1.0

Average 7.7 6.4 5.3 1.1 5.6 18.3 7.5 6.4 5.3 1.1 6.2 18.2 18.3

F6 63Sn37Pb 6.0 10.0 0.0 0.0 10.0 20.0 10.0 10.0 0.0 0.0 10.0 24.0 22.0

96.5Sn3.5Ag 10.0 6.3 10.0 2.0 7.0 22.7 10.0 5.3 10.0 2.0 7.0 21.7 22.2

99.3Sn0.7Cu 10.0 6.0 0.0 6.0 8.0 21.0 9.0 7.5 0.0 6.0 8.0 21.5 21.3

95.5Sn3.8Ag0.7Cu 9.0 6.7 10.0 6.0 7.0 23.3 10.0 6.3 10.0 6.0 7.0 23.9 23.6

93.6Sn4.7Ag1.7Cu 10.0 6.7 8.0 6.0 6.0 23.3 10.0 6.0 8.0 6.0 6.0 22.6 23.0

96.2Sn2.5Ag0.8Cu0.5Sb 9.0 6.1 10.0 6.0 5.0 21.9 10.0 5.1 10.0 6.0 5.0 21.9 21.9

91.7Sn3.5Ag4.8Bi 10.0 7.1 10.0 6.0 5.0 23.9 10.0 6.8 10.0 6.0 5.0 23.6 23.8

90.5Sn7.5Bi2Ag 9.0 8.3 2.0 6.0 5.0 21.7 10.0 6.5 2.0 6.0 5.0 20.9 21.3

58Bi42Sn 8.0 7.1 0.0 6.0 2.0 17.7 10.0 5.7 0.0 6.0 9.0 21.1 19.4

95Sn5Sb 8.0 5.0 0.0 4.0 5.0 16.2 10.0 5.7 0.0 4.0 5.0 18.9 17.6

89Sn8Zn3Bi 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Average 8.1 6.3 4.5 4.4 5.5 19.2 9.0 5.9 4.5 4.4 6.1 20.0 19.6

15.1

0

F7 63Sn37Pb 9.0 10.0 8.0 0.0 10.0 25.4 10.0 10.0 8.0 0.0 10.0 26.4 25.9

96.5Sn3.5Ag 10.0 1.0 8.0 0.0 4.0 15.0 10.0 1.0 8.0 0.0 4.0 15.0 15.0

99.3Sn0.7Cu 10.0 4.5 10.0 0.0 6.0 19.9 10.0 5.3 10.0 0.0 8.0 21.5 20.7

95.5Sn3.8Ag0.7Cu 10.0 5.1 10.0 0.0 6.0 20.5 10.0 5.3 10.0 0.0 6.0 20.7 20.6

93.6Sn4.7Ag1.7Cu 10.0 5.5 4.0 0.0 6.0 19.1 10.0 5.0 4.0 0.0 6.0 18.6 18.9

96.2Sn2.5Ag0.8Cu0.5Sb 10.0 4.8 10.0 0.0 5.0 19.8 10.0 5.3 10.0 0.0 5.0 20.3 20.1

91.7Sn3.5Ag4.8Bi 10.0 8.7 10.0 0.0 5.0 23.7 10.0 6.8 10.0 0.0 5.0 21.8 22.8

90.5Sn7.5Bi2Ag 10.0 8.1 10.0 0.0 5.0 23.1 10.0 7.1 10.0 0.0 5.0 22.1 22.6

58Bi42Sn 9.0 6.3 10.0 0.0 9.0 21.9 9.0 5.0 10.0 0.0 9.0 20.6 21.3

95Sn5Sb 10.0 3.0 6.0 0.0 5.0 16.8 10.0 4.5 6.0 0.0 5.0 18.3 17.6

89Sn8Zn3Bi 0.0 0.0 8.0 0.0 0.0 2.4 0.0 0.0 8.0 0.0 0.0 2.4 2.4

Average 8.9 5.2 8.5 0.0 5.5 18.9 9.0 5.0 8.5 0.0 5.7 18.9 18.9

F8 63Sn37Pb 10.0 10.0 8.0 6.0 10.0 28.2 10.0 10.0 8.0 6.0 10.0 28.2 28.2

96.5Sn3.5Ag 10.0 7.3 10.0 6.0 2.0 22.9 10.0 6.3 10.0 6.0 5.0 23.1 23.0

99.3Sn0.7Cu 9.0 1.0 10.0 6.0 8.0 18.0 10.0 7.0 10.0 6.0 3.0 23.0 20.0

95.5Sn3.8Ag0.7Cu 10.0 5.0 10.0 8.0 4.0 22.0 9.0 7.0 10.0 8.0 2.0 22.2 22.1

93.6Sn4.7Ag1.7Cu 10.0 7.0 10.0 6.0 6.0 24.2 10.0 5.8 10.0 6.0 5.0 22.6 23.4

96.2Sn2.5Ag0.8Cu0.5Sb 8.0 6.1 10.0 6.0 6.0 21.3 10.0 5.0 10.0 6.0 5.0 21.8 21.6

91.7Sn3.5Ag4.8Bi 10.0 7.0 10.0 8.0 5.0 24.4 10.0 7.5 10.0 8.0 5.0 24.9 24.7

90.5Sn7.5Bi2Ag 10.0 7.3 10.0 6.0 5.0 24.1 10.0 7.6 10.0 6.0 5.0 24.4 24.3

58Bi42Sn 10.0 7.6 6.0 8.0 3.0 23.0 10.0 4.5 6.0 8.0 3.0 19.9 21.5

95Sn5Sb 10.0 5.2 10.0 8.0 5.0 22.6 10.0 6.5 10.0 8.0 5.0 23.9 23.3

89Sn8Zn3Bi 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Average 8.8 5.8 8.5 6.2 4.9 21.0 9.0 6.1 8.5 6.2 4.4 21.3 21.2

(Continues)

15.1

1

TABLE 15.9 Compatibility Between Fluxes and Solder Alloys (Continued)



F9 63Sn37Pb 10.0 8.8 10.0 0.0 9.0 25.4 10.0 8.6 10.0 0.0 9.0 25.2 25.3

96.5Sn3.5Ag 1.0 0.0 10.0 0.0 4.0 5.6 1.0 0.0 10.0 0.0 4.0 5.6 5.6

99.3Sn0.7Cu 0.0 0.0 10.0 0.0 0.0 3.0 1.0 0.0 10.0 0.0 1.0 4.4 3.7

95.5Sn3.8Ag0.7Cu 1.0 0.0 10.0 0.0 1.0 4.4 1.0 0.0 10.0 0.0 1.0 4.4 4.4

93.6Sn4.7Ag1.7Cu 1.0 0.0 10.0 6.0 1.0 6.2 1.0 0.0 10.0 6.0 1.0 6.2 6.2

96.2Sn2.5Ag0.8Cu0.5Sb 0.0 0.0 10.0 2.0 0.0 3.6 0.0 0.0 10.0 2.0 0.0 3.6 3.6

91.7Sn3.5Ag4.8Bi 10.0 6.5 10.0 0.0 5.0 21.5 10.0 7.5 10.0 0.0 5.0 22.5 22.0

90.5Sn7.5Bi2Ag 10.0 6.5 10.0 2.0 5.0 22.1 10.0 5.7 10.0 2.0 5.0 21.3 21.7

58Bi42Sn 9.0 6.5 10.0 2.0 8.0 22.3 10.0 8.1 10.0 2.0 9.0 25.3 23.8

95Sn5Sb 0.0 0.0 10.0 0.0 0.0 3.0 1.0 0.0 10.0 0.0 1.0 4.4 3.7

89Sn8Zn3Bi 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Average 3.8 2.6 9.1 1.1 3.0 10.6 4.1 2.7 9.1 1.1 3.3 11.2 10.9

F10 63Sn37Pb 10.0 10.0 6.0 6.0 9.0 27.2 10.0 10.0 6.0 6.0 10.0 27.6 27.4

96.5Sn3.5Ag 6.0 6.1 8.0 6.0 5.0 18.3 7.0 7.5 8.0 6.0 8.0 21.9 20.1

99.3Sn0.7Cu 10.0 5.0 0.0 6.0 7.0 19.6 9.0 7.5 0.0 6.0 8.0 21.5 20.6

95.5Sn3.8Ag0.7Cu 8.0 6.5 4.0 6.0 7.0 20.3 8.0 8.0 4.0 6.0 8.0 22.2 21.3

93.6Sn4.7Ag1.7Cu 10.0 6.3 6.0 6.0 8.0 23.1 9.0 7.0 6.0 6.0 8.0 22.8 23.0

96.2Sn2.5Ag0.8Cu0.5Sb 8.0 6.1 4.0 6.0 5.0 19.1 9.0 7.0 4.0 6.0 5.0 21.0 20.1

91.7Sn3.5Ag4.8Bi 9.0 7.6 6.0 6.0 6.0 22.6 9.0 7.5 6.0 6.0 8.0 23.3 23.0

90.5Sn7.5Bi2Ag 9.0 8.5 10.0 6.0 5.0 24.3 9.0 7.8 10.0 6.0 8.0 24.8 24.6

58Bi42Sn 1.0 4.5 4.0 6.0 8.0 11.7 1.0 4.5 4.0 6.0 8.0 11.7 11.7

95Sn5Sb 10.0 6.0 0.0 6.0 5.0 19.8 9.0 6.6 0.0 6.0 7.0 20.2 20.0

89Sn8Zn3Bi 1.0 5.0 0.0 2.0 2.0 7.4 10.0 4.6 0.0 2.0 2.0 16.0 11.7

Average 7.5 6.5 4.4 5.6 6.1 19.4 8.2 7.1 4.4 5.6 7.3 21.2 20.3

15.1

2

15.1

3

TABLE 15.10 Summary of Compatibility of Solder Alloy Systems with a Variety of Fluxes

Cool profile Warm profile Average

Alloy SBI WI SLI TTI SAI C SBI WI SLI TTI SAI C SBI WI SLI TTI SAI C

63Sn37Pb 9.2 9.9 7.2 4.8 9.8 26.6 9.8 9.7 7.2 4.8 9.9 27.1 9.5 9.8 7.2 4.8 9.9 26.8

96.5Sn3.5Ag 7.0 4.6 6.6 5.0 5.0 17.1 6.9 4.8 6.6 5.0 6.3 17.7 7.0 4.6 6.6 5.0 5.7 17.4

99.3Sn0.7Cu 7.8 4.4 6.8 4.6 6.3 18.2 8.1 6.0 6.8 4.6 6.9 20.3 8.0 5.2 6.8 4.6 6.6 19.3

95.5Sn3.8Ag0.7Cu 7.6 5.3 9.4 5.0 6.4 19.8 7.8 5.5 9.4 5.0 6.4 20.2 7.7 5.4 9.4 5.0 6.4 20.0

93.6Sn4.7Ag1.7Cu 8.0 5.0 6.8 5.6 5.4 18.9 8.1 5.6 6.8 5.6 5.6 19.6 8.1 5.3 6.8 5.6 5.5 19.3

96.2Sn2.5Ag0.8Cu0.5Sb 7.4 5.1 9.2 5.2 4.8 18.8 8.0 5.2 9.2 5.2 5.3 19.6 7.7 5.2 9.2 5.2 5.1 19.2

91.7Sn3.5Ag4.8Bi 9.5 7.0 9.0 5.2 5.6 23.0 9.7 6.7 9.0 5.2 5.6 22.9 9.6 6.8 9.0 5.2 5.6 22.9

90.5Sn7.5Bi2Ag 9.2 7.1 8.4 5.0 5.8 22.6 9.4 6.9 8.4 5.0 6.2 22.8 9.3 7.0 8.4 5.0 6.0 22.7

58Bi42Sn 6.6 6.2 6.4 5.8 7.5 19.5 7.0 5.7 6.4 5.8 8.3 19.7 6.8 6.0 6.4 5.8 7.9 19.6

95Sn5Sb 8.2 4.2 6.6 5.2 5.2 18.0 8.1 5.3 6.6 5.2 5.9 19.3 8.2 4.7 6.6 5.2 5.6 18.7

89Sn8Zn3Bi 0.2 0.5 0.8 1.2 0.2 1.4 1.1 0.5 0.8 1.2 0.2 2.2 0.7 0.5 0.8 1.2 0.2 1.8

Average 7.3 5.4 7.0 4.8 5.6 18.5 7.6 5.6 7.0 4.8 6.1 19.2 7.5 5.5 7.0 4.8 5.8 18.9

TABLE 15.11 Summary of Compatibility of Flux Systems with a Variety of Alloys

Cool profile Warm profile Average

Flux SBI WI SLI TTI SAI C SBI WI SLI TTI SAI C SBI WI SLI TTI SAI C

F1 (NC, air, no-X, probe) 7.4 5.4 8.4 7.6 5.7 19.8 7.5 5.0 8.4 7.6 6.6 19.9 7.4 5.2 8.4 7.6 6.2 19.9

F2 (NC, air, no-X, probe) 6.9 5.4 7.6 6.5 6.3 19.0 7.4 5.7 7.6 6.5 6.5 19.9 7.1 5.5 7.6 6.5 6.4 19.5

F3 (NC, air, X) 6.9 5.5 6.9 7.6 7.2 19.7 7.3 6.4 6.9 7.6 7.4 21.0 7.1 6.0 6.9 7.6 7.3 20.3

F4 (NC, air, X) 7.4 5.0 6.9 7.6 6.5 19.3 7.5 5.9 6.9 7.6 7.2 20.7 7.5 5.4 6.9 7.6 6.9 20.0

F5 (RMA, air, X) 7.7 6.4 5.3 1.1 5.6 18.3 7.5 6.4 5.3 1.1 6.2 18.2 7.6 6.4 5.3 1.1 5.9 18.3

F6 (NC, air, no-X) 8.1 6.3 4.5 4.4 5.5 19.2 9.0 5.9 4.5 4.4 6.1 20.0 8.5 6.1 4.5 4.4 5.8 19.6

F7 (NC, N2, no-X, low R) 8.9 5.1 8.5 0.0 5.5 18.9 9.0 4.9 8.5 0.0 5.7 18.9 9.0 5.0 8.5 0.0 5.6 18.9

F8 (NC, N2, no-X, 8.8 5.7 8.5 6.2 4.9 21.0 9.0 6.1 8.5 6.2 4.4 21.3 8.9 5.9 8.5 6.2 4.6 21.2ultra-low R)

F9 (WS, air, no-X, med. 3.8 2.6 9.1 1.1 3.0 10.6 4.1 2.7 9.1 1.1 3.3 11.2 4.0 2.6 9.1 1.1 3.1 10.9Temp)

F10 (WS, air, X, high temp) 7.5 6.5 4.4 5.6 6.1 19.4 8.2 7.1 4.4 5.6 7.3 21.2 7.8 6.8 4.4 5.6 6.7 20.3

Average 7.3 5.4 7.0 4.8 5.6 18.5 7.6 5.6 7.0 4.8 6.1 19.2 7.5 5.5 7.0 4.8 5.8 18.9

Low R, low residue; no-X, no halide; WS, water soluble.

15.1

4

15.1.2.2 Compatibility of Fluxes. As shown in Table 15.11, the compatibilityof fluxes with alloys is very comparable for almost all of the fluxes studied, withcompatibility values ranging from 18.2 to 21.3. The only exception is flux F9, awater-soluble flux, mainly due to poor soldering performance. This may be relatedto the poor thermal stability of this flux when reflowed under higher-temperatureconditions.

15.1.2.3 Effect of Temperature on Compatibility of Alloys. Generally speaking,a higher reflow temperature results in a better compatibility, as shown in Table 15.10.Since a higher reflow temperature will favor a higher reaction rate between solderand the base metal, this effect is very much expected.The SnAgBi systems appear tobe one exception, for reasons that are unclear.

15.1.2.4 Effect of Temperature on Compatibility of Fluxes. Although a higherreflow temperature often results in a greater compatibility for fluxes, as shown inTable 15.11, several exceptions are observed, including fluxes F1, F5, and F7. In gen-eral, the flux reaction rate increases with increasing temperature,5 and therefore abetter compatibility will be expected for a higher reflow temperature. However,some fluxes may start to burn off or decompose at a higher temperature. This phe-nomenon may offset the reaction rate factor and cause a flat or declining compati-bility with increasing temperature.


FIGURE 15.2 Compatibility of alloys with reflow soldering using warm profile.

15.1.2.5 Cross Section of Solder Bumps FormedMacrovoiding. Although the solder appearance often is a good indication of

solder joint quality, the results of cross section show that properties such asmacrovoiding may not be reflected by the surface appearance. It appears that thereis negligible correlation between macrovoiding and solder appearance. This phe-nomenon is illustrated in Table 15.12, where a system with a low SAI value (such as5) may exhibit less macrovoiding (13 percent) than a system with a high SAI value(such as 8, with 56 percent voiding).


TABLE 15.12 Relationship Between Macrovoiding, SAI, and IMC Thickness

IMC VoidingFlux Alloy Profile SAI (µm) (%)*

F5 91.7Sn3.5Ag4.8Bi Warm 2 2.7 13

F10 89Sn8Zn3Bi Warm 2 2.0 100

F8 58Bi42Sn Cool 3 0.7 79

F5 91.7Sn3.5Ag4.8Bi Cool 5 1.7 13

F2 90.5Sn7.5Bi2Ag Cool 6 1.5 75

F5 90.5Sn7.5Bi2Ag Cool 7 2.0 15

F5 90.5Sn7.5Bi2Ag Warm 7 2.0 50

F5 95.5Sn3.8Ag0.7Cu Warm 8 2.0 27

F5 95.5Sn3.8Ag0.7Cu Cool 8 2.0 56

F5 63Sn37Pb Cool 10 1.0 0

* Percentage of cross-sectioned bumps exhibiting macrovoiding.

IMC Thickness. Also shown in Table 15.12 are examples of intermetallic com-pound (IMC) thickness. It is interesting to note that most of the lead-free alloys,except 58Bi42Sn (0.7 µm in IMC layer thickness), display a thicker intermetalliclayer (1.5 to 2.7 µm) than 63Sn37Pb (1 µm). The intermetallic thickness appears toincrease with either increasing reflow temperature or increasing Sn content. How-ever, for the solders studied here, the reflow temperature increases roughly withincreasing Sn content. Thus the reflow temperature is the lowest for 58Bi42Sn (withthe lowest Sn content), medium for 63Sn37Pb (with medium Sn content), and high-est for the rest of the lead-free alloys (with Sn content no less than 89 percent). Intheory, both high Sn content and high reflow temperature could promote formationof a thicker IMC layer. Data here are insufficient to clarify the relative effect ofreflow temperature versus Sn content on the thickness of IMC. However, they dosuggest that for those high-tin-content lead-free alloys that exhibit a melting tem-perature higher than that of eutectic SnPb solder, IMC thickness may tend to begreater than that for eutectic SnPb system, therefore posing some concern about thereliability of reflow applications.

15.1.3 ADDITIONAL FACTORS TO BE CONSIDERED

15.1.3.1 Significance of SBI, WI, and SAI. To assess the compatibility of alloyswith reflow applications, it is essential to recognize all of the crucial independent

performance parameters. For soldering performance, it may appear that wettingability, solder balling, and solder appearance are all related to each other and may bereflected by a single parameter.The relation among those three parameters is exam-ined by plotting WI vs. SBI (see Fig. 15.3) and SAI vs.WI (see Fig. 15.4) for all of themeasurements conducted in this work. Results indicate that the relation among wet-ting ability, solder balling, and solder appearance is very weak—if any exists—andthat all three properties have to be monitored in order to assess the compatibility ofalloys.

However, it should also be pointed out that while the smoothness of solderappearance may serve as a very good indicator of solder joint quality for an alloysystem, the same criteria may not be applicable when comparing solders with differ-ent compositions. Therefore, an alloy with some crystalline surface texture may bemore reliable than another alloy with a smooth surface texture. For this reason, thesolder appearance factor is weighed less in this study than other properties such assolder balling and wetting.

15.1.3.2 WI. For systems not fully reflowed, the WI value assigned is alwayszero, even if a partial wetting may be observed on the copper pad.

15.1.3.3 Potential of Alloys. The reflow profiles used in this study allow themaximum temperature to be 30°C above the liquidus temperature. In some indus-trial practice, a profile with a higher peak temperature, such as 40°C above the liq-uidus temperature, may be possible. Under those conditions, most of the lead-free


FIGURE 15.3 Relationship between SBI and WI.

alloy systems investigated here may have better compatibility than what has beendemonstrated in this study.

15.1.3.4 Solder Paste Deposition. In this work, the printability and dispensabil-ity of solder paste are not evaluated. The impact on the compatibility study is con-sidered minimal, since deposition performance is mainly a function of the fluxrheology, not of alloy type.

15.1.3.5 Testing Without Components. Since no components were used in theexperiment, certain specific process defects such as tombstoning, wicking, solderbeading, and skewing cannot be predicted by the work here.As to the voiding, whichis often a function of coverage area by the components,6 the performance will be dif-ficult to predict as well.

15.1.3.6 Surface Finishes. The surface finish is confined to OSP coating in thiswork. Varying the surface finish may not affect the results reported on solderballing and solder appearance, but may alter the conclusion on wetting ability. Sur-face finishes containing lead pose special concerns about reliability for Bi-containing alloys,3,4 which may outweigh the compatibility advantage in terms ofsoldering capability and handling ability displayed by SnAgBi solder systems forreflow applications.


FIGURE 15.4 Relationship between WI and SAI.

15.1.3.7 Beyond Reflow Soldering. The compatibility results are applicable tosolder paste applications only. Alloys being ruled out due to solder paste stabilityproblems, such as 89Sn8Zn3Bi, may still be promising for other applications such aswave soldering. On the other hand, alloys compatible with reflow soldering may suf-fer defects such as fillet lifting at wave soldering.3

15.1.4 COMPATIBILITY ASSESSMENT

The prospects of 10 major lead-free solder alloys for being widely used in reflow sol-dering are studied in this work. Compatibility of those alloys with a variety of repre-sentative flux chemistries is considered essential, and is determined for performancein handling ability, including shelf life and tack time, and soldering capability, includ-ing solder balling, wetting, and solder joint appearance. Results indicate that thecontrol 63Sn37Pb is still the most compatible alloy, rated 27.1 out of 30 in compati-bility when using warm profile.The primary factor that distinguishes 63Sn37Pb fromthe other alloys is soldering performance, particularly wetting and solder appear-ance. As to solder balling, although 63Sn37Pb is also the best, it is fairly close to thebest lead-free systems. Among the lead-free options, both SnAgBi alloys studiedhere, 91.7Sn3.5Ag4.8Bi and 90.5Sn7.5Bi2Ag, turn out to be at the top of lead-freesystems, rated 22.9 and 22.8, respectively. This is mainly attributed to their betterwetting and solder balling performance. Shelf life and tack time of the SnAgBi sys-tems are also fairly good, while the solder appearance is at best considered average.99.3Sn0.7Cu, 95.5Sn3.8Ag0.7Cu, 93.6Sn4.7Ag1.7Cu, 96.2Sn2.5Ag0.8Cu0.5Sb,58Bi42Sn, and 95Sn5Sb show fairly comparable performance, with compatibilityranging from 19.3 to 20.3. In general, the whole group displays a quite noticeablypoorer wetting than SnAgBi systems. 58Bi42Sn exhibits a fairly poor solder ballingperformance, but an outstanding solder appearance among lead-free systems.96.2Sn2.5Ag0.8Cu0.5Sb shows a relatively poor performance in both wetting andsolder appearance among these six alloys. 96.5Sn3.5Ag, rated 17.1 in compatibility, isranked below the other alloys described, mainly due to poor performance in solderballing and particularly poor wetting. 89Sn8Zn3Bi, rated only 2.2 in compatibility,falls far short in every category when compared with all other alloy systems. Obvi-ously, this is attributable to the very reactive nature of zinc, which results in excessiveoxidation of metal and excessive reaction with fluxes, and consequently a definitelyunacceptable performance for solder paste applications. High-tin-content lead-freealloys seem to display a thicker IMC layer than eutectic SnPb when reflowed.

15.2 IMPLEMENTING LEAD-FREE

WAVE SOLDERING

The challenge for lead-free wave soldering is no less than that for reflow solder-ing. Due to the relative reduction in solder wetting and fluxing ability, lead-freewave soldering experiences a higher level of defects, particularly in the case ofthrough-hole penetration and bridging. Diepstraten7 studied the potential factorsaffecting wave soldering yield using the Taguchi experimental design method. InDiepstraten’s work, four process factors were selected: solder temperature, con-tact time, preheat temperature of topside PCB, and wet flux amount. Table 15.13shows the three levels selected for each factor. The solder alloy used was94.9Sn3.8Ag0.8Cu0.5Sb, which has a melting point of 217°C. Flux used was a volatile


organic compound–free, halide-free low solid (<2 percent) synthetic flux applied byspray nozzle. Board finish was OSP coating.

Figure 15.5 shows the bridging performance relative to process factors. Contacttime was most influential, followed closely by preheat temperature. A lower settingis desired for both incidences.The effect of process factors on through-hole penetra-tion is shown in Fig. 15.6. Here preheat temperature is found to be most influential,with a relatively low temperature being more desirable. The effects of the rest fac-tors are about equal. Diepstraten concluded that the overall best settings for hisexperiment were as follows:

Solder temperature = 275°C; to avoid thermal damage, between 265° and 270°C Contact time = 1.8 s Preheat temperature (topside) = 110°C Wet flux volume = 474 mg/dm2

Barbini investigated the effect of nitrogen on wave soldering, and concluded thatnitrogen over wave allows for better wetting.8


TABLE 15.13 Process Factors and Test Conditions Used in TaguchiExperimental Design

Process Factors Level 1 Level 2 Level 3

A Solder temperature (°C) 250 260 275

B Contact time (s) 1.8 3.0 4.2

C Preheat temperature of 90 110 130topside PCB (°C)

D Wet flux amount 355 474 639(mg/dm2)

FIGURE 15.5 Nonbridging performance relative to process factors. The higherthe number, the higher the quality (200 = no bridging at all). Control time and pre-heat temperature were most influential.7

15.3 EFFECT OF REFLOW PROFILE

ON LEAD-FREE SOLDERING

The effect of reflow profile on soldering has been analyzed by Lee based on defectmechanism analysis.5 Listed in Table 15.14 are the major reflow-related defect type,mechanisms of defect formation, and desired profile features, as well as the break-down of the desired profile elements for each of the subjects under discussion. Anoptimized profile should favor minimizing most of the defects, even if it may not bethe best choice for reducing certain defects. For the heating zone, 13 defects preferto have a low ramp-up rate, and none prefer to have a high ramp-up rate. In otherwords, 100 percent of all defect types will benefit from a low ramp-up rate. For thecooling zone, two defects favor a low ramp-down rate, and five favor a high ramp-down rate, or 71 percent of relevant defect types favor a high ramp-down rate. Forpeak temperature, five favor a low temperature, while one favors a high peak tem-perature. These results can be summarized in Fig. 15.7. Therefore, the dominanttrend can be summarized as follows: a slow ramp-up rate to a low peak temperature,followed by fast cooling rate. Combining with the timing considerations discussedabove, the optimized profile can be represented by Fig. 15.8. Here the temperatureramps up slowly at a rate between 0.5 and 1°C/s until reaching about 180°C. Thetemperature is then gradually raised further to 186°C within about 30 s, then raisedquickly at a rate of about 2.5 to 3.5°C/s until reaching about 220°C. After that, thetemperature is brought down at a rapid cooling rate no higher than 4°C/s.

By examining Fig. 15.8, it can be noticed that the small soaking shoulder onlycauses a ripple in the ramp-up path.The effect of that small shoulder may not be sig-nificant, and a linear ramp-up path may be more favorable due to ease of setting upon the oven. Figure 15.9 represents a profile with this linear ramp-up until peak tem-perature followed by quick cool-down. Due to the shape of this profile, resemblingthat of a tent, it can also be called the tent profile. In general, a linear ramp-up pro-file followed by rapid cooling is considered the most desirable for most applications,and is recommended as a startup profile. Depending on the design of the PCB andthe solder paste chemistry employed, the profile can be tweaked for better results.


FIGURE 15.6 Effect of process factors on through-hole penetration. The higher thenumber, the higher the quality (4662 = 100 percent penetration for all holes). Preheattemperature is most influential.7

TABLE 15.14 Desired Profile Features for Minimizing Defects5

Ramp- Peak CoolingSubject Defect mechanism Desired profile feature up rate temperature rate

Parts cracking Too high an internal Slow down temperature change rate Slow Slowstress due to fasttemperature change rate

Tombstoning Uneven wetting at both Use slow ramp-up rate at temperature near and above Slowends of chip solder melting point to minimize the temperature

gradients across the chip

Skewing Uneven wetting at both Use slow ramp-up rate at temperature near and above Slowends of chip solder melting point to minimize the temperature

gradients across the chip

Wicking Leads hotter than PCB Slow ramp-up rate to allow the board and components to Slowreach temperature equilibrium before solder melts;more bottom side heating

Solder balling Spattering Slow ramp-up rate to dry out paste solvents or moisture Slowgradually

Excessive oxidation Minimize heat input prior to reflow (slow ramp-up rate, Slowbefore solder melting no plateau at soaking zone) to reduce oxidation

Hot slump Viscosity drops with Slow ramp-up rate to dry out paste solvent gradually Slowincreasing temperature before viscosity decreases too much

Bridging Hot slump Slow ramp-up rate to dry out paste solvent gradually Slowbefore viscosity decreases too much

Solder beading Rapid outgassing under Slow ramp-up rate prior to reflow to slow down the Slowlow standoff outgassing rate of pastecomponents

15.2

2

Opens Wicking Slow ramp-up rate to allow the board and components to Slowreach temperature equilibrium before solder melts;more bottom side heating

Nonwetting Minimize heat input prior to reflow (minimize soaking Slowzone, or use linear ramp-up from ambient to soldermelting temperature) to reduce oxidation

Poor wetting Excessive oxidation Minimize heat input prior to reflow (minimize soaking Slowzone, or use linear ramp-up from ambient to soldermelting temperature) to reduce oxidation

Voiding Excessive oxidation Minimize heat input prior to reflow (minimize soaking Slowzone, or use linear ramp-up from ambient to soldermelting temperature) to reduce oxidation

Flux remnant too high in Cooler reflow profile to allow more solvents in flux Lowviscosity remnant

Charring Overheating Lower temperature, shorter time Low Fast

Leaching Overheating at temperature Minimize heat input at temperature above solder melting Low Fastabove solder melting point point by using lower temperature or shorter time

Dewetting Overheating at temperature Minimize heat input at temperature above solder melting Low Fastabove solder melting point point by using lower temperature or shorter time

Cold joints Insufficient coalescence Use high enough peak temperature Medium

Excessive Too much heat input above Lower peak temperature and use shorter time Low Fastintermetallics solder melting point

Large grain size Annealing effect due to Fast cooling rate Fastslow cooling rate

15.2

3


FIGURE 15.7 Summary of relative preference on profile characteristics based onpercentage of defect types benefited.

FIGURE 15.8 Optimized profile via defect mechanism analysis.

FIGURE 15.9 Tent profile with linear ramp-up to peak, thenrapid cool-down.

Since absolute material properties are not required for analysis, the defect mech-anism analysis just discussed and the profile desired are applicable to all surface-mount assembly processes and should be insensitive to the solder alloy type. In otherwords, the conclusion derived is expected to be applicable to lead-free reflow sol-dering as well. Indeed, this is found to be true. Figure 15.10 shows linear ramp-upprofiles successfully adopted for the assembly process.9 The top one, with a 220°Cpeak temperature, is a 63Sn37Pb profile, while the bottom one, with a 249°C peaktemperature, is used for 95.5Sn3.9Ag0.6Cu reflow process.An example of a tweakedlinear ramp-up profile is shown in Fig. 15.11, which is reported by Motorola for the95.5Sn3.8Ag0.7Cu reflow process.10

Shina et al.11 conducted a design of experiment investigating, among other objec-tives, the effect of reflow profile on lead-free soldering defect rate, with resultsshown in Fig. 15.12.As expected, the linear ramp-up profiles3,4 yielded a lower defectrate than the conventional profile with a profound soaking zone,1,2 thus confirmingthe predictions based on the defect mechanism analysis.


FIGURE 15.10 Linear ramp-up profile. (a) 63Sn37Pb paste (220°C peak); (b) 95.5Sn3.9Ag0.6Cu(249°C peak).

(b)

(a)

15.4 FLUX DESIRED FOR LEAD-FREE

PASTE SOLDERING

For soldering performance, the SnAgBi systems are considerably better than allother lead-free alloys, while SnZnBi is totally unacceptable as a solder. The remain-ing alloys fit in between, with SnAg measurably poorer than the other alloys.

Carrol and Warwick12 reported that addition of 0 to 4 percent Bi to 60Sn40Pbcauses a nonlinear fall in surface tension, presumably caused by the low surface ten-


FIGURE 15.11 Profile used for 95.5Sn3.8Ag0.7Cu solder paste reflow process.

FIGURE 15.12 Linear ramp-up profiles show a lower defect ratethan conventional profile with a profound soaking zone.

sion of Bi. A similar effect may also prevail in the lead-free systems, hence explain-ing the fact that the SnAgBi system exhibits the best soldering performance amongall of the lead-free alloys tested.

None of the alloys are as good as eutectic SnPb. The superior soldering perform-ance of eutectic SnPb can be at least partially explained by its surface tension factor.Since Sn is higher in surface tension than Pb, replacing Pb with Sn naturally resultsin a higher surface tension for the new alloy, and accordingly contributes to a poorerwetting.

The following list shows the surface tensions (dyn/cm) of some alloys:

63Sn37Pb: 380 at 260°C 96.5Sn3.5Ag: 460 at 260°C 95Sn5Sb: 470 at 280°C 50Sn50In: 590 at 215°C 58Bi42Sn: 300 at 195°C

The superior soldering performance of 63Sn37Pb shown in Table 15.15 indicates thatthe fluxes employed nowadays for eutectic SnPb systems have to be upgraded inorder to deliver a satisfactory result for lead-free solder alloys. The direction ofchange in fluxes required is discussed and speculated on in the following text.

SnBi eutectic solders (melting temperature 138°C) may have a better solderingperformance, as implied by the low surface tension value of the alloy, if fluxes ade-quate for low-temperature soldering are more readily available. Most of the fluxesavailable in the industry are developed for eutectic SnPb processing temperaturesand are not active enough for 58Bi42Sn. In other words, a flux with a low activationtemperature will be desired for 58Bi42Sn. However, the wetting capability is notonly affected by surface tension alone. Humpston and Jacobson13 reported that theability to promote solder spreading follows the order of Sn > Pb > Ag > In > Bi, basedon the test results of a series of binary solders. The inferior wetting ability of Bi ver-sus Pb at high content level may be caused by a metallurgical factor that overridesthe surface tension factor. These spreading data suggest that the flux desired for


TABLE 15.15 Summary of Compatibility of Flux Systems with a Variety of Alloys

Sum of SBI Sum of SLIFlux SBI WI and WI SLI TTI and TTI

F1 (NC, air, no-X, probe) 7.4 5.2 12.6 8.4 7.6 16.0

F2 (NC, air, no-X, probe) 7.1 5.5 12.6 7.6 6.5 14.1

F3 (NC, air, X) 7.1 6.0 13.1 6.9 7.6 14.5

F4 (NC, air, X) 7.5 5.4 12.9 6.9 7.6 14.5

F5 (RMA, air, X) 7.6 6.4 14.0 5.3 1.1 6.4

F6 (NC, air, no-X) 8.5 6.1 14.6 4.5 4.4 8.9

F7 (NC, N2, no-X, low R) 9.0 5.0 14.0 8.5 0.0 8.5

F8 (NC, N2, no-X, ultra-low R) 8.9 5.9 14.8 8.5 6.2 14.7

F9 (WS, air, no-X, med. temp) 4.0 2.6 6.6 9.1 1.1 10.2

F10 (WS, air, X, high temp) 7.8 6.8 14.6 4.4 5.6 10.0

Courtesy of Benlih Huang and Ning-Cheng Lee, Indium Corporation of America, published in IMAPS’99—Chicago.

58Bi42Sn should exhibit not only a lower activation temperature, but also a higherflux activity to compensate for the difference caused by the alloy.

For SnZnBi solder, the poor performance is caused by the high oxide content aswell as the high oxidizing activity of solder at reflow. Flux needed for this alloyshould have considerably high flux capacity and high oxygen barrier ability. The lat-ter is required to minimize the formation of new oxide, as reported by Lee.14 In Table15.9, the only flux that provided marginal performance for this alloy was F10. F10was formulated to handle air reflow for temperature up to more than 300°C, hencesatisfying the requirement of both high flux capacity and high oxygen barrier ability.Fluxes containing tin metalloorganic compounds may also help wetting by decom-posing and plating a tin layer on top of copper during soldering.15 However, beforeadopting this flux chemistry, the potential for circuit shorts or leakage currentshould be closely examined. Other approaches may also be available. At least oneflux has been reported to perform acceptably at SMT application.16

As for the rest of the lead-free alloys, such as SnAgCu or SnAgBi, the demand forflux ability is less than that for SnZnBi, but follows the same direction. In general, anincrease in flux capacity or oxygen barrier ability on the top of existing flux systemswill be needed. This may be achieved relatively easily by improving current RMA,high-solid-content no-clean, and of course water-washable flux systems, as demon-strated by fluxes F5, F6, and F10 in Table 15.15. All three fluxes are among the topperformers of existing fluxes, and therefore are the most promising systems to besuccessfully upgraded. It should be noted that the oxygen barrier ability can be sub-stituted by employing an inert reflow atmosphere, as confirmed by the high per-formance of fluxes F7 and F8, and was elucidated in theory by Lee.14 Figure 15.13indicates that an improved soldering performance can be achieved by employingeither a flux with a lower K value or a reflow atmosphere with a lower oxygen par-tial pressure. A lower K value means a higher rosin or resin content, which serves asan oxygen barrier.


