Micro- and Opto-Electronic Materials, Structures, and Systems · Moisture sorption process refers...

Micro- and Opto-Electronic Materials, Structures,and Systems

Series EditorE. SuhirUniversity of California, Santa Cruz, CA, USA

For further volumes:http://www.springer.com/series/7493

X.J. Fan · E. SuhirEditors

Moisture Sensitivityof Plastic Packagesof IC Devices

Foreword by C.P. Wong

123

EditorsX.J. FanDepartment of Mechanical EngineeringLamar University

[email protected]

E. SuhirERS Co.Alvina Court 727

[email protected]

ISBN 978-1-4419-5718-4 e-ISBN 978-1-4419-5719-1DOI 10.1007/978-1-4419-5719-1Springer New York Dordrecht Heidelberg London

Library of Congress Control Number: 2010927324

© Springer Science+Business Media, LLC 2010All rights reserved. This work may not be translated or copied in whole or in part without the writtenpermission of the publisher (Springer Science+Business Media, LLC, 233 Spring Street, New York,NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use inconnection with any form of information storage and retrieval, electronic adaptation, computer software,or by similar or dissimilar methodology now known or hereafter developed is forbidden.The use in this publication of trade names, trademarks, service marks, and similar terms, even if they arenot identified as such, is not to be taken as an expression of opinion as to whether or not they are subjectto proprietary rights.

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Springer is part of Springer Science+Business Media (www.springer.com)

Beaumont,Texas 94024 Los Altos, California

Foreword

Moisture, according to the Merriam-Webster dictionary, is defined as “liquid dif-fused or condensed in relatively small quantity.” It is also defined, by Longmandictionary, as “small amounts of water that are present in the air, in a substance,or on a surface.” Both definitions emphasize on the small quantity, but presumablythe state of moisture is in liquid form. In fact, moisture, a small quantity of H2Omolecules, can be in vapor, liquid, or solid phase in air or in any substance.

It is interesting to note that moisture sorption is different from water sorption.Moisture sorption process refers to a process in a humid air environment, whilewater sorption refers to a complete immersion into water. Hydrophobic or superhy-drophobic (with water contact angle >150◦) materials can effectively prevent waterliquid from penetrating through surface, but not for the transmission of moisture(water vapor) in a humid air environment. This interesting phenomenon is illustratedin the first chapter of the book, although the entire book is focused on “moisture”sensitivity of plastic packages of integrated circuit (IC) devices.

Most of polymeric materials in IC packaging absorb moisture from an environ-ment. The presence of moisture in plastic materials alters thermal stress throughthe alteration of thermo-mechanical properties, induces hygroscopic stress throughdifferential swelling, induces vapor pressure that is responsible for the eventualpopcorn cracking, reduces interfacial adhesion strength, induces electrical-chemicalmigration-induced corrosion, and finally alters dielectric properties of materials.Despite the pivotal role of moisture, research activities in moisture-induced failureremain relatively low, compared to the thermally induced failure in IC packaging.This is in part due to the lack of material data and aggravated by the lack of fun-damental understanding of moisture transport, material characterization techniques,and procedures. These are reflected in the limited publications and the near absenceof such properties from the material vendors.

The development of three-dimensional (3D) microelectronics packaging withthrough-silicon via (TSV), ultrathin, and multi-die stacking technology has becomeessential to increase functionality with higher memory capacity in more complexand efficient architectures. Wafer thinning is required for such ultrathin die develop-ment from the original thickness of 750 μm down to as low as 30 μm. Consequently,new assembly and manufacturing processes must be invented to overcome thinwafer handling and cracking issues. Many new materials such as wafer-level coating

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vi Foreword

films have emerged. As a result, cohesive film rupture may occur due to moistureduring reflow. Therefore, one of the major challenges for a practical realization of3D microelectronics packaging concept is to design materials and meet reliabilityrequirements without cohesive failures subjected to moisture loads.

This book provides information on the state-of-the-art technologies and method-ologies related to moisture issues in plastic packages. The book covers the wideaspects including moisture diffusion and desorption, characterization and model-ing, hygroscopic swelling, interfacial adhesion degradation, accelerated moisturesensitivity/reflow test, electrical-chemical migration, moisture-aging effect on long-term reliability, and several finely selected real-world case studies on various failuremechanisms due to moisture. This is the first book ever to cover the full spectrum ofmoisture-induced failure mechanisms in IC packages. It is a timely and importantcontribution to the technical literature for researchers, engineers, and practitionersboth in academia and in electronics industry.

The editors of the book, Dr. Fan and Dr. Suhir, have rich experience inboth theoretical development and industrial practice. They have been offeringthe professional development courses at various IEEE (Institute of Electrical andElectronics Engineers) CPMT (components packaging and manufacturing technolo-gies) Society conferences, and hundreds of participants have attended their lectures.They have succeeded in bringing together well-recognized experts in this field andpresent a fine collection of papers covering the full spectrum of the related topics.They are to be congratulated for bringing this very important topic forth in a timelymanner.

Atlanta, GA C.P. WongNovember, 2009

Preface

Since moisture-sensitive plastic materials were introduced in integrated circuit (IC)device packaging several decades ago, moisture has been one of the major concernsfor package designers and reliability engineers. With the recent development of thethree-dimensional (3D) microelectronics packaging with through-silicon via (TSV)and multi-die stacking technologies, moisture-induced failures have become evenmore prominent due to the new materials employed and the overall reduction inpackage size and thickness. This book provides a comprehensive state-of-the-artand in-depth review of the fundamental knowledge and methodologies in the fieldof material and structural (“physical”) behavior and performance of various typesof moisture-sensitive plastic packages of IC devices.

The book consists of 21 chapters divided into six sections: (1) moisture diffusion,absorption and desorption, and adhesion degradation (Chapters 1, 2, 3, and 4); (2)hygroscopic swelling characterization and analysis (Chapters 5, 6, and 7); (3) inte-grated hydrothermal and thermal stress modeling (Chapters 8, 9, 10, 11, and 12);(4) case studies and applications (Chapters 13, 14, 15, 16, 17, 18, and 19); (5)electro-chemical migration (Chapter 20); and (6) molecular dynamics modeling andcharacterization (Chapter 21). Brief description of the chapter contents is set forthbelow.