FIGURE 15.13 Theoretical relation between soldering performance Sand oxygen partial pressure P of reflow atmosphere. K = R0/R, where R0 isrosin or resin content (50 percent) of regular RMA flux and R is that of theflux under analysis. (Courtesy of N. C. Lee.14)

Besides regulating the oxygen barrier content or oxygen partial pressure, the sol-dering performance can also be enhanced by using a more effective activator sys-tem. Among all of the activators, halides are considered the most effective in termsof fluxing performance per unit flux volume, and accordingly are the top candidatesfor improving fluxes. Other organic chemicals may also be considered. However, thehigh activator solid volume required tends to cause too high a viscosity for solderpastes.

A self-evident requirement is the thermal stability. Thermally decomposed oroxidized organic chemicals will lose the properties exhibited at a lower temperature.Without pertinent thermal stability, none of the features mentioned can be realized.In general, all of the flux ingredients should be able to survive the whole reflowprocess.


PASTE HANDLING

The handling performance is dictated by (1) reactivity of solder surface with fluxesand (2) solder surface texture and shape.A high reactivity will cause increasing pasteviscosity and decreasing flux capacity. On the other hand, a rough surface texture orirregular powder shape may promote excessive surface adsorption of chemicalsused in fluxes, hence altering the composition of flux medium in solder paste. Exceptfor SnZnBi alloy, almost all of the lead-free solder alloys have comparable or slightlybetter handling performance than 63Sn37Pb. This suggests that there is virtually noneed to further improve the flux chemistry for handling most of the lead-free solderpastes. As to SnZnBi solder system, obviously a highly stable flux system is neededin order to retard the reaction between Zn and fluxes. At least one flux has beenreported to perform properly for handling.16

15.6 CLEANING PERFORMANCE

OF LEAD-FREE SOLDER PASTE

Lead-free solder pastes face challenges not only from soldering and handling, butalso from a cleaning perspective. Similar to eutectic Sn-Pb solder systems, therequirement for cleaning may also be demanded for no-clean solder pastes forsome applications. Examples include (1) recovering boards suffering solderballing defects, (2) a single paste for both no-clean and cleaning customers for con-tract manufacturers, (3) high-frequency radio frequency applications, (4) inte-grated circuit packages, (5) military applications, (6) automotive applications, (7)aerospace applications, and (8) medical applications. In order to understand theimpact of lead-free flux technology on cleaning, 25 commercial lead-free solderpastes were tested with eight cleaner chemistries and several cleaning processes.The 25 pastes tested are shown in Table 15.16, and are categorized into threegroups—no-clean hard residue, no-clean soft residue, and water washable.The softresidue group also includes probe-testable pastes. The alloys tested include SnAg,SnAgCu, and SnAgCuSb, with melting temperature estimated to be approxi-mately 215 to 221°C.


15.6.1 CLEANING RESULTS

The cleaning results are shown in Table 15.17. Note the cleaning processes are cate-gorized as (1) semiaqueous and/or aqueous-solvent sprayable cleaning, (2) saponi-fied aqueous spray cleaning, and (3) solvent boil-rinse-vapor degrease. The cleaningperformance is further displayed in Tables 15.18 and 15.19 to show the effect of fluxchemistry or cleaning chemistry/process on cleaning.


TABLE 15.16 Solder Pastes Used in the Cleaning Study

No-clean hard residue Alloy

1 No-clean, amber hard residue SnAgCuSb

2 No-clean SnAgCu

3 No-clean, halide-free rosin, high-reliability electronics SnAgCu

4 No-clean, same as 3 with slight halide content SnAgCu

5 RMA, synthetic rosin type SnAgCu

6 No-clean RMA highly active resin/rosin-based formulation Sn96.5Ag3.5

7 No-clean mildly activated resin-based formulation SnAgCuSb

8 No-clean mildly activated resin-based formulation SnAgCuSb

9 No-clean Sn95Ag5

10 No-clean, extended work life Sn95Ag5

11 RMA, extended work life Sn95Ag5

12 No-clean Sn95.5Ag4.0Cu.5

13 RMA, mildly activated resin paste flux Sn95.5Ag4.0Cu.5

No-clean soft residue Alloy

14 No-clean solder paste Sn95.8Ag3.5Cu.7

15 No-clean, clear light soft residue Sn95.5Ag3.9Cu.6

16 No-clean solder paste, soft residue Sn95.5Ag4.0Cu.5

17 No-clean, clear light soft residue Sn95.5Ag4.0Cu.5

18 No-clean, pin-penetrable low residue SnAgCu

19 No-clean, pin-penetrable low residue SnAgCuSb

20 No-clean, pin-penetrable low residue SnAgCu

21 No-clean pin probe–testable mildly activated resin-based SnAgCuSb formulation

Water soluble Alloy

22 Water soluble Sn95.5Ag3.9/Cu.6

23 Water soluble, amber semiliquid Sn95.5Ag4.0Cu.5

24 Water soluble Sn95Ag5

25 Water soluble, polymer/dendrimer activator system Sn95.5Ag4.0Cu.5


RESIDUE CLEANING

Flux chemistry with a soft residue appears to exhibit a better cleanability than thatwith a hard residue, as shown in Table 15.18. This can be attributed to the lack ofcrystal formation in the soft residue, thus allowing it to be dissolved relatively easilyinto the cleaner. For hard residues, the crystal formation is fairly common and thedissolution process would have to overcome the crystallization energy before theresidue molecules can be pulled away from the main residue body. Water-washableflux residues also display a relatively good cleanability, presumably due to the samefactor. Although the solder alloy composition does vary from SnAg to SnAgCu toSnAgCuSb, its impact on flux cleanability is considered minimal due to the compa-rable process temperature and the dominant constituent being the same (Sn).

Although not supported with experimental data, it has been noted that lead-freesolder pastes pose a greater difficulty in cleaning than their eutectic SnPb counter-parts.17 The authors concur with this statement and attribute this phenomenon tothree factors. The first is a higher reflow temperature, which causes more side reac-tions within the flux such as oxidation or cross-linking reaction. The second is thegreater flux activity needed to boost the wetting of lead-free solders.This higher fluxactivity may induce more side reactions, thus causing greater difficulty in cleaning, asexemplified by the slightly poorer cleanability of paste 4 than paste 3 in Table 15.17.According to the paste manufacturer, pastes 3 and 4 have virtually the same fluxchemistry, except that paste 4 contains halide and exhibits a slightly higher activitythan paste 3. The third factor is the greater amount of tin salt formation at reflowdue to the use of high-tin solders. Tin salts seem to cause greater difficulty in clean-ing than lead salts, and thus may result in more white residues.This stipulation is sup-ported by the observation that the white residues encountered with 63Sn37Pb solderpastes often are not observed for high-lead solder pastes, even if the same flux isemployed for both occasions.

This reduced cleanability associated with lead-free solder pastes does not have tobe an unsolvable problem.Although use of high reflow temperature and fluxes withhigher flux activity is still inevitable, the thermal stability of flux ingredients still hasroom for improvement. Many organic chemicals exhibit thermal stability up to300°C (or even up to 350°C), thus allowing plenty of area to be explored.

15.8 CLEANING CHEMISTRY/PROCESS DESIRED

FOR LEAD-FREE RESIDUE CLEANING

By reviewing Table 15.19, referring back to Table 15.17, it is clear that the most effec-tive cleaning system is saponified aqueous with spray (average 3.49).The least effec-tive system is the solvent boil-rinse-vapor degrease process (average 3.03).Semiaqueous and/or aqueous solvent sprayable cleaning processes fall in between(average 3.27). However, it is interesting to note that the best two cleaning systems(4.00 and 3.98) all turn out to be within this solvent and/or aqueous spray category,as shown in Table 15.17. The fact that both top performers in cleaning belong to thesemiaqueous and/or aqueous solvent sprayable process demonstrates that the fluxresidues of lead-free solder pastes are still fairly soluble and do not have to rely onthe saponification reaction in order to pull the residues into the cleaner. The clean-ability is highly dependent on the solvency of cleaner used, with results ranging from


TABLE 15.17 Results of Lead-Free Solder Paste Flux Residue Cleaning Study

1 No-clean, amber hard residue 2.25 3.25 4 4 4 3.5 2.75 4 4 4 3 1 3.31

2 No-clean 2.5 3 1.25 4 4 4 2.5 4 4 3.5 2 1 2.98

3 No-clean, halide-free rosin, high-reliability electronics 3 2.25 1 4 4 4 3 4 4 4 3 1 3.1

4 No-clean, same as 3 with slight halide content 2.5 3.5 1 4 4 3 3.75 4 4 3.5 1.25 1 2.96

5 RMA, synthetic rosin type 3 2 2 4 4 3.75 4 4 4 4 3 2.25 3.33

6 No-clean RMA highly active resin/rosin-based formulation 3.5 4 4 4 4 3.5 3 4 4 4 3 1.25 3.52

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15.3

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7 No-clean mildly activated resin-based formulation 2 2 1 4 4 4 2 4 4 4 2 1 2.83

8 No-clean mildly activated resin-based formulation 2 1.25 1.25 4 4 3 2 4 4 4 2 1 2.71

9 No-clean 3 3.25 1 4 4 4 3.5 4 4 4 3.4 2 3.35

10 No-clean, extended work life 1.75 1 1.25 4 4 3.75 3.5 4 4 4 2.25 1 2.88

11 RMA, extended work life 2.25 1 1.25 4 4 3.25 3.5 2.25 4 4 2.9 1 2.78

12 No-clean 3.5 4 4 4 4 4 3.5 3.9 4 4 3.5 2.25 3.72

13 RMA, mildly activated resin paste flux 2 1.75 1 4 4 4 2.25 4 4 4 1 1 2.75

14 No-clean solder paste 4 2.25 0.75 4 4 4 4 4 4 4 3 1.25 3.27

15 No-clean, clear light soft residue 2 2.5 1.75 4 4 2 3.25 4 4 4 2 1 2.88

16 No-clean solder paste, soft residue 2.75 4 4 4 4 2.5 3.5 3.5 3.9 3.25 2.25 1 3.22

17 No-clean, clear light soft residue 3.65 4 4 4 4 2.5 3.5 3 3 3 3.25 1.25 3.26

18 No-clean, pin-penetrable low residue 3.5 3 3 4 4 4 4 4 4 4 3.4 2 3.58

19 No-clean, pin-penetrable low residue 4 3.25 2.75 4 4 4 4 4 4 4 4 2 3.67

20 No-clean, pin-penetrable low residue 2.25 2 2 4 4 4 3 4 4 4 3.5 1.75 3.21

21 No-clean pin probe–testable mildly activated resin-based 2.5 4 4 4 4 3 4 4 4 4 3.4 2.5 3.62formulation

22 Water soluble 4 4 4 4 4 4 4 1 4 1.75 3 1.9 3.3

23 Water soluble, amber semiliquid 4 4 4 4 4 4 4 2 3.5 2.25 3 2.25 3.42

24 Water soluble 4 4 4 4 4 4 4 0.5 4 3 3.25 1 3.31

25 Water soluble, polymer/dendrimer activator system 4 4 4 3.5 4 4 4 2 3 1.5 2.25 1.9 3.18

Average 2.96 2.93 2.49 3.98 4 3.59 3.38 3.45 3.9 3.59 2.74 1.46 3.21

Grading scale: 0 = no cleaning, 1 = significant residue, 2 = medium residue, 3 = low residue, 4 = totally clean.

15.3

3

fairly poor (2.49) to totally clean (4.00).The reason that this category falls below thesaponified aqueous with spray category can be attributed to varying process param-eters of time, temperature, solvency, and impingement energy.

There is another factor working against the saponification aqueous spray system.The SMT trend is driving toward a no-clean process, with no-clean already playing adominant role in the industry as of today. Since all no-clean flux residues are expectedto be hydrophobic and hardly dissolve in water, the water-based saponificationapproach really is in a poor starting position if the goal is to clean all types of lead-free paste residues. For water-washable solder paste systems, the saponification aque-ous spray system is still believed to be one of the top choices, as shown in Table 15.17,where all water-washable pastes received scores of 4.0 on cleaning. Perhaps it can beconcluded that, with proper choice of solvents, the solvent and/or aqueous spray sys-tem is the most desirable cleaning approach for residues of lead-free solder pastes.

Spray is critical for the success of cleaning. It is the primary difference betweenthe semiaqueous and/or aqueous solvent sprayable system and the solvent boil-rinse-vapor degrease system. The results appear to be insensitive to spray-in-air orspray-under-immersion, as long as spray is applied. Ultrasonic appears to be sec-ondary in effectiveness. Table 15.20 shows the comparison of systems with and with-out ultrasonic. Systems with ultrasonic aid display a considerably better cleaningperformance.


TABLE 15.18 Effect of Flux Chemistry on Cleaning Performance

Flux chemistry Cleaning performance

No-clean, hard residue 3.11

No-clean, soft residue 3.35

Water washable 3.30

TABLE 15.19 Effect of Cleaning Chemistry/Process on Cleaning Performance

Cleaning chemistry/process Cleaning performance

Semiaqueous and/or aqueous solvent 3.27sprayable cleaning

Saponified aqueous spray cleaning 3.49

Solvent boil-rinse-vapor degrease 3.03

TABLE 15.20 Effect of Ultrasonic Agitation on Cleaning Performance

Cleaning Ultrasonic Cleaningchemistry/process on/off performance

Solvent vapor degreaser On 3.90

#2 Off 3.59

Solvent vapor degreaser On 2.74

#3 Off 1.46

TABLE 15.21 Guidelines for Selecting Lead-Free Solder Paste

Characteristics to beProperty Test method examined Remark

Solderability Reflow through a Spreading, solder balling, Wide profile matrix andproduction reflow furnace solder appearance of a fine air reflow recommended.

dot print Peak temp. range 230–260°C.

Corrosion Cu corrosion test at Discoloration of copper J-STD-004 Cu corrosion40°C/93% relative humidity coupon test.

SIR SIR SIR reading, dendritic J-STD-004 SIR test.formation

Cleanability (for cleaning Production cleaning Ionics, flux residue Ionics measurement is notrequired applications) process meaningful for no-clean

flux or solder paste.

Printability Print speed up to 200 Print definition for fine dots Type 4 powder may bemm/s, aperture down to needed for 10 mil diameter10 mil diameter aperture.

Open time Print, tack check, place Printability, tack, 80% relative humidity component, reflow solderability with recommended for

increasing open time exposure atmosphere.

Assembly yield Actual production Defects such as solder Consult with paste supplierassembly process balling, bridging, regarding handling and

tombstoning, nonwetting, profile.voiding, etc.

Reliability Temp. cycling, vibration Electrical continuity Test condition applicationdependent.

Testing sequence starts from top.

15.3

5

15.9 SELECTION OF LEAD-FREE SOLDER PASTE

Implementing lead-free soldering requires a full scope of concerted upgradingefforts, including solder materials, reflow equipment, components, boards, andinspection equipment. In the case of solder paste materials, the primary challenge isdelivering a good soldering performance, since the prevailing lead-free solders allperform more poorly than eutectic SnPb.To further aggravate the wetting difficulty,the most wettable surface finish, hot-air solder leveling, is gradually fading awayfrom the industry due to the overall migration toward fine-pitch design. At thisstage, none of the lead-free surface finishes can provide a wettability as good as thatof hot-air solder leveling. Since the move toward use of lead-free solder alloys andlead-free finishes is inevitable, the industry can only hope that the intrinsic poor sol-derability associated with Pb-free solders and finishes can be compensated by a“better” flux, or a flux enabling the Pb-free soldering quality matching that of eutec-tic SnPb systems.

Unfortunately, this is a wish that cannot be easily met. Although it is possible todevelop a Pb-free solder paste with soldering performance matching that of eutecticSnPb, the natural trade-off is typically a high corrosivity and a poor cleanability.Other features such as rheology or solder paste handling often are less affected bythe migration toward lead-free flux chemistry. Selection of lead-free solder paste hasto be a very cautious process as regards not only testing the obvious requirement butalso examining the most likely compromises in performance, which often are notobvious. The recommended guideline for selecting lead-free solder paste can besummarized in Table 15.21.

For eutectic SnPb solder paste evaluation, the corrosion and surface insulationresistance (SIR) often are evaluated at a later stage. However, for lead-free solderpaste evaluation, the two properties are recommended to be investigated once thepaste has passed the solderability test. This is due to the high probability that manypastes may have overcompromised the noncorrosion feature in order to deliver thesoldering performance. This selection plan should be regarded as a guideline only.Detailed testing items and test conditions can be modified according to the require-ments of each assembly operation.

REFERENCES

1. Buetow, M., “The Latest on the Lead-Free Issue,” Technical Source, IPC 1999 Spring/Summer Catalog.

2. Bradley, E.,“Overview of No-Lead Solder Issue,” NEMI meeting,Anaheim, CA, February23, 1999.

3. “Lead-Free Solder Project Final Report,” NCMS Report 0401RE96, Ann Arbor, MI,August 1997.

4. Richards, B. P., C. L. Levoguer, C. P. Hunt, K. Nimmo, S. Peters, and P. Cusack, “An Analy-sis of the Current Status of Lead-Free Soldering,” British Department of Trade and Indus-try Report, April 1999.

5. Lee, N. C., “Optimizing Reflow Profile via Defect Mechanisms Analysis,” Proceedings ofIPC Printed Circuits Expo, 1998.

6. Hance,W. B., and N. C. Lee,“Voiding Mechanisms in SMT,” Proceedings of the 17th AnnualElectronics Manufacturing Seminar, China Lake, CA, 1993.

7. Diepstraten, G.,“Analyzing Lead-Free Wavesoldering Defects,” SMT’s Guide to Lead-FreeSoldering, 2–5, June 2001.


8. Barbini, D., “Wave Soldering with Lead-Free Alloys,” NEMI meeting, January 17, 2001.

9. Prasad, S., F. Carson, G. S. Kim, J. S. Lee, P. Roubaud, G. Henshall, S. Kamath, A. Garcia, R.Herber, and R. Bulwith,“Board Level Reliability of Lead-Free Packages,” SMTA Interna-tional, Chicago, IL, September 24–28, 2000.

10. Butterfield, A., V. Visintainer, V. Goudarzi, “Lead Free Solder Flux Vehicle SelectionProcess,” SMTA International, Chicago, IL, September 20–24, 2000.

11. Shina, S., H. Belbase, K. Walters, T. Bresnan, P. Biocca, T. Skidmore, D. Pinsky, P. Provencal,and D. Abbott, “Design of Experiments for Lead Free Materials, Surface Finishes andManufacturing Processes of Printed Wiring Boards,” SMTA International, Chicago, IL,September 20–24, 2000.

12. Carrol, M. A., and M. E. Warwick, “Surface Tension of Some Sn-Pb Alloys: Part 1—Effectof Bi, Sb, P, Ag, and Cu on 60Sn-40Pb Solder,” Materials Science and Technology, 3:1040–1045, December 1987.

13. Humpston, G., and D. M. Jacobson, “Principles of Soldering and Brazing,” ASM Interna-tional, Materials Park, OH, 1993.

14. Lee, Ning-Cheng, “A Model Study of Low Residue No-Clean Solder Paste,” Nepcon West,Anaheim, CA, 1992.

15. Vaynman, S., and M. E. Fine, “Fluxes for Lead-Free Solders Containing Zinc,” SMTAInternational, Chicago, IL, September 20–24, 2000.

16. Showa Denko, “Development of Sn-Zn Solder Paste of High Reliability,” IPCWorks’99,Minneapolis, MN, October 27, 1999.

17. Bivins, B. A., A. A. Juan, B. Starkweather, N. C. Lee, and S. Negi, “Post-Solder Cleaning ofLead-Free Solder Paste Residues,” SMT International 2000, Chicago, IL, September 2000.


CHAPTER 16CHALLENGES FOR

LEAD-FREE SOLDERING

The implementation of lead-free soldering is not a smooth ride. Challenges surfaceone by one in all aspects, including surface finishes, soldering processes, and reliabil-ity. In general, most of the challenges can be answered with certain approaches,although some of those approaches remain hypothetical or theoretical. However,some challenges still remain unanswered. In this chapter, most of the challengesencountered in lead-free soldering are listed and discussed.

16.1 CHALLENGES FOR SURFACE FINISHES

SnPb plating and hot-air solder leveling (HASL) have been used by the electronicsindustry for decades. Although technical challenges still exist, in general most of thebugs have been worked out, and applications and usages of SnPb surface finishes areregarded as a routine operation with the major emphasis on maintaining control.The same cannot be said for lead-free surface finishes. Due either to the specificchemistry utilized or to the short history of these finishes, virtually every lead-freesurface finish exhibits some challenges. In this section, several of these challengesare listed and discussed.

16.1.1 BLACK PAD

Electroless nickel/immersion gold (ENIG) is one of the prevailing lead-free surfacefinishes. It has been used by the industry for years due to its excellent solderabilityfor fine-pitch surface-mount technology and ball grid array (BGA) package devices.This is particularly true for thick boards, where the electroplating process experi-ences difficulty in providing even plating for through-holes. However, sporadic sol-der joint failure may occur due to joint weakness. A number of investigations havebeen conducted.1–6 Puttlitz1 first reported a phenomenon called black pad associatedwith defective joints, in which the pads exhibit a dark gray to black appearance thatwill only partially wet. This observation was confirmed by later investigators.2–6 Anexample of black pad is shown in Fig. 16.1.

Biunno4 studied the types and formation mechanisms of black pads. He classifiedthe black pad phenomenon into eight categories, listed by increasing extent of the syn-drome: (1) minimal immersion gold (IG) spike penetration, (2) deep IG spike pene-tration, (3) shallow spreading IG penetration, (4) deep spreading IG penetration, (5)IG separation of electroless nickel nodules, (6) small section black band, (7) cornersection black band, and (8) large section black band. Biunno concluded that the blackpad defect was caused by a hyperactive “corrosive” IG process that changes the near-surface microstructure of PNi into a black band, with a marginal to total nonwettingstate.The black pad defect is classified in terms of hyperactive corrosive activity.

16.1


The black pad defect does not occur in a random or sporadic manner; rather, itusually shows a clean separation at the transformed Ni surface. Virtually no NiSnintermetallic is observed at the Ni surface or on the component lead, as shown in Fig.16.2.The back surface has a “mud-cracked” appearance. In addition, the phosphorusconcentration exceeds 10 wt%.Volume enrichment of phosphorus by defect activityoccurs via the removal of Ni atoms into a depth of the near surface, and at the sametime no gold is deposited during the defect corrosion process.There is a nearly com-plete absence of NiSn intermetallic. Obviously, the presence of either black band orphosphorus inhibits the formation of NiSn intermetallics, thus reducing the bondstrength of solder joints. However, phosphorus enrichment also occurs through nat-ural intermetallic formation. Here Ni is depleted from the near surface due to for-mation of intermetallic. Therefore, phosphorus enrichment should not be regardedas a direct cause of weakened solder joints.

High-magnification scanning electron microscopy (SEM) suggests that blackband is voided or less dense than the underlying bulk Ni layer. This is supported bythe results from focused ion beam microprobe study. Figure 16.3 shows a micrographof black band. The ion beam milling produced some damage and preferential etch-ing. However, it can be seen that the near-surface structure of black band is voidedand less dense, as indicated by the side wall structure of the grove produced by thegallium ion beam slicing.

Advanced corrosion of the nickel surface by the IG chemistry may be induced byan electric charge imbalance between the printed circuit board (PCB) and ionicspecies within the IG plating bath. This stipulation is supported by experimentswhere black pads were produced by applying +1 V during the plating process, asshown in Fig. 16.1. Highly accelerated gold plating occurred on all the pads con-nected to ground, and large cubic gold crystals were observed throughout the orangepad surface. The average grain size for the orange pads was more than 1000 timesthat of normal IG. The orange color of these pads is due to the large grain size andsurface roughness.

High phosphorus content is not always associated with black pad, as reported byMei et al.5 Instead, an extraordinarily high carbon content is observed. The cause of

16.2 CHAPTER SIXTEEN

FIGURE 16.1 Black pads produced by applying +1 V during the plating process. The four blackpads on the right were connected to +1 V, and the lighter pads from the left to middle were connectedto ground. Under this test condition, all pads connected to +1 V turned black and all pads connectedto ground turned orange.

CHALLENGES FOR LEAD-FREE SOLDERING 16.3

FIGURE 16.2 Plane view of the surface structure of the black pad after solder joint failure.4

FIGURE 16.3 Focused ion beam microprobe micrograph of a spike/spread defect region.4

this high carbon content is not understood yet. However, the small electrical poten-tial bias as the proposed root cause of black pad by Biunno may still be applicable inMei’s work. In Mei’s failed boards, the failed packages were only a few plastic quadflat pack packages, and the failure occurred more frequently on a particular PQFPas well as on certain lead locations. Mei et al. speculate that there were small differ-ences in electrical potential among packages, and among all leads of the plastic quadflat pack, due to the complex circuitry design of the board.This small electric poten-tial difference may have induced different kinds or different degrees of chemicalreactions in the IG bath.

Overall, the black pad phenomenon has been studied in depth regarding its phys-ical structure. Although understanding of the formation mechanism in terms ofchemistry is still lagging behind, black pad can be controlled through a tight controlof plating conditions.

16.1.2 EXTRANEOUS/SKIP PLATING

For the ENIG system, critical properties of the surface on which plating will occurinclude contamination, organics, roughness, residual copper, residual solder mask,oxidation, and residual tin. Lack of control of these critical properties can causeeither extraneous plating or skip plating. The example of extraneous plating shownin Fig. 16.4 was caused by residual copper between traces.7 Excessive extraneousplating may cause circuit shorts. Skip plating is seen on some boards as areas that donot plate nickel, and has been discussed by Young,8 who proposes that static is cre-


FIGURE 16.4 Example of extraneous plating for ENIG system.

ated in the solder mask operation on certain capacitive areas of the circuitry. Thisstatic attracts volatiles during the mask cure operation and is the underlying cause ofskip plating.

16.1.3 TIN WHISKER

In the autumn of 1998, the NASA Goddard Space Flight Center (GSFC) wasinformed of an on-orbit commercial satellite failure attributed to a tin whisker–induced short circuit. The source of the tin whiskers was a pure tin–plated relay.9

Tin whisker is a growth of tin crystal protrusion on the surface of a tin-basedmetal, including tin-based surface finish. The shape of a tin whisker may vary fromfiberlike to highly irregular. Due to its threat of inducing circuit shorts, tin whiskerhas been the subject of numerous investigations since the early 1950s.10–58

Tin whisker can be formed not only on pure tin surfaces but also on some tinalloys. Figure 16.5 shows the tin whisker formed on 98Sn2Cu surface finish plated oncopper. The laminated copper can be recognized by its grain orientation. The inter-metallics are concentrated at the tin-copper interface and grain boundaries.59 In astudy of ChipPAC, Prasad et al.60 also observed whisker formation on pure tin,SnCu, and SnBi surface finishes, as shown in Fig. 16.6.

In order to understand and prevent the formation of tin whisker, Lee and Lee61

studied the spontaneous growth mechanism of tin whisker. The authors theorizedthat the generation of compressive stress in tin film is caused by the diffusion of cop-per from the substrate into the tin along its grain boundaries and the subsequent for-mation of Cu6Sn5. The whisker growth is attributed to the compressive stress.

Diffusion of copper from the substrate into the tin as a cause of tin whisker for-mation is supported by the findings of Boguslavsky of Shipley.62 In that work, tinwhisker development was studied for samples with eutectic SnCu surface finish witha thickness of 10 to 12.5 µm. In one set of samples, a Ni underlayer about 1.5 µmthick was applied between the SnCu surface finish and the copper base. In another


FIGURE 16.5 Focused ion beam images of tin whisker before and after gallium ion beam millingfor 98Sn2Cu plating on copper base.

set of samples, no Ni underlayer was applied. Both sets of samples were stored at55°C in dry heat. Results indicate that the samples without the Ni underlayer dis-played whiskers in a few days, while the samples with the Ni underlayer showed nowhisker growth. SEM/electron diffraction analysis indicates development of semi-continuous copper diffusion from the Cu base toward the SnCu surface finish forsamples without the Ni underlayer. On the other hand, no copper diffusion can bediscerned at all for samples with a Ni underlayer, as shown in Fig. 16.7.Apparently aNi underlayer effectively stops copper diffusion into nickel. Without the Ni under-layer, there is copper diffusion, presumably through the grain boundaries. The lightgray spots dispersed from the interface toward the dark background are presumablyCuSn intermetallics. The dark background color is the 99.3Sn0.7Cu surface finish.

Lee and Lee’s theory that the whisker growth is caused by compressive stress waschecked by Fan at Lucent EC&S.63 Fan studied the whisker growth of bright tin and


FIGURE 16.6 Whisker photos for pure tin, SnCu, and SnBi surface finishes.

FIGURE 16.7 SEM/EDS for 99.3Sn0.7Cu surface finish with and without Ni underlayer ontop of copper base.62

satin bright tin finish by monitoring the stress of surface finishes by x-ray diffraction.Both finishes showed zero stress after plating. After 4 months of room temperaturestorage, a compressive stress of about −10 MPa for bright tin or −7 MPa for satinbright tin in the tin plated directly on copper was measured. On the other hand, atensile stress of about 10 MPa for bright tin or 7 MPa for satin bright tin plated on anickel underlayer over copper was registered. After 4 to 18 months, the stress levelsdid not show significant change. Whiskers were found on the tin without a nickelunderlayer, while no whiskers were found for tin with a nickel layer. Similar resultswere seen for the tin finishes after 18 months of aging at 50°C.Again, no whisker wasseen for the finish with the nickel underlayer. The findings of Fan indicate that acompressive stress aggravates while a tensile stress hinders the whisker growth, thusconcurring with the theory of Lee and Lee. The use of a Ni underlayer generatedtensile stress, thus depressing formation of whiskers.

It should be pointed out that although Cu diffusion is believed to cause compres-sive stress and thus whisker growth, formation of intermetallic Cu6Sn5 may notcontribute to whisker formation. Table 16.1 shows the volume of Cu, Sn, and inter-metallic Cu6Sn5. The actual molar volume of Cu6Sn5 is smaller than the calculatedmolar volume if the volume is assumed to be additive. In other words, formingintermetallic Cu6Sn5 will result in a reduction in volume and thus presumably a ten-sile stress. Perhaps the compressive stress caused by the copper dissolved in solderoverrides the tensile stress generated by Cu6Sn5 and consequently results in a netcompressive stress. This net compressive stress is considered to be relievable byreflow with a reasonable cooling time. This allows the intermetallics to be formedmore readily, and would explain a decrease in the tendency for whisker formation.64

Besides copper diffusion, increased carbon content may also cause whiskergrowth. Ohkawara and Muroi65 studied the whisker growth rate from zinc platingversus chemical species in baths. They observed that high concentrations of cyanideand alkali in baths lowered whisker growth in zinc plating, but the mechanism andsuitable bath composition remain to be clarified. Their work concluded that the car-bon content of zinc plating, and the amount of zinc cyanide complex and free cyanideions in baths, are related to whisker growth. On the other hand, zincate ions arerelated to the electrodeposition rate but not to whisker growth. Ohkawara andMuroi66 continued the mechanism investigation work and studied the influence ofinternal stress and crystal structure on whisker growth from zinc plating. In a zinccyanide system, internal stress, lattice distortion (strain), and the carbon content ofplating are found to relate to one another. Figure 16.8 shows the effect of carbon con-tent on lattice distortion and whisker growth. Figure 16.9 shows the effect of latticedistortion on internal stress and whisker growth.The value of these physical and met-


TABLE 16.1 Volume of Cu, Sn, and Intermetallic Cu6Sn5

Atomic or formula Density Molar volumeMaterial weight (gm/mol) (gm/cm3) (cm3/mol) Notes

Cu 63.54 8.9 7.14 Measured

Sn 118.71 7.3 16.26 Measured

Cu6Sn5 974.79 8.28 117.73 Measured

— 124.14 Calculated, assumingadditive volume


FIGURE 16.8 Effect of carbon content on lattice distortion andwhisker growth. M ratio represents NaCN/Zn of the zinc plating baths.66

FIGURE 16.9 Effect of lattice distortion on internal stress andwhisker growth. M ratio represents NaCN/Zn of the zinc plating baths.66

allurgical properties of plating decreased with an increase in the M ratio (NaCN/Zn)of the baths, and whiskers did not grow below a particular value (i.e., internal stress55 MPa; lattice distortion (strain) 0.25 percent; carbon content 0.06 percent). Itappears that a higher carbon content in the plated zinc layer results in a higher latticedistortion, which in turn results in a higher internal stress. With carbon serving as animpurity inclusion, this internal stress very likely is compressive in nature. The paral-lel relationship between zinc and tin systems regarding compressive stress versuswhisker growth not only supports the compressive stress model, but also suggests thattin whisker might also be induced by high carbon content in the tin surface finishes.

Tin whisker growth is also affected by the grain size of tin. Zhang of Lucent67

reported that a large-grained coating is favored for reducing whisker growth, since alarge-grained coating exhibits fewer grain boundaries for copper to diffuse through.It also typically has zero or very low compressive stress to start with. When drivingforce (compressive stress) is present, the large grain also requires more energy to besqueezed out than fine grains do.

The effect of aging conditions on whisker growth rate has been a controversialsubject. The GSFC conducted a literature survey to determine optimal conditionsfor producing tin whiskers, and found that brass substrates with “bright” tin electro-plate of approximately 200 µin were highly prone to whisker formation. In addition,storage at 50°C was also considered to be an accelerating factor.9 The GSFC’s resultsindicate that a higher density of whisker growth is observed on samples stored underroom ambient conditions (∼22°C, 30 to 70 percent relative humidity) when com-pared to samples stored at 50°C. Furthermore, straight heat aging of the parts (with-out thermal cycling) at +90°C for 400 h generated no whiskers on parts from thesame lot.

Thermal cycling is very efficient in generating whisker. Kadesch and Leidecker9

observed that thermal cycling between −40 and +90°C for 331 cycles producedwhisker, and that the whisker length increased when the number of cycles wasincreased to 500. Brusse of NASA68 also reported that 100-µm-long whiskers wereseen on a Sn finish (6 µm thick) plated on a nickel underplate layer (6.5 µm thick)over a silver frit substrate after going through 100 thermal cycles of −40 to 90°C.More whiskers were found when the number of thermal cycles was increased. Thesame finish did not show whiskers after 90°C aging.

The case just described indicates that the presence of a Ni underlayer does notguarantee freedom from whiskers. Motorola, Shipley, and FCI have also observedwhiskers on Ni barrier samples in some of their experiments. It seems that if Ni isinvolved in the growth of whiskers, either the Ni plating process should be betterunderstood or Ni may merely delay the onset of whisker growth.

NASA has investigated the possibility of eliminating whisker growth with theuse of conformal coating.9 Results indicate that conformal coating does not slowdown the whisker growth rate. Instead, for the first year of the experiment, nodulesformed more rapidly and in greater numbers under the conformally coated sidethan the nonconformally coated side. Later, the density of nodules was essentiallyequal on both sides. However, conformal coating does delay the dielectric break-down, since the whisker would buckle before penetrating the coating on an adja-cent surface.

Whisker growth may be affected by introducing other elements into tin. Prasad etal.60 evaluated plating chemicals and the reliability of Pb-free leadframe packages.When compared with pure Sn and SnCu, SnBi systems exhibit minimum whiskers;however, no plating system is whisker-free. Whisker growth rate is found to berelated to substrate materials, with brass C194/C151 > C7025. Alloy 42 leadframe


did not seem to experience whisker problems with Pb-free plating chemical systemswithin the time frame studied.

16.1.4 SURFACE FINISH CLEANING RESISTANCE

Although flux residue cleaning is known as a challenge in the Pb-free solderingprocess,69,70 cleaning unreflowed solder paste on PCB can also pose a challenge. Forsolder paste processes, the PCB often has to be cleaned if the printed paste misreg-isters, smears, or dries out.This is not a problem for any metallization, such as HASLor NiAu, since no solvent can remove any of those metal finishes. However, in thecase of organic solderability preservatives, board cleaning can result in removal oforganic surface finishes. For single-sided PCB, this is not an issue. However, for dou-ble-sided PCB, removal of surface finishes may cause wetting difficulty after onethermal excursion, whether the subsequent soldering is reflow, wave solder, orrework.

16.2 CHALLENGES FOR SOLDERING

Both reflow and wave soldering face many challenges. Examples include inter-metallic compounds, dross, wave solder composition, lead contamination, fillet lift-ing, poor wetting, voiding, and rough joint appearance.These will be discussed in thefollowing sections.

16.2.1 INTERMETALLIC COMPOUNDS

All of the common base materials form tin intermetallic compounds. The amount ofintermetallic compound at an interface or within the solder joint is highly dependenton the base material. Other than tin itself, gold dissolves most rapidly into tin-basedsolders, due to the high solubility of gold in tin—approximately 15 wt% at 250°C.71

However, the solubility of gold in tin and lead in the solid state is very low and vir-tually all of the gold dissolved subsequently precipitates as AuSn4. Gold is wellknown as being in a class of its own in terms of its ability to embrittle joints. This ismainly due to the high dissolution rate of gold in solder. Another factor is the stoi-chiometry of the compounds formed. For example, 3 at% gold in the liquid phasegives rise to 15 at% AuSn4, while 3 at% silver or copper yields only 4 at% Ag3Sn and5.5 at% Cu6Sn5, respectively.72 Silver also dissolves quite readily in solder. The solu-bility of silver at 250°C is about 6 wt% in pure tin73 and about 3.5 wt% in eutecticSnPb.74

Pd does not dissolve as rapidly as Au or Ag in Sn. However, similarly to Au, Pdforms intermetallic PdSn4, and thus a small amount of Pd can also rapidly generatelarge quantities of intermetallics due to the stoichiometry factor. This is particularlya concern for reflow soldering on a thick layer of Pd surface finish, where the soldervolume in contact with the base’s metallization is very limited. Additional exposureto temperature cycling inevitably will aggravate the formation of intermetallic com-pounds. Figure 16.10 shows a secondary electron micrograph of the cross section ofa Pb-free solder joint between a small-outline integrated circuit (SOIC) lead and aPd-finished PCB after 2500 thermal cycles.75 The solder alloy used here isSn2.5Ag0.8Cu0.5Sb (CASTIN). A large quantity of PdSn4 can be seen everywhere



FIGURE 16.10 Secondary electron micrograph of cross section of a solder joint between a SOIClead and a Pd-finished PCB after 2500 thermal cycles.75

within the solder joint. The presence of a large quantity of PdSn4 in solder jointsafter reflow, even prior to thermal cycling treatment, is responsible for weak jointstrength in devices.75

Both Au and Pd are among the favorite Pb-free surface finishes. A thin layer ofAu or Pd will not pose any concern in terms of intermetallic formation. However,since a relatively thick layer of Au or Pd may be needed for wire bondability appli-cations, care should be taken in balancing the need for both wire bondability andsolder joint reliability. Copper is the most commonly used base material. The solu-bility of copper in tin at 250°C is about 1.5 wt%.74 Although Cu dissolves in Sn fasterthan in 63Sn37Pb, the intermetallic compound formation rate in Sn or Pb-free high-Sn alloys often is about equal to or slower than that in eutectic SnPb, as shown inTable 16.2.72 The initial CuSn intermetallic formation is faster for Pb-free alloysdoped with copper than for those without copper dopant. In all incidences, theintermetallic formation rate declines rapidly with increasing thickness of the inter-metallic layer and thus does not pose process or reliability concerns.

16.2.2 DROSS

For SnAg, SnCu, and SnAgCu systems, dross formation is not a major concern.Table16.3 compares the oxide thickness development of seven tin-based alloys at 140°Cabove the melting temperature of the alloys.75 Both SnCu and SnAg compare favor-

ably with eutectic SnPb in terms of oxide formation rate. On the other hand, alloyscontaining Bi, Sb, Zn, or In all oxidize more rapidly than 63Sn37Pb in the moltenstate, suggesting a higher dross formation rate at wave soldering than that for eutec-tic SnPb. Indeed, Lotosky76 reported that SnAgBiCu alloy suffers a higher soldermass loss due to dross removal, as shown in Fig. 16.11. On the other hand, SnAg iscomparable with eutectic SnPb while SnAgCu or SnCu are lower in solder mass lossrate than eutectic SnPb, consistent with the findings of Miric and Grusd.77 The high


TABLE 16.2 Quantity of Intermetallic Formed at Copper Interfaces (µm) and CopperDissolved (µm) During Soldering at Various Temperatures/Times with Three Different Alloys

5 s 15 s 30 s 60 s

IMC Dissolved IMC Dissolved IMC Dissolved IMC Dissolved

95.5Sn3.5Ag

235°C 0.3 (5) 0.5 (6) 1.0 (9) 1.5 (11)

245°C 0.4 (5) 0.5 (6) 1.0 (9) 1.5 (12)

260°C 0.8 (5) 1.2 (6) 1.5 (9) 1.7 (10)

99.3Sn0.7Cu

235°C 1.0 (2) 1.3 (3) 1. (3) 2.0 (5)

245°C 1.0 (2) 1.0 (6) 1.8 (6) 2.5 (6)

260°C 1.0 (5) 1.0 (6) 2.1 (7) 2.5 (9)

63Sn37Pb

235°C 0.5 (3) 1.0 (4) 1.4 (4) 1.8 (5)

260°C 0.5 (5) 1.5 (5) 1.5 (7) 1.8 (9)

Figures in parentheses indicate the quantity of copper dissolved into the solder matrix.

TABLE 16.3 Oxide Thickness: Initial and After Oxidizing the Solder Preform in Air at140°C Above the Melting Point of the Alloy

Oxidation Oxide thickness (Å) DominantAlloy temperature (°C) Initial After 10 min After 50 min oxide type

Sn99.3Cu0.7 367 20 50 50 Sn oxide

Sn96.5Ag3.5 361 30 50 50 Sn oxide

Sn63Pb37 323 30 50 500 Sn oxide

Bi58Sn42 278 350 800 Sn oxide

Sn95Sb5 380 20 875 1425 Sn oxide

Sn91Zn9 339 70 200 325 Zn oxide

52In48Sn 257 20 175 600 In oxide

Before oxidizing the preform in air, the initial oxides were removed by heating the preform in nitrogen to500°C and then holding it for 10 min in a flow of hydrogen (hydrogen reduces oxides); afterward, the pre-form was cooled in nitrogen to a temperature 140°C above the solder’s melting point. Then a nitrogen flowwas switched to air flow to start oxidation. Finally, the preform was cooled to room temperature in nitrogenand the oxide thickness was measured using auger electron spectroscopy.

dross formation rate associated with alloys containing Bi, Sb, Zn, and In can beaddressed by inerting the wave zone, thus minimizing the cost of using those alloys.

16.2.3 WAVE SOLDER COMPOSITION

Unlike in reflow soldering, where the alloy composition of solder paste remains con-stant, wave solder composition is constantly changing due to the leaching process.Upon contact with the solder wave, the metallization of PCB dissolves into themolten solder.Where copper is the base material, copper in the molten solder is con-verted into solid intermetallic Cu6Sn5 once it reaches the limit of solubility.As shownin Table 16.4, this intermetallic compound can be removed easily by skimming thesurface of the molten eutectic SnPb bath, due to the relatively lower density of


FIGURE 16.11 Loss of solder quantity due to dross removal at wave soldering.76

Grams per minute of mass removed during typical dross removal per minute of wave runtime. Data based on minimum of four separate 2- to 4-h experiments.

TABLE 16.4 Density of Metal MaterialsRelated to Soldering

Material Density (gm/cm3)

Cu6Sn5 8.28

Cu3Sn 8.9

Ni3Sn4 8.65

63Sn37Pb 8.4

95.5Sn3.8Ag0.7Cu 7.5

96.5Ag3.5Ag 7.5

99.3Sn0.7Cu 7.3

Sn 7.3

Ni 8.9

Cu 8.9

Cu6Sn5 (8.28 gm/cm3) vs. eutectic SnPb (8.4 gm/cm3). As a result, the bath composi-tion can be maintained easily by adding pure tin to compensate for the loss of Sn dueto the removal of intermetallic Cu6Sn5.

Unfortunately, this simple bath maintenance practice cannot be utilized whenperforming Pb-free soldering. As shown in Table 16.4, all of the major Pb-free sol-ders, including SnAg, SnCu, and SnAgCu, exhibit a much lower density than theintermetallic Cu6Sn5. Consequently, the intermetallic Cu6Sn5 formed tends to pre-cipitate. Although the bath composition is still maintained by addition of pure Sn,removal of the solid Cu6Sn5 from the bottom of the bath becomes a tedious task.

16.2.4 LEAD CONTAMINATION

Pb contamination is very likely, particularly at the early phase of transition to lead-free soldering. The presence of Pb may appear in the solder materials as impurity, inthe surface finish of components or PCBs, or as solder deposits, such as solder ballson BGA. Table 16.5 shows examples of composition and impurity analysis data forsome Pb-free solder alloys.78 For the 10 samples analyzed, the highest level of Pbimpurity is 265 ppm. One of the primary sources of Pb contamination is tin. Table16.6 shows impurity analysis data for two typical 99.9 percent Sn lots, with one lotexhibiting 150 ppm Pb impurity.78 Although solders with lower Pb impurity levelsare achievable, the cost associated with the process is considered prohibitive.

The presence of lead contamination often results in a drop in melting tempera-ture. Bieler79 studied the effect of Pb contamination on the properties of eutecticSnAg. SnAg alloy doped with three levels of Pb was investigated, with compositionshown in Table 16.7. The effect of Pb content on melting behavior was studied withdifferential scanning calorimetry (DSC). Addition of Pb introduces a small newmelting peak with the onset of melting at 179°C, as shown in Fig. 16.12. An increasein Pb content not only increases the proportion of the low melting phase, but alsoshifts the high melting phase toward a lower temperature. The National ElectronicsManufacturing Initiative (NEMI) also reported that 1 percent Pb contamination willlower solidus temperature by 40 to 50°C, as shown in Table 16.8.80

For Bi-containing alloys, the sensitivity of melting temperature toward Pb con-tamination increases with increasing Bi content. Toyoda81 studied the effect of Pbcontamination on the melting behavior of a SnAgCuBi system and found that theimpact of Pb contamination becomes most significant at Pb content greater than 0.5percent. At Pb content higher than 2 percent, no additional drop in solidus can bediscerned, as shown in Fig. 16.13.

The impact of Pb contamination on Pb-free soldering is much more than reduc-tion in melting temperature.While the solder wetting appears to be insensitive to Pbcontamination, as reported by Vianco et al.,82 the mechanical strength and fatigueresistance of Pb-free solders turn out to be extremely sensitive to the presence of Pb.Baggio et al.83 investigated the effect of Pb contamination on the fracture strength ofSn3.5Ag3Bi alloys for reflow applications. Results indicate that at Pb contentgreater than 0.2 percent, the fracture strength drops drastically, with only 20 percentfracture strength remaining at 1 percent Pb contamination, as shown in Fig. 16.14.

The failure mechanism induced by Pb contamination was first investigated byMei et al.84 on a eutectic SnBi system. Figure 16.15(a) shows the as-solidifiedmicrostructure of 58Bi42Sn solder joints between a hot-air leveled 63Sn37Pb padand an 80Sn20Pb-coated component lead. Figure 16.15(b) shows the microstructureafter 400 cycles between −45 and +100°C over 16 days. The Pb dissolves into moltenBiSn during the soldering process, resulting in the formation of a 52Bi30Pb18Sn ter-


TABLE 16.5 Composition and Impurity Analysis Data for Some Lead-Free Solder Alloys

Alloy Sn Ag Bi Cd Cu Fe In Mg Ni Pb Sb Tl

58Bi42Sn 5946 41.80% 3 ppm 58.19% 1 ppm 5 ppm 10 ppm 5 ppm 0.3 ppm 15 ppm 50 ppm 40 ppm

58Bi42Sn 5939 42.80% 30 ppm 57.18% 2 ppm 5 ppm 15 ppm 2 ppm 0.0 ppm 10 ppm 75 ppm 30 ppm

93.5Sn3.5Ag3Bi 93.41% 3.50% 3.06% 3 ppm 5 ppm 30 ppm 10 ppm 0.2 ppm 8 ppm 150 ppm 75 ppm 1 ppm

90.5Sn7.5Bi2Ag 90.01% 2.13% 7.84% 2 ppm 5 ppm 30 ppm 5 ppm 0.0 ppm 40 ppm 50 ppm 75 ppm 1 ppm

91.8Sn3.4Ag4.8Bi 91.55% 3.47% 4.97% 2 ppm 3 ppm 20 ppm 5 ppm 0.0 ppm 10 ppm 50 ppm 40 ppm 1 ppm

93.6Sn4.7Ag1.7Cu 93.20% 5.07% 75 ppm 3 ppm 1.70% 20 ppm 50 ppm N/A 5 ppm 60 ppm 40 ppm 1 ppm


99.3Sn0.7Cu 99.27% 0.5 ppm 30 ppm 1 ppm 0.7% 20 ppm 5 ppm 0.3 ppm 15 ppm 30 ppm 50 ppm 1 ppm


96.5Sn3.5Ag 96.28% 3.68% 55 ppm 5 ppm 12 ppm 24 ppm 10 ppm N/A N/A 265 ppm 25 ppm N/A

16.1

5

nary eutectic structure in the solidified solder joint. The solder joints became weakin mechanical strength when subjected to (1) thermal cycling at temperaturesgreater than 96°C, because of low melting ternary eutectic phase accelerated graingrowth and phase agglomeration; or (2) long-time aging at 85°C, probably becauseof the eutectoid decomposition of the X phase in the ternary eutectic structure.Addition of small amounts of indium to 58Bi42Sn solder may eliminate the forma-tion of the ternary eutectic phase, as indicated by DSC measurements.

The deterioration mechanism of Pb-free solder joints by Pb contamination is alsoapplicable to non-Bi-containing alloys. Seelig and Suraski85 reported that investiga-tion of a field failure of SnAgCu solder joints showed that the failure is caused byintergranular separation driven by the lead in the solder. Figure 16.16 shows a dis-tinct phase between the normal grains. Pb formed a ternary SnPbAg phase, presum-ably with a melting temperature of 179°C. This low-melting phase surrounds thePb-free grains and exhibits poor adhesion to the Pb-free alloy, thus causing the grainseparation.

Besides confirming the applicability of a low-melting phase—ternary SnAgPbin this case—at intergranular space as failure mode, Seelig and Suraski also pro-posed a Pb concentration mechanism via the zone refining principle. They pro-posed that, upon cooling, the Pb gradually enriched at the last solidified spot dueto the low-melting nature of the ternary eutectic SnPbAg alloy, as illustrated inFig. 16.17. This enriched low-melting phase pocket serves as void, and very likely


TABLE 16.6 Impurity Analysis Data for Two Lots of 99.9 Percent Sn (ppm)

Lot Ag Bi Cd Cu Fe In Mg Ni Pb Sb Tl

A 2 50 2 1 20 5 0.3 1 150 50 1

B N/A 30 1 10 50 20 0.3 50 40 75 1

TABLE 16.7 Composition (wt%) of EutecticSnAg Solder and Three Ternary SnAgPb Alloys

Alloys Sn Ag Pb

Ternary alloy A 94.61 3.43 1.96

Ternary alloy B 91.91 3.33 4.76

Ternary alloy C 89.35 3.24 7.41

Eutectic alloy E 96.5 3.5 —

TABLE 16.8 Effect of 1 Percent Lead Contamination on the SolidusTemperature of Alloys

Solidus of alloy with 1% PbAlloy Melting temperature (°C) contamination (°C)

99.3Sn0.7Cu 227 183

96.5Sn3.5Ag 221 179

58Bi42Sn 138 96

FIGURE 16.12 DSC heating curves for three ternarySnAgPb alloys.79

FIGURE 16.13 Solidus line of SnAgCuBi + Pb.81

FIGURE 16.14 Effect of Pb contamination level on the fracturestrength of Sn3.5Ag3Bi alloy.83

16.17


FIGURE 16.15 (a) As-solidified microstructure of 58Bi42Sn solder joints between a hot-air lev-eled 63Sn37Pb pad and an 80Sn20Pb-coated component lead; (b) the microstructure after 400cycles between −45 and +100°C over 16 days.84


FIGURE 16.16 The Pb-free materials comprise the lighter areas, with the darker SnPb area sur-rounding them (3500× micrograph).85

FIGURE 16.17 A lead-free solder joint may be contaminated by the SnPb surface finish of com-ponent leads. Pb will enrich and settle at the last area to cool—under the lead at the PCB interface.85

may initiate joint failure. Figure 16.18 shows a micrograph of a lead-free solderjoint fracture resulting from Pb contamination and displaying Pb pockets. Similarto the case for Bi-containing systems, the presence of Pb contamination alsocauses early failure in fatigue tests for SnAgCu systems, as shown in Table 16.9 forbulk solder testing.


FIGURE 16.18 Micrograph of a lead-free solder joint frac-ture resulting from Pb contamination and displaying Pbpockets.85

TABLE 16.9 Effect of Pb Contamination on Low-CycleFatigue Testing ASTM E606 Performance of95.5Sn4Ag0.5Cu Alloy in Bulk Solder Testing

Sample Cycles to failure Result

95.5Sn4Ag0.5Cu 13,400 Pass

0.5% Pb contamination 6,320 Fail

1% Pb contamination 3,252 Fail

16.2.5 FILLET LIFTING

The National Center for Manufacturing Sciences (NCMS) has reported a phenome-non called fillet lifting associated with some Pb-free solder joints.86 Fillet lifting atwave soldering consists of solder fillet pulling away from the copper land on theboard.The separation occurs mostly at the solder to intermetallics, with cracks stop-ping at the knee on the land side, as shown in Fig. 16.19. Although mostly observedat wave soldering, fillet lifting may also occur at reflow soldering.

The direct driving force for fillet lifting is a mismatch in thermal coefficient ofexpansion (TCE) between solder and PCB, as illustrated in Fig. 16.20. Upon cooling,the solder shrinks more in the x-y direction, while the PCB shrinks faster in the zdirection. As a result, a lifting force is generated in both the x-y plane and the zdirection. However, this driving force is not sufficient to create fillet lifting, since vir-tually all solder joints experience a similar mismatch in TCE, but many alloys such

as 63Sn37Pb do not have fillet lifting problems. Apparently, driving forces beyond amere mismatch in TCE are required for fillet lifting to occur.

In the NCMS report, fillet lifting occurred more frequently with pasty alloys,including Bi-containing alloys and Pb-contaminated high-tin alloys. Suganuma87

studied the mechanism of fillet lifting in lead-free soldering. In his analysis, solidifi-cation of a through-hole fillet propagates rapidly from the top surface to the innerregion. Cu lead serves as a heat sink. The heat flow from the inside of the through-hole propagates through the Cu sleeve and land and keeps the narrow layer of sol-der facing the Cu land pad in a liquid state. Handwerker88 noted that for pastymaterials, the liquid can no longer redistribute to accommodate the stresses whenthere is more than about 90 percent solid. Upon cooling, the maximum stress is gen-erated at fillet tip due to mismatch in TCE. Since the solder at the interface withPCB cools more slowly than the rest of the fillet joint, hot tearing of the semiliquidsolder then occurs at this interface and results in fillet lifting. Handwerker has donesome in situ studies that have shown that fillet lifting occurs before the eutectic tem-perature of the system is reached. This is exactly the same as hot tearing duringmetal casting.88


FIGURE 16.19 Cross-sectional view of fillet lifting.86

FIGURE 16.20 The driving force for fillet liftingis a mismatch in TCE.

Suganuma’s work on Bi-bearing alloys pointed out two more possible factorscontributing to fillet lifting. First is Bi enrichment. Suganuma reported that uponsolidification, Bi is enriched into the liquid interface region between solder fillet andCu land by the formation of tin-rich dendrites. The diffusion distance is only a fewmicrometers, and a substantial amount of Bi along the interface remains in a liquidstate [see Fig. 16.21(a)]. Heat flow through the Cu land retards cooling of the inter-face region, as shown in Fig. 16.21(b), and the mismatch in TCE between solder andsubstrate results in fillet lifting. The second factor is the skeleton wicking effect. Thedendrite formation not only promotes Bi segregation, but also provides a skeletonthat may suck in residual liquid and aggravate the fillet lifting process. This skeletonwicking effect may also cause fillet surface cracking in other high-tin alloy systems.Harrison and Vincent89 studied SnAgCuSb and reported that 60 to 80 percent ofdendrites formed during solidification, causing liquid wicking back and shrinkagecracking on the final surface to solidify, as shown in Fig. 16.22.

Fillet lifting can also occur at reflow soldering. In this case, instead of partial filletlifting, the whole solder joint is lifted, as demonstrated by the work of Nakatsuka etal.90 (See Fig. 16.23). By analyzing the solder joint microstructure of lifted or weak-ened joints, it was observed that, similar to the fillet lifting situation in wave solder-


FIGURE 16.21 Schematic of fillet lifting mechanismfor Bi-bearing alloys. (a) Liquid Bi enrichment due toSn-rich dendrite formation. (b) Mismatch in TCE andslow cooling at solder-pad interface result in hot tearingof liquid Bi phase and fillet lifting.87

ing, Bi enrichment also occurred at the interface between solder and Cu land, asshown in Fig. 16.24. In order to understand the driving force of Bi enrichment, aredistribution experiment was conducted on a SnAg32Bi alloy system. The high Bicontent was expected to augment the potential Bi redistribution capability. First, thealloy was allowed to fill the gap between two copper plates. The two copper plateswere then cooled down at different rates in order to generate temperature gradients.A band of redistributed Bi (probably Sn1Ag57Bi, ternary eutectic, melting point137°C, the lowest-melting-point composition in the SnAgBiCu system) was notedon the inner higher-temperature side. The amount of redistribution decreases withdecreasing temperature gradient, as shown in Fig. 16.25.The plates on the left side ofFig. 16.25(a) and (b) are higher in temperature, and the sample in (a) has a greatertemperature gradient than the sample in (b). It was concluded by Nakatsuka et al.that the component joint lifting was caused by warping of PCB and a strengthdecrease resulting from the hardness and brittleness of a vicinity of eutectic compo-sition with low melting point that was redistributed at the last cooled location withinthe solder. The Bi enrichment process can be minimized by applying a coolingprocess with minimal temperature gradient between components and PCB via equalheating from both the top and bottom.87 However, it should be noted that eutecticSnBi, although hard and brittle, has been successfully used in electronic assembly formany decades without problems. Therefore, it is the author’s opinion that the jointlifting may have actually occurred before the joint was fully solidified, and a hottearing mechanism similar to that responsible for fillet lifting in wave solderingresults in the component separation.


FIGURE 16.22 SEM of surface shrinkage cracking on fillet of SnAgCuSb joint caused by den-drite formation.89

Fillet lifting was also observed on the top side only for SnPb finished componentswave-soldered with 99.3Sn0.7Cu. Suganuma87 studied this phenomenon and ob-served that Pb enrichment occurred at the interface between solder and lead as wellas between solder and copper pad, as shown in Fig. 16.26. The mechanism proposedby Suganuma postulates that SnCu solder touches the lead wire of the bottom of aPCB and flows up to the top side through the through-hole. The surface SnPb coat-ing dissolved by the SnCu liquid flow is conveyed to the top side. SnCu with Pbexhibits a lower solidus temperature (see Fig. 16.27). Presumably the low-meltingPb enriched at the last cooling spot through a zone refining mechanism.85

In summary, fillet lifting can be minimized by the following approaches:

1. Avoiding Pb contamination2. Rapid cooling to depress dendrite growth3. Even cooling of both top side and bottom side4. Minimizing the pasty range of solders


FIGURE 16.23 Solder joint lifting in the vicinity of SnAgBiCu solder/Cu pad interface.90

16.2.6 POOR WETTING

Perhaps poor wetting is the first draw-back noticed by the industry when try-ing to implement lead-free soldering.This poor wetting can be caused by thecomponent finish, the pad surface finish,or the solder itself. In the reflow case,the joint of TQFP64 with NiPd lead fin-ish and Sn3.3Ag3Bi1.1Cu solder exhibitspoor wetting at both toe and heel loca-tions.91 The same component with SnPbcomponent finish displays a very goodwetting, with solder wicking up at bothtoe and heel locations. The poor wettingperformance of Pb-free solders can bequantitatively reflected by the wettingtime study of Toyoda,81 as shown in Fig.16.28. Here the eutectic SnPb wets thebest, followed by SnAgCuBi family andthen by SnAgCu family, with eutecticSnCu exhibiting the poorest wetting.

The poor wetting of Pb-free alloysmay be an inherent characteristic, sincethe surface tension of high-tin alloys typ-ically is higher than that of eutectic SnPb.


FIGURE 16.24 Cross-sectional view of weakened solder joints. Only (a) ruptured joints or (b)weak joints (<3 N vs. >6 N) showed Bi redistribution in the vicinity of the solder/Cu pad interface.90

FIGURE 16.25 Results of redistribution exper-iment with SnAg32Bi. TL, TH, and t are the tem-peratures of points L and H and the time,respectively. (a) A band of redistributed Bi wasnoted on the inner higher-temperature side. (b)The amount of redistribution decreases withdecreasing temperature gradient.90

(b)(a)

On the other hand, the wetability of eutectic SnPb HASL is insurmountable whencompared with those of all lead-free finishes. This is attributed to the fact that onlycoalescence of molten solder is needed in order to wet to HASL surfaces. For allother surface finishes, such as organic solderability preservatives or NiAu finishes,metallurgical diffusion is required in order to achieve solder wetting.

16.2.7 VOIDING

Poor voiding is another shortcoming noticed by the industry when dealing with Pb-free soldering. Voiding is a phenomenon commonly associated with solder joints.Generally the voids are caused by the outgassing of flux entrapped in the sand-wiched solder during reflow. This is especially true when reflowing a solder paste insurface-mount technology applications. The void content increases with decreasingsolderability.92 With decreasing solderability, the substrate oxide can be cleaned lessreadily, thus allowing more opportunity for the flux to be entrapped to form voids.As discussed in the previous section, Pb-free soldering suffers from poor wetting,which inevitably results in poor voiding. Surface finish appears to be a more criticalfactor in affecting voiding than the solder alloy.76

During the transition stage of implementing Pb-free soldering, the use of Pb-freematerials together with SnPb materials is very likely. Jessen93 studied the effect of


FIGURE 16.26 Cross-sectional view of solder joint with fillet lifting occurring on top side only forSnPb finished components wave-soldered with 99.3Sn0.7Cu. Pb enrichment is observed at the inter-face between solder and lead as well as between solder and copper pad.87


solder material on BGA voiding per-formance, with results shown in Fig.16.29. His results indicate that void con-tent in the BGA joints decreases in thefollowing order: SnAgCu paste/SnPbsolder ball > SnAgCu paste/SnAgCu sol-der ball > SnPb paste/SnPb solder ball.This relative voiding tendency can beexplained by the model shown in Fig.16.30. For the BGA attachment process,if the melting temperature of solder ballis higher than that of the solder paste, noflux fumes will be able to penetrate intothe solder ball and form voids. However,if the ball exhibits a lower melting tem-perature than the paste, as shown in Fig.16.30(b), voiding will be a big problem.As soon as the ball reaches melting tem-perature, a large quantity of the fluxvolatiles generated will enter the moltensolder and form voids rigorously. Thisrigorous void-forming process will con-tinue until the solder paste coalesces,which in turn will cause the flux to beexpelled from the interior of the moltensolder. The voiding action will then sub-side due to shortage in fresh supply ofvolatiles. It is interesting to note that thismodel also explains the voiding behav-ior involving Sn62 and Sn63. It has beennoted that the BGA assembly with Sn62ball and Sn63 paste yielded much morevoiding than the system with Sn63 ball

FIGURE 16.27 SnCu solder touches the leadwire of the bottom of a PCB and flows up to thetop side through the through-hole. The surfaceSnPb coating dissolved by the SnCu liquid flowis conveyed to the top side and redistributed atthe interface.87

FIGURE 16.28 Meniscograph of wetting time test results of solders.81

and Sn62 paste.94 Since Sn62 exhibits a solidus temperature of 179°C while Sn63melts at 183°C, the inferior voiding performance of Sn62 ball/Sn63 paste becomeseasily understandable.

The model in Fig. 16.30 dictates that a mixed alloy system may be tolerable onlyif the ball does not have a lower melting point than the paste. Violating this rule willresult in unacceptable voiding in the joints.

16.2.8 ROUGH JOINT APPEARANCE

The solder joints in the lead-free process typically exhibit a rough, grainy appear-ance. Figure 16.31 shows lead-free solder joints from a cellular phone using no-cleansolder paste (with 95.5Sn3.8Ag0.7Cu, type 3, 89.3 percent). Compared with the typ-


FIGURE 16.29 Voiding performance of CSP169 and CBGA256. TSSOP48 and R2512 are rated byinsufficient solder volume instead of voiding.93

FIGURE 16.30 Voiding mechanism of BGA assembly process with alloy A for solder ball andalloy B for solder paste. Voiding becomes significant if the melting point of alloy A is less than thatof alloy B.

(b)(a)

ical shiny, smooth eutectic SnPb solder joints, the joints’ appearance is fairly rugged.This is mainly attributed to the high tin composition of Pb-free alloys, which tend todevelop dendrites easily, as shown in Fig. 16.22. The primary impact of a rough jointappearance is inspection. A new criterion has to be established in order to differen-tiate a good rough joint from a bad rough joint.

16.3 CHALLENGES FOR RELIABILITY

Although the dust gradually settles in terms of solder alloy selection, challenges forreliability still exist. This is true even for the prevailing Pb-free alloys. Examplesinclude tin pest, intermetallic compound platelet, trace fracture, thermal damage,conductive anodic filament (CAF), and flux residue removal.

16.3.1 TIN PEST

Tin pest is growth of pestlike formations on the surface of tin, as demonstrated byFig. 16.32. It is caused by transformation of β-tin to α-tin. Figure 16.33 shows SEMimages of the microstructures of β-tin and α-tin. The loose structure of α-tinundoubtly poses a major threat to the reliability of solder joints. The tin pest phe-nomenon has occurred on solder 99.5Sn0.5Cu in as cast form, as reported by Karlya


FIGURE 16.31 Solder joints of cellular phone using no-clean solder paste (with 95.5Sn3.8Ag0.7Cu,type 3, 89.3 percent).

et al.95 Although no tin pest has been observed on solder joints in PCB thus far, theconditions for formation of tin pest should be assessed in order to assure the reli-ability of Pb-free solder joints.

16.3.2 INTERMETALLIC COMPOUND PLATELET

SnAgCu is the most attractive system, mainly due to its overall superior reliabilityand acceptable soldering property and cost. SnAg is also a favorite choice. Reliabil-ity of SnAg can range from poor to good, and is highly dependent on applications.Its soldering performance is marginally acceptable, as discussed in Chap. 13.Microstructure study of solder joints from both alloys shows presence of Ag3Sn andCu6Sn5. However, large primary Ag3Sn precipitate was found to appear inSn3.9Ag0.6Cu when the sample was cooled slowly, as shown in Fig. 16.34.96 Thislarge, sharp Ag3Sn precipitate degrades the tensile property of this alloy, thus com-promising the reliability of solder joints. Development of large Ag3Sn precipitate hasalso been observed in solder joints of 62Sn36Pb2Ag on Cu base.72 Here the Ag3Snnucleated on the Cu6Sn5 layer and grew into large platelets.


FIGURE 16.32 Tin pest development progress with increasing aging on 99.5Sn0.5Cu.95

FIGURE 16.33 SEM of grip end of 99.5Sn0.5Cu specimen aged at 255 K for 7 months.95

The large Ag3Sn platelets can also appear in Pb-free solder joints. Lee et al.97

studied the microstructural stability of flip chip solder bumps on Cu pads. Fouralloys were investigated: eutectic SnAg, SnAgCu, SnCu, and SnPb. While a small orthin Cu6Sn5 phase is present in every alloy system, huge Ag3Sn platelets are foundin eutectic SnAg and Sn3.8Ag0.7Cu systems, as shown in Fig. 16.35. For theSn3.8Ag0.7Cu sample, the solder was etched away to expose the platelet structure.The presence of large Ag3Sn platelets can cause solder joints of an area array pack-age to split and slide along the interface between solder and platelet, and conse-quently result in early failure. The formation of Ag3Sn platelets can be prevented bylowering the Ag content in solder. In Suganuma’s work, Sn3Ag0.5Cu is recom-mended as a reliable choice.

16.3.3 STIFF JOINT

Trace fracture has been observed for BGA with SnAg solder balls assembled withSnPb paste after 2877 −50 to 150°C cycles (see Fig. 16.3698).This is attributable to the


FIGURE 16.34 SEM and electron probe microanalysis (EPMA) images of mildly cooledSn3.9Ag0.6Cu. A large primary Ag3Sn precipitate is found in the Sn matrix.96


FIGURE 16.35 SEM micrographs showing different interfacial intermetallic formation as afunction of alloy.97

FIGURE 16.36 Flat section of a trace failure on an NSMD test board pad at 2877 −50 to 150°Ccycles. Part had SnAg solder balls and was assembled with SnPb paste.98

high rigidity of SnAg solder ball. Upon thermal cycling, the stress caused by mis-match in TCE between BGA and PCB cannot be absorbed by the compliance of sol-der joints, and consequently results in trace fracture. For Pb-free soldering, this is acommon issue, since many Pb-free alloys exhibit a high stiffness, such as SnAgBi,SnAgBiCu, SnAgCuSb, and SnSb systems. Although the trace fracture problemcould be addressed with solder mask–defined pad design or a widened trace at junc-tion point with pad, the loss in trace routine spacing will limit its acceptance in high-density interconnect applications.

16.3.4 THERMAL DAMAGE

Pb-free alloys typically exhibit a higher melting temperature than eutectic SnPb.Thisconsequent higher soldering temperature inevitably may induce some thermal dam-age to the components or boards. Figure 16.37 shows a cross section of 144LQFPpackage after level 2a/260°C, indicating a crack in the molding compound.99 Thecrack initiates from the leadframe paddle and propagates along a diagonal paththrough the molding compound toward the bottom of the package, although it doesnot extend to the external package boundary. Similar damage on plastic ball gridarray (PBGA) caused by Pb-free soldering is exemplified by Fig. 16.38, where thecross-sectional view of the two-layer PBGA package after level 2a/260°C stressingindicates delamination within the die attach layer and internal substrate layers.99

Besides the mechanical cracks in components, the high-temperature soldering


FIGURE 16.37 Cross section of 144LQFP package after level 2a/260°C indicating crack in moldingcompound.99

process can also cause damage to the board materials, as discussed in the followingsection.

16.3.5 FLUX RESIDUE CLEANING

Cleaning the flux residue of lead-free solder pastes is more challenging than clean-ing that of SnPb systems.69,70 This is primarily due to (1) higher reflow temperature;(2) higher flux capacity, and therefore higher flux-induced side reactions; and (3)more tin salt formation. Bivins et al.69 studied lead-free flux residue cleanability.Theresults indicate that visual cleanability decreases from no-clean soft-residue fluxesto water-washable fluxes to no-clean hard residue fluxes. Improvement in visualcleanability may link to improvement in the thermal stability of fluxes. Semiaqueousand/or aqueous solvent sprayable cleaners yield the best visual cleaning efficiency, ifthe solvent is selected properly. Spray is the most critical mechanical agitation,regardless of whether it is spray in air or spray under immersion. Ultrasonic aid alsoimparted a significant positive effect. Saponified aqueous spray is one of the topchoices for cleaning water-washable residues. Better solvency and spray are thedirections for further improving cleaning. Improvement in flux thermal stability willhelp both soldering and cleaning.

Another study on postsolder cleaning of lead-free solder paste residues used sur-face insulation resistance (SIR) and ionic contamination as criteria for cleanability.70

From these experiments it was concluded that:

1. Cleaning efficiency is highest for water-soluble paste and lowest for no-clean,halide-free, ultra-low-residue paste. Halide-containing, full residue no-cleanpaste is slightly better than low-residue paste.


FIGURE 16.38 Cross section of the two-layer PBGA package after level 2a/260°C stressing indi-cating delamination within the die attach layer and internal substrate layers.99

2. Reflow temperature does not significantly affect cleaning of flux residues fromlead-free solders.

3. Cleaning chemistry has no effect on ionic contamination. For SIR performance,vapor degreasing cleaning chemistry shows the poorest performance.

4. Physical approaches, including cleaning time and ultrasonic agitation, have negli-gible effect on cleaning efficiency, while the chemical approach shows significanteffect. The latter is demonstrated by the major improvement in efficiency whenalkaline cleaner is added to water. This negligible effect of physical approachesreflects the greater difficulty in removing the residue of Pb-free solder pastesthan eutectic Sn-Pb systems, presumably due to the higher reflow temperatureused for Pb-free reflow processes.

5. Cleaning efficiency based on ionic contamination is virtually independent ofcleaning efficiency based on SIR.

16.3.6 CONDUCTIVE ANODIC FILAMENT

Conductive anodic filament (CAF) formation is a failure mode for printed wiringboards in which a conductive filament forms along the epoxy-glass interface grow-ing from anode to cathode. Figure 16.39 shows an example of CAF.The white region


FIGURE 16.39 Cross section of a printed wiring board showing CAF growing along theepoxy-glass interface.100


FIGURE 16.40 CAF appears as dark shadows coming from the copper anode to the cathode whena backlighting is applied.100

indicates a copper-containing filament growing along the epoxy-glass interface.Using backlighting, CAF appears as dark shadows coming from the copper anode tothe cathode, as shown in Fig. 16.40.

CAF is closely associated with reflow temperature. Turbini et al.100 studied CAFoccurrence frequency and SIR value as a function of reflow temperature using 21fluxes. The results indicate that a higher board-processing temperature results inincreased numbers of CAFs for most of the fluxes tested, as shown in Table 16.10.The findings here pose a great concern for electrical reliability, since virtually all Pb-free soldering requires a high-temperature soldering process. Perhaps the problemcan be resolved by using board materials with a higher thermal stability, althoughcosts will be higher.

16.4 UNANSWERED CHALLENGES

Up to this point, many challenges associated with lead-free soldering have been dis-cussed.Although with certain difficulty, in general those issues can be addressed oneway or the other. However, there are some challenges for which the answers have

still not been found. For instance, Pb-free alternatives to high-Pb solders, such as97Pb3Sn or 90Pb10Sn, are still not available. The higher cost of implementing Pb-free soldering is another issue. This includes solder materials, component and boardredesigns, and equipment upgrades. Toxicity and recycling are other unansweredchallenges. Table 16.11 shows the environmental impact of lead-free solders com-pared to SnPb.101 Toxicity of Ag and recycling of Bi are examples for which the solu-tion has not been identified yet.

Overall, although quite some progress has been made in lead-free soldering,many issues still have to be addressed, and the battle is far from being over.