Chapter 1 presents an overview of moisture-induced failures in plastic packagesof IC devices, and illustrates the fundamental characteristics of moisture diffu-sion, hygroscopic swelling, and adhesion degradation. Chapter 2 describes thelatest investigations of anomalous moisture diffusion and the corresponding adhe-sion behaviors in epoxy molding compounds. Chapter 3 provides a method anddetailed analysis for real-time moisture absorption and desorption in thin films.Chapter 4 reviews the existing methodologies of moisture diffusion modeling andwhole-field vapor pressure analysis. Chapters 5, 6, and 7 describe several character-ization methods and techniques for hygroscopic swelling, such as photomechanicsmeasurement techniques and point measurement method using thermo-mechanicaland thermo-gravimetric analyzers. Chapters 8, 9, 10, 11, and 12 provide a collec-tion of the most advanced analysis and methods for integrated hydrothermal stressmodeling. Chapter 8 describes recent progress in modeling of moisture diffusionand moisture-induced stresses in semiconductor and MEMS packages. Chapter 9presents a novel methodology for integrated vapor pressure, hygro-swelling, and

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viii Preface

thermo-mechanical stress modeling of IC packages. Chapter 10 describes a fail-ure criterion for moisture sensitivity of plastic packages based on the theory ofthin flexible plates of large deflections. Chapter 11 develops a continuum theoryand describes its application to moisture-induced failures in IC packages. Chapter12 reviews recent efforts to develop micromechanics-based failure theories/modelsand computational tools for material and process selection in the design and fabri-cation of plastic IC packages and provides recommendations for the improvementof their reliability under the anticipated service conditions. Chapters 13, 14, 15,16, 17, 18, and 19 are dedicated to several case studies on moisture-induced fail-ures in a wide range of package types, such as QFP (quat flat package), QFN(quat flat no-lead), and D2Pak (Chapter 14), QFN package (Chapter 15), system-in-packages and BGA (ball grid array) packages (Chapter 16), flip chip BGA packages(Chapter 17), and ultrathin 3D stacking die packages (Chapter 18). From mate-rial perspectives, epoxy molding compounds, die attach adhesives, and underfillmaterials are all covered in these case studies. Chapter 13 describes a new method-ology for an equivalent acceleration of the IPC/JEDEC moisture sensitivity levels.Chapter 19 reviews an automated simulation system to perform moisture-relatedmodeling for various package types. Chapter 20 describes the fundamentals ofthe phenomenon of the electrochemical migration (ECM), primarily manifestedas bridging metallic dendrites. Chapter 21 shows how molecular dynamics sim-ulation and the nano-scale characterization methods could be used to obtain aninsight into the moisture-induced failure modes and mechanisms at the atomisticlevel.

The original scope of the book was based on the professional developmentcourse notes of one of the editors, Dr. Fan, on moisture-related reliability in elec-tronic packaging, which has been presented at the IEEE (Institute of Electrical andElectronics Engineers) CPMT (components packaging and manufacturing technolo-gies) Society-sponsored conferences. To present a complete coverage on the latestdevelopment and the most recent advances in this field, we have invited experts inthis field to bring together a full spectrum of moisture-induced failure mechanismsin IC packages. We are grateful to all the authors from the industry and the academiafor their in-depth contributions and their efforts to bring this book to the readers.

The first editor, Dr. Fan, would like to express his gratitude to many of hisex-colleagues at Intel, Philips Research, and the Institute of Microelectronics inSingapore. Many of the book chapters reflect the results of numerous collabora-tive efforts and extensive team work. Without this work our book would never bepossible.

The second editor, Dr. Suhir, expresses his deep appreciation to his friends andformer colleagues at Bell Laboratories, Physical Sciences and Engineering ResearchDivision, at Murray Hill, NJ, and Allentown, PA, for introducing him about 25 yearsago to the subject of, and the challenges in, the exciting field of polymeric materialsin general and plastic packages of IC devices in particular. Dr. Suhir would like totake this opportunity to acknowledge, with thanks, his collaborations, for almost 20years, during the “golden age” of Bell Labs, with Shiro Matsuoka, Phil Hubbauer,Lloyd Shepherd, Louis Manzione, Don Dahringer, Harvey Bair, C.P. Wong, Arturo

Preface ix

Hale, John Segelken, Alan Lyons, Bonnie Bachman, Charles Cohn, Quazi Ilyas,and many other top-notch materials scientists, physicists, chemists, and chemicalengineers.

Beaumont, TX X.J. FanSanta Cruz, CA E. SuhirNovember 2009

Series Preface

This title is the second book in the series. The series encompasses a broad areaof micro-, opto-electronic, and photonic engineering, with particular emphasis onmaterials, physics, mechanics, design, reliability, and packaging. The titles in theseries feature eminent engineers and scientists as authors and/or editors focused onaddressing major issues in the above areas of engineering. Our objective is to havea comprehensive series on the materials, mechanics, physics, packaging, functionalperformance, and reliability as they pertain to micro- and opto-electronics.

The audience for these volumes are those who work in micro- and opto-electronics and photonics, as well as those in many related areas of applied scienceand engineering. The expected readers are practitioners and professionals, scientistsand researchers, along with senior-level undergraduate and graduate students. Thesevolumes can serve as expanded encyclopedias in the field of the mechanics of micro-and opto-electronic materials and structures. Selected titles could also serve as text-books, reference works, and as general guidance works for those interested in thesesubjects. The series contains both descriptions of the state-of-the-art developmentsin particular fields, as well as new results obtained by authors, editors, and their col-leagues. The authors also identify and address crucial, but still unresolved, issuesthat come up when discussing new developments and issues within the discussedtopics.

I am thankful to Dr. Fan, the editor of this title, who did the major work by bring-ing together an excellent team of experts and by putting together many outstandingchapters in this title. It has been a pleasure working with him.

I would also like to take this opportunity to thank the authors and editors of thebooks that are now in the process of being written as well as those authors who havealready completed their volume for this series. Potential authors, editors, and thosespecialists interested in making contributions to the current state of knowledge in aparticular field of engineering or applied science within the scope of this book seriesare invited to send their book proposals to me.