TABLE 16.10 Comparison of SIR Levels and Number of CAFs Associated with Two DifferentReflow Temperatures

SIR (Ω) SIR (Ω) #CAF at #CAF at Flux 201°C reflow 241°C reflow 201°C reflow 241°C reflow

Polyethylene glycol-600 (PEG) <106 <106 90 55

PEG/HCl <106 High 108 None None

PEG/HBr <106 High 108 None None

Polypropylene glycol 1200 (PPG) >1010 >1010 None 455

PPG/HCl >1010 >1010 None 379

PPG/HBr >1010 >1010 1 423

Polyethylene propylene glycol 1800(PEPG 18) High 109 High 109 1 406

PEPG 18/HCl High 109 High 109 10 135

PEPG 18/HBr 1010 High 109 9 279

Polyethylene propylene glycol 2600(PEPG 26) High 109 High 109 None 91

PEPG 26/HCl High 109 High 109 6 218

PEPG 26/HBr 1010 High 109 None 51

Glycerine (GLY) >1010 High 109 None 56

GLY/HCl >1010 High 109 None 583

GLY/HBr >1010 High 109 3 104

Ocyl phenol ethoxylate (OPE) Low 109 Low 109 None 83

OPE/HCl Low 109 Low 109 14 62

OPE/HBr >1010 High 109 2 599

Linear Aliphatic Polyether (LAP) Low 109 Not tested None Not tested

LAP/HCl Low 109 Low 109 15 203

LAP/HBr Low 109 Low 109 None 272

TABLE 16.11 Overview of the Environmental Impact of Lead-Free Solders Compared to SnPb

TPI Acute toxicity Ecotoxicity Metal products Manufacturing Recycling Disposal

SnPb 100% Pb: Highly toxic; Pb: Accumulates; 100% Optimized SnPb solder Pb leaching(SnPb37) teratogenic; highly toxic to process retrieval at 40 ppm Pb

mutagenic ? many organisms secondary in leachatecancerogenic ? Cu smelters

SnAg 29% Ag: Argyria Ag: Toxic to 7% High energy Up to 10% Sn <0,1 ppm Ag(SnAg3.5) microorganisms but demand tolerated at in leachate

low bioavailability precious metalrefining

SnAgCu 32% Ag: Argyria Ag: Toxic to 8% High energy Up to 50% Cu at ?(SnAg4Cu0.5) microorganisms but demand PMR; Cu leaching

low bioavailability only 1% Ag atCu smelting

SnCu 14% Cu: Low toxicity Cu: Toxic to aquatic life 2% High energy Up to 10% Sn ?(SnCu0.7) to mammals but low content demand tolerated at Cu leaching

Cu smelting

SnBi 6% Bi: Lower toxicity ? ? Bi not wanted Bi leaching(SnBi58) than Pb Lower bioavailability 62% Process not by Cu smelters 3,9 ppm Bi

than Pb yet evaluated in leachate

SnAgBi 29% Ag: Argyria Ag: Low bioavailability 12% Lower energy Bi not wanted Bi leaching(SnAg3.5Bi4.8) Bi: Low content demand than by Cu smelters expected

SnAg

SnZn 14% Zn: Low toxicity; no Zn: Toxic to some 1% Aggressive flux Only up to 1% Zn ?(SnZn9) lethal intoxications plants and aquatic and cleaning tolerated at PMR Zn leaching

reported organisms agents and Cu smelting

16.3

8

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40. Kakeshita, T., R. Kawanaka, and T. Hasegawa, “Grain Size Effect of Electro-Plated TinCoatings on Whisker Growth,” Journal of Materials Science, 17:2560–2566, 1982.

41. Dunn, B. D., “The Fusing of Tin-Lead Plating on High Quality Printed-Circuit Boards,”Transactions of the Institute of Metal Finishing, 58:26, 1980.

42. Hada,Y., O. Morikawa, and H.Togami,“Study of Tin Whiskers on Electromagnetic RelayParts,” 26th Annual National Relay Conference, pp. 9.1–9.15, April 25–26, 1978.

43. Smith, G. A., “How to Avoid Metallic Growth on Electronic Hardware,” Circuits Manu-facturing, pp. 66–72, July 1977.

44. Zakraysek, L., D. B. Blackwood, W. Brouillette, W. Leyshon, A. Tardone, C. Byrns, and F.Poe, “Whisker Growth from a Bright Acid Tin Electrodeposit,” Plating and Surface Fin-ishing, 64:38–43, March 1977.

45. Dunn, B. D., “Whisker Formation on Electronic Materials,” ESA Scientific and TechnicalReview, 2(1):1–22, 1976.


46. Lindborg, U., “A Model for the Spontaneous Growth of Zinc, Cadmium, and TinWhiskers,” Acta Metallurgica, 24:181, 1976.

47. Sabbagh, N. A. J., and H. J. McQueen, “Tin Whiskers: Causes and Remedies,” Metal Fin-ishing, March 1975.

48. Britton, S. C., “Spontaneous Growth of Whiskers on Tin Coatings: 20 Years of Observa-tion,” Transactions of the Institute of Metal Finishing, 52:95–102, April 3, 1974.

49. Kehrer, H. P., and H. G. Kadereit, “Tracer Experiments on the Growth of Tin Whiskers,”Applied Physics Letters, 16(11):411–412, June 1, 1970.

50. Key, P. L., “Surface Morphology of Whisker Crystals of Tin, Zinc and Cadmium,” IEEEElectronic Components Conference, pp. 155–157, May 1977.

51. Furuta, N., and K. Hamamura, “Growth Mechanism of Proper Tin-Whisker,” Journal ofApplied Physics, 8(12):1404–1410, December 1, 1969.

52. Walker, R., “Internal Stress in Electrodeposited Metallic Coatings,” Metal FinishingMonograph, p. 32, 1968.

53. Arnold, S. M., “Repressing the Growth of Tin Whiskers,” Plating, 53:96–99, 1966.

54. Besancon, R. M., The Encyclopedia of Physics: Electrical Discharges in Gases, pp.189–193, Reinhold, New York, 1966.

55. Glazunova, V. K., and N. T. Kudryavtsev, “An Investigation of the Conditions of Sponta-neous Growth of Filiform Crystals on Electrolytic Coatings,” Zhurnal Prikladnoi Khimii,36(3):543–550, March 1963 (translated).

56. Arnold, S. M., “Growth of Metal Whiskers on Electrical Components,” Proceedings ofElectrical Components Conference, pp. 75–82, 1959.

57. Frank, F. C., “On Tin Whiskers,” Philosophical Magazine, 44:854, 1953.

58. Zhang,Y.,“Electroplated Pure Tin—A Lead Free Alternative,” IPCWorks’99, Minneapo-lis, MN, October 27, 1999.

59. Baudry, I., and G. Kerros, “Focused Ion Beam in Microelectronics Packaging Applica-tions—Leadfree Plating Analysis,” STMicroelectronics, Grenoble, France, 2001.

60. Prasad, S., F. Carson, G. S. Kim, J. S. Lee, Y. C. Park, Y. S. Kim, K. S. Min, S. S. Lu, L. Hui, X.Hai, S. H. Khor, and C. L. Tan, “Plating Chemical Evaluations and Reliability of Pb-FreeLeadframe Packages,” Pan Pacific, February 13, 2001.

61. Lee, B. Z., and D. N. Lee,“Spontaneous Growth Mechanism of Tin Whiskers,” Acta Mater,46:3701–3714, 1998.

62. Boguslavsky, I., meeting minutes, 8th NEMI Whisker Modeling Group conference call,October 18, 2001.

63. Fan, C., meeting minutes, 10th NEMI Whisker Modeling Group conference call, October31, 2001.

64. Oberle, B., e-mail discussion, “Re: Minutes and Action Items: 16th NEMI Whisker Mod-eling Group Conference Call,” October 30, 2001.

65. Ohkawara, Y., and Muroi, “Whisker Growth from Zinc Plating Versus Chemical Speciesin Baths,” Surface Finishing Japan, 49, 1998.

66. Ohkawara, Y., and Muroi, “Influence of Internal Stress and Crystal Structure on WhiskerGrowth from Zinc Plating,” Surface Finishing Japan, 51, 2000.

67. Zhang, Y., meeting minutes, 8th NEMI Whisker Modeling Group conference call, Octo-ber 18, 2001.

68. Brusse, J., e-mail comment, NEMI Whisker Modeling Group conference call, October 31,2001.

69. Bivins, B.A.,A.A. Juan, B. Starkweather, N.-C. Lee, and S. Negi, “Post-Solder Cleaning ofLead-Free Solder Paste Residues,” SMT International 2000, Chicago, IL, 2000.

70. Lee, N.-C., and M. Bixenman, “Lead-Free: How Flux Technology Will Differ for Lead-Free Alloys and Its Impact on Cleaning,” Etronics, Anaheim, CA, March 2001.


71. Denman, R. D., “Soldering to Gold Coatings,” ITRI publication no. 736, 1996.

72. Harris, P. G., and K. S. Chaggar,“The Role of Intermetallic Compounds in Lead-Free Sol-dering,” Soldering and Surface Mount Technology, 10(3):38–52, 1998.

73. Manko, H. H., Solders and Soldering, McGraw-Hill, New York, 1964.

74. Steen, H.A.H.,“The Effect of Impurities on the Microstructure and Solidification Behav-iour of Eutectic Sn-Pb Solders,” Swedish Institute for Metals Research, report no. IM-1643, 1982.

75. Ray, U., I. Artaki, D. W. Finley, G. M. Wenger, T. Pan, H. D. Blair, J. M. Nicholson, and P. T.Vianco, “Assessment of Circuit Board Surface Finishes for Electronic Assembly withLead-Free Solders,” SMI 96, San Jose, CA, September 10–12, 1996.

76. Lotosky, P.,“Lead-Free Update,” tutorial at IMAPS-Brazil, Sao Paulo, Brazil,August 1–3,2001.

77. Miric, A. Z., and A. Grusd, “Lead-Free Alloys,” Soldering and Surface Mount Technology,10(1):19–25, 1998.

78. Indium Corporation of America internal data.

79. Bieler, T., “Lead Effect on SnAg,” Soldering and SMT, 13(2), 2001.

80. NEMI Lead Free Solder Meeting, Chicago, IL, May 25, 1999.

81. Toyoda, Y., “The Latest Trends in Lead-Free Soldering,” Proceedings of the InternationalSymposium on Electronic Packaging Technology, pp. 434–438, Beijing, China, August8–11, 2001.

82. Vianco, P., J. Rejent, I. Artaki, U. Ray, D. Finley, and A. Jackson, “Compatibility of Lead-Free Solders with Lead Containing Surface Finishes as a Reliability Issue in ElectronicAssemblies,” ECTC, 1996.

83. Baggio, T. J., K. Suetsugu, and T. Okumura, “Challenges and Solutions for Lead-Free Sol-dering of Large PCB Assembly,” Apex 2000, Long Beach, CA, March 2000.


85. Seelig, K., and D. Suraski,“Pb Contamination in Pb-Free Assembly,” Surface Mount Tech-nology, pp. 70–73, October 2001.

86. “Lead-Free Solder Project Final Report,” NCMS Report 0401RE96, August 1997.

87. Suganuma, K., “Mechanism and Prevention of Lift-Off in Lead-Free Soldering,” IMAPS,pp. 325–329, Boston, MA, September 20–22, 2000.

88. Handwerker, C., e-mail communication, “Information on Fillet Lifting with SnCu andSnAgCu with Ag Surface Finishes,” NEMI Lead-Free Task Group discussion, July 27,2000.

89. Harrison, M. R., and J. H. Vincent, “IDEALS: Improved Design Life and Environmen-tally Aware Manufacturing of Electronics Assemblies by Lead-Free Soldering,” SSTC,1999.

90. Nakatsuka, T., K. Serizawa, T. Soga, H. Shimokawa, and A. Nishimura, “Reliability of Pb-Free Solder Joints of Surface-Mounted LSI Packages After Flow-Soldering,” IMAPS, pp.330–335, Boston, MA, September 20–22, 2000.

91. Feldmann, K., and M. Reichenberger, “Assessment of Lead-Free Solders for SMT,” Apex2000, Long Beach, CA, March 2000.

92. Hance, W. B., and N.-C. Lee, “Voiding Mechanisms in SMT,” China Lake’s 17th AnnualElectronics Manufacturing Seminar, China Lake, CA, February 2–4, 1993.

93. Jessen, J., “X-ray Imaging of Lead Free Solder,” Etronix, Anaheim, CA, March 1, 2001.

94. Ladhar, H., private communication on Solectron BGA assembly experience, June, 2001.

95. Karlya, Y., C. Gagg, and W. J. Plumbridge, “Tin Pest in Lead Free Solders,” Soldering andSurface Mount Technology, 13(1):39–40, 2000.


96. Suganuma, K., K. S. Kim, and S. H. Huh, “Selection of Sn-Ag-Cu Lead-Free Alloys,”IMAPS, Baltimore, MD, 2001.

97. Lee,T.Y.,W. J. Choi, K. N.Tu, J.W. Jang, S. M. Kuo, J. K. Lin, D. R. Frear, K. Zeng, and J. K.Kivilahti, “Morphology, Kinetics, and Thermodynamics of Solid State Aging of EutecticSnPb and Pb-Free Solders (Sn3.5Ag, Sn3.8Ag0.7Cu, and Sn0.7Cu) on Cu,” to be pub-lished, 2001.

98. Mawer,A., and K. Levis,“Automotive PBGA Assembly and Board-Level Reliability withLead-Free Versus Lead-Tin Interconnect,” SMTA International, Chicago, IL, September24–28, 2000.

99. Vaccaro, B.T., R. L. Shook, and D. L. Gerlach,“The Impact of Lead-Free Reflow Temper-atures on the Moisture Sensitivity Performance of Plastic Surface Mount Packages,”SMTA International, Chicago, IL, September 24–28, 2000.

100. Turbini, L. J., W. R. Bent, and W. J. Ready, “Impact of Higher Melting Lead-Free Solderson the Reliability of Printed Wiring Assemblies,” SMTA International, Chicago, IL, Sep-tember 20–24, 2000.

101. Griese, H., J. Muller, and K.-H. Zuber, “Toward Green Electronic Packaging Technolo-gies,” Proceedings of the Fourth International Symposium on Electronic Packaging Tech-nologies, pp. 59–66, Beijing, China, August 8–11, 2001.


CHAPTER 17INTRODUCTION TO

CONDUCTIVE ADHESIVES

This chapter gives a brief overview of packaging technologies, trends in electronicspackages, and conductive adhesive technology, and also introduces the scope andnature of the following chapters.The intent is to provide a fundamental understand-ing of ECA technology and to develop high-performance conductive adhesives forsolder replacement. This chapter also includes a detailed outline of the researchgoals and selected approaches to gaining a fundamental understanding and devel-oping solder replacement conductive adhesives.

17.1 ELECTRONICS PACKAGING:

A BRIEF OVERVIEW

Packaging of electronic circuits is the science and art of establishing interconnectionand a suitable operating environment for predominantly electrical circuits toprocess or store information.1 Packaging has four main functions:

1. Signal distribution, involving mainly topological and electromagnetic considera-tions

2. Power distribution, involving electromagnetic, structural, and materials aspects3. Heat dissipation (cooling), involving structural and materials considerations4. Protection (mechanical, chemical, and electromagnetic) of components and inter-

connections

Levels of electronic packaging are defined in Fig. 17.1. In first-level packaging,the integrated circuit is assembled into a package such as a quad flat pack, pin gridarray, or ball grid array using wiring bonding, tape automated bonding, or flip chipbumping assembly techniques. The packaged device is then attached either directlyto a printed circuit board or to another type of substrate, which is defined as second-level packaging. The next level of packaging (third or higher) may be the outer shellof a small piece of equipment.

The packaging concepts and technologies in all packaging levels are undergoingquick evolution because of the dramatic change in the computer, telecommunica-tions, automotive, and consumer electronics industries to low cost, portability, highperformance, diverse functions, and environmental and user friendliness. Figure 17.2illustrates the historic packaging evolution for each of the major packaging hierar-chy technologies from integrated circuit interconnection to first-level packaging andsecond-level packaging during the last four decades and the expected future trend.2

In general, packaging has evolved from dual inline, wire-bonding, and through-holein printed wiring board technologies in the 1970s to ball grid array, chip-scale, and

17.1


17.2 CHAPTER SEVENTEEN

FIGURE 17.1 Levels of electronics packaging.

FIGURE 17.2 Electronics packaging evolution.

INTRODUCTION TO CONDUCTIVE ADHESIVES 17.3

surface-mount technologies in the 1990s. The numbers of discrete components havealso decreased significantly, primarily due to advances in semiconductor technology.The future trend is to incorporate all these components onto a common substrateand form a single-level integrated module.As an example, Fig. 17.3 shows the system-level integrated module (SLIM) that has been proposed by the Packaging Research

FIGURE 17.3 General view of SLIM concept proposed byGeorgia Tech Packaging Research Center.

FIGURE 17.4 Silicon efficiency of various types of packaging.

Center at Georgia Tech to satisfy the next-generation packaging needs in the areasof digital, analog, optoelectronics, radio frequency, and mixed-signal applications.Thus, the growing trends of semiconductor, packaging, and other technologies areexpected to go hand in hand in the future.

The best measure of packaging technology is the analysis of silicon efficiency,which is defined as the ratio of silicon area to board area. Silicon efficiency continuesto increase with package evolution, as shown in Fig. 17.4.As can be seen from this fig-ure, packaging efficiency increased from less than 2 percent in the 1970s with dualinline packages to about 10 percent in the 1980s with quad flat packages, to approxi-mately 20 percent with ball grid array in the 1990s, and to 40 percent with multichipmodules and chip-scale packages, and will reach 80 percent with SLIM-type system-level integrated packages. Recent advances in paper-thin die and three-dimensionalpackaging will exceed 100 percent silicon efficiency in modern high-performancepackages.

17.2 OVERVIEW OF CONDUCTIVE

ADHESIVE TECHNOLOGY

ECAs are composites of polymeric matrices and electrically conductive fillers. Poly-meric matrices have excellent dielectric properties and thus are electrical insulators.The conductive fillers provide the electrical properties and the polymeric matricesprovide mechanical properties. Therefore, electrical and mechanical properties areprovided by different components, which is different from the case for metallic sol-ders that provide both electrical and mechanical properties. ECAs have been with usfor some time. Metal-filled thermoset polymers were first patented as ECAs in the1950s.3–5 There are two kinds of conductive adhesives: anisotropically conductiveadhesives (ACAs) and isotropically conductive adhesives (ICAs).

17.2.1 ACAs

ACAs represent the first major division of polymer bonding agents. The anisotropicclass of adhesives provides unidirectional electrical conductivity in the vertical or z-axis (out of plane). This directional conductivity is achieved by using a relativelylow-volume loading of conductive filler (5 to 20 volume percent),6–8 which is insuffi-cient for interparticle contact and prevents conductivity in the x-y plane (in plane)of the adhesive. The z-axis adhesive, in film or paste form, is interposed between thesurfaces to be connected. Application of heat and pressure to this stack causes con-ductive particles to be trapped between opposing conductor surfaces on the twocomponents. Once electrical continuity is produced, the dielectric polymer matrix ishardened by chemical reaction (thermosets) or by cooling (thermoplastics). Thehardened dielectric polymer matrix holds the two components together and helpsmaintain the pressure contact between component surfaces and conductive parti-cles.A cross section of a joint formed by an ACA between two components is shownin Fig. 17.5.

ACAs have been developed for use in electrical interconnections, and variousdesigns, formulations, and processes have been patented in Europe, Japan, and theU.S.An analysis of published patents by nationality of the owning company is shownin Fig. 17.6.8


ACAs fall into two broad categories: those that are anisotropically conductingbefore processing and those whose anisotropy arises as a result of processing. Theircharacteristics can be summarized as follows:

Preprocessing anisotropic. These materials are characterized by an ordered systemof conductor elements interspersed in an adhesive matrix film. They are always intape or sheet form and are evidently complicated to manufacture, requiring anadhesive film to be laser-drilled or etched and then filled with conducting materi-als. They should provide predictable contacts and may be applied to the substrateas preforms.

Postprocessing anisotropic. This category consists of materials that are a homoge-neous mix of conductive fillers and adhesive matrix and that have no internalstructure or order prior to processing. All adhesive pastes and some tapes fall intothis category.

17.2.1.1 Adhesive Matrix. The adhesive matrix is used to form a mechanicalbond at the interconnection. Both thermosetting and thermoplastic materials areused. Thermoplastic adhesives are rigid (glassy) at temperatures below the glasstransition temperature Tg of the polymer. Above this temperature, polymer flow


Component

Substrate

Conduct ive fillerPolymer Matrix

FIGURE 17.5 Cross section of an ACA joint.

USA

Europe

Japan

FIGURE 17.6 Patents applicable to ACAs by region.

occurs. Thus the Tg must be sufficiently high to avoid polymer flow during use con-ditions, but low enough to prevent thermal damage to the electronic circuits duringassembly. The principal advantage of thermoplastic adhesives is the relative easewith which the interconnection can be disassembled for repair operations.9,10 How-ever, there are many disadvantages of thermoplastic ACAs. The adhesion is notstrong enough to hold the conductive particles in position, and the contact resistanceincreases after thermal shocks.9,10 Moreover, a phenomenon called springbackincreases the contact resistance while the adhesive layer recovers from the stresscaused by the pressing of the ACA onto the components during bonding. This phe-nomenon, originating from the creep of thermoplastic elastomer, occurs more thana few weeks after the film has been heated to connect circuits. In the springback phe-nomenon, the contact resistance sometimes increases to more than three times theinitial resistance.9

Thermosetting adhesives, such as epoxies and silicones, form a three-dimensionalcross-linked structure when cured under specific conditions. Cure techniquesinclude heat, ultraviolet light, and added catalysts.As a result of this irreversible curereaction, the initial un-cross-linked material is transformed into a rigid solid. Thethermosetting ACAs are stable at high temperatures and, more importantly, give lowcontact resistance. The compressive force on the conductive particles is locked in bythe adhesive after cure. The additional shrink force caused by the cure reactionaccomplishes the low contact resistance with long-term stability.The ability to main-tain strength at high temperature and robust adhesive bonds are the principaladvantages of these materials. However, because the cure reaction is not reversible,rework or repair of interconnections might be a problem.9,10 The choice of adhesivematrix and its formulation is critical to the long-term life properties of the compos-ite. In practice, many options exist for the adhesive matrix. Acrylics can be used inlow-temperature applications (under 100°C), while epoxies are more robust and canbe used at higher temperatures (up to 200°C). Polyimide is used in the harshest envi-ronments, where the temperature approaches or exceeds 300°C.8

17.2.1.2 Conductive Fillers. Conductive fillers are used to provide the adhesivewith electrical conductivity. The simplest fillers are metal particles, e.g., gold, silver,nickel, indium, copper, chromium, and lead-free solders (SbBi).8,9,11–13 The particlesare usually spherical or are referred to as grain. Particle sizes of the fillers of ACAsare generally in the range of 3 to 15 µm.14 The terms needles or whiskers are alsoquoted in some patents.8

Some other ACA systems employ nonconductive particles with a thin metal coat.The core material is either plastic or glass and the metals can be gold, silver, nickel,aluminum, or chromiun. The basic particle shape is spherical. Plastic-cored particlescan be deformed between the opposing contact surfaces and thus provide a largecontact area. The core can be polystyrene. Because the thermal expansion coeffi-cient of metal-coated polystyrene beads is very close to that of thermoset adhesive,the combination of epoxy resin and metal-plated polystyrene beads results in a largeimprovement in thermal stability.9 In order to obtain fine pitch connection, anotherkind of filler has been developed. A metal sphere or metal-coated plastic sphere iscoated with an insulating resin; the insulating resin layer is broken only under pres-sure to expose the conducting surfaces. This kind of filler is called microcapsulefiller. For this kind of filler, a higher filler loading can be used to achieve finer pitchwithout producing an electrical short circuit between two pitches.9,15 A drawing of across section of an interconnection using this kind of filler is shown in Fig. 17.7.Glass-cored particles, however, lead to a controlled bond line thickness. Since theconductive particle size is known, the conductivity of the joint can be predicted.


17.2.1.3 Applications of ACAs. ACAs have been primarily used in liquid crys-tal display (LCD) panels for a long time.14,16,17 Today, tape automated bonding usingACAs is the predominant packaging approach for large-area LCDs. In LCD panels,ACAs are used to connect the output lead electrode of the tape automated bonding(which is mounted to the driver integrated circuit) to the transparent indium tinoxide electrode of the LCD panels. However, there is a trend toward increasing thepackaging density as well as reducing the material consumption by moving to chip-on-glass technology.14

Recently, intensive research and development work has been carried out in thefield of flip chip technology using ACA films as an alternative to soldering.18–23

ACAs have been used for bonding flip chip on both rigid and flexible substrates.Theelectric current passing through electrical conductive particles in ACAs is the domi-nant conduction path24 (Fig. 17.8). Anisotropic (or z-axis) adhesives offer severalattractive advantages, e.g., very high resolution potential (pitch down to <50 µm dueto the possibility of nonspecific application), fast curing, and low process tempera-tures, as well as lead-free, fluxless bonding that eliminates the need for postassemblycleaning. In addition, the polymer matrix provides an underfill for flip chip andthereby eliminates further process steps.23,25 However, there are still difficulties toovercome before adhesive flip chip will be fully utilized in mass production. Forexample, thermosetting ACAs require a relatively long time to cure the matrix prop-erly, and most ACAs must be bonded under relatively high pressure. As comparedwith soldering, adhesive flip chip needs very accurate component alignment andplacing systems because no self-alignment occurs. Humidity in the environment maycause problems, especially with certain metallizations, because polymers take upwater, resulting in unstable electrical contacts.26,27 Swelling of the polymer matrixcan also separate physically the filler particles from the pads.23

In addition, ACAs were investigated for possible use for fine-pitch application insurface-mount technology to replace SnPb solders. Results showed that none of theACAs tested could pass temperature cycling from −55 to 125°C for 1000 cycles, eventhough some did show stable resistance in the 85°C/85 percent relative humidityenvironment.28


metal

AdhesiveInsulating layer

FIGURE 17.7 Cross section of an interconnection using an ACA filled withmicrocapsule filler.

IC

Substrate

Bump

particlePad

Conductive

FIGURE 17.8 Cross section of an ACA flip chip interconnection.

17.2.2 ICAs

ICAs, also called polymer solder, are composites of polymer resin and conductivefillers. The conductive fillers provide the composite with electrical conductivitythrough contact between the conductive particles. With increasing filler concen-trations, the electrical properties of an ICA transform it from an insulator to aconductor. Percolation theory has been used to explain the electrical properties ofthe composites. At low filler concentration, the resistivity of an ICA decreasesgradually with filler concentration. However, the resistivity drops dramaticallyabove a critical filler concentration Vc, which is called percolation threshold. It isbelieved that at this concentration, all the conductive particles contact each otherand form a three-dimensional network. The resistivity decreases only slightly withfurther increased filler concentrations.29–31 A schematic explanation of the resistiv-ity change of ICAs using percolation theory is shown in Fig. 17.9. In order toachieve desirable conductivity, the volume fraction of the conductive filler in an

ICA should be equal to or slightlyhigher than the critical volume frac-tion. ICAs provide the dual functionsof electrical connection and mechani-cal bond. In an ICA junction (Fig.17.10), the polymer resin providesmechanical interconnection and theconductive filler provides electricalconductivity. Filler loadings that aretoo high deteriorate the mechanicalproperty of the adhesives. Therefore,the challenge in an ICA formulation isto use as much conductive filler as pos-sible to achieve high electrical conduc-tivity without affecting the mechanicalproperties adversely. In a typical ICAformulation, the volume fraction of theconductive filler is about 25 to 30 per-cent.6,7

17.8 CHAPTER SEVENTEENR

esi

stiv

ity

Volume fraction of filler (%)

Vc

FIGURE 17.9 Schematic explanation of resis-tivity versus volume fraction of filler.

Component Polymer matrix

Conductive filler

Electric conduction

Substrate

FIGURE 17.10 Cross section of an ICA junction.

17.2.2.1 Adhesive Matrix. Polymer matrices of ICAs are similar to those ofACAs.An ideal matrix for ICAs should have the following properties: long shelf life(good room-temperature latency), fast cure, relatively high Tg, low moisture pickup,and good adhesion.25

Both thermoplastic and thermoset resins can be used for ICA formulations. Themain thermoplastic resin used for such formulations is polyimide resin.An attractiveadvantage of thermoplastic ICAs is that they are reworkable, e.g., can be easilyrepaired. A major drawback, however, is the degradation of adhesion at high tem-perature. Another drawback of polyimide-based ICAs is that they generally containsolvents. During heating, voids probably are formed when the solvent is evaporated.Most of the commercial ICAs are based on thermosetting resins. Epoxy resins aremost commonly used in thermoset ICA formulations because epoxy resins havesuperior balanced properties. Silicones, cyanate esters, and cyanoacrylates are alsoemployed in ICA formulations.32–36

Most commercial ICAs must be kept and shipped at a very low temperature—usually −40°C—to prevent the material from curing. Shelf life is a very importantfactor for users of ICAs. In order to achieve desirable latency at room temperature,epoxy hardeners have to be carefully selected. In some commercial ICAs, solid cur-ing agents are used.This solid does not dissolve in the epoxy resin at room tempera-ture. However, it can dissolve in the epoxy at a higher temperature (curingtemperature) and react with the epoxy resin.Another approach to achieving latencyis to employ encapsulated imidazole as a curing agent or a catalyst. An imidazole isencapsulated inside a very fine polymer sphere. At room temperature, the polymersphere does not dissolve or react with the epoxy resin. But at a higher temperature,after the polymer shell is broken, the imidazole is released from the sphere and curesthe epoxy or catalyzes the cure reaction.

Fast cure is another attractive property of a desirable ICA. Shorter cure time canpotentially lower the processing cost. In epoxy-based ICA formulations, properhardeners and catalysts such as imidazoles and tertiary amines can be used toachieve fast cure.

Conductive adhesives with low Tgs might lose electrical conductivity during ther-mal cycling aging.37 Electrical conductivity in metal-powder-filled conductive adhe-sives is produced by the contact of adjacent metal particles with each other, thusproducing electrical interconnection continuity between the component lead andthe metallization pad.When the joint is subjected to thermal cycling, repeated cyclicshear motion of the lead relative to the substrate occurs. The amount of shear strainis primarily dependent on the thermal cycling conditions and thermal expansionmismatch between the component and the substrate. The shear strain produced isaccommodated by viscoelastic or viscoplastic deformation of the conductive adhe-sive. When the conductive adhesive deforms to accommodate the shear strain pro-duced, metal particles move, thus changing the position of contact point(s) betweenadjacent metal particles. If the organic matrix is too compliant, it will flow to fill thearea left behind the moving metal particle. When the direction of the shear strain isreversed (thermal cycling), adjacent metal particles move back to find the originalcontact spots partially covered with the compliant and dielectric organic matrix. Asthe number of thermal cycles increases, the contact resistance between adjacent par-ticles increases, thus increasing the interconnection joint resistance.37

Moisture absorption of conductive adhesives can influence the reliability of con-ductive adhesive interconnection joints. Studies on polymer composites have shownwithout doubt that moisture adversely affects both mechanical and electrical prop-erties of epoxy laminates.38,39 In the electronic packaging arena, studies relating toreliability and moisture sensitivity indicate similar degradative effects. Moisture


absorption might cause increased contact resistance, especially if the metallizationsystems on both pads and components consist of non-noble metals.40 In order toachieve high reliability, conductive adhesives with low moisture absorption areneeded. High adhesion strength to pads and component metallizations is a desirableproperty for a conductive adhesive. Epoxy-based ICAs tend to have better adhesionstrength than polyimide- and silicone-based ICAs. However, the silicone matrixtends to have lower moisture absorption than epoxy resins.33

17.2.2.2 Conductive Fillers for ICAs. Because polymer matrices are dielectricmaterials, conductive fillers in ICA formulations provide the material with electricalconductivity. In order to achieve high conductivity, the filler concentration must beat least equal to or higher than the critical concentration predicted by percolationtheory.

Silver (Ag) is by far the most popular conductive filler, although gold (Au), nickel(Ni), copper (Cu), and carbon are also used in ICA formulations. Silver is uniqueamong all of the cost-effective metals by nature of its conductive oxide (Ag2O).Oxides of most common metals are good electrical insulators, and copper powder,for example, becomes a poor conductor after aging. Nickel- and copper-based con-ductive adhesives generally do not have good resistance stability, because bothnickel and copper are easily oxidized. Even with antioxidants, copper-based con-ductive adhesives show an increase in volume resistivity on aging, especially underhigh-temperature and -humidity conditions. Silver-plated copper has commercialapplications in conductive inks, and this type of filler should work in adhesives aswell. While composites filled with pure silver particles often show improved electri-cal conductivity when exposed to elevated temperature and humidity or thermalcycling, this is not always necessarily the case with silver-plated metals, such as cop-per flake. Presumably, the application of heat and mechanical energy allows the par-ticles to make more intimate contact when pure silver is used, but the silver-platedcopper may have coating discontinuities that allow oxidation of the copper and thusreduce electrical paths.6 The most common morphology of conductive fillers usedfor ICAs is flakes, because flakes tend to have large surface areas, more contactspots, and thus more electrical paths than spherical fillers. The particle size of thefillers of ICAs generally ranges from 1 to 20 µm. Larger particles tend to provide thematerial with a higher electrical conductivity and lower viscosity.41 Recently, a newclass of silver particles—porous nano-sized silver particles—was introduced in ICAformulations.42,43 ICAs made with these particles showed improved mechanicalproperties, but the electrical properties were not as good as those of ICAs filled withsilver flakes.

In addition, short carbon fibers have been used as conductive fillers in conductiveadhesive formulations.29,44 However, carbon-based conduction adhesives show muchlower electrical conductivity than silver-filled ones.

A new copper powder with a specific structure was introduced as a conductivefiller for conductive adhesives in 1992.45 This powder consists of two metallic com-ponents: copper and silver. Silver is highly concentrated on the surface of the filler,with the concentration gradually decreasing from the surface into the inside, thoughit contains a small amount of silver in itself. Conductive adhesive paste filled withthis powder showed excellent properties: high oxidation resistance, enough to befired under conditions of high oxygen content (about 100 ppm) added to a nitrogenatmosphere; higher solderability than commercially available copper pastes; suffi-cient adhesion strength even after heating and/or cooling test; and very low migra-tion, almost to the same degree as pure copper paste.45

In order to improve electrical and mechanical properties, low-melting-point alloyfillers have been used in ICA formulations.A conductive filler powder is coated with


a low-melting-point metal. The conductive powder is selected from a group consist-ing of Au, Cu, Ag, Al, Pd, and Pt. The low-melting-point metal is selected from agroup of fusible metals such as Bi, In, Sn, Sb, and Zn. The filler particles are coatedwith the low-melting-point metal, which can be fused to achieve metallurgical bond-ing between adjacent particles and between the particles and the contact surfacesthat are joined using the adhesive material.46,47 A relative new class of conductiveadhesives, called transient liquid-phase sintering conductive adhesives, is a hybrid ofsolder and conductive adhesive joining technologies. Two kinds of metal filler areused in these adhesives: a high-melting-point metal (such as Cu) and a low-melting-point alloy (such as SnPb). At a certain temperature, the low-melting-point alloyfiller melts into liquid. The liquid phase dissolves the high-melting-point particles.The liquid exists only for a short period of time and then forms an alloy and solidi-fies. Electrical connection is established through a plurality of metallurgical con-nections formed in situ from powders of the high-melting-point metal and thelow-melting-point alloy in an adhesive-flux polymer binder. The binder fluxes boththe metal powders and the metal surfaces to be joined, thus allowing an interparti-cle and particle-to-surface metallurgical network to be formed through a processknown as transient liquid-phase sintering. A schematic of a joint formed by the adhe-sive is shown in Fig. 17.11. In this kind of conductive adhesive, the organic mixturefacilitates the transient liquid-phase bonding of the powders to form a stable metal-lurgical network for electrical conduction while also forming an interpenetratingpolymer network providing adhesion.48,49


Component

Substrate

Metallic network

Polymer binder

FIGURE 17.11 A Schematic of a Joint Formed Using Transient Liq-uid Phase Sintering Conductive Adhesives.

17.2.2.3 Applications of ICAsDie Attach Adhesives. The principal function of die attach adhesives is to

mechanically attach the integrated chip (IC) to the substrates in a highly reliablemanner. Die attach can be accomplished using one of several materials. A good dieattach material must have attributes fitted to the desired functionality, which areoften governed by mechanical, thermal, and electrical properties. Sufficient adhe-sion is required to ensure that the die remains fixed in place when it is subjected toassembly processing or during actual device service. Thermally, it must impart theleast stress during expansion and effectively accommodate transfer of heat gener-ated in the die to the package. Until recently, hermetic die attach was accomplishedusing inorganic adhesives such as silver-filled glass or gold-silicon eutectic. Eutecticdie attach is a low-throughput manual method that cannot be easily adapted for

high-speed automation. Although silver-glass die attach has provided some process-ability improvements over the eutectic process, it still requires a lengthy and precisetemperature profile in order to remove the organic vehicle at a controlled rate. Asdie sizes become larger, the silver-glass firing process becomes difficult to control. Inhermetic packages, the high-temperature eutectic and silver-filled glass die attachraise concern about mechanical, thermal, and diffusional stresses that greatly affectdevice performance, thereby prompting the use of a polymeric substitute processedat low temperatures. Good dispensibility is certainly a process concern, as it is nec-essary to minimize nonuniform die attach, especially in large die. Nonuniform dieattach can enhance die (package) stresses (causing the die cracks), thus becoming areliability issue. Polymer adhesives are used extensively in the attachment of ICs toa variety of electronic packages. The polymer-based adhesives offer many advan-tages, such as lower stresses on IC die, ease of use in a manufacturing environment,and low cost compared to inorganic adhesives.32,33,50 For certain cases, the attachmentof some bare IC die also requires electrical contact to the back side of the die. Inthese applications, a conductive die attach adhesive is used. The electrical reliabilityof polymer adhesive is crucial to the operation of certain IC devices and is related tothe thermal treatments to which the adhesive is subjected during the fabrication ofthe electronic package. For example, wire bonding and lid sealing (brazing) are twosteps that expose the die attach adhesive to temperatures on the order of 300°C.50

Another factor that affects not only the die attach process but also device func-tioning is outgassing. Evolution of solvents in solvent-based die attach material inthe succeeding steps (e.g., curing and sealing) produces voids that are consideredrejectable if they exceed 15 percent of the die attach area. Voids are consideredstress raisers and bring some degree of inhomogeneity in the die attach, alteringsome operational properties. In some hermetic packages, outgassing of dissolvedmoisture increases moisture content in the package headspace, consequently has-tening a number of physiochemical mechanisms that eventually lead to device fail-ures. Thus, control of moisture in electronic packages is very important.34

Most conventional epoxy-based die attach adhesives currently in use for plasticpackaging have reached their limits due to trends toward larger die sizes, thinnerpackages in a wide diversity of configurations including thin small-outline package,thin quad flat package, ball grid array, power quad packages, and so on. Die attachdelamination and “popcorn” cracking are among the most troublesome problemswith these types of packages. To date, attempts to prevent problems focused onimprovements to epoxy compounds, reduced moisture absorption, dimpled lead-frames, and dry pack bags, have been pursued with some success.The die attach adhe-sive still remains the weak link and epoxy die attach is inadequate for the mostdifficult package configurations. Cyanate-ester-based die attach adhesives havefound increased popularity for many applications, including hermetic as well as plas-tic molded IC packages.The principal advantages of cyanate thermosets are high heatresistance, low outgassing of volatiles, and easy modification to satisfy various appli-cation requirements.34,35 The unmodified cyanate-ester-silver material has gainedrapid acceptance in the industry for solder seal hermetic applications because itoffers both ease of processing and high reliability. New product development includesformulations with silver loading as high as 87 percent to enhance electrical and ther-mal performance. With its high modulus, this adhesive is not suitable for most plasticpackage applications. Copolymerization of polycyanate thermoset and a thermoplas-tic elastomer yields a toughened and flexible composite suitable for use in the for-mulations with low-stress die attach adhesive for plastic packages. With increasingchip sizes, popcorn cracking is among the most common of the reliability problems.Modified cyanate-ester-based adhesives can offer a unique combination of properties


such as lower modulus, good hot strength, and less moisture outgassing at reflow tem-perature, providing a solution to the persistent popcorn problem.

Flip Chip InterconnectionFlip Chip Bumping Using ICAs. A variety of bumping compositions is cur-

rently being used for forming electrical and structural interconnects between chipand package. The bumps can be composed of a single metal, an alloy or a composi-tion of metals, groups of alloys, and polymers. During recent years, there has been anincreased interest in developing a low-cost, lead-free, fluxless flip chip technology.Printed conductive adhesive bumps can offer an attractive alternative to the otherbumping technologies in terms of cost and manufacturability. The printing processtypically involves a screen or stencil with openings through which bumps aredeposited.A screen consists of an interwoven wire mesh with emulsion patterned tomatch the bump sites. Stencils are made of metal foil. Holes for bump deposition aremade by etching, electroforming (plating), or laser drilling.51,52

During the printing process, the paste is typically dispensed some distance awayfrom the stencil apertures.A schematic of the printing process is shown in Fig. 17.12.Typically, the stencil is separated from the substrate by the snap-off distance. Thesqueegee is lowered, resulting in contact of the stencil with the substrate or wafersurface. As the squeegee moves across the stencil surface, a stable flow patterndevelops in the form of a paste roll. The consequent hydrodynamic pressure in thepaste pushes the paste into the stencil holes. On the trailing edge of the squeegee, thestencil lifts away from the substrate surface, leaving the paste on the substrate. Oncethe paste has been deposited on the surface, it must be cured at an elevated temper-ature. In some instances, depending on the paste chemistry and filler loading, it maybe possible to cure the paste using ultraviolet radiation. With the polymer flip chipprocess the wafer is bumped and cured by an ICA in the first stencil printing.


squeegee

stencil

substrate

ICA paste

FIGURE 17.12 Schematic of a printing process.

Flip Chip Bonding Using ICAs. ICA materials use a much higher loading ratethan ACAs to give electrical conduction isotropically (in all directions) throughoutthe material. In order for these materials to be used for flip chip applications, it isnecessary to apply them selectively onto those areas that are to be electrically inter-connected, and to ensure that spreading of the materials does not occur during

placement or curing, which would cause electrical shorts between the separate path-ways. ICAs are generally supplied in paste form. To precisely deposit the ICA paste,screen or stencil printing is most commonly used. However, to do this to the scaleand accuracy required for flip chip bonding would require very accurate patternalignment. To overcome this need, the transfer method may be used. For this tech-nique, raised studs or pillars are required on either the die or the substrate.The ICAis then selectively transferred to the raised area by contacting the face of the die orthe substrate to a flat, thin film of the ICA paste, which adheres to the prominentsurfaces (see Fig. 17.13). This thin film may be produced by screen printing and thetransfer thickness may be controlled by changing the printed film thickness. Thismethod confines the paste to the area of the contact surfaces, and the quantity maybe adequately controlled so as to prevent spreading between pathways when the dieis placed. Pressure during bonding is not required for this technique, which providesthe option of oven-curing the assembly.

In a high-volume environment, high-precision screen-printing techniques can beused to print the ICA paste directly onto the input-output pads of the substrate.Thiswould remove the requirement for stud pillars on the substrate track terminations,and quite possibly the need for bumping of the flip chip pads. Once such a process isin place, the ICA technique can compete with the ACA method on the basis of speedand ease of processing; however, substantial improvements in bond strength willneed to be made before the technique can be realistically considered. Unlike thecase for ACA flip chip bonding, however, a separate underfilling step would berequired with ICA flip chip bonding to improve long-term reliability of the bond. Itseems that reliability is quite good with ICA flip chip joining on rigid substrate.53 The


Bumped chip

Contact bumps to an ICA layer

Chip with ICA on bumps

Place chip on substrate andcure the ICA

Underfill and cure the underfill

bump

ICA layer

FIGURE 17.13 Flip chip bonding process using ICAs.

difficulties with the ICA flip chip joining technology are the poor processability andsmall process window in handling of the flip chip module directly after assembly.53

Surface-Mount Applications. Surface-mount technology is the main techniquefor interconnecting chip components to substrate in the form of lap shear. Figure17.14 shows a schematic of several different components interconnected by surface-mount technology. Tin-lead (Sn-Pb) solder has been exclusively used as the inter-connection material in surface-mount technology.


FIGURE 17.14 Schematic of surface-mount interconnection.

Recently, due to the extreme toxicity of lead, legislation has been proposed toreduce the use of and even ban lead from electronics. Three alternatives have beenidentified: conductive adhesives, lead-free solder, and nonconductive adhesives. Asone of the major alternatives, ICAs are proposed to be used as interconnection mate-rials for surface-mount technology. However, a good deal of research work has provedthat current commercial conductive adhesives cannot be used as drop-in replacementsfor solder. Many reliability issues are associated with current conductive adhesives.More details will be discussed in later chapters. Only ICAs are studied in Chaps. 17through 20.Therefore, in later chapters, the term ECAs refers to ICAs only.

17.3 PROPOSED APPROACHES FOR

FUNDAMENTAL UNDERSTANDING OF

CONDUCTIVE ADHESIVE TECHNOLOGY AND

DEVELOPING CONDUCTIVE ADHESIVES FOR

SOLDER REPLACEMENT

ECAs have been with us for some time and have been used in die attach and otherlow-end applications. Recently ECAs have been identified as a potential alternativeto lead-containing solders. However, current commercial ECAs cannot be used asdrop-in replacements for tin-lead (Sn-Pb) solders, mainly due to their lower electri-cal conductivity, unstable contact resistance, and poor impact strength. Before newconductive adhesives can be developed, these reliability issues of current conductiveadhesives must be fully understood.

Both organic lubricants of silver flake (conductive fillers for ECAs) surface andpolymer matrices might affect the bulk electrical conductivity of an ECA. Toimprove the bulk electrical conductivity of ECA materials, the chemical nature andbehavior of the organic lubricants of silver flakes will be investigated. Also the con-ductivity mechanism of ECAs will be studied by elucidating the roles of silver flakelubricants and polymer matrices on electrical conductivity. Based on this study, sev-eral approaches to improving the electrical conductivity of ECAs will be discussed.

Two possible mechanisms (simple oxidation and corrosion) have been proposedin the literature as cause of the unstable contact resistance phenomenon of ECAs.Systematic experiments are designed to differentiate these two mechanisms and elu-cidate which is the dominant one. Based on the findings from this investigation, sev-eral approaches (different additives) are used to stabilize contact resistance.

In order to improve the impact strength of ECAs, rubber-modified epoxy resinsare used to formulate ECAs. Also, some epoxide-terminated polyurethane resinsare synthesized and used in ECA formulations. Potential resin formulations withhigh impact strength are identified based on the results from this study.

Based on this mechanistic study on bulk conductivity, unstable contact resistance,and impact strength, solder replacement conductive adhesives can be developed byselecting proper epoxy resin formulations to increase impact strength, and also byintroducing additives to improve the conductivity and stabilize the contact resist-ance of ECA materials.

17.4 RESEARCH OBJECTIVES/GOALS

The objectives of Chaps. 18 through 20 are to conduct a fundamental study on reli-ability issues of current conductive adhesive technology and to develop conductiveadhesives that have satisfactory electrical conductivity, stable contact resistance, anddesirable impact strength and that can potentially be used for solder replacement.The goals of this research are manifold and are highlighted in the following list, fol-lowed by detailed research objectives. The research goals are as follows:

1. To conduct a fundamental study on the chemical nature and behavior of theorganic lubricants of silver flakes

2. To elucidate the dominant conductivity mechanism of ECAs by investigating theroles of silver flake lubricants and cure shrinkage of the polymer matrices of ECAs,and to identify effective approaches to improving the conductivity of ECAs

3. To identify the main mechanisms underlying the unstable contact resistance ofECAs on non-noble metal surfaces and to explore approaches to stabilizing con-tact resistance

4. To identify resins that can provide ECAs with improved impact strength5. To develop conductive adhesives that show satisfactory conductivity, stable con-

tact resistance, and desirable impact strength

17.4.1 FUNDAMENTAL STUDY OF THE CHEMICAL NATURE AND

BEHAVIOR OF ORGANIC LUBRICANTS ON SILVER FLAKES

There is a layer of organic lubricants on the commercial silver flakes. This organiclubricant layer can affect the electrical conductivity of ECAs. Therefore, in order toimprove the conductivity of ECAs, it is essential to elucidate the chemical natureand behaviors of the lubricants during the cure of the ECAs.Tasks of this part of theresearch work are as follows:

1. Characterizing the chemical nature of the lubricants on commercial silver flakesusing different approaches such as thermal analysis and Fourier transforminfrared spectroscopy


2. Lubricating silver flakes and then determining the chemical interactions betweenthe silver flake surface and the lubricants

3. Studying the effects of organic lubricants of silver flakes on electrical conductiv-ity of ECAs

4. Investigating the effects of additives on lubricant removal and electrical conduc-tivity of ECAs

17.4.2 INVESTIGATION OF THE CONDUCTIVITY MECHANISM

OF CONDUCTIVE ADHESIVES

ECA pastes are nonconductive before curing. Lubricant removal during thermalcuring of ECAs and intimate contact between silver particles caused by cure shrink-age may be responsible for the establishment of conductivity in ECAs. The roles oflubricant removal and cure shrinkage have been, and, from this study, the main con-ductivity mechanism of ECAs has been elucidated. Tasks of research work on theconductivity mechanism are as follows:

1. Studying and comparing the conductivity establishment of silver flakes andECAs filled with these silver flakes during heating. The role of the polymericresin is determined from the comparison.

2. Investigating the correlation between lubricant removal and the establishment ofconductivity in ECAs by studying conductivity development in ECAs filled withblank Ag particles (without organic lubricants).

3. Investigating the establishment of conductivity in ECAs that are cured at roomtemperature. At room temperature, the organic lubricants cannot be thermallyremoved, and thus the ECAs achieve conductivity through the closer contactbetween silver particles.

4. Studying the correlation between conductivity establishment and changes inother properties, including cure shrinkage of ECAs during both dynamic andisothermal cure of the ECAs.

5. Investigating the effects of shrinkage of electrical conductivity of ECAs by usingresin formulations that have different cure shrinkages.

17.4.3 IDENTIFICATION OF THE MAIN MECHANISMS

UNDERLYING THE UNSTABLE CONTACT RESISTANCE OF ECAs

ON NON-NOBLE METALS AND APPROACHES TO STABILIZATION

OF CONTACT RESISTANCE

Silver flake–filled ECAs generally show dramatically increased contact resistanceon non-noble metal.This has been the bottleneck of conductive adhesive technologyfor solder replacement. Two possible mechanisms are proposed in the literature, butno prior work has been done to identify which one is dominant and to stabilize con-tact resistance. Tasks of research work in this area include:

1. Designing a series of experiments to differentiate the two mechanisms (simpleoxidation and corrosion) and elucidate which is the dominant mechanism.

2. Observing metal oxide formation at the interface between ECAs and non-noblemetals using transmission electron microscopy (TEM).


3. Employing different approaches to stabilizing contact resistance and identifyingeffective additives to stabilize contact resistance. The approaches include usingpure ingredients (epoxy resins, hardeners, catalysts, etc.), minimizing the mois-ture absorption of the formulations, and adding proper additives (oxygen scav-engers).

17.4.4 DEVELOPMENT OF CONDUCTIVE ADHESIVES WITH

SATISFACTORY CONDUCTIVITY, STABLE CONTACT RESISTANCE,

AND DESIRABLE IMPACT STRENGTH

A solder replacement conductive adhesive should have high electrical conductivity,stable contact resistance, and good impact strength. Based on the results from previ-ous sections, solder replacement ECAs will be formulated. Tasks of this researchwork include:

1. Formulating ECAs with high impact strengths. Different resins including rubber-modified epoxies will be employed in ECA formulations. New resins such asepoxide-terminated polyurethane resins will be synthesized and used in ECAformulations. Potential resins will be identified from this study.

2. Formulating ECAs with high impact strength and desirable conductivity.Approaches identified in Sec. 17.4.2 will be used to improve conductivity.

3. Formulating ECAs with desirable conductivity, high impact strength, and stableresistance. ECAs are formulated using the resins selected from (1) and additivesthat are identified in Secs. 17.4.2 and 17.4.3. The contact resistance and impactstrength of these formulated ECAs will be evaluated.


Study chemical nature andbehaviors of silver flakelubricants

Investigate roles oflubricant removal andcure shrinkage onconductivity establishmentof ECAs

Formulate ECAs withimproved conductivity

Investigate mechanismsunderlying unstable contactresistance of ECAs

Use different approaches tostabilize contact resistance

Formulate ECAs withstable contact resistance

Improve impact strengthusing different modifiedresins

Formulate ECAs withimpact strength

Develop ECAs with desirableconductivity, stable contactresistance and high impactstrength

FIGURE 17.15 Schematic outline of the research work.

17.5 OUTLINE OF RESEARCH

In Chap. 18, the electrical conductivity mechanisms of ECAs are studied by investi-gating the roles of lubricant removal of the silver flake and cure shrinkage of poly-meric matrix on the establishment of conductivity in ECAs during thermal curing. Inaddition, effects of cure shrinkage on conductivity are studied. In Chap. 19, system-atic experiments are designed to differentiate two possible mechanisms underlyingthe unstable contact resistance of conductive adhesives on non-noble metals. InChap. 20, different approaches are explored to stabilize the contact resistance andeffective additives are identified. The effects of these additives on other propertiesof ECAs are also studied. A schematic layout of the overall research work is out-lined in Fig. 17.15.

ACKNOWLEDGMENT

C. P. Wong would like to specially acknowledge his former Ph.D. student, Dr. D. Lu,who has published many excellent articles and an outstanding thesis on ECAs thatprovide the basis of this chapter. His brilliant, hard work and persistence in pursuingthe fundamental understanding of ECAs have made significant contributions to thisimportant field.

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44. Pramanik, P. K., D. Khastgir, S. K. De, and T. N. Saha, Journal of Materials Science,25:3848–3853, 1990.

45. Yokoyama, A., T. Katsumata, A. Fujii, and T. Yoneyama, “New Copper Paste for CTFApplications,” IMC 1992 Proceedings, pp. 376–381, 1992.

46. S. K. Kang, R. Rai, and S. Purushothaman,“Development of High Conductivity Lead (Pb)-Free Conducting Adhesives,” Proceedings of the 47th Electronic Components and Technol-ogy Conference, pp. 565–570, 1996.

47. Kang, S. K., R. Rai, and S. Purushothaman,“Development of High Conductivity Lead (Pb)-Free Conducting Adhesives,” IEEE Transactions on Components, Packaging, and Manu-facturing Technology, Part A, 21(1):18–22, March 1998.

48. Gallagher, C., G. Matijasevic, and J. F. Maguire,“Transient Liquid Phase Sintering Conduc-tive Adhesives as Solder Replacements,” Proceedings of the 47th Electronic Componentsand Technology Conference, pp. 554–560, 1997.

49. Gallagher, C., G. Matijasevic, and A. Capote,“Transient Liquid Phase Sintering ConductiveAdhesives,” U.S. Patent 5,853,622, 1998.

50. Krishnamurthy,V., K. Paik, and D. Lester,“Characterization of Polymer Die-Attach Adhe-sives on Au and Al Surfaces,” ISHM’92 Proceedings, pp. 719–724, 1992.


51. Lin, J. K., J. Drye,W. Lytle,T. Scharr, R. Subrahmanyan, and R. Sharma,“Conductive Adhe-sive Bump Interconnects,” Proceedings of the 1996 Electronic Components and TechnologyConference, pp. 1059–1067, 1996.

52. Rosner, B., J. Liu, and Z. Lai, “Flip-Chip Bonding Using Isotropically Conductive Adhe-sives,” Proceedings of the 1996 Electronic Components and Technology Conference, pp.578–581, 1996.

53. Liu, J., Z. Lai, H. Kristiansen, and C. Khoo, “Overview of Conductive Adhesive JoiningTechnology in Electronics Packaging Applications,” Proceedings of the 3rd InternationalConference on Adhesive Joining and Coating Technology in Electronics Manufacturing, pp.1–17, Binghamton, NY, 1998.


CHAPTER 18CONDUCTIVITY

ESTABLISHMENT OFCONDUCTIVE ADHESIVES

18.1 INTRODUCTION

Electrically conductive adhesives (ECAs) are composed of an insulating polymericmatrix and conductive fillers, generally silver flakes. The properties of compositesystems are understood in terms of percolation phenomena: when a sufficientamount of conductive filler is loaded into the insulating matrix, the compositetransforms from an insulator to a conductor as a result of continuous linkages offiller particles.As the volume fraction of the conductive filler is increased, the prob-ability of continuity increases until the critical volume fraction is reached. Beyondthis the electrical conductivity is high and only increases slightly with increasingvolume fraction.1

Percolation theory predicts that insulators containing 20 vol% or higher loadingsof dispersed silver particles should be electrically conductive. Based on this theory,25 to 30 vol% of silver flake is used in almost all isotropic conductive adhesives(ICAs) in order to achieve high conductivity. However, one interesting phenomenonof ECAs is that ECA pastes generally have very high bulk resistance before curingbut the resistance decreases dramatically after the polymeric matrix is cured andsolidifies. Before curing, all the silver particles should contact each other and formcontinuous electrical paths, based on the prediction of percolation theory. Not muchprior work has been conducted to elucidate what really happens during the cure andsolidifying of the ECAs.

There is a thin layer of organic lubricant layer, which is electrically insulating, onthe silver flakes. Lovinger2 believed that (1) the initial high bulk resistance of ECApastes is due to the insulating lubricant coating on the silver flakes; (2) the onset ofelectrical conduction during cure is the result of removal of this coating; and (3)shrinkage of the epoxy matrix does not play a significant role either in the develop-ment or in the final value of conductivity of the ECAs. In Lovinger’s study, a siliconeoil was used to investigate the effects of shrinkage on conductivity development ofan ECA. Because silicone oil does not provide any compressive stress (due to itsnoncompressible nature) when cooled from a high temperature to a low tempera-ture, the role of shrinkage in conductivity development was not really investigated inLovinger’s study. During the curing of an ECA, many possible processes may causeconductivity establishment. Conductivity initiation may be related to the silver flakelubricant layer, or to cure shrinkage of the polymeric matrix, or to both. The maingoals of this study were to examine the roles of the silver flake lubricant layer andcure shrinkage of the adhesive matrix in the conductivity establishment of an ECAand to identify possible approaches to improving the electrical conductivity ofECAs.

18.1


18.2 EXPERIMENTS

18.2.1 MATERIALS

Two silver flakes with an organic lubricant layer on their surfaces and a silver powderwithout a lubricant were purchased from Degussa Corporation. A commercial ECA,ECA-A (a Ag flake–filled epoxy adhesive), was used in the study of conductivityestablishment during cure. A bisphenol F–type epoxy resin, RSL1738, was purchasedfrom Shell Chemical Company. 1-cyanoethyl-2-ethyl-4-methylimidazole (2E4MZCN)from Shikoku Chemical Company was employed as the hardener. Methanol, tetrahy-drofuran (THF), and diethylene glycol butyl ether were purchased from AldrichChemical Company.A trifunctional epoxy resin, MY500, was supplied by Ciba-Geigy.A hardener, 1,2-cyclodianhydride, and diethlyene glycol diethyl ether were also pur-chased from Aldrich Chemical Company.All the chemicals were used as received.

18.2.2 TRANSMISSION ELECTRON MICROSCOPY (TEM) STUDY

OF ECAs

The interparticle contact between silver particles of an ECA was observed using atransmission electron microscope from JEOL (model 4000EX). The cured ECAsample was embedded into an epoxy resin. After being polished, the sample was cutinto pieces 50 to 100 nm thick. These pieces were fixed on copper grids and thenwere studied by TEM.

18.2.3 CONDUCTIVITY ESTABLISHMENT DURING CURE

The test device, depicted in Fig. 18.1, consisted of a glass slide onto which two cop-per strips were bonded with an epoxy adhesive.Two strips of tape were applied ontothe slide with 0.1 in of distance between them. The conductive adhesive pastes andAg flakes were spread on the gap by a doctor blade, and then the tape strips wereremoved.The specimen was then placed on a hot plate.The resistance change of thesamples during heating was measured from the two copper strips by a Keithley 2000multimeter. The temperature of the glass slide surface was monitored by a type Kthermocouple thermometer (model 650, Omega Engineering, Inc.).

18.2.4 MEASUREMENTS OF CURE SHRINKAGE

18.2.4.1 Cure Shrinkage of Resins. Cure shrinkage of a resin was calculatedfrom the densities of the resin before and after cure. The density of the resin before

18.2 CHAPTER EIGHTEEN

glass slide thermocouple

copper strips

sample

FIGURE 18.1 Test device for conductivity establishmentduring heating.

CONDUCTIVITY ESTABLISHMENT OF CONDUCTIVE ADHESIVES 18.3

Probe

Cover glass

ECA paste

Sample platform

FIGURE 18.2 Measurement setup of dimensionchange of an ECA during thermal cure.

cure was measured using a 10-ml gravity bottle. The density of the resin after curewas measured in a 50-ml burette. Diethlyene glycol diethyl ether was employed asthe medium for measuring the volume of the cured resins. Shrinkage of a resin wascalculated based on the following equation:

Cure shrinkage (%) = × 100

where d1 = density of a resin before cureds = density of a resin after cure

18.2.4.2 Cure Shrinkage of Conductive Adhesive Pastes. The preceding methodcould not be used to measure the shrinkage of conductive adhesive pastes, becausethey were too viscous. Instead, dimension changes of conductive adhesive pastesduring cure were investigated with a thermomechanical analyzer (TMA) from TAInstruments (model 2923). Cure shrinkage of the ECA pastes could be observedfrom the dimension change recorded by the TMA. Measurement setup is shown inFig. 18.2. A small amount of an ECA paste was held by two pieces of microscopecover glass. The static force applied on the probe in the course of measurement wasminimized to 0.01 N to prevent the paste from being squeezed out. Dimensionchanges of the ECA sample with temperature or time were recorded.

18.2.5 CONDUCTIVITY DEVELOPMENT OF Ag PARTICLES AND

ECA PASTES WITH EXTERNAL PRESSURES

The test device, shown in Fig. 18.3, con-sisted of an insulating tube with a 3⁄8-ininner diameter and two Al bars with 3⁄8 inin diameter. Ag powders and ECApastes were placed in the tube, afterwhich pressure was applied through thetwo Al bars with a RIMAC press (pur-chased from Rinck-Mcilwaine, Inc.).The resistance change was measuredfrom the Al bars with a Keithley multi-meter.

1/d1 − 1/ds

1/d1

Al bar tube Ag powders orICA pastes

FIGURE 18.3 Test device for conductivitychange with external pressures.

18.2.6 CONDUCTIVITY ESTABLISHMENT OF A CONDUCTIVE

ADHESIVE AND LUBRICANT BEHAVIOR OF THE Ag FLAKE

A conductive adhesive was formulated with an epoxy resin (RSL1738), a hardener(2E4MZCN), and Ag flake.The weight ratio of the epoxy to the hardener was 94 to6, and the loading of the Ag flake was 80 wt%. This adhesive was cured at 25°C for1 week. Bulk resistance of the cured sample was measured using a Keithley multi-meter with a four-point probe. Bulk resistivity was calculated based on bulk resist-ance. The measurement setup and calculation method can be found in previoussections.

In order to simulate the Ag flake lubricant behavior in the conductive adhesiveduring cure, the same Ag flake was mixed with RSL1738 and 6 wt% 2E4MZCNmethanol solution, respectively. These samples were also kept at 25°C for 1 weekand then washed with THF three times to remove the epoxy and hardener. Afterdrying under vacuum at room temperature, the treated Ag flakes were studied by adifferential scanning calorimeter (DSC) from TA Instruments (model 2923).Exothermic peak areas (∆H, J/g) of the DSC curves were used to estimate theamounts of the lubricants.

18.2.7 MEASUREMENTS OF MODULUS CHANGE DURING CURE

Modulus change of a conductive adhesive during cure was studied with a rheometerfrom TA Instruments (model AR1000-N). Test fixtures used were high-temperaturedisposable plates (4 cm in diameter). Modulus was measured under oscillation modeand storage modulus change with temperature or time was reported.

18.2.8 CURE STUDY OF CONDUCTIVE ADHESIVES

Heat generated during cure of conductive adhesives was studied with a DSC fromTA Instruments (model 2920). An adhesive sample of about 10 mg was placed in ahermetic aluminum DSC pan. In dynamic cure study, the sample was heated in theDSC cell from 25 to 250°C at a heating rate of 5°C/min. In isothermal cure, the sam-ple was placed promptly into the DSC cell, which had been preheated to a set tem-perature. Then the DSC was started to collect data.

18.2.9 MEASUREMENTS OF CROSS-LINKING DENSITY

Cured epoxy resin samples were cut into proper dimensions—approximately 3 cmlong, 1 cm wide, and 0.1 cm thick. Modulus changes of the samples with temperaturewere studied with a dynamic mechanical analyzer (DMA) from TA Instruments(model 2980). A single cantilever clamp was used in the measurements.

The cross-linking density ρ(E′) of a sample was calculated from DMA results byusing the kinetic theory of rubber elasticity as follows:3

ρ(E′) = E′/3φ RT

where E′ is storage elastic modulus of cured resin at peak temperature of tan δ +10°C, φ is a front factor (assumed as φ = 1), R is the gas constant, and T is the absolutetemperature.


18.3 RESULTS AND DISCUSSION

18.3.1 OBSERVATION OF INTERPARTICLE CONTACT

BETWEEN SILVER FLAKES

It is generally believed that all the particles should contact each other when the par-ticle volume fraction in a composite is higher than a critical value (percolationthreshold). In all of the commercial silver flake–filled conductive adhesives, the vol-ume fraction of the silver flakes is 25 to 30 percent, which is equal to or higher thanthe threshold value, to ensure high electrical conductivity.Therefore, the silver flakesshould physically contact each other after the conductive adhesives are cured. Con-tact between silver particles of a commercial ECA was studied using TEM, and theresult is shown in Fig. 18.4. From this figure, it can be observed that the silver parti-cles did contact each other.


Silver

Silver

Silver

SilverEpoxy resin

FIGURE 18.4 TEM image of a silver flake–filled ECA.

18.3.2 CONDUCTIVITY ESTABLISHMENT OF CONDUCTIVE

ADHESIVES DURING CURE

The resistance changes with temperatures of Ag flake and an ECA (ECA-A), whichwas filled with this Ag flake, are shown in Fig. 18.5(a) and (b). Both samples had veryhigh resistance before heating. However, their resistance decreased dramaticallyabove certain temperatures (Tcond). As can be seen in the figure, the Tcond of the Agflake (230°C) is much higher than that of the adhesive (130°C). These results indi-cate that the epoxy resin lowered the Tcond. Tcond of the Ag flake is consistent with theonset of lubricant decomposition of the Ag flake lubricant from our previous DSCstudy, which is shown in Fig. 18.6.Also, it was observed that the Ag particles agglom-erated together after heating.This may be caused by the decomposition of the lubri-cant on the Ag flake at high temperature. It can be seen that, without any externalpressure, Ag flakes could become conductive after the lubricants decomposed at atemperature higher than 230°C.

The Tcond of the ECA (about 130°C) is much lower than the decomposition tem-perature (approximately 230°C) of the Ag flake lubricant. Therefore, at the Tcond ofthis ECA, the lubricant certainly did not decompose. The lubricant either (1) dis-

solved and/or reacted with the epoxy resin or (2) remained on the Ag flake surfaceafter cure.

During the curing and solidifying of the ECA, the epoxy resin shrinks.Therefore,the Ag flakes in the adhesive experience a compressive stress caused by resin cureshrinkage. The dimension changes with heating of this ECA and the sample holder(microscope cover glass) were studied using a TMA. The results are shown in Fig.


1.00E+00

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5 0 7 0 9 0 110 130 150 170 190 210 230 2 50 270 290

Temperature (°C)

Res

ista

nce

(Ω)

FIGURE 18.5 Resistance change during heating of Ag flake (a) and an ECA (b).

(b)

(a)

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nce

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18.7. This figure shows that the cover glass did not undergo obvious dimensionchange, but the ECA showed significant dimension decrease (cure shrinkage) at thesame temperature range as the Tcond of the ECA. This study indicates that resin cureshrinkage may play a very important role during conductivity establishment in aconductive adhesive.


FIGURE 18.6 DSC curve of Ag flake in air.

FIGURE 18.7 Dimension changes of an ICA measured using a TMA.

18.3.3 STUDY OF THE RELATIONSHIP BETWEEN SILVER FLAKE

LUBRICANT LAYER AND CONDUCTIVITY IN ECAs

18.3.3.1 Comparison of Conductivity in ECAs Filled with Blank Ag Powders andLubricated Ag Flakes. Two experiments were designed to investigate the role ofthe lubricant layer on conductivity establishment in a conductive adhesive. In the firstexperiment, the conductivity establishment of an ECA filled with a blank silver pow-der was investigated. There was no organic lubricant on the silver powder surfaces.The filler concentration of this ECA was 75 wt%, which was higher than the percola-tion threshold. The conductivity establishment during heating of an ECA filled withthe blank silver powder is shown in Fig. 18.8. It can be observed that initial resistanceof the ECA was still high. Also, the resistance dropped significantly below a certaintemperature. The fact that this ECA showed similar behavior to the ECA filled withlubricated silver flakes suggested that the organic lubricant layer on the silver flakesurface was not strongly related to the conductivity initiation of the ECA.

In the second experiment, silver flake without organic lubricant was prepared bywashing silver flake with acetic acid (HAc). The amount of organic lubricant of thesilver flake was determined by the peak area of a DSC curve.The DSC curves of thesilver flake before and after washing with HAc are shown in Fig. 18.9.As can be seenfrom the figure, the washed silver flake showed a much smaller exothermic peakthan the original silver flake, indicating that most of the organic lubricant waswashed away on the silver flake.Therefore, the washed Ag flake could be considereda blank silver flake.The washed silver flake and the original silver flake were used toformulate ECAs using the same resin. The conductivity establishment of the ECAsis shown in Fig. 18.10. Clearly, both ECAs showed similar conductivity developmentbehavior during curing. Again, the result suggested that the lubricant layer was notstrongly related to conductivity initiation of the ECA.


1

10

100

1000

0 20 40 60 80 100 120 140 160 180

Temperature (°C)

Res

ista

nce

(Ω)

FIGURE 18.8 Conductivity establishment of an ECA filled with blank Ag powder.


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)

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Ag flakeAg flake washed with HAc

Universal V2.5H TA InstrumentsExo Up

FIGURE 18.9 DSC dynamic scan of Ag flake before and after washing with HAc.

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ECA filled with Ag flake

ECA filled with washed Ag flake

FIGURE 18.10 Conductivity establishment of two ECAs filled with Ag flake and Ag flakewashed with Hac.

18.3.3.2 Conductivity in a Conductive Adhesive and the Lubricant Behavior of AgFlakes. A Ag flake–filled conductive adhesive was cured at 25°C in order to ensurethat the lubricant was not thermally removed after cure.The resistivity of the adhesivebefore cure was high and was beyond the measurement limits of the multimeter.Aftercure, the resistivity changed to about 4 × 10−3 Ω/cm. Because the ECA was cured atroom temperature, the lubricant on the Ag flakes was not thermally removed.

The shrinkage of the resin part of this adhesive was calculated from the densitiesof the uncured and cured resin. The densities of the resin before cure and after curewere measured by a gravity bottle. Diethylene glycol butyl ether was used as themedium in the density measurement of the cured resin. Assuming no significantweight change during the cure of the resin, resin cure shrinkage can be calculatedusing the following equation:

Shrinkage = × 100

where d1 is the density of the uncured resin and ds is the density of the cured resin.Based on this equation, the resin cure shrinkage of this adhesive is 3.42 percent.

In order to find out if the Ag flake lubricant remained on the Ag flake surfacesafter the adhesive was cured at room temperature (25°C), another parallel study wasconducted. The same Ag flake was mixed with the epoxy resin (RSL1837) and 6wt% hardener (2E4MZCN) methanol solution, respectively. Methanol was used asa solvent because it would not remove the Ag flake lubricants. The mixtures werekept at 25°C for the same period of time as the cure time of the ECA.Then, the mix-tures were washed with THF three times to remove the resin and hardener. Previousstudy showed that THF also did not wash away the lubricants. After drying, therecovered Ag flakes were studied by DSC in an air atmosphere and compared withthe same Ag flake washed three times with THF. The peak area (∆H, J/g) of theexothermic peak in the DSC curves of the Ag flakes was used to estimate theamount of the lubricants semiquantitatively. For each Ag flake, three samples werestudied. The average enthrope ∆H and standard deviation for each of the Ag flakesare given in Table 18.1.

1/d1 − 1/ds

1/d1


TABLE 18.1 DSC ∆H Values of Ag Flakes

Average StandardAg flakes ∆H (J/g) deviation

Recovered from epoxy 50.0 0.4

Recovered from 2E4MZCN 46.6 0.5methanol solution

Untreated 48.4 0.8

As can be seen from Table 18.1, ∆H values of the recovered Ag flakes are the sameas those of the original Ag flake within experimental error. The result indicates thatthe Ag flake lubricant was not removed after mixing with the epoxy or hardenerunder the cure condition of the ECA (1 week at room temperature). The results ofthis study indicate that the lubricant remained on the Ag flake surfaces after the ECAwas cured at room temperature for 1 week. Therefore, this conductive adhesiveachieved conductivity with the lubricant layer remaining on the Ag flake surfaces,meaning that the silver flake lubricant layer is not strongly related to conductivity ini-tiation of an ECA. However, lubricant removal might further improve conductivity.


18.3.4 STUDY OF THE RELATIONSHIP BETWEEN CURE

SHRINKAGE AND CONDUCTIVITY ESTABLISHMENT

Results from Sec. 18.3.3.2 indicated that the lubricant layer of the silver flake wasnot strongly related to the conductivity establishment of an ECA during thermalcuring. In the following sections, roles of cure shrinkage of the polymeric resin of anECA in conductivity development of the ECA were investigated.

18.3.4.1 Conductivity Development of Ag Powders and ICA Pastes with Exter-nal Pressures. The purpose of this study is to simulate the Ag flake behaviorscaused by resin cure shrinkage in the ICA formulations by investigating the rela-tionship between conductivity of Ag flakes and external pressures.

Two commercial Ag flakes (Ag A and B), both of which have lubricants, and a Agpowder without lubricant (Ag C), were tested.The results are shown in Fig. 18.11.TheAg particles were packed very loosely in the tube of the test device at the initial stage,when they were first placed into the tube without any external pressure. After a verysmall force was applied through the bars, the Ag particles were packed more tightlyand very low resistance values were obtained. The resistance decreased only slightlywith further increase in external pressures. These tests were done at room tempera-ture; therefore, the lubricants of the Ag flakes were not thermally removed. Also,under these low pressures, the lubricants were not mechanically removed either. Thefact that blank Ag powder (Ag C) showed similar resistance behavior to these two Agflakes suggests that lubricants do not affect electrical conductivity significantly in thiscase.Therefore, the conductivity establishment of these Ag particles was the result ofintimate contacts between the Ag particles caused by the small external pressures.The results indicate that the conductivity of the Ag flakes could be achieved just byapplying small pressures to the material. The compressive force pushes the Ag parti-cles and forces them to penetrate the organic lubricant layer to form an electricalpath. The Ag flakes will experience compressive force generated from resin cure

0

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nce

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Ag AAg CAg B

FIGURE 18.11 Resistance change of some Ag particles with external pressure.

shrinkage during the curing of an ECA. The results from this study imply that resincure shrinkage of an ECA may be related to conductivity establishment.

18.3.4.2 Property Changes of an ECA During Dynamic Cure. Propertiesincluding cure profile, modulus, shrinkage, and conductivity establishment of a Agflake–filled ECA during a dynamic thermal cure were investigated, and the resultsare shown in Figs. 18.12 through 18.15.

DSC dynamic study (Fig. 18.12) showed that the cure reaction of the ECA hap-pened mainly from 120 to 145°C. According to the figure, storage modulus of theECA sample increased greatly at temperatures from 120 to 145°C. As can be seenfrom the TMA curve in Fig. 18.13, the dimensions of the ECA sample decreased(shrank) dramatically, also at temperatures from 120 to 145°C. Therefore, at thesetemperatures, this conductive adhesive paste was cured, solidified, and shrank.

Conductivity establishment of this ECA during heating is shown in Fig. 18.15.Theresistance of the ECA was very high at low temperatures, then decreased graduallyfrom 50 to 108°C, then increased a little from 108 to 128°C. An explanation for theresistance increase lies in the opposing effects of (1) thermal expansion and (2)cross-linking causing the decreases in resistance.4 At low temperature, (1) over-comes (2), and therefore the resin expands and the distance between Ag particlesincreases slightly.The increase of distance causes slight resistance increase.Again, attemperatures between 120 and 145°C, the resistance decreased dramatically. After150°C, cure reaction nearly finished and thus the resistance leveled off above 150°C.

Based on these results, it can be seen that, at the same temperature range, whenthe ECA was cured, it shrank and compressed the Ag particles more closely and thusthe resistance of the ECA decreased.


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DSC

Universal V2.3C TA Instruments

FIGURE 18.12 Cure profile of an ECA during a dynamic thermal cure.


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FIGURE 18.13 Modulus change of an ECA during a dynamic thermal cure.

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TMA

FIGURE 18.14 Dimension change of an ECA during a dynamic thermal cure.

18.3.4.3 Properties Change of an ECA During Isothermal Cure. Propertychanges of the ECA during an isothermal cure (at 100°C) were also investigated.Figures 18.16 through 18.19 show the results.