Santa Cruz, CA E. Suhir, Ph.D.Series Editor

xi

Contents

1 Fundamental Characteristics of Moisture Transport,Diffusion, and the Moisture-Induced Damagesin Polymeric Materials in Electronic Packaging . . . . . . . . . . 1X.J. Fan and S.W.R. Lee

2 Mechanism of Moisture Diffusion, Hygroscopic Swelling,and Adhesion Degradation in Epoxy Molding Compounds . . . . 29M.H. Shirangi and B. Michel

3 Real-Time Characterization of Moisture Absorptionand Desorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71Y. He and X.J. Fan

4 Modeling of Moisture Diffusion and Whole-Field VaporPressure in Plastic Packages of IC Devices . . . . . . . . . . . . . 91X.J. Fan, T.Y. Tee, X.Q. Shi, and B. Xie

5 Characterization of Hygroscopic Deformations by MoiréInterferometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113E. Stellrecht, B. Han, and M. Pecht

6 Characterization of Interfacial Hydrothermal Strengthof Sandwiched Assembly Using PhotomechanicsMeasurement Techniques . . . . . . . . . . . . . . . . . . . . . . . 131X.Q. Shi, X.J. Fan, Y.L. Zhang, and W. Zhou

7 Hygroscopic Swelling of Polymeric Materialsin Electronic Packaging: Characterization and Analysis . . . . . . 153J. Zhou, T.Y. Tee, and X.J. Fan

8 Modeling of Moisture Diffusion and Moisture-InducedStresses in Semiconductor and MEMS Packages . . . . . . . . . . 181C. Jang and B. Han

9 Methodology for Integrated Vapor Pressure,Hygroswelling, and Thermo-mechanical Stress Modelingof IC Packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221T.Y. Tee

xiii

xiv Contents

10 Failure Criterion for Moisture-Sensitive Plastic Packagesof Integrated Circuit (IC) Devices: Application andExtension of the Theory of Thin Plates of Large Deflections . . . . 245E. Suhir and X.J. Fan

11 Continuum Theory in Moisture-Induced Failuresof Encapsulated IC Devices . . . . . . . . . . . . . . . . . . . . . . 279X.J. Fan, J. Zhou, G.Q. Zhang, and A. Chandra

12 Mechanism-Based Modeling of Thermal-and Moisture-Induced Failure of IC Devices . . . . . . . . . . . . 301H.B. Chew, T.F. Guo, and L. Cheng

13 New Method for Equivalent Acceleration of IPC/JEDECMoisture Sensitivity Levels . . . . . . . . . . . . . . . . . . . . . . 333B. Xie, X.J. Fan, and X.Q. Shi

14 Moisture Sensitivity Level (MSL) Capabilityof Plastic-Encapsulated Packages . . . . . . . . . . . . . . . . . . 359J.K. Fauty

15 Hygrothermal Delamination Analysis of Quad FlatNo-Lead (QFN) Packages . . . . . . . . . . . . . . . . . . . . . . . 389M.S. Zhang, S.W.R. Lee, and X.J. Fan

16 Industrial Applications of Moisture-Related ReliabilityProblems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 411W.D. van Driel, D.G. Yang, C.A. Yuan, and G.Q. Zhang

17 Underfill Selection Against Moisture in Flip ChipBGA Packages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435X.J. Fan, T.Y. Tee, C.Q. Cui, and G.Q. Zhang

18 Moisture Sensitivity Investigations of 3D Stacked-DieChip-Scale Packages (SCSPs) . . . . . . . . . . . . . . . . . . . . 461X.Q. Shi, X.J. Fan, and B. Xie

19 Automated Simulation System of Moisture Diffusionand Hygrothermal Stress for Microelectronic Packaging . . . . . 479Y. Liu

20 Moisture-Driven Electromigrative Degradationin Microelectronic Packages . . . . . . . . . . . . . . . . . . . . . 503L.F. Siah

21 Interfacial Moisture Diffusion: Molecular DynamicsSimulation and Experimental Evaluation . . . . . . . . . . . . . . 523H. Fan, E.K.L. Chan, and M.M.F. Yuen

About the Editors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 551

Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 555

Contributors

E.K.L. Chan Department of Mechanical Engineering, Hong Kong Universityof Science and Technology, Clear Water Bay, Kowloon,Hong Kong, [email protected]

A. Chandra Department of Mechanical Engineering, Iowa State University,Ames, IA 50011, USA, [email protected]

L. Cheng Department of Mechanical Engineering, National University ofSingapore, Singapore, Singapore 117576, [email protected]

H.B. Chew Department of Mechanical Engineering, National University ofSingapore, Singapore, Singapore 117576, [email protected]

C.Q. Cui Compass Technology Co., Ltd., Shatin, Hong Kong, People’s Republicof China, [email protected]

H. Fan Department of Mechanical Engineering, Hong Kong University of Scienceand Technology, Clear Water Bay, Kowloon, Hong Kong,[email protected]

X.J. Fan Department of Mechanical Engineering, Lamar University, PO Box10028, Beaumont, TX 77710, USA, [email protected]

J.K. Fauty 46641 East Decatur Street, Mesa, AZ 85205, USA, [email protected]

T.F. Guo Department of Mechanical Engineering, National University ofSingapore, Singapore, Singapore 117576, [email protected]

B. Han Department of Mechanical Engineering, CALCE Electronics Products andSystems Center, University of Maryland, College Park, MD 20742, USA,[email protected]

Y. He Intel Corporation, Assembly Test & Technology Development, 5000 W.Chandler Blvd., Chandler, AZ 85226, USA, [email protected]

C. Jang Department of Mechanical Engineering, University of Maryland, GlenMartin Hall, College Park, MD 20742, USA, [email protected]

xv

xvi Contributors

S.W.R. Lee Department of Mechanical Engineering, Hong Kong University ofScience and Technology, Clear Water Bay, Kowloon, Hong Kong, [email protected]

Y. Liu Fairchild Semiconductor Corp., 82 Running Hill Rd, South Portland, ME04106, USA, [email protected]

B. Michel Department of Micro Materials Center Berlin (MMCB), FraunhoferInstitute for Reliability and Microintegration (IZM), Volmerstraße 9B, 12489Berlin, Germany, [email protected]

M. Pecht Department of Mechanical Engineering, CALCE Electronics Productsand Systems Center, University of Maryland, College Park, MD 20742, USA,[email protected]

X.Q. Shi Applied Science & Technologies Research Institute (ASTRI) 5/F,Photonics Center, Science Park, Shatin, Hong Kong, People’s Republic of China,[email protected]

M.H. Shirangi Department of Micro Materials Center Berlin (MMCB),Fraunhofer Institute for Reliability and Microintegration (IZM), Volmerstraße 9B,12489 Berlin, Germany; Robert Bosch GmbH, Automotive Electronics,Development ASIC & Power Packages, Reutlingen, Germany,[email protected]

L.F. Siah Assembly Technology Development Q&R – Malaysia (ATD Q&R-M),Intel Technology (M) Sdn. Bhd., Bayan Lepas FTZ Box 121, 11900 Pulau Pinang,Malaysia, [email protected]

E. Stellrecht Department of Mechanical Engineering, CALCE ElectronicsProducts and Systems Center, University of Maryland, College Park,MD 20742, USA, [email protected]

E. Suhir Department of Electrical Engineering, University of California, 1156High Street, SOE2, Santa Cruz, CA 95064-1077; Department of MechanicalEngineering, University of Maryland, College Park, MD, ERS Co. LLC, Los altos,CA, USA, [email protected]

T.Y. Tee Blk 408, Hougang Ave 10, #07-1082, Singapore, Singapore 530408,[email protected]

W.D. van Driel Philips Lighting, Mathildelaan 1,5611 BD Eindhoven,The Netherlands; Delft University of Technology, Mekelweg 2,2628 CD Delft,The Netherlands, [email protected]

C.P. Wong Materials Science and Engineering, Georgia Institute ofTechnology, 771 Ferst Drive, N.W. Atlanta, GA 30332-0245, USA,[email protected]

B. Xie Applied Science & Technologies Research Institute (ASTRI) 5/F,Photonics Center, Science Park, Shatin, Hong Kong, People’s Republic of China,[email protected]

Contributors xvii

D.G. Yang Guilin University of Electronic Technology, Jinji Road 1, Guilin,China, [email protected]

C.A. Yuan TNO IenT, Eindhoven, The Netherlands, [email protected]

M.M.F. Yuen Department of Mechanical Engineering, Hong Kong University ofScience and Technology, Clear Water Bay, Kowloon, Hong Kong, [email protected]

G.Q. Zhang Philips LightLabs, Mathildelaan 1, 5611 BD Eindhoven,The Netherlands; Delft University of Technology, Mekelweg 2, 2628 CD Delft,The Netherlands, [email protected]

M.S. Zhang Department of Mechanical Engineering, Hong Kong University ofScience and Technology, Clear Water Bay, Kowloon, Hong Kong,[email protected]

Y.L. Zhang School of Mechanical & Production Engineering, NanyangTechnological University, 50 Nanyang Avenue, Singapore, Singapore 639798,[email protected]

J. Zhou Department of Mechanical Engineering, Lamar University, PO Box10028, Beaumont, TX 77710, USA, [email protected]

W. Zhou School of Mechanical & Production Engineering, NanyangTechnological University, 50 Nanyang Avenue, Singapore, Singapore 639798,[email protected]

Chapter 1Fundamental Characteristics of MoistureTransport, Diffusion, and the Moisture-InducedDamages in Polymeric Materials in ElectronicPackaging

X.J. Fan and S.W.R. Lee

1.1 Introduction

Many failures in microelectronic packages can be traced back to moisture [1, 2]. Themechanical behavior of polymer systems is affected significantly by the absorptionof atmospheric moisture [3–8]. In general, there are three types of failure mecha-nisms when an electronic package is exposed to humidity conditions [8]. The firsttype of failure is often referred to as “popcorn failure” [4–7]. Packaged microelec-tronic devices are exposed to factory environmental conditions after the openingof protective dry-bags for surface mounting to printed circuit board (PCB). Whenmoisture, present in the factory environment, is absorbed by the polymeric packag-ing materials, it condenses in free volumes or micro-/nanopores in the bulk materialsand along interfaces. The condensed moisture will vaporize and produce high pres-sure during the surface mount process. The board and devices are entirely placed ina reflow oven, in which the peak temperature ranges typically from 220 to 260◦C.The reflow process is completed within a few minutes. Polymer materials, such asdielectric films, adhesives, encapsulants, and plastic printed circuit boards, becomeextremely compliant when the temperature exceeds their glass transition tempera-tures. In addition, the interfacial adhesion strength drops substantially. As a result,delamination may occur due to the combined effects of thermo-mechanical stresses,hygroscopic stresses, vapor pressure, material softening, and adhesion degradation.An audible sound may be produced if water vapor is suddenly released due topackage cracking. The second type of moisture-induced failure mechanism con-cerns the package reliability during a lifelong service period under various fieldenvironmental conditions. Polymeric materials in packages will swell upon mois-ture absorption. Hygroscopic swelling creates dimensional changes of materials.Similar to thermal stresses due to the mismatch of thermal expansion, hygroscopicstresses are induced. Moisture will also substantially affect the interfacial adhesion.In addition, aging effect becomes a concern for a long-period exposure to humid-ity conditions. Consequently, delamination may take place along weaker interfaces

X.J. Fan (B)e-mail: [email protected]

1X.J. Fan, E. Suhir (eds.), Moisture Sensitivity of Plastic Packages of IC Devices,Micro- and Opto-Electronic Materials, Structures, and Systems,DOI 10.1007/978-1-4419-5719-1_1, C© Springer Science+Business Media, LLC 2010

2 X.J. Fan and S.W.R. Lee

to cause electrical failures of devices. The third type of moisture-assisted failureis due to electrochemical migration (corrosion) in the presence of both electricalbias and moisture (see Chapter 20). There are two kinds of corrosion in electronicpackages. The first kind is metal dendritic growth at the cathode side on the sub-strate surface (e.g., copper trace). Electrolytic dissolution of copper at the anodecreates metal ions. Metal ions then migrate to the cathode side as moisture providestransport path (e.g., moisture absorbed by the solder resist). When metal ions reachthe cathode, dendritic growth occurs. Continuous reduction of dissolved metal ionsleads to the nucleation and growth of metal dendrites, and eventually the formationof anode–cathode short failure. The second kind of corrosion, the so-called conduc-tive anodic filament (CAF) growth, occurs at the sub-surface associated with glassfibers/epoxy resin interface. The CAF grows from anode to cathode along delami-nated fiber/epoxy interfaces when moisture is present. A CAF is made from solublecopper salt at the anode and built at the anode by turning to insoluble salt due topH effect. Dendrites occur as a result of solution at the anode and plating at thecathode.