From DSC study results in Fig. 18.16, we can see that the cure reaction of theECA mainly occurred from the 10th to 20th min at 100°C. Figure 18.17 shows thatthe modulus increased greatly between the 15th and 20th min. In the TMA study, thetemperature was raised from room temperature to 100°C at a heating rate of10°C/min and then kept at 100°C.The temperature and dimension change with timeare given in Fig. 18.18. After the period of time for raising the temperature fromroom temperature to 100°C (about 8 min) was deducted, dimension change (shrink-ing) also happened from the 10th to the 20th min. These results indicate that, at100°C, the ECA was cured, solidified, and shrank from the 10th to the 20th min.

The conductivity development of the ECA during isothermal cure is given in Fig.18.19. The resistance increased a little before the 10th min and then decreased sig-nificantly from the 10th to the 20th min. The initial resistance increase probably wasalso due to thermal expansion of the resin.The resistance drop between the 10th and20th min was due to more intimate packing between Ag particles when the ECAcured and shrank.

It can be concluded that at 100°C, between the 10 and 20th min, the resin of theECA shrank during thermal cure and compressed the Ag flakes more tightly, thuscausing resistance drop.

18.3.4.4 Relationship Between Shrinkage and Conductivity. The precedingresults indicate that resin cure shrinkage of an ECA is one of the factors that initiateelectrical conductivity of the ECA. Therefore, it is very important to study the rela-tionship between resin cure shrinkage and electrical conductivity of ECAs. The


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FIGURE 18.15 Conductivity establishment of an ECA during a dynamic thermal cure.


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Exo Up Unversal V2.3C TA Instruments

FIGURE 18.16 Cure profile of an ECA during isothermal cure (at 100°C).

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a)

FIGURE 18.17 Modulus change of an ECA during isothermal cure (at at 100°C).


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FIGURE 18.18 Dimension change (cure shrinkage) of an ECA during isothermal cure (at 100°C).

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1.00E+04

1.00E+05

0 105 15 20 25 30 35 40

Time (min)

Res

ista

nce

(Ω)

FIGURE 18.19 Conductivity establishment of an ECA during isothermal cure (at 100°C).

lubricant of the Ag flakes also influences conductivity. In order to eliminate theeffect of lubricant and elucidate the effects of shrinkage on conductivity, a blank Agpowder without lubricant was used in this study.

A trifunctional epoxy was employed to vary the cross-linking density and shrink-age of ECA formulations. Two and 10 wt% of the trifunctional epoxy, respectively,was introduced into two resin formulations. The shrinkages of a resin formulationwithout the trifunctional epoxy (ECA1) and two other resin formulations with thetrifunctional epoxy (ECA2 and ECA3, which contained 2 percent and 10 percent tri-functional epoxy, respectively) were calculated from densities of the resins beforeand after cure.The storage modulus changes with temperature of these resin formu-lations are shown in Fig. 18.20.

Three ECAs were formulated with these three resin formulations and the blankAg powder (filler loading was 70 wt%).The bulk resistance of these ECAs after curewas measured and compared. Cross-linking density, cure shrinkage, and bulk resis-tivity of these three ECAs are given in Table 18.2.As can be seen from the table, for-mulations with higher cross-linking density had higher cure shrinkage and lower


0

500

1000

1500

2000

2500

3000

Sto

rage

Mod

ulus

(M

Pa)

30 55 80 105 130 155 180

Temperature (°C)

ICA1ICA2ICA3


FIGURE 18.20 Storage modulus change with temperature of three ECAs.

TABLE 18.2 Cross-Linking Density, Cure Shrinkage, and Volume Resistivity of Three ECA Formulations

Cross-linking density Shrinkage Bulk resistivityFormulation (10−3 mol/cm3) (%) (mΩ/cm)

ICA1 4.50 2.98 3.0

ICA2 5.33 3.75 1.2

ICA3 5.85 4.33 0.58

resistivity. Because there was no lubricant on the Ag particles, the bulk resistancedifference was only due to the different cure shrinkages of these samples.

18.4 CONCLUSIONS

Ag flakes can become electrically conductive after the organic lubricant thermallydecomposes and agglomerates at a temperature higher than 230°C. However, theECA filled with this Ag flake achieves high conductivity at a temperature muchlower than the decomposition temperature of the Ag flake. Therefore, the polymermatrix of the ECA plays an important role in the conductivity establishment of theECA.

ECAs filled with blank silver particles also show high initial resistance and sig-nificant resistance decrease above certain temperatures during heating. Also, ECApastes can achieve high conductivity with a lubricant layer remaining on silver flakesurfaces.These results indicate that the organic lubricant layer is not strongly relatedto the conductivity initiation of ECAs.

Ag flakes with lubricants and Ag powder without lubricant become highly elec-trically conductive after they are packed more intimately together by small externalcompressive forces, and their resistances do not change much with further increasedcompressive forces. This result suggests that a compressive force can cause conduc-tivity achievement of an ECA.

During dynamic heating, at the same temperature range, an ECA was cured,shrank, and achieved high conductivity. Similarly, during an isothermal cure, at thesame period of time, the ECA was cured, shrank, and became highly conductive.Therefore, in both cases, the ECA becomes highly electrical conductive when theresin is cured and shrinks. The results strongly indicate that resin cure shrinkage isone of the factors that initiate the electrical conductivity of ECAs during curing.ECAs with higher shrinkage show lower resistance or better conductivity.Therefore,another approach to improving electrical conductivity of an ECA is to increase thecure shrinkage of the resin by introducing multifunctional epoxy resins.

REFERENCES

1. Ruschau, G. R., S. Yoshikawa, and R. E. Newnham, “Resistivities of Conductive Compos-ites,” Journal of Applied Physics, 73(3):953–959, 1992.

2. Lovinger,A. J., “Development of Electrical Conduction in Silver-Filled Epoxy Adhesives,”Journal of Adhesion, 10:1–15, 1979.

3. Tobolsky, A. V., in Properties and Structure of Polymers, Wiley, New York, 1960.

4. Miller, B.,“Polymerization Behavior of Silver-Filled Epoxy Resins by Resistivity Measure-ments,” Journal of Applied Polymer Science, 10:217, 1966.


CHAPTER 19MECHANISMS UNDERLYING

THE UNSTABLE CONTACTRESISTANCE OF ECAs

19.1 INTRODUCTION

Electrically conductive adhesives (ECAs) will be used as a replacement for tradi-tional tin-lead (SnPb) solders. Compared to SnPb solder, ECA technology offersnumerous advantages including environmental friendliness, fine pitch capability,better thermomechanical properties, fewer processing steps, no chlorofluorocarbonsolvents, and low processing temperature.1–4 However, ECA technology is still in itsinfancy, and concerns and limitations do exist. While current conductive adhesiveshave adequate electrical conductivity for most applications, almost all showincreased contact resistance between the adhesive and nonnoble metal finishedcomponents when they are continuously exposed to high-temperature and high-humidity conditions, particularly 85°C/85% relative humidity (RH).1,5–7 While a cir-cuit can be designed to accommodate cumulative junction resistances, any change inresistance with time will probably have a detrimental effect on overall electrical per-formance. There has been tremendous effort to investigate the contact resistance ofECAs on different metallizations.

Changes in resistance are dependent on the termination material of the compo-nents and the substrates. A greater change in resistance for components with SnPbterminations, especially in combination with SnPb finishing of the board, is generallyobserved. The metallizations may form metal oxides that are not conducting exceptfor Ag2O.These metals or their metal oxides also form hydroxides such as Cu(OH)2,Ni(OH)2, or Pt(OH)2.These metal hydroxides are only weakly bonded to the metalsand thus can easily tear off to cause debonding of a joint.8

Gaynes et al.5 evaluated contact resistance variations for several isotropic ECAson a copper substrate whose surface was plating finished with Pd alloy, Au, Sn, andNi during environmental tests such as thermal cycling, thermal aging, and tempera-ture and humidity conditioning. It was found that the Pd alloy surface provided anelectrically superior joint compared to Au, Sn, and Ni. The reason for the unstableresistance of the Au surface was not clear. One possible explanation for the increas-ing electrical resistance is oxidation of the Ni that is used to harden Au.

Jagt et al.4 discussed the electrical and mechanical behavior of conductive adhe-sives for bonding R 1206 jumpers with SnPb or AgPd terminations on bare copperand on SnPb- or Au-plated boards, both directly after bonding and after climate test-ing. The influence of the component terminations was found to be dominant. Con-ductive adhesives gave good and reliable electrical connections if used in combinationwith AgPd-terminated components and Cu or Au metallization on the printed cir-cuit board (PCB). The oxidation of the SnPb surface was confirmed by x-ray photo-electron spectroscopy (XPS) measurements. An increased oxide layer was found atthe component side after the sample was cleaved in the bondline after aging. Auger

19.1


electron spectroscopy indicated some increase of PbO oxide layer at the componentsurface. Moreover, another very possible cause of resistance increase after humidity-heat testing—i.e., Ag depletion in the adhesive within a border layer to the compo-nent of more than 50 nm thick—was mentioned.Apparently, diffusion of Ag towardthe SnPb layer took place. No clear correlation between mechanical and electricalproperties after various tests was found.

Botter7 studied the factors that influence the electrical contact resistance ofisotropic conductive adhesive (ICA) joints during climate chamber testing. Theinfluence of different types of metallizations of components and PCB, size and shapeof the components, and adhesive and curing conditions was investigated. The resultsshowed that the amount of Sn from the metallization of the component and PCB isthe main influencing factor. The deterioration process is enhanced when a Au met-allization of the PCB is used in combination with a Sn metallized component. Met-allizations of pure Cu and Pb showed no or only a little increase in electrical contactresistance. Other results showed that a small contact area is more sensitive to dete-rioration than a large contact area and that the adhesive and curing conditions usedonly influence the speed and the severity of the deterioration process, but do notalter the process itself. Based on all these results, a failure mechanism was defined inwhich electrochemical corrosion of the metallization plays an important role.

Failure mechanisms of conductive adhesives on Sn, Cu, and Au metallizationswere studied by Liu et al.9 After testing of Cu and SnPb joints under temperatureand humidity (85°C/85%RH), an increase in electrical contact resistance was foundwith increasing testing time. Oxide formation on the Cu surface was confirmed bytransmission electron microscopy (TEM), and the oxide was found to be Cu2O. Thesurprising result was that no Sn oxide layer was found on SnPb finish but PbO wasconfirmed by TEM.

Bosch et al.10 found that the initial bond strength of components with SnPb ter-minations was in general high compared to that of other termination materials, andthat the change in adhesive strength is smaller in the case of SnPb terminations. Butthere was no explanation for these results.

The electrical and mechanical behaviors of three Ag-filled epoxy ECAs werestudied by Keusseyan et al.11 Copper plates coated with Ni or AuNi were adhesive-bonded to AgPt, Cu, or Au thick-film conductors. The test structures and moduleswere subjected to elevated-temperature and -humidity aging (85°C/85%RH). Theextent to which the adhesive joints were affected during aging was a function ofadhesive choice and adherend metallizations. During aging the electrical resistanceof the adhesive bonds was affected, sometimes resulting in a nonconducting joint.The effect of surface metallization on die bond reliability was profound duringaging.The die-bonded metal plates performed well with all test adhesives when onlyAu surfaces were present. When Ni-plated metal plates were die-bonded to Authick-film conductors, the electrical resistance of the structure was no longer stableduring elevated-humidity and -temperature aging even though all bonds remainedconducting. When Ni-plated metal plates were die-bonded to Cu and AgPt thick-film conductors, the aging effects became rather evident.

Even though these investigations did not show consistent results, they did indi-cate one importance issue: contact resistance increase is mainly due to oxide forma-tion at the interface of conductive adhesives and nonnoble metals. However, thereare two possible mechanisms of oxide formation: simple oxidation and electrochem-ical corrosion of the nonnoble metals.

A brief introduction to oxidation and electrochemical corrosion, especially gal-vanic corrosion, is given here. Oxidation is a reaction between a material (metals inthis case) and oxygen. It can happen under either dry or wet conditions and gener-

19.2 CHAPTER NINETEEN

ally at high temperatures.12 However, a galvanic corrosion process will happen onlywhen the following conditions are met: (1) two metals with different electrochemi-cal potentials are present and connected, (2) an aqueous phase with electrolyteexists, and (3) one of the two metals has an electrochemical potential lower than thepotential of the reaction (H2O + 4e− + O2 = 4 OH−), which is 0.4 eV or below stan-dard conditions. When two different metals contact under wet conditions, galvaniccorrosion (electrochemical corrosion) occurs. The less noble metal (with an electro-chemical potential less than 0.4 V) acts as an anode and loses electrons. The anodemetal becomes ions (M − ne− = Mn+) and dissolves in the aqueous medium.The noblemetal (with a higher electrochemical potential) acts as a cathode and, under manyconditions, the reaction on this electrode is generally H2O + 4e− + O2 = 4 OH−. In thismechanism, oxygen is also involved in the reaction but does not directly react withthe anode metal.12 The metal ion Mn+ will combine with the OH− and form a metalhydroxide, which is usually unstable and becomes a metal oxide. Therefore, electro-chemical corrosion can only occur under wet conditions between two different met-als. Theoretically, if only one metal is involved or under dry conditions, galvaniccorrosion is insignificant.12

Clearly simple oxidation and electrochemical corrosion are two differentprocesses. In order to prevent oxide formation at the interface between an ECA andnonnoble metals, it is of critical importance to differentiate these two mechanismsand elucidate which mechanism is the dominant one. No prior work has been doneto elucidate the main mechanism and no prior work has been conclusive.

19.2 EXPERIMENTS

19.2.1 MATERIALS

A bisphenol F–type epoxy resin used in this study was supplied by Shell ChemicalCompany. Hardeners for the epoxy resin were purchased from Aldrich ChemicalCompany. All of the chemicals were used as received. Ag flakes were obtained fromDegussa Corporation. Ni flakes were obtained from Novamet Company. Metalwires including Ni, Sn, Cu, Ag, Au, and Pt wires (all about 0.25 mm in diameter and99.99 percent pure) were purchased from Aldrich Chemical Company. EutecticSnPb wire (0.25 mm diameter) was obtained from Hisco Company. The commercialECA used in this study is a Ag flake–filled epoxy adhesive (Ablebond 8175A). Allthe chemicals were used as received.

19.2.2 STUDY OF BULK RESISTANCE SHIFTS

The bulk resistance of conductive adhesives was obtained from specimens with spe-cific dimensions. The dimensions of the specimens were controlled using the follow-ing procedures: (1) applying two strips of an adhesive tape on a precleaned glassslide with a 2.54-mm distance between the two strips; (2) spreading a conductiveadhesive paste on the glass slide within the gap with a doctor blade; (3) removing theadhesive tape and curing the specimen; and (4) measuring the resistance of the spec-imen using a Keithley multimeter with a four-point probe (refer to Fig. 19.1). Threespecimens were tested for each sample. The specimens were aged under either85°C/85%RH or 85°C/dry conditions. The bulk resistance of each specimen wasrecorded periodically and the results for the specimens were reported.

MECHANISMS UNDERLYING THE UNSTABLE CONTACT RESISTANCE OF ECAs 19.3

19.2.3 STUDY OF CONTACT RESISTANCE SHIFTS

Contact resistance was measured using an in-house testing device that is shown inFig. 19.2. This device consisted of metal wire segments (about 1 cm long) separatedby approximately 1-mm gaps. The metal wires were used to simulate the metalliza-tion of PCB and the surface finish of surface-mount technology components. Con-ductive adhesive pastes were applied to the gaps to connect the metal wire segments.After cure, the total contact resistance of a specimen was measured with a Keithley2000 multimeter. Three specimens were tested for each sample. The specimens wereaged either under 85°C/85%RH (in a temperature and humidity chamber fromLunaire Environmental, model CEO932W-4) or 85°C/dry (in a Blue M oven) condi-tions. The contact resistance of each specimen was measured periodically duringaging. The results for these specimens were reported.


ECA

probe

FIGURE 19.1 Setup for measurement of bulk resistance of ECAs.

ECA dot Metal wire segment

FIGURE 19.2 In-house contact resistance test device.

19.2.4 STUDY OF OXIDE FORMATION

Metal oxide formation at the interface between an ECA and SnPb after85°C/85%RH aging was observed using TEM. The TEM samples were preparedbased on the following procedure: (1) preparing and curing an ECA joint sample asshown in Fig. 19.3; (2) aging the sample at 85°C/85%RH for 1000 h; (3) embeddingthe sample in epoxy and then curing the epoxy; (4) cutting cross-sectional specimensabout 50 to 80 nm thick from the embedded sample using a microtomy technique;and (5) putting the specimens on a copper grid. Images of the interface between theECA and SnPb were studied. To investigate the oxide layer growth and the chemi-cal composition of the oxide layer, TEM with energy dispersive x-ray (EDX) withultrathin window technique was used.


50–80 nm

ECA

Metal

cut

FIGURE 19.3 Schematic of a sample for TEM study.


19.3.1 CONTACT RESISTANCE SHIFT PHENOMENON

19.3.1.1 Study of Bulk Resistance Shifts. The total contact resistance of anECA joint consists of the bulk resistance of metals Rmetal, the bulk resistance of theECA material RECA, and the interfacial resistance Rinterface between the ECA and themetal (refer to Fig. 19.4). The bulk resistance of the metals does not change duringaging.Therefore, changes of bulk resistance of an ECA and the interfacial resistancewill cause total contact resistance shifts. The change of bulk resistance of an ECAmaterial during aging was studied first.

Rint

Rbulk

Rint

Component metallization

Substrate metallization

Silver flake

Polymeric resin

FIGURE 19.4 Contact resistance of an ECA joint.

Bulk resistance shifts of five commercial conductive adhesives (ECA-1, ECA-2,ECA-3, ECA-4, and ECA-5) during 85°C/85%RH aging were studied, and resultsare shown in Fig. 19.5. All of the five ECAs are silver flake–filled and epoxy-basedconductive adhesives and are from four different manufacturers. As can be seenfrom this figure, the bulk resistance of all the ECAs decreased slightly in the earlystage of the aging and remained stable thereafter. The initial decrease in bulk resist-ance may be due to further cure of the ECAs. The bulk resistance of silver-filledECAs did not change during aging because silver flakes do not tend to oxidize orcorrode and silver oxide is still highly electrically conductive even after silver is oxi-dized. It can be concluded from this study that a conductive adhesive showed stablebulk resistance during an elevated-temperature and -humidity aging as long as theECA was filled with silver flakes. In other words, silver flake–filled conductive adhe-sives have stable bulk resistance during aging.

19.3.1.2 Contact Resistance Shifts. The five previously mentioned commercialconductive adhesives were also used in this study. Contact resistance between theseECAs with different metal wires was studied using the contact resistance test devicedescribed in the previous section. Shifts of the contact resistance during85°C/85%RH aging were recorded. After 500 h of aging, if the increase in the con-tact resistance was larger than 20 percent, then the contact resistance was defined asunstable, but if the increase was less than 20 percent, the contact resistance was con-sidered stable.6 It was found that all five ECAs exhibited the same trend in contactresistance change during aging. As an example, results for one of those ECAs aregiven in Table 19.1. As can be seen from the table, this ECA showed stable contactresistance on the noble metals Ag, Pt, and Au but showed significant contact resist-ance shifts on the nonnoble metals Ni, Sn, and SnPb. The results are consistent withthose from other researchers, which indicate that our in-house test vehicle is validand reliable.5–7


0.00E+00

1.00E-04

2.00E-04

3.00E-04

4.00E-04

5.00E-04

6.00E-04

7.00E-04

8.00E-04

0 200 400 600 800 1000

Aging Time (h)

Res

istiv

ity (

Ω-c

m)

ECA-1ECA-2ECA-3ECA-4ECA-5

FIGURE 19.5 Bulk resistance shifts of ECAs during aging.

The fact that the bulk resistance of all these ECAs remained stable and the con-tact resistance of these ECAs on nonnoble metals increased during aging indicatedthat the contact resistance increase was caused by the increase of the interfacialresistance Rinterface between the ECA and the metal. Therefore, the main task in thischapter is to elucidate the reasons for the increase in Rinterface during aging.

19.3.2 INVESTIGATION OF MECHANISMS UNDERLYING

THE UNSTABLE CONTACT RESISTANCE PHENOMENON

19.3.2.1 Base Resin Formulation. In order to focus on mechanism study, theECA formulations used in this study were kept as simple as possible. A base resinwas composed of just a bisphenol F–type epoxy, a hardener, and a catalyst. Othercomponents such as adhesion promoters, conductivity enhancers, and diluents werenot included in the formulation.

19.3.2.2 Contact Resistance of a Ni-Filled ECA with Different Metals. AnECA was formulated using the preceding base resin formulation. The filler was a Niflake and filler loading was 70 wt%. In order to test contact resistance of this ECAwith different metals, different metal wires (Ni, Sn, Cu, and Ag) were selected andemployed in the test vehicles. The contact resistance changes of these samples dur-ing 85°C/85%RH aging are shown in Fig. 19.6. It was found that all resistanceincreased with aging time but the contact resistance with Ag wire increased muchmore than that with Ni, Sn, and Cu, suggesting that the contact resistance change wasnot due to oxidation of the nonnoble metals. If simple oxidation were the dominantmechanism, the contact resistance with Ni, Sn, and Cu should have increased fasterthan that with Ag wire.

19.3.2.3 Contact Resistance Shifts During Different Aging Conditions. The Niflake–filled ECA in the preceding section was also used in this study. In order to dif-ferentiate between simple oxidation and galvanic corrosion mechanisms, two differ-ent metal wires (Ni and Ag) were selected. Therefore, two different samples weretested here: Ni flake–filled ECA with Ni wire (called NiNi combination in later sec-tions) and Ni flake–filled ECA with Ag wire (called NiAg combination). Six speci-mens were prepared for each sample. Three of the specimens were aged under85°C/85%RH and the other three under 85°C/dry. The contact resistance of eachspecimen was collected periodically during aging. Results were reported for each


TABLE 19.1 Shifts in Contact Resistance in an ECA withDifferent Metals

Metal wires Contact resistance change during aging

Pt Stable

Au Stable

Ag Stable

Sn Unstable

SnPb Unstable

Ni Unstable


1

10

100

1,000

10,000

100,000

0 100 200 300 400 500 600

Time (h)

Res

ista

nce

(Ω)

NiAg

NiSnNiNi

NiCu

FIGURE 19.6 Shifts of contact resistance of nickel-filled ECAs on different metal wires.

1

10

100

1,000

10,000

0 100 200 300 400 500 600

Time (h)

Res

ista

nce

(Ω)

NiNi

NiAg

FIGURE 19.7 Contact resistance shifts of a Ni flake–filled ICA with Ni and Ag wires under85°C/dry aging conditions.

sample. The resistance changes of the samples under 85°C/dry and 85°C/85%RHaging are shown in Figs. 19.7 and 19.8, respectively.

As can be seen from Fig. 19.7, under 85°C/dry aging conditions, both NiNi andNiAg combinations showed no significant contact resistance change. From Fig. 19.8it was found that, under 85°C/85%RH aging, contact resistance of the NiNi combi-nation showed a slight increase but contact resistance of the NiAg combinationincreased dramatically. According to conditions for oxidation and galvanic corro-sion, oxidation can happen under either wet or dry conditions but galvanic corrosionhappens only under wet conditions. Under 85°C/dry aging conditions, no galvaniccorrosion happens, and therefore simple oxidation of the metals is the only possiblemechanism. Insignificant contact resistance shifts for both samples under 85°C/dryaging conditions indicated that simple oxidation is not dominant at 85°C. Under85°C/85%RH aging conditions, both oxidation and galvanic corrosion can happen.If simple oxidation dominated, then the NiNi combination should have had a largerresistance shift than NiAg combination because Ni is very easy to oxidize but Ag isnot. The fact that the NiAg combination showed very significant resistance shiftsindicates that simple oxidation of the metals is not a dominant mechanism for resis-tance shifts under 85°C/85%RH aging either.

19.3.2.4 Bulk Resistance Shifts Under Different Aging Conditions. In order tofurther prove that simple oxidation is not the dominant mechanism for unstablecontact resistance, another experiment was conducted. Three conductive adhesives(ECA-1, ECA-2, and ECA-3) were formulated with the base resin formulation men-tioned in the previous section.These three ECAs are exactly the same except for thecomposition of their fillers. ECA-1, ECA-2, and ECA-3 were filled with Ni flake,Agflake, and a mixture of Ni flake and Ag flake (weight ratio of Ni flake to Ag flake 95


1

10

100

1,000

10,000

0 100 200 300 400 500 600

Time (h)

Res

ista

nce

(Ω)

NiNi

NiAg

FIGURE 19.8 Contact resistance shifts of a Ni flake–filled ICA with Ni and Ag wires under85°C/85%RH aging conditions.

to 5), respectively. Again, six specimens were tested for each sample. Three of themwere aged under 85°C/dry conditions and the other three under 85°C/85%RH con-ditions. The bulk resistance of each specimen was measured periodically during theaging. Results for these three specimens for each sample were reported. Shifts in thebulk resistance of these three samples under 85°C/dry and 85°C/85%RH conditionsare shown in Figs. 19.9 and 19.10, respectively. It can be seen that (1) under dry agingconditions, both samples showed relatively stable resistance, and (2) under wet agingconditions, the sample filled with the mixture of Ni and Ag flakes had a much largerresistance increase than the sample filled with only Ni flake.

It can be summarized that (1) under 85°C/dry aging conditions, regardless of themetals involved, all samples showed relatively stable contact resistance even thoughthe metals involved included an easily oxidizable metal such as Ni; and (2) under85°C/85%RH conditions, if only one type of metal was involved, the samplesshowed stable contact resistance, even for the commonly known oxidizable metalNi. However, the contact resistance increased dramatically if two different metalswere involved. These results strongly indicate that electrochemical (galvanic) corro-sion rather than direct metal oxidation was the dominant mechanism for oxide for-mation at the interface and the unstable contact resistance phenomenon duringhigh-temperature and -humidity aging.

These experimental results perfectly match the electrochemical (galvanic) corro-sion mechanism. A schematic explanation of a corrosion process is given in Fig.19.11. When a Ag flake contacts a nonnoble metal under wet conditions, moistureand oxygen diffuse into the interface and then the moisture condenses into water.The condensed water could dissolve some impurities from the resin or the metal


0.01

0.1

1

10

100

100 200 300 400 500 600 700 800

Time (h)

Bul

k R

esis

tanc

e (Ω

)

Ag

Ni

Ni+Ag

0 900

FIGURE 19.9 Bulk resistance shifts of three ICAs with different fillers under 85°C/dry aging con-ditions.

flake and form an electrolyte solution.Therefore, all the requirements for a galvaniccorrosion are met. The nonnoble metal M acts as an anode, loses electrons, andbecomes Mn+.The reaction can be represented as M − ne− = Mn+.The noble metal Agacts as a cathode, and the reaction on this electrode was H2O + 4e− + O2 = 4 OH−. Mn+

will combine with the OH− and form a metal hydroxide that is usually unstable andbecomes metal oxide. Therefore, a thin layer of metal oxide is formed at the inter-face. Because this oxide layer has much higher resistance than the nickel, the contactresistance increased significantly after aging.

Normal electrochemical potentials of some metals are listed in Table 19.2.13

Even though the corrosion behavior of a corrosion cell is determined by the actualpotentials of the anode and cathode, normal potentials can still be used to predict


1.00E-02

1.00E-01

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+07

00 200 300 400 500 600 700 80

Time (h)

Bul

k R

esis

tanc

e (Ω

)

Ag

Ni

Ni+Ag

0 9000 1

FIGURE 19.10 Bulk resistance shifts of three ICAs with different fillers under 85°C/85%RH agingconditions.

Silver flake

Nonnoble metal (M)

Condensed water solution

Cured epoxy resin

FIGURE 19.11 Schematic explanation of galvanic corrosion at the interface betweenmetal flake (such as Ni) and metal wire (such as Ag) during 85°C/85%RH aging.

corrosion behavior roughly. Electrochemical corrosion can happen only if one ofthe metals has a potential that is lower than the potential of the cathodic reaction,H2O + 4e− + O2 = 4 OH−, which is 0.4 eV below standard conditions. According toTable 19.1, the normal potentials of Ag, Au, and Pt are higher than the potential ofthe cathodic reaction. Therefore, no corrosion occurs and the contact resistancebetween the Ag-filled ECA with these noble metals remains stable during aging.On the contrary, the other metals, Ni, Sn, and SnPb, all have lower potentials thanthe cathodic reaction. Therefore, electrochemical corrosion occurred and the con-tact resistance between the Ag flake–filled ECA with these metals increased dra-matically during aging.

All of these results indicate that unstable contact resistance between an ECA andnonnoble metal finished surface-mount components is mainly due to metal oxideformation resulting from galvanic corrosion of the nonnoble metal.

19.3.3 OBSERVATION OF METAL OXIDE FORMATION

Metal oxide formation resulting from galvanic corrosion has been identified as themain mechanism for unstable contact resistance. In this study, metal oxide formationat the interface between an ECA and a nonnoble metal (SnPb was selected here)during aging was observed using TEM with EDX with ultrathin window technique.Two images of a cross-sectional interface between an ECA and SnPb are shown inFig. 19.12. From the images, it can be observed clearly that a metal oxide layer wasformed at the interface.

The oxide layer formation was confirmed using EDX. Two EDX spectra weretaken in the SnPb region and the oxide layer region and are shown in Fig. 19.13(a)and (b), respectively. By comparing these two spectra, it was found that the oxygenpeak was much higher in the spectrum of the oxide region than in that of the SnPbregion.This confirmed metal oxide formation at the interface between the ECA andSnPb after aging.


TABLE 19.2 Normal Potentials of Some Electrode Reactions13

Normal potential (eV)Electrode reaction (normal hydrogen scale)

Au − 3e = Au3+ 1.50

Pt − 2e = Pt2+ 1.20

Ag − 1e = Ag+ 0.80

H2O + O2 + 4e = 4OH− 0.40

Cu − 1e = Cu+ 0.34

Cu − 2e = Cu2+ 0.52

Pb − 2e = Pb2+ −0.13

Sn − 2e = Sn2+ −0.14

Ni − 2e = Ni2+ −0.25

19.4 CONCLUSIONS

The bulk resistance of silver flake–filled ECAs did not change during aging. Thecontact resistance of silver flake–filled ECAs on nonnoble metals increased dramat-ically during aging.

Results from this systematic study strongly indicate that electrochemical (gal-vanic) corrosion rather than simple oxidation of a nonnoble metal was the dominantmechanism for the oxide formation at the interface between ECAs and nonnoblemetals and for the unstable contact resistance between conductive adhesives and thenonnoble metal finished surface-mount components. TEM and EDX results con-firmed that a metal oxide layer was formed at the interface between an ECA andSnPb metal.


ECA

Sn/Pb

Oxide layer

FIGURE 19.12 TEM images of a cross-sectional interfacebetween an ECA and SnPb.

ECA

Sn/Pb

Oxide layer


FIGURE 19.13 EDX spectra taken in SnPb region (a) and oxide region (b).

(b)

(a)

REFERENCES

1. Jagt, J. C.,“Reliability of Electrically Conductive Adhesive Joints for Surface Mount Appli-cations: A Summary of the State of the Art,” IEEE Transactions on Components, Packag-ing, and Manufacturing Technology, Part A, 21(2):215–225, 1998.

2. Nguyen, G., J. Williams, F. Gibson, and T. Winster, “Electrical Reliability of ConductiveAdhesives for Surface Mount Applications,” Proceedings of the International ElectronicPackaging Conference, pp. 479–486, 1993.

3. Nguyen, G., J. Williams, and F. Gibson, “Conductive Adhesives: Reliable and EconomicalAlternatives to Solder Paste for Electrical Applications,” ISHM Proceedings, pp. 510–517,1992.

4. Jagt, J. C., P.J.M. Beric, and G.F.C.M. Lijten, “Electrically Conductive Adhesives: AProspective Alternative for SMD Soldering?” IEEE Transactions on Components, Packag-ing, and Manufacturing Technology, Part B, 18(2):292–298, 1995.

5. Gaynes, M.A., R. H. Lewis, R. F. Saraf, and J. M. Roldan,“Evaluation of Contact Resistancefor Isotropic Electrically Conductive Adhesives,” IEEE Transactions on Components,Packaging, and Manufacturing Technology, Part B, 18(2):299–304, 1995.

6. Zwolinski, M., J. Hickman, H. Rubon, and Y. Zaks, “Electrically Conductive Adhesives forSurface Mount Solder Replacement,” Proceedings of the 2nd International Conference onAdhesive Joining and Coating Technology in Electronics Manufacturing, pp. 333–340,Stockholm, Sweden, June 3–5, 1996.

7. Botter, H., “Factors That Influence the Electrical Contact Resistance of Isotropic Con-ductive Adhesive Joints During Climate Chamber Testing,” Proceedings of the 2nd Inter-national Conference on Adhesive Joining and Coating Technology in ElectronicsManufacturing, pp. 30–37, Stockholm, Sweden, June 3–5, 1996.

8. Rusanen, O., et al., “The Effect of Moisture on Die Attach Joints Made with Silver FilledEpoxy,” Microelectronics International, 37:25–27, 1995.

9. Liu, J., et al., “Surface Characteristics, Reliability and Failure Mechanisms of Tin, Copperand Gold Metallizations,” IEEE Transactions on Components, Packaging, and Manufactur-ing Technology, Part A, 20(1):21–30, 1997.

10. Bosch, D.V.A., et al., “Conductive Adhesives: A Feasible Challenge?” Proceedings of the2nd International Conference on Adhesive Joining and Coating Technology in ElectronicsManufacturing, pp. 160–163, Stockholm, Sweden, June 3–5, 1996.

11. Keusseyan, R. L., J. L. Diiday, and B. S. Speck,“Electric Contact Phenomena in ConductiveAdhesive Interconnections,” International Journal of Microcircuits and Electronic Packag-ing, 17(3):236–242, 1994.

12. Evans, U. R., The Corrosion and Oxidation of Metals: Scientific Principles and PracticalApplications, Edward Arnold, London, 1960.

13. Milazzo, G., Electrochemistry: Theoretical Principles and Practical Applications, p. 157,Elsevier, New York, 1963.


CHAPTER 20STABILIZATION OF CONTACT

RESISTANCE OF CONDUCTIVEADHESIVES

20.1 INTRODUCTION

20.1.1 FACTORS AFFECTING GALVANIC CORROSION

Experimental results in Chap. 19 strongly indicate that metal oxide formation result-ing from galvanic corrosion of a nonnoble metal at the interface between an electri-cally conductive adhesive (ECA) and the metal was the main mechanism for theunstable contact resistance of conductive adhesives. In this chapter, variousapproaches will be used to stabilize the contact resistance.

Several factors, such as moisture absorption and concentration of electrolyte, canaffect the rate of galvanic corrosion. One of the critical requirements for galvaniccorrosion is the presence of electrolyte solution.Without aqueous solutions, galvaniccorrosion does not happen.An ECA formulation with lower moisture pickup shouldhave less condensed water at the interface between the ECA and the nonnoblemetal. Such an ECA is expected to show less serious galvanic corrosion at the inter-face and less increase in contact resistance.

Electrolytes provide electrical conductivity, which is also required for a galvaniccorrosion process. Without electrolytes, galvanic corrosion is very slow. The elec-trolytes in this case mainly come from impurities in the resin.Therefore, purer resinsshould provide ECAs with better resistance to galvanic corrosion.

20.1.2 ADDITIVES TO PREVENT GALVANIC CORROSION

20.1.2.1 Oxygen Scavengers. Oxygen scavengers are chemicals that are added towater solutions to inhibit oxygen corrosion. As their name implies, they react withdissolved oxygen in aqueous solution. However, to assume that they act as inhibitorsof oxygen corrosion by “scavenging” the oxygen from the water is too simple a view.Their reactions with oxygen are important, but so are their properties as corrosioninhibitors. These depend on complex chemical interactions between the metal, theoxygen, the oxygen scavenger, and other variables of water chemistry such as pHand dissolved substances. While not every detail of these interactions is fully under-stood, the evidence is strong that oxygen scavengers are true corrosion inhibitors, ifthe term corrosion inhibitor is taken to mean that corrosion inhibition is a conse-quence of chemical reactions between metal species and a corrosion-inhibitingchemical at the metal-water interface.1

The main mechanism for corrosion inhibition by oxygen scavengers is thecathodic mechanism, which is based on the lowering of oxygen concentrations.1 Inthe system consisting of metal, water, oxygen, and oxygen scavengers, the cathodic

20.1


reaction of O2 competes with its chemical reduction. If the overall corrosion processis cathodically controlled, the reduction in corrosion rate depends on the relativemagnitudes of the rates of the electrochemical cathodic process Rel and the chemicalprocess Rch. The overall rate of oxygen reduction Rox is given by the sum of the ratesof the two processes:

Rox = Rel + Rch (20.1)

Since the rate of corrosion Rcorr is equal to the rate of the electrochemical cath-ode process Rel, the corrosion rate is given by:

Rcorr = Rox – Rch (20.2)

Equation (20.2) shows that the greater the rate of the chemical reduction of oxy-gen, the smaller the corrosion rate. Therefore, the reactivity of an oxygen scavengerwith oxygen is an important one of its properties.

There are several kinds of oxygen scavengers. Some commonly used oxygen scav-engers include sulfites (such as Na2SO4), hydrazine (H2N-NH2), carbohydrazide(H2N-NH-CO-NH-NH2) (CHZ), diethylhydroxylamine [(C2H5)2N-OH], and hydro-quinone (HO-C6H4-OH) (HQ).1–3

In order to prevent corrosion at the interface between an ECA and a nonnoblemetal, oxygen scavengers will be incorporated into ECA formulations. However, avery important consideration is that the oxygen scavengers should not adverselyaffect the properties of the ECA joints. Sulfites are not desirable because inorganicions such as sodium affect other properties of ECA joints. Another issue that mustbe considered is that the oxygen scavengers must not react with the epoxy resin orother components of the ECA formulations at and below the cure temperature ofthe ECA formulation. Otherwise, the oxygen scavengers will be consumed and losetheir effectiveness. Diethylhydroxylamine is not suitable for this application becauseit reacts with epoxy resins readily at low temperatures. In addition, due to itsextreme toxicity, hydrazine is not a desirable oxygen scavenger. Therefore, only twooxygen scavengers—CHZ and HQ—are selected in this study.