In order to address those moisture effects, three types of accelerated moisturesensitivity/reliability tests are often applied for package reliability qualifications [2].Moisture/reflow sensitivity test is required prior to environmental stresses for alldevices that are surface mounted to PCB. Moisture/reflow sensitivity test has beendocumented in the joint JEDEC/IPC industry standard J-STD-020D [9]. This testspecification has established exposure conditions of temperature, humidity, and timefor which the moisture sensitivity ranking of surface mount devices are classifiedand referenced. Moisture/reflow sensitivity test insures that the temperature, humid-ity, and/or the shipping requirements are met before assessing the reliability underoperation conditions. The so-called highly accelerated stress test (HAST) (withoutelectrical bias) is the second type of accelerated test to evaluate non-hermetic pack-aged devices in humid environments. The test employs temperature and humidity toaccelerate the penetration of moisture through external protective material or alongthe internal build-up or joined interfaces. Electrical bias is not applied in this testin order to ensure that the failure mechanisms, which may be overshadowed bythe bias, can be detected. This test is used to identify failure mechanisms inter-nal to package and is destructive. An autoclave test or pressure cooker test (PCT)is similar to HAST. PCT has a fixed temperature (121◦C) and fixed RH (100%).During HAST and PCT, the packages are placed in environmental chambers withhumidity and temperature controls for a certain period of time. Both HAST and PCTare industrial standards for qualification requirements. Lastly, the biased tempera-ture/humidity (TH) test is a moisture-related test used to evaluate the reliability ofa powered device at an elevated temperature and in a high-humidity environmentsubject to the static bias of the nominal operation state (ranging from 0.1 to 7 V).There are two standard forms of biased moisture test that can be employed. The tem-perature, humidity, bias (THB) test is performed under the atmospheric pressure, ata temperature between 30 and 85◦C, and with relative humidity (RH) from 50 to85%. Highly accelerated stress test with bias (BiHAST) is performed at less than 3atmospheres of pressure and at a temperature between 110 and 160◦C.

1 Fundamental Characteristics in Polymeric Materials in Electronic Packaging 3

In order to fully understand failure mechanisms associated with moisture, itbecomes exceedingly important to characterize the moisture absorption behaviorof polymeric materials involved in electronic packaging and to quantify polymermatrix damage and adhesion degradation induced by moisture transport. In thischapter, the fundamental characteristics of moisture behaviors in polymer systemsand the interactions of moisture with polymer matrix and along interfaces arereviewed. Both Fickian and non-Fickian kinetics of moisture absorption and desorp-tion are investigated. Saturated moisture concentrations at different humidity levelsas a function of temperature are discussed for different materials in a wide rangeof temperature. A specific experiment, which was designed to reveal the passageof moisture vapor through a hydrophobic membrane, is described. The difference ofwater sorption (immersion in water) and moisture sorption in humid air is discussed.Mercury intrusion method is introduced to measure the sizes of nanopores or freevolumes for different types of packaging materials. An approximate method to esti-mate the free volume fraction is proposed based on moisture weight gain test data.Moiré interferometry technique is applied to study the aging effect of hygroscopicswelling of a polymeric material as a function of time. Adhesion and fracture tough-ness characterization methods are applied to investigate the influence of moisture onthe interfacial fracture toughness or the adhesion strength. Finally, certain issues onthe state of moisture in polymer materials, effect of hygroscopic swelling, effects offillers, and the duration of moisture absorption at different conditions are discussed.

1.2 Fickian and Non-Fickian Moisture Diffusion

Fickian transport of moisture in polymers or polymer composites is described bythe following equation:

∂C

∂t= D

(∂2C

∂x2+ ∂2C

∂y2+ ∂2C

∂z2

). (1.1)

This equation is known as the general diffusion law or Fick’s second law ofdiffusion (Fick’s law, for short), C (kg/m3) is the moisture concentration, t (s) istime, and D (m2/s) is the diffusion coefficient or diffusivity (diffusion constant) ofthe moisture in the material.

The temperature dependence of the diffusion constant can be described by theBoltzmann–Arrhenius type of equation:

D = D0 exp

(−Ed

kT

), (1.2)

where D0 is a pre-factor, Ed is the activation energy, k = 1.38 × 10−23 J/K is theBoltzmann’s constant, and T is the absolute temperature. Note that the pre-factorD0 and the activation energy Ed are different below and above the glass transition


temperature [1]. In other words, there exist two sets of constants D0 and Ed for amaterial to fully describe the moisture diffusivity in it as a function of temperature.

Consider a one-dimensional case of an infinite plate of thickness h. The generalanalytical solution, giving the temporal and spatial moisture concentration, C, attime t and the distance x from the mid-plane, can be obtained

C(x,t) − C0

Ci − C0= 1− 4

π

∞∑n=0

( − 1)n

(2n + 1)exp

[− (2n + 1)2π2D

h2t

]·cos

(2n + 1)π

hx (1.3)

for the initial and boundary conditions

C = C0, − h/2 < x < h/2, t = 0 ,C = Ci, x = −h/2, x = h/2, t ≥ 0 ,

(1.4)

where Ci is the moisture concentration on the boundary, C0 is the initial moistureconcentration, D is the Fickian diffusion coefficient, and h is the thickness of theplate. Assuming the plate is initially dry and then placed into a humid environ-ment, C0 = 0 and Ci = Csat (the saturated moisture concentration), equation (1.3)yields

C(x, t) = Csat

[1 − 4

π

∞∑n=0

( − 1)n

(2n + 1)exp

[− (2n + 1)2π2D

h2t

]· cos

(2n + 1)π

hx

]

(1.5)

and describes moisture absorption process as a function of time. With the time pro-gressing, the plate will be fully saturated with a uniformly distributed moisture withthe concentration Csat. On the other hand, if C0 = Csat and Ci = 0, equation (1.3)yields

C(x, t) = Csat4

π

∞∑n=0

( − 1)n

(2n + 1)exp

[− (2n + 1)2π2D

h2t

]· cos

(2n + 1)π

hx (1.6)

and describes a desorption process, in which an initially fully saturated plate releasesmoisture into the dry enviroment. The moisture in the plate will be driven outcompletely as time goes on.

Since it is not possible to measure the moisture concentration at an arbitrary innerpoint experimentally, equation (1.3) should be integrated over the thickness of thebulk plate so that it would be possible to measure at the plate surfaces the ratio of theamount Mt of moisture at a given moment of time t to the amount M∞ of moistureat t → ∞. The total moisture mass in the specimen can be obtained as a function oftime as follows:


Mt

M∞= 1 − 8

π2

∞∑n=0

1

(2n + 1)2exp

[− (2n + 1)2π2D

h2t

]for moisture absorption ,

(1.7)

Mt

M∞= 8

π2

∞∑n=0

1

(2n + 1)2exp

[− (2n + 1)2π2D

h2t

]for moisture desorption ,

(1.8)

where Mt is the total mass of moisture at time t and M∞ is the mass related to thesaturated concentration Csat by the formula

Csat = M∞V

, (1.9)

where V is the total volume of the test specimen and M∞ is the saturated mass ofmoisture over the plate.

When analyzing the experimental data, D and M∞ are optimized so that the dif-ference between the experimentally determined mass Mt and its calculated valueobtained on the basis of equation (1.3) is minimized [10]. The change in the samplevolume caused by the moisture absorption–desorption is small and may be ignored.The accurate evaluation of the parameters Csat and D is critical to understand thephysics of the absorption/desorption process and the related reliability issues. Csatdetermines the moisture absorption capacity of the specimen and D determines howfast the moisture could diffuse into or out of the sample.

In equation (1.1) the Fickian diffusion is assumed to be independent of themoisture concentration. While the Fickian diffusion model provides a reasonableapproximation to the characteristics of moisture uptake, it rarely provides a fulldescription of the uptake data and is unable to account for the changes in the poly-mer due to relaxation, material degradation, or cracking. Non-Fickian behavior takesplace as the consequence of the relaxation process in polymer molecules and/or as aresult of an irreversible reaction between polymer and moisture, such as formationof hydrogen bonds and chemical reactions [11, 12]. It has been found that non-Fickian diffusion can take diverse forms [5]. Two-stage or dual-uptake diffusion hasbeen observed in some polymeric materials and is referred to in the literature [5] asan anomalous moisture uptake.

Moisture weight gain measurement is a common experimental technique to eval-uate the moisture uptake. In order to investigate the kinetics of the Fickian andnon-Fickian moisture absorption, moisture weight gain measurements were car-ried out for several typical packaging materials. The specimen geometries and thetemperature/humidity conditions were as follows:

1. A conventional underfill material. The test sample was a 1-mm-thick disk-likespecimen with a diameter of 50 mm. Moisture absorption under 85◦C/85% RHwas measured.


2. A thin bismaleimide-triazine (BT) 70-μm-thick core material with the dimen-sions 7 mm×7 mm. In this study, moisture absorption–desorption experimentswere performed at temperatures 30, 60, and 80◦C, respectively. At each isother-mal temperature, two relative humidity (RH) levels were chosen: 60 and 80%RH.In situ measurements were performed on this thin sample.

3. A 0.6-mm-thick bismaleimide-triazine (BT) core material. The sample sizewas 50 mm×50 mm. Moisture absorption was measured under 30◦C/60% RHconditions.

4. A conventional molding compound material sample. Two different samplegeometries from the same material in the form of molded disks were fabricated,one with a thickness of 1 mm (MC1) and a diameter of 50 mm and anotherwith a thickness of 2 mm (MC2) and a diameter of 100 mm, respectively. Thespecimens were exposed to the 85◦C/85% RH conditions in a humidity chamber.

To determine the dry weights, the samples were initially dried (“baked”) at 125◦Cfor 24 h. An electronic balance was used for the weight measurements, except forthe thin BT core, in which an in situ measurement was performed [10]. The sampleswere periodically removed and weighed and then returned to the chamber for furthersoaking.

Figure 1.1 is the plot of the Fickian fit for moisture uptake for an underfill samplecompared to the experimental data. To determine the diffusivity D and the saturatedweight gain M ∞, the least-squares fitting technique was used. In this approach, thesum of the square of the differences between the experimental weight gain and the

calculated one, (�M)2 = ∑ki=1

(Mexp

t,i − Mcalt,i

)2, was calculated based on equation

(1.7), using some initial estimated values of the parameters D and M∞. Mexpt,i was

Fig. 1.1 Fickian curve fit for a homogeneous underfill material under 85◦C/85% RH (sample size50 mm×50 mm×1 mm)


the ith point of the experimentally determined weight gain at the moment t of time,while Mcal

t,i was calculated on the basis of equation (1.7). Here k is the total number ofthe experimental points used when calculating (�M)2. Then, D and M∞ were varieduntil (�M)2 was minimized. The obtained D and M∞ are taken as the diffusivity andthe saturated moisture mass of the sample for that particular temperature/humidityconditions. As one could see from Fig. 1.1, the Fickian diffusion law predicts theexperimental behavior of the test specimen within the test period very well. Withthe increasing time t, the absorption curve smoothly levels off to the saturation levelM∞. The test results in a previous study on different types of underfill also exhibitedFickian behaviors [2].

An example of moisture absorption–desorption experiment for a thin BT core isshown in Fig. 1.2. The temperature was kept at 60◦C and the RH level was cycledbetween 0 and 60%. During the first 60 min, a 0% RH condition was imposed todrive residual moisture out of the sample. This was followed by a 200 min of mois-ture absorption at 60% RH, then a 200-min desorption at 0% RH. The followingconclusions could be drawn from Fig. 1.2:

(1) During the first 60 min, the moisture was not completely driven out of thesample. A longer time was needed to accomplish that.

(2) The subsequent absorption–desorption cycles were reciprocal: the samplereached during re-sorption approximately the same saturated moisture level thatthe weight was lost as the result of drying. This indicates that the process is com-pletely mechanical (“physical”) and there was no chemical reaction between thewater molecules and the material.

Figure 1.3 indicates, however, that the Fickian fit is not satisfactory. This could beattributed to the fact that the BT core has a highly inhomogeneous structure involv-ing glass fibers and resin matrix. Rapid moisture diffusion into or out of the substrateat the initial stages of the moisture diffusion might be due to the moisture diffusionthrough the polymer-based region located at the outer layers of the substrate at the

0 200 400 600 800 1000 1200

99.9

100.0

100.1

100.2

100.360°C

Time (min)

Wei

ght (

%)

Weight

0

10

20

30

40

50

60 Reference S

ensor RH

(%)

RH

Fig. 1.2 Moistureabsorption–desorptionexperiment conducted using asorption TGA at 60◦C withthe RH level cycled between0 and 60% [10]


0 2000 4000 6000 8000–25

–20

–15

–10

–5

0 Experimental Fitted (D = 1.65 µm2/s)

Wei

ght C

hang

e (μ

g)

Time (sec)

Fig. 1.3 Desorption curvesof a 70-μm BT core. Solidline represents the fittedweight loss versus time curveusing D = 1.65 × 10−8 cm2/sby Fick’s law [10]

beginning of the diffusion process. At the later stages, however, moisture diffusionwas forced to progress through fibers which are moisture insensitive. Therefore, theinner components of the substrate acted as a physical barrier against the moisturediffusion, thereby reducing the rate of moisture diffusion or diffusivity.

Figure 1.4 is a plot of the moisture weight gain for a 0.6-mm-thick BT core at30◦C/60% RH. After the extended hours of moisture absorption, the moisture uptakestarted to increase again. One could clearly see a two-stage moisture absorption. Thematerial behaved initially as Fickian, and then exhibited a non-Fickian diffusionbehavior afterward.