CHZ, a derivative of hydrazine, has a decomposition temperature of 153 to154°C, and its solubility in water is 32 g/100 ml H2O at 25°C. Its reaction with oxygenproceeds directly as shown in the following equation:

(H2N-NH)2CO + 2O2 = 2N2 + 3H2O + CO2

HQ is an organic solid with the properties of a weak acid, and its solubility inwater is 5.9 g/l at 15°C. Its reaction with dissolved oxygen is very fast even at ambi-

20.2 CHAPTER TWENTY

OH

OH

+ 1/2O2

O

O

hydroquinone benzoquinone

FIGURE 20.1 Reaction of HQ with oxygen to form benzoquinone.

ent temperature. Its fast reaction kinetics in ambient oxygenated water has beenproven in both industrial and utility applications. It is likely that HQ first reacts withoxygen to form benzoquinone (Fig. 20.1).

The overall reaction, however, involves more than the theoretical value of 0.5mole of oxygen per mole of HQ. Base catalysis leads to further oxidation (and hencescavenging), as represented in Fig. 20.2.1,3

20.2 EXPERIMENTS

20.2.1 MATERIALS

A bisphenol F–type epoxy resin, Epon 862, from Shell Chemical Company wasemployed as the base resin of ECA formulations.A cycloaliphatic epoxy resin, ERL-4299, was obtained from Union Carbide. A polyamide, 313B, was used as the hard-ener for the epoxy resin. CHZ, HQ, 8-hydroxyquinoline (HQL), 1,10-phenanthroline(PAL), sodium chloride, ammonium chloride, ammonium sulfate, and sodiumacetate, and all metal wires including Sn, Cu, and Ni, were purchased from AldrichChemical Company. All the chemicals were used as received.

20.2.2 CONTACT RESISTANCE TEST DEVICES

Two kinds of contact resistance test devices were used in this study. One type oftest device, which consisted of glass slides and metal wire segments, was describedin Chap. 19. The other kind of test device is shown in Fig. 20.3. This device con-sisted of metal patterns on a piece of printed circuit board. The metal patterncould be composed of SnPb, Sn, or Cu. Conductive adhesive pastes were dispensedon the gaps. After being cured and cooled down to room temperature, the totalresistance of one circle was measured using a Keithley multimeter with a four-point probe.

20.2.3 STUDY OF CURING BEHAVIORS OF ECAs

The curing behavior of ECAs was investigated using a differential scanningcalorimeter (DSC) (model 2970) from TA Instruments. The procedure for runningDSC samples is found in previous sections.

STABILIZATION OF CONTACT RESISTANCE OF CONDUCTIVE ADHESIVES 20.3

OH

OH

+ by-products CO× O2 2

FIGURE 20.2 Base catalysis of hydroquinone.

20.2.4 STUDY OF DYNAMIC PROPERTIES OF ECAs

Dynamic mechanical properties of the cured polymeric matrix of an ECA wereinvestigated using a dynamic mechanical analyzer (DMA) from TA Instruments(model 2980) with a single cantilever clamp. Sample preparation was the same asthermomechanical analyzer sample preparation except that the cured samples werecut into rectangles with dimensions of about 40 × 8 × 1.5 mm. In a nonisothermalscan, after a sample was mounted on the clamp, the temperature was raised from 25to 280°C at a heating rate of 3°C/min. The sample was studied under an oscillationmode with a frequency of 1 Hz. Storage and loss moduli with temperature wererecorded.

20.2.5 MEASUREMENT OF MOISTURE ABSORPTION

Moisture absorption of the cured polymeric matrix of an ECA was measured bymonitoring the mass changes of the cured resin with time in a chamber (modelCEO932W-4 from Lunaire Environmental) at 85°C/85% relative humidity (RH).The samples were prepared by curing approximately the same volume of each resinin an aluminum pan 1.5 in in diameter.After being cured and removed from the con-tainer, the samples were placed in the chamber and their mass changes with timewere recorded. Three specimens were tested for each sample. The average of mois-ture absorption of the three specimens was reported.

20.2.6 MEASUREMENT OF ADHESION STRENGTH

Die shear adhesion strength was measured at 25°C by using an adhesion tester fromRoyce Instruments (model 552).The size of the die was 2 × 2 mm.The dies used werenot passivated and substrates were eutectic SnPb. The thickness of the adhesivelayer between the die and substrate was controlled using 75-µm glass beads. Tenspecimens were tested for each ECA sample. The average adhesion strength andstandard deviation for each ECA sample were reported.The adhesion strength datawere not included in the calculation of the average adhesion strength if the die wasbroken or fractured during the die shear adhesion test.

20.4 CHAPTER TWENTY

Metal pattern

Gap

PCB

FIGURE 20.3 Contact resistance test device.


20.3.1 EFFECTS OF ELECTROLYTES ON CONTACT

RESISTANCE SHIFTS

The results in previous sections strongly indicate that galvanic corrosion is the mainmechanism for the unstable contact resistance of conductive adhesives on nonnoblemetals.As mentioned in the previous sections, electrolyte solution is one of the require-ments for galvanic corrosion. Electrolytes should increase the electrical conductivity ofthe solution, accelerate galvanic corrosion, and cause larger contact resistance increase.

The effects of four electrolytes—NaCl, NaAc, NH4Cl, and (NH4)2SO4—on con-tact resistance shifts of an ECA on SnPb metal were investigated.The concentrationof each electrolyte was 0.5 parts electrolyte per 100 parts ECA resin. The contactresistance of the ECAs with and without the electrolytes was measured periodicallyduring 85°C/85%RH aging. The results are shown in Fig. 20.4. As can be seen fromthe figure, the ECAs with the electrolytes showed a faster increase in contact resist-ance than the ECA without electrolytes. Electrolytes can increase electrical conduc-tivity of the solution and accelerate galvanic corrosion. It can be concluded from thisstudy that, in order to stabilize contact resistance during aging, resins, hardeners, andother ingredients with low impurity contents should be used to formulate ECAs.

20.3.2 EFFECTS OF MOISTURE ABSORPTION ON CONTACT

RESISTANCE SHIFTS

The water condensed from the absorbed moisture at the interface between an ECAand a metal formed the electrolyte solution required for galvanic corrosion. There-


-500.0

500.0

1500.0

2500.0

3500.0

4500.0

5500.0

6500.0

7500.0

0 100 200 300 400 500 600 700 800 900 1000

Aging Time (h)

Con

tact

Res

ista

nce

Shi

ft (%

)

Without ElectrolytesSodium ChlorideAmmonium SulfateAmmonium ChlorideSodium Acetate

FIGURE 20.4 Effects of electrolytes on contact resistance shift of ECAs on SnPb.

fore, an ECA with lower moisture absorption should show slower contact resistanceshift during aging due to its slower corrosion rate at the interface.

Three ECAs were formulated with different epoxy resins but the same hardenerand catalyst. The detailed compositions of the resins of these ECAs are shown inTable 20.1. These three ECAs had similar properties but different moisture absorp-tion.The moisture absorption of the cured resins of these ECAs is shown in Fig. 20.5.As can be seen from this figure, ECA-III had the highest moisture absorption andECA-I showed the lowest moisture absorption.

20.6 CHAPTER TWENTY

0.00

0.50

1.00

1.50

2.00

2.50

3.00

0 100 200 300 400 500 600 700 800

Aging Time (h)

Moi

stur

e A

bsor

ptio

n (%

)

ECA-IECA-IIECA-III

FIGURE 20.5 Moisture absorption of ECA-I, ECA-II, and ECA-III.

TABLE 20.1 Compositions of the Resins of Three ECAs

Resin Epon-862 ERL-4299 MHHPA 2E4MZ

ECA-I 5.0 0.0 4.18 0.090

ECA-II 3.0 3.0 4.7 0.11

ECA-III 0.0 6.0 4.2 0.10

The contact resistance shifts of these ECAs on SnPb were compared and areshown in Fig. 20.6. Comparing Figs. 20.5 and 20.6, it was found that the ECA with thehighest moisture absorption (ECA-III) showed the fastest contact resistance shiftand the ECA with the lowest moisture absorption (ECA-I) showed the slowest con-tact resistance shift during aging. This indicates a correlation between moistureabsorption and contact resistance shift. Therefore, one of the approaches to formu-lating an ECA with more stable contact resistance is to select epoxy and hardenercombinations that can provide ECAs with the lowest moisture absorption.

20.3.3 STABILIZATION OF CONTACT RESISTANCE

USING ADDITIVES

20.3.3.1 Formulations of ECAs. In order to investigate the effects of additives(corrosion inhibitors and oxygen scavengers) on the contact resistance, the ECAformulations used in this study were kept as simple as possible. The base resin for-mulation used in this study consisted only of a bisphenol F epoxy resin (Epon 862),a polyamide hardener (313B), and an additive. All the additives were solids. Addi-tives were ground into superfine powders and, to ensure uniform dispersion, theresin mixture was stirred using a mixer with high shearing. ECAs were formulatedby adding a certain amount of silver flake into the base resin.The filler loading of allthe ECA formulations was kept at 80 wt%.

20.3.3.2 Effects of Oxygen ScavengersEffects of Oxygen Scavengers on Contact Resistance Shift. A small amount (5

parts per 100 parts of the resin) of CHZ and HQ was introduced into the base ECAformulation. Shifts of the contact resistance of the base ECA formulation, the ECAwith CHZ, and the ECA with HQ on eutectic tin-lead (SnPb) solder were moni-tored during 85°C/85%RH aging. In the short term, the component finish will still beSnPb due to the current infrastructure; therefore, SnPb alloy was selected to be stud-ied. The effects of these additives on other metals should be similar.

The contact resistance change during 85°C/85%RH aging of these ECAs isshown in Fig. 20.7.As can be seen from the figure, the contact resistance of the ECAswith these two oxygen scavengers increased more slowly than that of the base ECAwithout additives. It can be concluded the oxygen scavengers can slow contact resist-ance shift. It also can be seen that CHZ is more effective than HQ. This may be


0.0

500.0

1000.0

1500.0

2000.0

2500.0

0 100 200 300 400 500 600 700 800

Aging Time (h)

Con

tact

Res

ista

nce

Shi

ft (%

)

ECA-IECA-IIECA-III

FIGURE 20.6 Contact resistance shift of ECA-I, ECA-II, and ECA-III.

because CHZ is more alkaline than HQ.After CHZ was dissolved by the condensedwater, a more alkaline electrolyte solution was formed at the interface. Under alka-line conditions, the galvanic corrosion was somewhat inhibited. Another possiblereason was that CHZ and HQ may have different tendencies to absorb onto anSnPb surface.

Absorption of Oxygen Scavengers on Metals. The absorbance of each oxygenscavenger solution was scanned between wavelengths of 400 and 190 nm. Theabsorbance versus wavelength of CHZ and HQ is shown in Fig. 20.8(a) and (b),respectively. From the figure, it can be seen that CHZ showed the strongestabsorbance at a wavelength of 191 nm and HQ had the strongest absorbance at 196nm. The two wavelengths were selected as the fixed wavelengths in the measure-ment of the concentrations of the oxygen scavengers.

A small amount (2 g) of a SnPb powder was placed into 8 ml aqueous solution ofCHZ and HQ (concentration 3 × 10−5 g/ml). The mixtures were kept at room tem-perature for 24 h.After the SnPb powders settled down, the upper clear solution wasstudied using the UV-visible spectrometer.The absorbances of the original solutionsof these two oxygen scavengers and the upper clear solutions from the mixtureswere measured and compared. The results are shown in Table 20.2. As can be seenfrom the table, the absorbance of CHZ solution that had been mixed with SnPbpowder was much lower than that of the original CHZ solution. However, theabsorbance of the HQ solution that had been mixed with the SnPb powder was sim-ilar to that of the original HQ solution. From this study, it can be concluded thatCHZ absorbed on the SnPb surface much more readily than HQ. This result mayexplain why CHZ was more effective than HQ in slowing down contact resistance ofECAs on an SnPb surface.

Effects of Oxygen Scavengers on Curing Behavior of ECAs. The effects of thesetwo oxygen scavengers on curing behavior of the base formulation were investi-

20.8 CHAPTER TWENTY

0

1000

800

600

400

200

1200

1400

1600

1800

2000

0 100 200 300 400 500 600

Aging Time (h)

Per

cent

age

of C

onta

ct R

esis

tanc

e S

hift

(%)

( R-R

o)/R

o*1

00

Without Oxygen Scavengers

5% CHZ

5% HQ

FIGURE 20.7 Effects of two oxygen scavengers on contact resistance shift.

Abs

orba

nce

Wavelength (nm)190 400

196

gated. In order to find out the interaction between these additives (CHZ and HQ)and the epoxy resin, the additives (10 wt%) were mixed with only the epoxy resinfirst. The mixtures were thermally scanned using a DSC. The results are shown inFig. 20.9.

As can be seen from Fig. 20.9, neither oxygen scavenger reacted with the epoxyresin below 150°C. CHZ decomposes after 150°C; therefore several peaks appearabove 150°C. HQ does not react with epoxy resin until 200°C. These results suggestthat ECAs with these additives should be cured below 150°C to ensure the additivesremain in the cured ECA systems and continue to be effective.

The oxygen scavengers were also added to the base resin formulation to investi-gate their effects on its curing behavior.The concentration of the oxygen scavengerswas kept at level of 5 wt% based on the total weight of the resin. The DSC curecurves of the base resin, the base resin with CHZ, and the base resin with HQ areshown in Fig. 20.10. The glass transition temperature Tg of these materials was alsomeasured using DSC under modulated mode, and the results are given in Fig. 20.11.From these two figures, it can be seen that CHZ and HQ shifted the curing peak ofthe base resin formulation to lower temperature ranges but did not affect the glasstransition temperature of the cured resin appreciably.

STABILIZATION OF CONTACT RESISTANCE OF CONDUCTIVE ADHESIVES 20.9A

bsor

banc

e

Wavelength (nm)190 400

191

FIGURE 20.8 Absorbance of CHZ (a) and HQ (b) in the wavelength range from 190 to 400 nm.

(b)(a)

TABLE 20.2 Absorbance of CHZ and HQ Solutions

CHZ (at 191 nm) HQ (at 196 nm)

CHZ solution before CHZ solution HQ solution before CHZ solution aftermixing with SnPb after mixing with mixing SnPb with mixing with SnPb

powder SnPb powder powder powder

1 2.096 0.884 3.024 3.288

2 2.117 0.882 3.102 3.173

3 2.105 0.882 3.030 3.186

Average 2.106 0.883 3.052 3.215

Effects of Oxygen Scavengers on Mechanical Properties of ECAs. The baseresin formulation, the resin formulation with 5 wt% CHZ, and the resin formulationwith 5 wt% HQ were cured at 140°C for 40 min. The dynamic properties of curedresins were studied using a DMA. The storage modulus and tan δ of these curedresins are shown in Figs. 20.12 and 20.13, respectively. As can be seen from these

20.10 CHAPTER TWENTY

-0 .5

0 .0

0 .5

1 .0

1 .5

40 90 140 190 240 290

Temperature (°C)

Hea

t Flo

w (

W/g

)

With CHZWith HQ

Universal V2.5H TA InstrumentsExo Up

FIGURE 20.9 Interactions between oxygen scavengers and an epoxy resin.

–0.5

0.0

0.5

1.0

1.5

Hea

t Flo

w (

W/g

)

0 50 100 150 200

With HQWith CHZWithout additives

250 300

Temperature (°C)Exo Up Universal V2.5H TA Instruments

FIGURE 20.10 Effects of oxygen scavengers on curing behavior of the base formulation.

results, CHZ slightly decreased the storage modulus (before Tg) of the cured resinbut the oxygen scavengers did not have significant effects on the tan δ of the baseresin formulation.

Effects of Oxygen Scavengers on Moisture Absorption. The moisture absorp-tions of cured resins with and without oxygen scavengers were investigated andcompared. The moisture absorptions of ECAs with and without these additives are


74.44°C

80.63°C(I)

85.53°C

90.03°C

84.48°C

77.20°C

84.14°C(I)

79.71°C(I)

74.78°C

0.00

–0.05

Rev

Hea

t Flo

w (

W/g

)

30

Temperature (°C)Exo Up Universal V2.5H TA Instruments

–0.10

–0.15

–0.2040 50 60 70 80 90 100 110 120 130

Without AdditivesWith CHZWith HQ

FIGURE 20.11 Effects of oxygen scavengers on glass transition temperatures.

4000

3000

Sto

rage

Mod

ulus

(M

Pa)

20

Temperature (°C) Universal V2.5H TA Instruments

2000

1000

04030 50 60 70 80 90 100 110 130120 140

Without AdditivesWith CHZWith HQ

FIGURE 20.12 Storage moduli of cured resins with and without oxygen scavengers.

shown in Fig. 20.14. From this figure, it can be concluded that CHZ and HQ do nothave significant effects on moisture absorption of the ECA materials.

Effects of Oxygen Scavengers on Adhesion Strength. The effects of oxygen scav-engers on adhesion strength were studied by measuring die shear adhesion strengthof ECAs containing these additives on a SnPb surface. The die shear strength


1.0

0.8

Tan

δ

20

Temperature (°C) Universal V2.5H TA Instruments

0.6

0.4

0.2

0.040 60 80 100 120 160140 180

Without Oxygen Scavengers

With CHZ

With HQ

FIGURE 20.13 Tan δ of cured resins with and without oxygen scavengers.

3.00

2.50

Moi

stur

e A

bsor

ptio

n (%

)

0

Time (h)

2.00

1.50

1.00

0.50

0.0050 100 150 200 250 300 350

Without Additives

With CHZ

With HQ

FIGURE 20.14 Effects of two oxygen scavengers on moisture absorption.

(expressed in kilograms) of these ECAs is shown in Fig. 20.15.This figure shows thatCHZ and HQ clearly did not have significant effects on the die shear adhesionstrength of conductive adhesive materials.

20.4 CONCLUSIONS

ECAs formulated with high-purity resins showed more stable contact resistance.ECAs based on resins with low moisture absorption exhibited more stable contactresistance.

The oxygen scavengers CHZ and HQ could slow down the contact resistanceincrease of conductive adhesives on SnPb, but CHZ was more effective than HQ.CHZ and HQ shifted the curing peak of the base resin formulation to lower tem-perature ranges, but they did not affect glass transition temperature, dynamic prop-erties, moisture absorption, or adhesion strength of the ECAs.

20.5 SUMMARY

In Chaps. 17 through 20, fundamental studies on reliability issues of current conduc-tive adhesives technology are reviewed and, based on the resulting understanding,conductive adhesives with improved performance are developed. Current conduc-tive adhesives are not suitable for solder replacement, mainly due to their lowerelectrical conductivity, unstable contact resistance on nonnoble metal metallizedpads, and poor impact performance. Full understanding of most of these reliability


Without Additives Without CHZ

ECAs

With HQ

12

Adh

esio

n S

tren

gth

(kg) 8

6

10

4

2

0

FIGURE 20.15 Die shear adhesion strength (kg) of ECAs with and without oxygen scavengers (diesize: 2 × 2 mm)

issues of current conductive adhesive technology is key to development of high-performance solder replacement materials. A qualified solder replacement conduc-tive adhesive should have: (1) a high electrical conductivity (resistivity lower than 1 × 10−3 Ω/cm); and (2) stable contact resistance on a nonnoble metal (less than 20percent increase after 500 h of aging at 85°C/85 percent relative humidity). The pro-posed conductive adhesive meets all the National Center for Manufacturing Science(NCMS) requirements for solder replacement.

To begin with, the chemical nature and thermal behaviors of the organic lubri-cants of silver flakes (fillers in conductive adhesives) are investigated. The roles ofthe silver flake lubricant layer and resin cure shrinkage on conductivity establish-ment of an electrically conductive adhesive (ECA) are studied. Based on this study,it is found that the silver flake lubricant layer is not strongly related to the initiationof conductivity of ECAs, but that resin cure shrinkage is one of the factors that ini-tiate electrical conductivity of ECAs. Several approaches, including the use of addi-tives (short alkyl chain carboxylic acids) and multifunctional epoxy resins, arediscovered to improve electrical conductivity of an ECA.

To clarify the dominant mechanisms underlying the unstable contact resistanceof current commercial ECAs on nonnoble metals, a series of experiments wasdesigned. The results clearly indicate that metal oxide formation resulting from gal-vanic corrosion of the nonnoble metallization contacts is the main mechanism forthe unstable contact resistance phenomenon. Effects of electrolytes and moistureabsorption on contact resistance shifts of ECAs are studied.Also, several approachesare identified to stabilize contact resistance.A few effective additives, including oxy-gen scavengers and corrosion inhibitors, are identified based on the study.

In order to improve the impact strength of an ECA, many modified resins includ-ing rubber-modified epoxy resins are introduced into ECA formulations. Also, twoepoxide-terminated epoxy resins are synthesized and used in ECA formulations.The loss factor (tan δ) value of ECAs based on these resins is measured and impactperformance of these ECAs is evaluated using a drop test. Based on this study, effec-tive resins are identified.

By combining effective resins for improving impact strength, effective additivesfor stabilizing contact resistance, and effective approaches to improving electricalconductivity, conductive adhesives with high performance are developed. TheseECAs meet all the NCMS requirements for solder replacements. For further refer-ence, the excellent work of D. Lu of Georgia Tech is highly recommended.4–32 Theusefulness of present ECAs for high-performance electronic applications with highcurrent density and self-alignment capability is still limited. The development ofhigh-performance ECAs will require collaborative efforts between materials scien-tists, chemists, chemical engineers, and electrical engineers in the coming years.

REFERENCES

1. Noack, M. G., “Oxygen Scavengers,” Corrosion’89, paper no. 436, New Orleans, LA, April17–21, 1989.

2. Reardon, P. A., and W. E. Bernahl, “New Insight into Oxygen Corrosion Control,” paperno. 438, San Francisco, CA, March 9–13, 1987.

3. Romaine, S., “Effectiveness of a New Volatile Oxygen Scavenger,” Proceedings of theAmerican Power Conference, pp. 1066–1073.

4. Lu, D., Study of Electrically Conductive Adhesives, Ph.D. thesis, Georgia Institute of Tech-nology, Atlanta, GA, 2000.


5. Lu, D., and C. P. Wong, “Novel Conductive Adhesives for Surface Mount Applications,”Journal of Applied Polymer Science, 74:399–406, 1999.

6. Lu, D., C. P. Wong, and Q. Tong, “Conductivity Mechanisms of Isotropic Conductive Adhe-sives,” IEEE Transactions on Components, Packaging, and Manufacturing Technology, PartC, 22(3):223–227, 1999.

7. Lu, D., C. P.Wong, and Q.Tong,“Mechanisms Underlying the Unstable Contact Resistanceof Conductive Adhesives,” IEEE Transactions on Components, Packaging, and Manufac-turing Technology, Part C, 22(3):228–232, 1999.

8. Lu, D., C. P. Wong, and Q. Tong, “A Study of Lubricants of Ag Flake for MicroelectronicsConductive Adhesives,” IEEE Transactions on Components, Packaging, and Manufactur-ing Technology, Part A, 22(3):365–371, 1999.

9. Lu, D., and C. P. Wong, “Effect of Shrinkage on Conductivity of Isotropic ConductiveAdhesives,” International Journal of Adhesives and Adhesion, 20:189–193, 2000.

10. Lu, D., and C. P. Wong, “Characterization of Lubricants of Ag Flakes,” Journal of ThermalAnalysis and Calorimetry, 59:729–740, 2000.

11. Lu, D., and C. P. Wong, “High Performance Conductive Adhesives,” IEEE Transactions onElectronics Packaging Manufacturing, 22(4):324–330, 1999.

12. Lu, D., and C. P. Wong, “Properties of Conductive Adhesives Based on Anhydride-CuredEpoxy Systems,” Journal of Electronics Manufacturing, 9(4):241–248, 2000.

13. Lu, D., and C. P. Wong, “Thermal Decomposition of Silver Flake Lubricants,” Journal ofThermal Analysis and Calorimetry, 59:729–740, 2000.

14. Shi, S. H., D. Lu, and C. P.Wong,“Study on Relationship Between the Surface Compositionof Copper Pads and No-Flow Underfill Fluxing Capability,” IEEE Transactions on Elec-tronics Packaging Manufacturing, 22(4):268–273, 1999.

15. Lu, D., and C. P.Wong,“A Study of Contact Resistance of Conductive Adhesives Based onAnhydride-Cured Epoxy Systems,” IEEE Transactions on Components, Packaging, andManufacturing Technology, 23(3):440–446, 2000.

16. Lu, D., C. P.Wong, and Q.Tong,“Mechanisms Underlying the Unstable Contact Resistanceof Conductive Adhesives,” Proceedings of the 49th Electronic Components and TechnologyConference, pp. 342–346, 1999.

17. Tong, Q. K., G. Fredrickson, R. Kuder, and D. Lu, “Conductive Adhesives with SuperiorImpact Resistance and Stable Contact Resistance,” Proceedings of the 49th ElectronicComponents and Technology Conference, pp. 347–352, 1999.

18. Lu, D., and C. P. Wong, “Conductive Adhesives with Improved Properties,” Proceedings ofthe 2nd International IEEE Symposium on Polymeric Electronics Packaging, pp. 1–8, 1999.

19. Lu, D., and C. P. Wong, “Conductive Adhesives Based on Anhydride-Cured Epoxy Sys-tems,” Proceedings of the 2nd International IEEE Symposium on Polymeric ElectronicsPackaging, pp. 27–34, 1999.

20. Lu, D., Q. Tong, and C. P. Wong, “A Fundamental Study on Silver Flakes for ConductiveAdhesives,” 1998 International Symposium on Advanced Packaging Materials, pp.256–260, Braselton, GA, 1998.

21. Wong, C. P., D. Lu, S.Vona, and Q. K.Tong,“Fundamental Study of Electrically ConductiveAdhesives,” Proceedings of the 1st IEEE International Symposium on Polymeric Electron-ics Packaging, pp. 80–85, Norrkoping, Sweden, 1997.

22. Lu, D., Q. K. Tong, and C. P. Wong, “Conductivity Mechanisms of Isotropic ConductiveAdhesives (ICAs),” Proceedings of the 4th International Symposium and Exhibition onAdvanced Packaging Materials, pp. 2–10, Braselton, GA, March 17–19, 1999.

23. Lu, D., and C. P. Wong, “Effect of Shrinkage on Conductivity of Isotropic ConductiveAdhesives,” Proceedings of the 4th International Symposium and Exhibition on AdvancedPackaging Materials, pp. 295–301, Braselton, GA, March 17–19, 1999.


24. Lu, D., and C. P. Wong, “Novel Conductive Adhesives for Surface Mount Applications,”Proceedings of the 4th International Symposium and Exhibition on Advanced PackagingMaterials, pp. 288–294, Braselton, GA, March 17–19, 1999.

25. Wong, C. P., D. Lu, and Q. Tong, “Lubricants of Silver Flake for Conductive AdhesiveApplications,” 3rd International Conference on Adhesive Joining and Coating Technologyin Electronic Manufacturing, pp. 184–192, Binghamton, NY, September 28–30, 1998.

26. Shi, S. H., D. Lu, and C. P.Wong,“Study on Relationship Between the Surface Compositionof Copper Pads and No-Flow Underfill Fluxing Capability,” Proceedings of the 4th Inter-national Symposium and Exhibition on Advanced Packaging Materials, pp. 325–332,Braselton, GA, March 17–19, 1999.

27. Lu, D., and C. P. Wong, “Development of Conductive Adhesives Filled with Low-Melting-Point Alloy Fillers,” Proceedings of the 5th International Symposium and Exhibition onAdvanced Packaging Materials, pp. 7–13, Braselton, GA, March 6–8, 2000.

28. Lu, D., and C. P. Wong, “Conductive Adhesives for Solder Replacement in ElectronicsPackaging,” Proceedings of the 5th International Symposium and Exhibition on AdvancedPackaging Materials, pp. 24–31, Braselton, GA, March 6–8, 2000.

29. Lu, D., and C. P.Wong,“Development of High Performance Conductive Adhesives for Sur-face Mount Applications,” Proceedings of the 50th Electronics Components and Technolo-gies Conference, pp. 892–898, Las Vegas, May 2000.

30. Lu, D., and C. P. Wong, “Effects of Curing Agents on the Properties of Conductive Adhe-sives,” Proceedings of the 5th International Symposium and Exhibition on Advanced Pack-aging Materials, pp. 311–318, Braselton, GA, March 6–8, 2000.

31. Shimada, Y., D. Lu, and C. P. Wong, “Electrical Characterizations and Considerations ofElectrically Conductive Adhesives (ECAs),” Proceedings of the 5th International Sympo-sium and Exhibition on Advanced Packaging Materials, pp. 335–342, Braselton, GA, March6–8, 2000.

32. Lu, D., and C. P. Wong, “Conductive Adhesives for Solder Replacement,” IEEE Transac-tions on Components, Packaging, and Manufacturing Technology, Part A, in press.


AAcid rain, 8.4Acid rosin flux, 7.10Adhesives:

conductive, 1.12 (see also specific types)for die attach, 17.11–17.13polymer-based, 17.12vs. solders, 4.17, 5.42thermoplastic, 17.6, 17.9thermosetting, 17.6

Alumina, hydrated, 8.11Aluminum hydroxide, as flame retardant,

6.12–6.13Amkor, CABGA technology of, 6.22Ammoniacal etchants, 10.13Analytic hierarchy process (AHP) matrix,

10.5, 10.6Anisotropic conductive adhesives (ACAs),

1.12, 5.23–5.29, 8.11, 17.4–17.785°C/85% RH results for, 5.26–5.27adhesive matrix of, 17.5–17.6applications for, 17.7categories of, 17.5conductive fillers for, 17.6formulation of, 5.23measurement results for, 5.23–5.26nonconductive fillers in, 5.22–5.23pros/cons of, 17.7thermal cycling results for, 5.27–5.29

Anisotropic conductive films (ACFs), 1.12assembly yield of, factors in, 5.5disadvantages of, 5.5FCOB assemblies with, 5.4–5.5nonconductive fillers in, 5.3, 5.22–5.23

Anisotropic conductive pastes (ACPs), 1.12Antimony, 1.12

cost of, 12.4oxide, 8.1

Antimony (Cont.):properties of alloys containing, 16.12toxicity of, 12.2

Astatine, 1.12Atomized spray coating, 10.19Automobile industry, 1.1–1.2

BBare board:

minimizing process waste in manufactureof, 10.2

optimizing, 10.7–10.8Basel Treaty, 8.5Batteries, recycling of, 8.15Benzimidazoles, 14.8–14.13

benefits of, 14.12–14.13fabrication of, 14.10–14.13vendors of, 14.8

Benzotriazole, 14.2–14.7fabrication of, 14.4–14.6performance of, 14.7

Bioaccumulation, 9.1Biopolymers, 8.21Bismuth:

availability of, 12.4and fillet lifting, 16.22–16.23properties of alloys containing, 12.33–12.35,

13.24, 13.26, 14.59, 14.69, 16.12regional preferences for, 12.33–12.35as solder additive, 11.9, 11.10, 13.54, 13.55,

15.26–15.27toxicity of, 12.2

Black pad, 14.24, 16.1–16.4Blue Angel, 8.24Boeing, Pb elimination plans of, 11.7Bond strength, effect of additives on, 12.6Brite-Euram project, Pb-free alloys

recommended by, 11.13, 12.13

I.1

INDEX


Brominated flame retardants (BFRs), 1.12,8.1–8.2, 8.8–8.9

breakdown products of, 9.1concerns about, 8.8–8.9health effects of, 9.1total use of, 9.2uses of, 8.8

Bromine, 1.11, 8.7vs. chlorine, 9.5flame-retarding action of, 8.1industry emissions of, 9.9toxicity of, 9.7

Brush coating, 10.19

CCable sets, recycling of, 8.15Cadmium, 8.7

recycling of, 8.15toxicity of, 12.2

Carbon, and black pad, 16.2–16.4Carbonization, 8.12CASTIN, 13.41, 13.53Categorical hazard score, 10.2, 10.6Charging devices, energy consumption of,

8.5–8.7ChipArrayBGA (CABGA), 6.22–6.28

factors affecting coplanarity of, 6.25–6.26halogen-free flame retardant resins for, 6.22moisture sensitivity tests on, 6.27–6.28

Chip cracking, causes of, 7.10Chip-level interconnects, solderless, 1.9Chip-on-flex (COF), 5.14ChipPAC, Pb elimination plans of, 11.7Chlorine, 1.11, 8.7

vs. bromine, 9.5Chromic-based etchants, 10.13CHZ, 20.2, 20.7–20.13Combustion, designing to prevent, 8.25Conductive anodic filament (CAF),

16.35–16.36Conductive fillers, 17.6, 17.10–17.11Conformal coatings, 10.17–10.21

chemistries of, 10.17–10.18curing of, 10.18–10.19dispensing of, 10.19–10.20epoxy-based, 10.18parylene, 10.18process issues of, 10.20–10.21silicone, 10.18solvent-based vs. solvent-free, 10.17and tin whisker, 16.9uses of, 10.17

Copper:catalysis of, 14.20as conductive filler, 17.10cost of, 12.4effect on solder grain structure, 12.11electroless, 10.15IMCs formed by, 16.11toxicity of, 12.2uses of, 12.4

Copper chloride etchants, 10.13Copper (solderless) bumps:

assemblies using, 5.4–5.5vs. Au bumps, 5.9–5.10vs. Ni-Au bumps, 5.5

Copper wires, electroplated, 4.9–4.10 (seealso Wire Interconnect Technology)

Corrosion, galvanic, 19.3, 19.10–19.12factors affecting, 20.1inhibitors of, 20.1–20.3oxide formation due to, 19.12

Creep, 12.9–12.10Cure shrinkage, 6.26

DDaylighting, 8.7Delco, Pb-free alloys used by, 11.14Delphi Delco, Pb elimination plans of, 11.7Denitrification, 9.9Denmark, Pb ban in, 11.2Department of Trade and Industry (UK),

Pb-free alloys recommended by,11.13, 12.13

Deserts, increase in, 8.5Design for the environment (DfE), 8.16–8.18

implementing, 10.16–10.17for PCB, 10.1–10.2

Design parameters:relationships between, 10.7–10.8, 10.11and waste, 10.8

DF-335–7 die attach film, 7.1–7.6characteristics of, 7.6

DF-400 die attach film, 7.6–7.9characteristics of, 7.7–7.9

Die attach adhesives, 17.11–17.13Die attach films, environmentally benign,

7.1–7.8effects of Ag filler content on, 7.4effects of modulus on, 7.7effects of moisture absorption on, 7.4–7.5effects of temperature on, 7.7effects of thermosetting resin content on,

7.2–7.4

I.2 INDEX

Die manufacturing, environmental issues of,10.11

Diethylhydroxylamine, 20.2Dip coating, 10.19Direct chip attach (DCA), 1.9

with solderless FCOB with ACF, 5.1–5.11Direct metallization, 10.15Dross, 16.11–16.13

EElectrically conductive adhesives (ECAs),

17.4Ag particle contact in, 18.5bulk resistance of, 19.5–19.6

and aging, 19.9–19.12composition of, 18.1conductivity of, 17.15–17.16, 18.1–18.18

and shrinkage, 18.14–18.18cure of, 18.1, 18.6–18.7, 18.12–18.14

oxygen scavengers and, 20.8–20.9influence of moisture on, 17.9–17.10printed bumps of, 17.13resistance of, 18.1, 18.5–18.7, 18.18as solder alternative, 17.15, 19.1unstable contact resistance of, 19.1–19.15

effect of electrolytes on, 20.5effect of moisture absorption on,

20.5–20.6oxygen scavengers and, 20.7–20.13

Electric Household Appliance RecyclingLaw, 1.3

Electricity, means of producing, 8.5Electronics:

chemical issues with, 8.7–8.14end-of-life management of, 1.13–1.14environmental concerns about, 8.2–8.7“green”:

manufacture of, 10.11–10.17popularity of, 1.3

life cycle of, 8.2overview of industry, 1.2–1.3packaging of, 17.1–17.4

functions of, 17.1levels of, 17.1

recycling of, 8.14–8.16reducing environmental impact of,

10.16–10.17reduction of toxic solvents in, 8.12–8.14reuse of components of, 8.15risky substances in, 8.7–8.9risky substances in manufacture of, 8.9–8.10waste recovery from, 1.3

Electrowinnowing, 10.14–10.15Encapsulants:

reworkable, 8.22–8.23underfill, 5.13

Energy:controlling consumption of, 8.5–8.7reducing use in PCB manufacture,

8.19–8.20Environmental labeling, 11.1Environmentally benign die attach films,

7.1–7.8Environmentally benign In-Sn die attach

bonding technique, 7.10–7.17Environmentally benign manufacturing

(EBM):of electronics, trends in, 1.6–1.14worldwide trends in, 1.3–1.6

education activity, 1.5government activity, 1.4industry activity, 1.4R&D activity, 1.5

Environmentally benign moldingcompounds, 6.1–6.30

for MAP-PBGA, 6.22–6.28for PBGA, 6.10–6.22for PQFP, 6.1–6.10

Environmental Protection Agency (EPA),history of, 1.1–1.2

Epoxy:as conformal coating, 10.18direct melting of, 8.13lignin-based, 8.13–8.14, 8.21

Epoxy resin, self-extinguishing,9.18–9.19

Ericsson, Pb-free alloys used by, 11.14Etchants, chromic-based vs.

ammoniacal/copper, 10.13Etch resist, Sn-Pb vs. Sn, 10.13Europe:

adoption of Pb ban in, 1.3favorite Pb-free alloys in, 12.13Pb-free initiative in, 11.13