The moisture absorption for a molding compound is plotted in Fig. 1.5 for spec-imens of two different thicknesses. An initial moisture uptake to quasi-equilibrium(the first stage of “virtual” saturation) is followed by a slower linear moisture uptake.This suggests that at least two different mechanisms are present in the moistureabsorption process. The first one is the absorption of the water molecules in freevolumes or nanovoids. The second mechanism is the hydrogen bonding forma-tion between the water molecules and the polymer chains due to their molecularpolarity. The former mechanism reaches a saturation point and is a reversible phe-nomenon in nature. The latter seems to be linear without a clear saturation, at least

00.020.040.060.080.1

0.120.14

0 24 48 72 96 120 144 168 192 216 240

Time (hr)

Wei

gh

t G

ain

(%

)

Fig. 1.4 Moisture weightgain curve for a thick BT coresample with the thickness of0.6 mm subjected to30◦C/60% RH


Fig. 1.5 Normalized moisture uptake of a conventional mold compound with different thicknessesat 85◦C/85% RH (x-axis is sqrt(time)/thickness)

for the timescale of observation in this study. The thicker sample (MC2) was furtherexposed to a humid environment for 8 months. The experimental data showed thatthe moisture weight gain reached the level of about 0.3% after 8 months comparedto the 0.2% at the Fickian saturation point. Our latest experimental data showed thatabout 40% of residual moisture content was kept for MC2 at 110◦C after a longperiod of desorption [11].

Although the composition and chemistry are different for the underfill material,BT, and the molding compound samples, it is not clear what particular mecha-nisms caused the difference in the moisture uptake for those materials. The moldingcompound specimen showed a stronger non-Fickian absorption kinetics than theunderfill specimen.

Vrentas and Duda [13] introduced a diffusion Deborah number (DEB)D to char-acterize the presence of the non-Fickian effects during absorption experiments.There are a number of non-Fickian diffusion kinetics [5], in which the two-stagenon-Fickian type of absorption is widely applied. The general manifestation of thetwo-stage sorption is characterized by a rapid initial uptake, then a quasi-equilibriumprocess, followed by a slow approach to the final, true equilibrium. Ultimately, com-plete saturation will be reached in all instances. A theory that satisfactorily describesthe features of a “two-stage” sorption has been suggested by Hopfenberg [14]. Intheir diffusion relaxation model they considered the absorption process to be com-posed of two empirical independent contributions: a diffusion part, MF (t), that isgoverned by Fick’s law, and a structural part, MR (t), resulting from polymer relax-ations. The total weight gain at time t may be expressed as the linear superpositionof these two contributions:

M (t) = MF (t) + MR (t) (1.10)


In this equation, MF (t) is given by the solutions to the diffusion equation (1.1). Itis assumed that more than one independent relaxation process might be possible, sothat the part MR (t) could be expressed by a series

M∞(t) =∑

i

M∞,i(1 − e−kit) , (1.11)

where M∞,i represents the equilibrium absorption due to the ith relaxation processand ki is the first-order relaxation constant of the ith relaxation process. It has beenshown that such a model is able to describe the non-Fickian behaviors [5].

Whether the non-Fickian effects are considered in the field conditions or in thetesting conditions depends on the duration of the time of exposure to moisture. Ifthe exposure time does not exceed the time before polymer relaxation commences,then Fick’s law can be accepted to describe the moisture diffusion. Otherwisenon-Fickian’s behavior must be considered. IPC/JEDEC standards require that theduration of moisture exposure at different humidity levels is usually 7–9 days.To determine the exposure time when Fickian law is valid, modeling shouldbe employed. Such modeling should consider material’s chemistry, specimen’sgeometry, and humidity level.

1.3 Saturated Moisture Concentration

Saturated moisture concentration Csat is a measure of the moisture absorption capac-ity under given humidity and temperature conditions. According to the definition ofthe relative humidity as

RH = p

psat= actual partial water vapor pressure of the air

saturated partial water vapor pressure of the air× 100% (1.12)

the partial water vapor pressure of the air for the given relative humidity and absolutetemperature is

p = RH × psat , (1.13)

where psat is the saturated water partial vapor pressure and is a function oftemperature. Assuming that Henry’s law applies, we have

p = Csat

S(1.14)

or

S = Csat

p, (1.15)


where S is the solubility, a material property, which is independent of the ambientrelative humidity. From equations (1.13) and (1.15), we obtain

Csat = (S × psat)RH . (1.16)

Equation (1.16) indicates that the solubility S can be obtained when Csat is mea-sured at different temperatures. Since the solubility and the saturated water vaporpressure are functions of temperature, the saturated moisture concentration is pro-portional to the relative humidity, as long as Henry’s law is fulfilled. Such a linearrelationship has been confirmed by the moisture weight gain test obtained for thethin BT sample from the RH step scan experiment, as shown in Fig. 1.6. This figureshows that, at a fixed temperature (30◦C in this case), Csat is linearly proportionalto the RH level.

The saturated moisture concentration in the BT sample is shown in Fig. 1.7 as afunction of temperature for two different RH levels. The data were obtained frommoisture absorption–desorption experiments. Figure 1.7 shows that the parameterCsat in BT core is essentially temperature independent.

For most polymer materials at the temperatures well below the glass transi-tion temperature, Csat is temperature independent. This implies that the solubilitydecreases with temperature according to equation (1.16). The situation is differentwhen the temperature exceeds the glass transition temperature, Tg. The saturatedmoisture concentration for a die-attach film in the range from 30 to 80◦C at 60%RH level is shown in Fig. 1.8 as a function of temperature. A strong depen-dence of temperature is evident. The tested film has a Tg at around 35◦C. Fortemperatures exceeding Tg, the saturated moisture concentration depends stronglyon the temperature. As a matter of fact, since the saturated moisture concentra-tion has strong dependence on the free volume fraction, the saturated moistureconcentration increases significantly when temperature exceeds the glass transi-tion temperature. It is important to investigate the saturated moisture concentration

0 10 20 30 40 50 60 70 80 900

1

2

3

4

5

6

7

8

9

Slope = 0.11

Experimental Data Linear Fit

Csa

t (m

g/cm

3 )

Relative Humidity (%)

Fig. 1.6 Csat as a function ofrelative humidity at 30◦C [10]


30 40 50 60 70 80

4

6

8

10

70 μm BT core

60% R.H. 80% R.H.