Extraneous plating, 16.4–16.5

FFillers, nonconductive, 5.3, 5.22–5.29Fillet lifting, 11.9, 12.33, 16.20–16.24

causes of, 16.20–16.21minimizing, 16.24

Finland, Pb ban in, 11.2FIRESEL, 8.25Flame resistance, means of achieving, 6.1

INDEX I.3

Flame retardancy:design for, 8.25factors in, 8.23–8.24

Flame retardants:additive, 6.2–6.4, 9.4–9.5

uses of, 9.4, 9.5antimony-containing, 8.1brominated (see Brominated flame

retardants)halogenated:

elimination of, 8.11–8.12health effects of, 8.24uses of, 8.23

halogen-free (see Halogen-free flameretardants)

novel resins, 6.4oligomeric, 9.5phenolic, 1.12reactive, 9.4

health effects of, 9.4Flip chip in a package (FCIP), solder-

bumped, 1.9Flip chip interconnection, 17.13–17.15Flip chip on board (FCOB) assemblies with

ACF, 5.4–5.5SIR test of, 5.10thermal cycling test of, 5.9–5.10

Flip Chip Technology, WLCSP testingprocedures of, 3.15–3.19

Flow coating, 10.19Fluorine, 1.12, 8.7Fluxes:

cleaning residue from, 16.34–16.35compatibility of, 15.15

effect of reflow temperature on, 15.15for Pb-free paste handling, 15.29for Pb-free paste soldering, 15.26–15.29for Pb-free residue cleaning, 15.31water-soluble, 10.21

Fluxless bonding, 7.10–7.13Ford Motor Co., Pb-free alloys used by, 11.14FormFactor, microspring technology of,

4.10–4.11, 5.12Fossil fuels, environmental effects of, 8.4, 8.5Freon, decomposition of, 8.4Fujitsu Computer Packaging Technologies:

Pb elimination plans of, 11.5Pb-free alloys used by, 11.14WIT technology of, 4.9–4.10, 5.11–5.12

Furukawa, Cu stud bump technology of,4.17–4.23

GGeorgia Institute of Technology, Cu SBB

technology of, 5.42–5.43Germany:

environmental labeling in, 11.1Pb-free alloys used in, 11.13

Glass transition temperature (Tg):controlling PBGA warpage with

dispersion of, 6.16–6.19effect on die attach material, 7.7effect on package coplanarity, 6.25of PCB, 1.9–1.10

Global warming, 8.2Gold, intermetallic compounds formed by,

16.10, 16.11Gold-gold metallic diffusion, 5.35Gold-gold thermocompression, optimal

parameters for, 5.41Gold (solderless) bumps:

assemblies using, 5.4–5.5vs. Cu bumps, 5.9–5.10

Grain structure, effect of additives on,12.9–12.11

Green card technologies, 8.19

HHadco, Pb elimination plans of, 11.7Halogen-free flame retardants, 1.11–1.12

availability of, 8.25for CABGA, 6.22hydroxides as, 6.12–6.16international driving forces for, 8.23–8.25non-phosphorus, 8.12toxicology of, 9.7–9.11

Halogens, 1.11–1.12elimination of, 8.11–8.12as flame retardants, 8.7–8.8

health effects of, 8.24HAL User Group, 11.6Health hazard assessment, 10.2–10.6Heat cure, 10.19Hitachi:

die attach films of, 7.1–7.9Pb elimination plans of, 11.5, 15.1Pb-free alloys used by, 11.14solder ball mounting method of, 2.12–2.20stress-relaxation layer study of, 3.1–3.5

Hitachi Chemical, double-layer ACF of,5.1–5.3

Holland, producer responsibility laws in, 11.2Home Electronics Recycling Law, 11.4

I.4 INDEX

Honeywell, Pb elimination plans of,11.7

Hot-air solder leveling (HASL), 14.1,14.62–14.63, 14.68

drawbacks of, 10.14etch resist used in, 10.13–10.14fabrication of, 14.62–14.63pros/cons of, 14.63Sn-Cu, 14.60–14.61wetability of, 15.36, 16.26

Housings, recycling of, 8.15HQ, 20.2–20.3, 20.7–20.13Hydrazine, 20.2

IIBM, Pb elimination plans of, 11.7IBM Research, diluted cleaning solution by,

8.18Iceland, Pb ban in, 11.2Imidazoles, 14.7–14.8Immersion bismuth surface finish,

14.36–14.37fabrication of, 14.36performance of, 14.36–14.37

Immersion gold process, 14.23–14.24Immersion gray tin, 14.51, 14.53Immersion silver surface finish,

14.26–14.36fabrication of, 14.26–14.28microetch of, 14.28–14.29performance of, 14.30–14.36plating chemistry of, 14.29–14.30

Immersion white tin, 14.50–14.51,14.52–14.53

Improved Design Life and EnvironmentallyAware Manufacture of ElectronicAssemblies by Lead-Free Soldering,Pb-free initiative of, 11.13

Indium:availability of, 12.4cost of, 12.4melting temperature of, 7.10properties of alloys containing, 12.36,

13.24, 13.29, 16.12regional preferences for, 12.36toxicity of, 12.2

Industrial waste, transfer of, 8.5Institute for Printed Circuits (IPC), Pb

policy of, 11.6Integrated circuits:

fabrication of, 1.9

Integrated circuits (Cont.):packaging of, 1.9waste in manufacture of, 1.9

Interconnects, Pb-free, qualifying, 11.7–11.8Intergovernmental Panel on Climate Change

(IPCC), global warming predictionsof, 8.2

Intermetallic compounds (IMCs),16.10–16.11

in Pb-free solders on Cu UBM, 2.31in Pb-free solders on electroless Ni-P-

immersion Au UBM, 2.27–2.29platelet formation by, 16.30–16.31pros/cons of, 2.3and reflow times, 2.22–2.24in surface finishes, 14.69thickness of, 15.16

International Project on Flame Retardancyin Electronics-Conceptual Study, 8.23

Iodine, 1.12Ionic testing, 10.21IPC Works ’99, 11.6Iron, effect on solder grain structure, 12.11Isothermal fatigue of WLCSPs, 3.5–3.8Isotropic conductive adhesives (ICAs), 1.12,

8.10–8.11, 17.8–17.15adhesive matrix for, 17.9–17.10applications for, 17.11–17.15conductive fillers for, 17.10–17.11conductivity in, 18.11–18.12contact resistance of, 19.2reworkability of, 5.13in SBB, 5.13vs. solders, 8.10–8.11

JJapan:

adoption of waste recovery in, 1.3dioxin problems in, 8.24effects of Pb elimination efforts in, 11.5elimination of halogens in, 8.11–8.12favorite Pb-free alloys in, 12.13Pb ban in, 11.2Pb-free initiative in, 11.13, 11.15Pb problems in, 11.1recycling laws in, 11.2, 11.4

Japanese Electronics Industry DevelopmentAssociation (JEIDA):

Pb-free alloys recommended by, 11.13,12.13

Pb-free roadmap of, 11.4

INDEX I.5

Japanese Institute of Electronic Packaging(JIEP), Pb-free roadmap of, 11.4

KKorea Advanced Institute of Science and

Technology (KAIST):ACAs by, 5.22–5.23electroless Ni-P plating process of, 2.3–2.6stencil printing process of, 2.20–2.26

Kulike & Soffa, SBB equipment made by, 4.17Kumgang Korea Chemical, CABGA

technology of, 6.22

LLamination, 5.4Lasers, use in PCB manufacture, 10.16Lead, 8.7

alternatives to, 17.15ban on in electronics, 1.3, 11.1–11.5

reasons for, 1.10contamination of solders by, 16.14–16.20cost of, 12.4disposal of, 11.1in electronic components, 11.12health problems caused by, 8.7, 11.1as impurity in Pb-free alloys, 11.7manufacturer elimination plans, 11.4–11.5,

11.7reporting threshold for, 11.6

opposition to, 11.7toxicity of, 12.2uses of, 11.1

Lead-based solder paste, cleaningperformance of, 15.31

Lead-based solders:compatibility of, 15.7, 15.19developing alternatives to, 11.13–11.14vs. ECAs, 19.1replacement of, 8.10

Lead-free paste handling, fluxes for, 15.29Lead-free paste soldering, fluxes for,

15.26–15.29Lead-free residue cleaning:

chemistry for, 15.31–15.34fluxes for, 15.31

Lead-free soldering:effect of reflow profile on, 15.21–15.25unanswered challenges for, 16.36–16.37

Lead-free solder paste:cleaning performance of, 15.29–15.30,

15.31, 16.34–16.35selection of, 15.36

Lead-free solders, 1.10–1.11, 8.11, 11.8–11.11alloys, 13.1–13.62 (see also specific alloys)

compatibility of, 15.7, 15.15, 15.19existing, 12.4–12.5modification of, 12.5–12.13performance of, 15.26–15.27properties of, 12.14–12.32regional preferences for, 12.33–12.36

challenges for reliability of, 16.29–16.36compatibility with SMT reflow process,

15.1–15.19cost of, 12.4criteria for, 11.8, 12.1eutectic Sn-Ag, 13.1–13.14

mechanical properties of, 13.1–13.6physical properties of, 13.1reliability of, 13.10–13.14wetting properties of, 13.6–13.10

eutectic Sn-Cu, 13.14–13.23mechanical properties of, 13.14physical properties of, 13.14reliability of, 13.17–13.23wetting properties of, 13.14–13.17

melting point of, 1.10, 6.1, 7.1, 13.14microball wafer bumping with,

2.6–2.12molding compounds compatible with,

6.7–6.10most favorable, 15.1patent issues with, 12.36–12.37Pb contamination of, 16.14–16.20problems with, 1.11reflow temperature of, 1.10–1.11, 6.1, 7.1safety of, 11.15Sn96.5/Ag3.5, 11.8Sn99.3/Cu0.7, 11.8–11.9SnAg, 15.7, 16.30Sn-Ag-Bi, 13.23–13.29, 15.7

physical/mechanical properties of,13.23–13.24

reliability of, 13.26–13.29wetting properties of, 13.24–13.26

Sn-Ag-Bi-In, 13.23–13.29physical/mechanical properties of, 13.24reliability of, 13.29

SnAgBiX, 11.9–11.10SnAgCu, 11.9, 13.31–13.53, 15.7, 16.30

mechanical properties of, 13.34–13.41physical properties of, 13.31–13.34reliability of, 13.45–13.53wetting properties of, 13.42–13.44

SnAgCuSb, 15.7

I.6 INDEX

SnAgCuX, 11.9, 13.31–13.53mechanical properties of, 13.34–13.41physical properties of, 13.31–13.34reliability of, 13.45–13.53wetting properties of, 13.42–13.44

SnBi, 11.11, 15.7SnSb, 11.10, 15.7Sn-Zn, 13.54–13.57

mechanical properties of, 13.55physical properties of, 13.54reliability of, 13.56–13.57wetting properties of, 13.55–13.56

Sn-Zn-Bi, 13.54–13.57, 15.7mechanical properties of, 13.55physical properties of, 13.54reliability of, 13.56–13.57wetting properties of, 13.55–13.56

SnZnX, 11.10thermal damage with, 11.12–11.13,

16.33–16.34wetting problems with, 16.25–16.26

Lead-free surface finishes, 14.1–14.82 (seealso individual finishes)

challenges for, 16.1–16.10for components, 14.70, 14.76–14.78pros and cons of, 14.68–14.69

Lead-free wave soldering, implementing,15.19–15.20

Life cycle assessment (LCA), 8.16, 10.11drawbacks of, 10.11

Lignin, epoxies containing, 8.13–8.14, 8.21Liquid crystal display (LCD) panels,

17.7Lucent Technologies, Pb elimination plans

of, 11.7

MMacrovoiding, 15.16Magnesium hydroxide, as flame retardant,

6.13–6.15, 8.11Matsushita:

Au-stud-bumped FCOF with ICA by,5.14–5.22

“green” products by, 1.3ICA by, 5.18LCP tape by, 5.17Pb elimination plans of, 15.1Pb-free alloys used by, 11.14Pb-free products by, 11.4

Melting point:of Pb-free solders, 1.10, 13.14of Sn-Pb solders, 1.10

Melting temperature, effect of additives on,12.6

Microballs:formation of, 2.6–2.8management of, 2.9–2.12problems with, 2.9–2.10pros/cons of, 2.6touch-up of, 2.12wafer bumping with Pb-free solders, 2.6–2.12

Microelectronics (see Electronics)Microsprings, 4.10–4.11

fabrication of, 4.11pros/cons of, 4.11with solders or adhesives on

PCB/substrate, 5.12Microvias, methods of forming, 10.16Microwave curing, 8.19–8.20Mitsubishi, Pb elimination plans of, 11.5Modulus, effect on die attach materials, 7.7Moisture absorption, effect on die attach

film, 7.4Moisture cure, 10.19Moisture sensitivity level (MSL)

performance, factors influencing, 6.28Mold array PBGA, environmentally benign

molding compounds for, 6.22–6.28Molding compounds:

conventional, components of, 6.1–6.2effects of reflow temperature on, 6.4–6.8environmentally benign (see Environ-

mentally benign molding compounds)halogen-free, 6.8–6.10Tg of, effect on package coplanarity, 6.25total shrinkage of, 6.16, 6.26

effect on package coplanarity, 6.25–6.26minimizing, 6.16–6.18

for use in Pb-free soldering, 6.7–6.10viscosity of, effect on package coplanarity,

6.26Motorola:

immersion plating process of, 14.59Pb elimination plans of, 11.7Pb-free alloys used by, 11.14stencil printing process of, 2.27–2.31WLCSP reliability testing process of,

3.5–3.14

NNational Center for Manufacturing Sciences

(NCMS):Pb-free alloys recommended by, 11.13, 12.33Pb-free initiative of, 11.13

INDEX I.7

National Electronics ManufacturingInitiative (NEMI):

Pb-free alloys recommended by, 11.13,12.33

Pb-free initiative of, 11.6, 11.13Pb-free roadmap of, 11.14–11.15

National Institute of Standards andTechnology (NIST), Pb-free initiativeof, 11.13

Natural resources, depletion of, 8.4NEC:

Pb elimination plans of, 11.5Pb-free alloys used by, 11.14SBB technology of, 5.37–5.38use of flame-retarding plastics by, 9.17

New Industry and Industrial TechnologyDevelopment Organization, Pb-freeinitiative of, 11.13

Nickel:electroless, interface with solders,

2.20–2.22as surface finish, 14.14–14.15

Nickel-gold (solderless) bumps:assemblies using, 5.4–5.5vs. Cu bumps, 5.5

Nickel-gold surface finishes, 14.14–14.26electroless Ni/electroless (autocatalytic)

Au, 14.25–14.26electroless Ni/immersion Au (ENIG),

14.18–14.25, 14.70–14.71, 16.1, 16.4chemistry of, 14.20–14.25fabrication of, 14.18–14.20performance of, 14.24–14.25,

14.70–14.71electrolytic Ni-Au (EG), 14.15–14.18

fabrication of, 14.15–14.16performance of, 14.16–14.18

Nickel surface finishes:electroless Ni/Pd, 14.71–14.72electroless Ni/Pd/(Au FLASH):

fabrication of, 14.43–14.44performance of, 14.44–14.45

electrolytic Ni/Sn, 14.55–14.56fabrication of, 14.56performance of, 14.56

Ni/Pd(X) surface finishes, 14.45–14.46electroless Ni/PdNi/Au FLASH,

14.45–14.46electrolytic Ni/PdCo/Au FLASH, 14.45

Nippon Steel, microball wafer bumpingprocess of, 2.6–2.12

Nitrogen, flame retarding action of, 8.12

Nitto Denko, molding compounds developedby, 6.10–6.12, 6.21

Nokia, Pb-free alloys used by, 11.14Nonconductive adhesives (NCAs), 1.12Nordic Swan, 8.24Nortel Networks:

Pb-free alloys used by, 11.14Pb-free initiative of, 11.3

North America, favorite Pb-free alloys in,12.33

Norway:Pb ban in, 11.2producer responsibility laws in, 11.2

NTT, Pb elimination plans of, 11.5

OOligomers, 9.5Optipad, 14.64–14.65

fabrication of, 14.64performance of, 14.65

Organic solderability preservatives (OSPs),11.11, 14.1–14.14, 14.69 (see alsospecific OSPs)

Outgassing, 17.12Oxidation, 19.2–19.3

resistance to, effect of additives on, 12.8Oxygen scavengers, 20.1–20.3

effects of, 20.7–20.13types of, 20.2

Ozone:at ground level, 8.5hole in, 8.4

PPalladium, intermetallic compounds formed

by, 16.10–16.11Palladium surface finishes, 14.38–14.43, 19.1

electroless (autocatalytic) Pd with/withoutimmersion Au, 14.42–14.44

fabrication of, 14.42performance of, 14.42–14.43

electrolytic Pd/Ni, 14.72electrolytic Pd with/without immersion

Au, 14.38–14.41, 14.71fabrication of, 14.38–14.39performance of, 14.39–14.41, 14.71

Panasert, SBB equipment made by,4.14–4.16, 5.14

Parylene, as conformal coating, 10.18Passivation cracking, in Ni-Au solder bumps,

4.2–4.6Percolation theory, 18.1

I.8 INDEX

Personal computers (PCs), numbers of, 1.2Phase matrix, 10.2, 10.6Phenolics, 1.12Phosphorus, 8.11

and black pad, 16.2–16.4in ENIG process, 14.23flame retarding action of, 8.12

Photodielectrics, 10.16Pick and place, 5.4PICOPAK, solder bump formation process

of, 4.2Plastic ball grid array (PBGA):

controlling warpage of:with stress-absorbing agents, 6.19–6.21with Tg dispersion, 6.10

conventional molding compounds for,6.16–6.17

die attach materials for, 7.6–7.9environmentally benign molding

compounds for, 6.10–6.22vs. PQFP, 6.10

Plastic quad flat pack (PQFP):die attach materials for, 7.1–7.6environmentally benign molding

compounds for, 6.1–6.10vs. PBGA, 6.10

Plastics:disposal/recycling of, 8.15–8.16flame-retardant, 9.11–9.19

Pollution, 8.4Polybrominated biphenyls (PBBs), 9.5

breakdown products of, 9.1, 9.5environmental effects of, 8.1, 9.5health effects of, 9.1, 9.5vs. PCBs, 9.5uses of, 9.8

Polybrominated dibenzodioxins (PBDDs),9.1

Polybrominated dibenzofurans (PBDFs), 9.1Polybrominated diphenyl ethers (PBDEs),

9.6breakdown products of, 9.1environmental effects of, 8.1, 9.5health effects of, 9.5uses of, 9.8

Polycarbonate resin, flame-retardant,9.12–9.17

Polychlorinated biphenyls (PCBs):health effects of, 9.5vs. PBBs, 9.5uses of, 9.8

Polyimides, moisture absorption of, 7.2

Polymers, 1.11combustion mechanism of, 8.8

Polytetrafluoroethylene (PTFE), 8.23Precision pad technology (PPT), 14.66


Preflux, 14.14Prepregs, 8.10

manufacture of, 8.19Print and etch, 10.12Printed board assemblies (PBAs), recycling

of, 8.15Printed circuit board (PCB), 1.9–1.10

cleaning of, 10.21DfE for, 10.1–10.2double-sided, 10.12environmental issues for, 8.1–8.27environmentally friendly, manufacture of,

10.1–10.22future trends in, 10.15

environmental research on, 8.18–8.22glass transition temperature of, 1.9–1.10halogen-free, prices of, 8.12manufacturing process of, 8.10, 10.1multilayer, 10.12with OCC pads, 5.5renewable resins for, 8.21single-sided, 10.12surface finishes for, 10.14

Pb-free, 11.11–11.12waste in manufacture of, 1.10, 10.12

Process chemistries, nonchelated, 10.14Process models:

for electronic packaging, 10.11for PCB, 10.1–10.2for semiconductor manufacturing, 10.11uses of, 10.2

Producer responsibility laws, 11.2

RRain forests, destruction of, 8.5Recycling:

of bismuth, 12.33of electronic products, 8.14–8.16laws about, 11.2, 11.4of manufacturing by-products, 10.14–10.15of water, 10.15

Reduction of Hazardous SubstancesDirective (ROHS), 1.2–1.3, 11.3

Reflow profile, effect on Pb-free soldering,15.21–15.25

Reflow soldering, and fillet lifting, 16.22–16.23

INDEX I.9

Reflow temperature:and CAF, 16.36and compatibility of fluxes/alloys, 15.15and deformities, 6.1effects on molding compounds, 6.4–6.8of Pb-free solders, 1.10–1.11, 6.1of Sn-Pb solders, 1.10, 6.1

Reflow time:effects on electroless Ni–solder interface,

2.20effects on solder bump shear strength,

2.24–2.26Resins (see also Preflux)

B-stage, 8.10, 8.19C-stage, 8.10renewable, 8.21water-based, 8.19–8.20

Rosins, 14.14

SSAF-A-LLOY, 13.41Samsung Group, “green” products of,

11.3Selective coating, 10.19Semiconductors:

environmental issues with, 10.11market for, 1.6–1.8technology trends in, 1.7

Sharp, bonding process of, 5.33–5.35Shipley Co. LLC, Pb elimination plans of,

11.7Silicone:

as conformal coating, 10.18as flame retardant, 9.12–9.13

Silver:advantages of, 14.26as conductive filler, 17.10cost of, 12.4effect on solder mechanical properties,

12.11IMCs formed by, 16.10toxicity of, 12.2uses of, 12.4

Silver filler content, effect on die attach film,7.4

Silver flake:vs. blank Ag powder, 18.8lubricant behavior of, 18.10resistance of, 18.5, 18.18

Silver pastes, 7.1Silver powder:

conductivity of, 18.11–18.12, 18.18

Sipad, 14.65–14.66fabrication of, 14.65performance of, 14.65–14.66pros/cons, 14.65

Site modeling, 10.2–10.3Skip plating, 16.4–16.5Solder balls, Sn-Ag-Cu, mounting on wafers,

2.12–2.20Solder bumps:

electroless Ni-Au:advantages of, 4.2fabrication of, 4.2passivation cracking in, 4.2–4.6

electroless Ni-P-immersion Au, 4.1–4.6electroplated Au, 4.6

formation of, 4.6microhardness of, 4.6specifications and measurement

methods, 4.6thin film structure of, 4.6

electroplated Cu, 4.8–4.9formation of, 4.8pros/cons of, 4.8–4.9

shear strength of, 2.24–2.262.30–2.31sizes of, 2.1

Solder cladding, 14.67fabrication of, 14.67performance of, 14.67

Soldering processes:challenges for, 16.10–16.11conventional, oxides in, 7.10

Solder jetting, 14.67fabrication of, 14.67performance of, 14.67

Solder joints, 16.14–16.16effects of Pb contamination on,

16.14–16.16In-Sn, 7–12–7.17minimizing stress in, 7.10reliability on WLCSPs, 3.1–3.19rough appearance of, 16.28–16.29stiff, 16.31–16.33

Solders:additives to:

effect on bond strength, 12.6effect on grain structure, 12.9–12.11effect on melting temperature, 12.6effect on oxidation resistance, 12.8effect on wetting, 12.5–12.6, 12.7–12.8

vs. adhesives, 4.17, 5.42vs. alternative attachment technologies,

5.1, 17.15

I.10 INDEX

Solders (Cont.):components of:

cost/availability of, 12.4toxicity of, 12.1–12.4

cost of, 11.11vs. ICAs, 8.10–8.11impurity tolerance of, 12.11–12.13In-Sn, die attach bonding using, 7.10–7.17

characterization of, 7.14–7.17design and process of, 7.12–7.14phase diagram for, 7.11–7.12

interface with electroless Ni UBM,2.20–2.22, 2.27–2.29

interface with Ti-Cu UBM, 2.31–2.34paste printing on wafers with Al-NiV-Cu

UBM, 2.34Pb-based (see Lead-based solders)Pb-free (see Lead-free solders)polymer (see Isotropic conductive

adhesives)Sn-Pb vs. Pb-free, 15.27

melting point and reflow temperatureof, 6.1

stencil printing of, 2.20–2.31surface tension of, 15.27for wave soldering, composition of,

16.13–16.14Solectron, Pb-free alloys used by, 11.14Solid solder deposition (SSD), 14.62–14.68Solvents:

changes in, 10.13concerns about, 8.9–8.10reducing use in PCB manufacture,

8.19–8.20toxic, reduction of, 8.12–8.14

Sony:Pb elimination plans of, 11.4–11.5Pb-free alloys used by, 11.14

Stannic oxide, 14.48State Fire Marshals Association, 8.2Stress-absorbing agents, controlling PBGA

package warpage with, 6.19–6.21Stress-relaxation layer, 2.15–2.20

effects on capacitance, 3.4thermal fatigue life of, 3.2–3.4

Stud bump bonding (SBB):adhesives used in, 5.42requirements for, 5.45using Cu:

vs. using Au, 5.43–5.45with Pb-free solders on PCB,

5.42–5.45

Stud bump bonding (SBB) (Cont.):using Au, 4.12–4.17

equipment used in, 4.14–4.17fabrication of, 4.12vs. using Cu, 5.43–5.45with ICA on PCB, 5.13–5.14with ICA on tape, 5.16–5.22with NCA on Au-plated flex, 5.37–5.41with NCA on Au-plated PCB, 5.32–5.35

Stud bumps:Au, 4.12–4.17

equipment used for, 4.14–4.17fabrication of, 4.12

Cu, 4.17–4.23application of, 4.17fabrication of, 4.21shear strength of, 4.23

Study of Critical Environmental Problems,1.1

Substrates, flexible, advantages of, 5.15Sulfites, 20.2Sumitomo, flame retardants studied by, 6.2Sun Microsystems, Pb elimination plans of,

11.7Super Solder, 14.67–14.68


Surface finishes:cleaning resistance of, 16.10Pb-free (see Lead-free surface finishes)SnPb, and fillet lifting, 16.24

Sweden:Pb ban in, 11.2producer responsibility laws in, 11.2

Sweden Environmental Quality Objectives,11.2

Switzerland, producer responsibility laws in,11.2

TTacking, 5.4TCO’95/TCO’99, 8.24Temperature, effect on die attach materials,

7.7Tetrabromobisphenol A (TBBA), 9.2, 9.6–9.7

breakdown products of, 9.6, 9.7environmental effects of, 9.8–9.9health effects of, 9.6–9.9uses of, 9.4, 9.8

Texas Instruments:Pb elimination plans of, 11.7Pb-free alloys used by, 11.14

INDEX I.11

Thermal fatigue:failure mechanisms in, 3.11–3.14of Sn-Ag, Sn-Ag-Cu, Sn-Ag-Cu-Sb, and

Sn-Ag-In-Cu WLCSPs on ceramicsubstrate, 3.15

of Sn-Ag-Cu WLCSP on PCB, 3.15of solder/UBM systems, 3.8–3.14and stress-relaxation layer, 3.2–3.5

Thermosetting resin content, effects on dieattach film, 7.2–7.4

Through-holes, in thick boards, 14.69, 16.1Tin:

characteristics of, 8.11, 14.46–14.47as etch resist, 10.14properties of alloys containing,

12.35–12.36pure, use in solder bumps, 1.9regional preferences for, 12.35–12.36toxicity of, 12.2

Tin-bismuth surface finishes, 14.59Tin-copper HASL surface finish, 14.60–14.61

fabrication of, 14.60performance of, 14.60–14.61

Tin pest, 14.69, 16.29–16.30Tin surface finishes, 14.46–14.55, 14.72

electrolytic Sn, 14.47–14.50fabrication of, 14.48–14.50performance of, 14.50

electrolytic Sn-Ag, 14.72–14.74fabrication of, 14.73performance of, 14.73–14.74

electrolytic Sn-Bi, 14.74–14.75fabrication of, 14.74performance of, 14.74–14.75

electrolytic Sn-Ni, 14.61fabrication of, 14.61performance of, 14.61

immersion Sn, 14.50–14.55applications of, 14.55fabrication of, 14.51performance of, 14.52

Sn-Cu, 14.75–14.76fabrication of, 14.76performance of, 14.736

Sn-Pb, resistance of, 19.1Tin whisker, 14.69, 16.5–16.10

causes of, 16.5–16.9growth rate of, 16.9–16.10

Toshiba:“green” products by, 1.3Pb elimination plans of, 11.5, 15.1Pb-free alloys used by, 11.14

Total shrinkage of molding compounds, 6.16,6.26

effect on package coplanarity, 6.25–6.26Toxicity, 9.8Transient liquid-phase sintering, 17.11Transition metal magnesium hydroxide

complex (TMMHC), as flameretardant, 6.14–6.16

Trichloroethane, 10.13Two-component mixing, 10.18

UUltraviolet (UV) light cure, 10.19Under-bump metallurgy (UBM), 2.1–2.6

Al-NiV-Cu, 2.6electroless Ni-Au, advantages of, 4.2electroless Ni-P immersion Au, 2.1–2.6types of, 2.1, 2.2

Underfill encapsulants, reworkability of, 5.13United States:

Pb-free initiatives in, 11.13Pb limits in, 11.1Pb reporting threshold levels in, 11.6

suit opposing, 11.7University of California-Irvine, low-

temperature fluxless bondingtechnique of, 7.1, 7.10, 7.12

VVisasystems, Pb elimination plans of, 11.7Viscosity, effect on package coplanarity,

6.26Voiding, 16.26–16.28

WWafer bumping:

companies that provide, 1.9methods of, 2.1

Wafer-level chip-scale packages (WLCSPs),2.12–2.20

with Au, Cu, or Ni-Au bumps, on PCBwith ACF, 5.1–5.11

with Au stud bumps diffused on Au-platedflex with NCA, 5.37–5.41

with Au stud bumps diffused on Au-platedPCB with NCA, 5.29–5.35

with Au stud bumps with ACA/ACF onPCB, 5.22–5.29

with Au stud bumps with ICA on flex,5.14–5.22

with Au stud bumps with ICA on PCB,5.12–5.14

I.12 INDEX

Wafer-level chip-scale packages (Cont.):with Cu stud bumps with Pb-free solders

on PCB, 5.42–5.45with Cu wire, with solders or adhesives on

substrate, 5.11–5.12reliability of solder joints on, 3.1–3.19Sn-Ag-Cu on PCB:

high-temperature storage of, 3.15–3.17shear strength of, 3.17

with stress-relaxation layer, 2.15–2.20using solder vs. adhesive, 5.43–5.45

Wafers, for Ni-Au, electroplated Au, andelectroplated Cu bumps, 4.1

Waste:in bare board manufacture, 10.2and design parameters, 10.8industrial, transfer of, 8.5in manufacture of ICs, 1.9in PCB manufacture, 1.10, 10.12recovery from electronics, 1.3

Waste Electrical and Electronic EquipmentDirective (WEEE), 1.2–1.3, 8.24,11.2–11.3

Waste streams, predicting, 10.2

Wastewater:ecological concerns about, 8.10methods of treating, 10.14

Water, reuse/recycling of, 10.15Wave coating, 10.19Wetting:

effect of additives on, 12.5–12.6, 12.7–12.8effect of surface finishes on, 13.6–13.7problems with, 16.25–16.26

Wire bonding, surface finishes for, 14.69Wire interconnect technology (WIT),

4.9–4.10, 5.11–5.12fabrication of, 4.10structure of, 4.9

ZZinc:

cost of, 12.4effect on solder mechanical strength,

12.11properties of alloys containing, 12.35, 15.7,

16.12regional preferences for, 12.35toxicity of, 12.2

INDEX I.13

ABOUT THE AUTHORS

John H. Lau received his PhD in theoretical and applied mechanics from the Uni-versity of Illinois, an MASc in structural engineering from the University of BritishColumbia, a second MS in engineering physics from the University of Wisconsin,and a third MS in management science from Fairleigh Dickinson University. He alsohas a BE in civil engineering from National Taiwan University. John is an intercon-nection technology scientist at Agilent Technologies, Inc. His current interests covera broad range of optoelectronic packaging and manufacturing technology.

Prior to coming to Agilent, Lau worked for Express Packaging Systems, Hewlett-Packard, Sandia National Laboratory, Bechtel Power Corporation, and ExxonProduction and Research Company. With more than 30 years of R&D and manu-facturing experience in the electronics, photonics, petroleum, nuclear, and defenseindustries, he has given over 200 workshops and invited presentations, authored andcoauthored over 200 peer-reviewed technical publications, authored more than 100book chapters, and is the author and editor of 14 books on IC packaging.

Lau has served on the editorial boards of IEEE Transactions on Components,Packaging, and Manufacturing Technology and ASME Transactions, Journal of Elec-tronic Packaging. He also has served as general chairman, program chairman, ses-sion chairman, and invited speaker at several ASME, IEEE, ASM, MRS, IMAPS,SEMI, and SMI International conferences. He has received many awards from theASME and IEEE for best proceedings and transactions papers and outstandingtechnical achievements and is one of the distinguished lecturers of the ASME andIEEE/CPMT. He is an ASME Fellow and IEEE Fellow and is listed in AmericanMen and Women of Science and Who’s Who in America.

C. P. Wong is a Regents’ Professor at the School of Materials Science and Engineer-ing and a Research Director at the NSF Packaging Research Center at the GeorgiaInstitute of Technology. He received his BS in chemistry from Purdue University,and his PhD in chemistry from Pennsylvania State University. Thereafter, he wasawarded a two-year postdoctoral fellowship at Stanford University with Nobel Lau-reate Professor Henry Taube.

Wong spent 19 years at AT&T Bell Labs and was elected a Bell Labs Fellow in1992. His research interests lie in the fields of polymeric materials, reaction mecha-nism, IC encapsulation, hermetic equivalent plastic packaging, electronic packagingprocesses, interfacial adhesions, PWB, SMT assembly, and component reliability.

He has received many awards, among which are the AT&T Bell LaboratoriesDistinguished Technical Staff Award (1987), the AT&T Bell Labs Fellow Award

A.1


(1992), the IEEE Components, Packaging and Manufacturing Technology (CPMT)Society Outstanding and Best Paper Awards (1990, 1991, 1994, 1996, 1998 and 2002),the IEEE Technical Activities Board (TAB) Distinguished Award (1994), the IEEECPMT Society’s Outstanding Sustained Technical Contribution Award (1995), theGeorgia Tech Outstanding Faculty Research Program Development Award (1999),the NSF-Packaging Research Center Faculty of the Year Award (1999), the GeorgiaTech Sigma Xi Faculty Best Research Paper Award (2000), the University Press(London, UK) Award of Excellence (2000), the IEEE Third Millennium Medal(2000), the IEEE EAB Education Award (2001), and the IEEE Exceptional Techni-cal Contributions Award (2002). He holds over 40 U.S. patents, numerous interna-tional patents, and over 450 technical papers in related areas.

Wong was elected a member of the National Academy of Engineering in 2000,and he is a Fellow of the IEEE, AIC, and AT&T Bell Labs. He served as technicalvice president (1990 and 1991) and president (1992 and 1993) of the IEEE-CPMTSociety, the IEEE TAB Management Committee (1993 to 1994), and chair of IEEETAB Design and Manufacturing Committee (1994 to 1996), the IEEE Nominationand Appointment Committee (1998 to 1999), and the IEEE Fellow Committee(2001–).

Ning-Cheng Lee is the vice president of technology of Indium Corporation ofAmerica. He has been with Indium since 1986. Prior to joining Indium, he was withWright Patterson Air Force Base Materials Laboratory (1981 to 1982), MortonChemical (1982 to 1984), and SCM (1984 to 1986). He has more than 18 years ofexperience in the development of fluxes and solder pastes for SMT industries. Inaddition, he also has very extensive experience in the development of high-temperature polymers, encapsulants for microelectronics, underfills, and adhesives.His current research interests cover advanced materials for interconnects and pack-aging for electronics and optoelectronics applications, with emphasis on both highperformance and low cost of ownership.

Lee received his PhD in polymer science on structure-property relationshipsfrom the University of Akron in 1981. He also studied organic chemistry at RutgersUniversity in 1976 and received a BS in chemistry from National Taiwan Universityin 1973.

Lee is the author and coauthor of several books on electronic packaging tech-nologies. He received two awards from SMTA and one from SMT magazine for bestproceedings papers of international conferences. He also served on the editorialadvisory boards of Soldering and Surface Mount Technology and Global SMT andPackaging. He has been published in numerous publications and frequently givespresentations, invited seminars, keynote speeches, and short courses worldwide atmany international conferences or symposiums.

Shi-Wei Ricky Lee received his BS in mechanical engineering from National TaiwanUniversity in 1981. After two years of military service, he joined the Yue LoongMotor Engineering Center as structural testing engineer. In 1986, he went to theUnited States for postgraduate studies, receiving an MS in engineering mechanicsfrom Virginia Polytechnic Institute & State University in 1987 and a PhD in aero-nautics and astronautics from Purdue University in 1992.Through years of intensiveresearch, he has developed expertise in computational modeling and experimentalmethods. Before taking a teaching position at the Hong Kong University of Science& Technology (HKUST) in 1993, he spent one year at Purdue University as post-doctoral research associate and visiting assistant professor.

A.2 ABOUT THE AUTHORS

Currently, Lee is associate professor of mechanical engineering and director ofEPACK Lab at HKUST. He is an associate editor of IEEE Transactions on Compo-nents and Packaging Technologies and also sits on the editorial advisory boards oftwo international journals: Soldering and Surface Mount Technology and SmartMaterials and Structures. In 1997 to 1998, he served as guest editor for Smart Materi-als and Structures, and published a special issue on piezoelectric motors/actuatorsand their applications. Lee is very active in professional societies. He is a member ofTau Beta Pi, the ASME, IMAPS, and a senior member of the IEEE. He was the vice-chair of the Hong Kong Section of ASME International (1997 to 1998) and is chairof the Hong Kong Chapter of the IEEE-CPMT Society (2001 to 2002). He is also amember of the executive committee of the Electronic and Photonic Packaging Divi-sion of the ASME. Lee has served as track organizer and session chair for manyinternational conferences and sits on the program committee (interconnections) ofthe Electronic Components and Technology Conference. Furthermore, he is quitekeen on continuing education for professional development. He has organized sev-eral workshops and short courses, and has been invited to deliver short courses andseminars around the world.

Lee’s recent research activities cover flip chip and CSP technologies, wafer-levelpackaging, high-density interconnects, and mechanics for sensors and actuators. Hehas published numerous technical papers in international journals and conferenceproceedings, and is the coauthor of three books. He is a two-time recipient of theJEP Best Paper Award (2000 and 2001), conferred by ASME Transactions: Journalof Electronic Packaging, and owns one U.S. patent.

ABOUT THE AUTHORS A.3

Electronics Manufacturing with Lead-Free Halogen-Free and Conductive-Adhesive

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