Csa

t (m

g/cm

3 )Temperature (°C)

Fig. 1.7 Saturated moisturecontent as a function oftemperature for two differentRH levels [10]

30 40 50 60 70 80

0.4

0.6

0.8

1.0

1.2

Y = 0.34 + 0.0084T

Measured Linear Fit

Sat

urat

ed M

oist

ure

Upt

ake

(%)

Temperature (°C)

Fig. 1.8 Saturated moistureconcentration as a function oftemperature for a die-attachfilm at 60% RH level (Tg ofthe film is 35◦C)

at reflow soldering temperatures since an “over-saturation” phenomenon mighttake place at the reflow soldering conditions [15]. “Over-saturation” is a situationthat the material might continue to absorb more moisture despite that desorp-tion takes places during soldering reflow. When saturated moisture concentrationis a function of temperature, the moisture diffusion modeling using normaliza-tion approach is no longer valid [2]. A direct concentration approach (DCA) hasbeen proposed [15, 16] to conduct moisture diffusion modeling at reflow solderingconditions.

1.4 Water Sorption and Moisture Sorption

When a polymer material is immersed into water, the sorption process is referredto as water sorption. For a material that is subjected to a humid air condition witha relative humidity less than 100%, it is referred to as moisture sorption process.


There are two distinct diffusion mechanisms involved in the transport of moisture:moisture transfer across the surface and through the bulk material, respectively.

In order to study the difference between water sorption and moisture sorption,an experiment of moisture sensitivity/reflow test for two groups of quad flat no-lead(QFN) plastic packages was conducted [17]. One group of the packages was coatedwith a hydrophobic membrane fabricated by a grafting method [18] to prevent wateruptake, while the other group was as received. Two types of moisture precondi-tioning were applied: immersion into water at a constant temperature of 60◦C andplacement in a humidity chamber at 60◦C/60% RH. Both sorption durations were192 h. Table 1.1 summarizes the weight gain data and the results after the reflow.The coated packages under water did not exhibit any weight change. However, thecoated packages in humid air condition gained almost the same amount of moistureas the uncoated packages either in water or in humid air. Furthermore, the reflowtest indicated that the coated packages at 60◦C/60% RH preconditioning had thesame failure rate as the uncoated packages in the same preconditioning conditions,while the coated packages immersed into water did not show any failures after thereflow. These results indicate that a hydrophobic polymer film is very effective inblocking liquid water from penetrating through the package surface, but not forwater vapor transmission (see Fig. 1.9). Several other hydrophobic polymeric mate-rials have been found to exhibit similar behavior, as far as water vapor passage inhumid air conditions is concerned (Winkler, P., of Badger Meter Inc. 2009, personalcommunications).

Table 1.1 Effect of hydrophobic membrane on moisture sorption versus water sorption

Sorption conditionHydrophobic filmcoated

Averaged weightgain (%)

Reflow testfailure rate

Water immersion (60◦C) Yes 0.03 0/12Water immersion (60◦C) No 0.32 7/12Moisture sorption (60◦C/60% RH) Yes 0.31 6/12Moisture sorption (60◦C/60% RH) No 0.31 8/12

1.5 Characterization of Pore Size, Porosity, and Free Volume

Porosity of, or free volume fraction in, polymers is a critical material propertyrelated to the moisture absorption. Polymer volume is divided into three elements:occupied volume (the “van der Waals” volume), interstitial free volume, and hole-free volume [19]. The hole-free volume is accessible for penetrant transport and maybe altered by absorption and desorption of penetrants. Changes in the total polymervolume are largely governed by changes in the hole-free volume.

Experimental technique known as mercury intrusion porosimetry characterizesmaterial’s porosity by applying various levels of pressure to a sample immersedin mercury [20]. The pressure required to intrude the mercury into sample’s pores


Water liquid molecules

Water vapor molecules

Water-proof coating film

Fig. 1.9 Schematic diagram for a hydrophobic coating film, which can prevent liquid phasemoisture, but not vapor phase moisture, from penetrating into the film

is inversely proportional to the size of the pores. Mercury intrusion porosimetry isbased on the capillary law governing liquid penetration into small pores. This law,in the case of a non-wetting liquid like mercury, is expressed by the Washburn’sequation

D = −4γ cos θ

P, (1.17)

where γ is the surface tension of mercury, θ is the contact angle between the mer-cury and the sample, P is the applied pressure, and D is the pore diameter, all inconsistent units. The volume V of mercury penetrating the pores is measured directlyas a function of the applied pressure. This P-V curve serves as a unique character-ization of the pore structure. The Washburn equation assumes that all pores arecylindrical. Although in reality pores or free volumes are rarely cylindrical, thisequation provides a practical representation of pore distributions, providing veryuseful results for most applications. As pressure increases during an analysis, poresize is calculated at each pressure point, and the corresponding volume of mercuryrequired to fill in these pores is measured. These measurements taken over a rangeof pressures provide the pore volume versus pore size distribution for the samplematerial. Mercury porosimetry can determine the pore size distribution quite accu-rately. Comprehensive data provide extensive characterization of sample porosityand density. Available results include total pore volume, pore size distribution, andpore diameter. Mercury porosimetry is typically applied over a capillary diameterrange from 50 nm to 360 μm.


Several types of polymer materials for electronic packaging applications, such asdie-attach films, solder mask, and underfills, were tested using the mercury intrusiontechnique [21]. Those materials were also examined for moisture weight gain testat 85◦C/85% RH. The results showed that the relative weight gain ranged from0.48 to 1.27% for those tested materials. However, no significant pore size down to∼50 nm was observed for all materials under investigation. This implies that the freevolumes, which are occupied by the absorbed moisture, are typically in nanometerrange. The same materials were then subjected to a simulated reflow process. Anexample of microstructures of a die-attach film before and after the reflow is shownin Fig. 1.10. It can be seen that the significant voiding of the film due to the internalvapor pressure was developed after the reflow process [22]. Soles et al. [23] usedpositron annihilation lifetime spectroscopy (PALS) to quantify the polymer networktopology, which produces a nanovoid volume fraction as a function of temperature.A strong correlation was observed between the absolute zero volume fraction andthe ultimate moisture uptake. Although the correlation is clear, the absolute moisturezero hole volume fraction is not sufficient to predict the ultimate moisture uptake.

An approximate estimate method to obtain the free volume fraction of polymersis proposed using moisture weight gain test. Consider a material with a saturatedmoisture concentration Csat at the given temperature and humidity. The initial freevolume fraction f0 can be found as

f0 = Csat

ρ, (1.18)

where ρ is the moisture density in the pores or free volumes. Since the density ofthe liquid water is 1.0 g/cm3, the moisture density in free volume, i.e., the ρ valuein equation (1.18), must be less than or equal to 1.0 g/cm3:

f0 ≥ Csat. (1.19)

Here Csat has the unit of g/cm3. In general, Csat depends on the relative humidityand temperature. The water liquid will fill in the free volume completely at 100%

Fig. 1.10 Microstructures of a soft die-attach film (a) before and (b) after reflow with moistureabsorption


RH [24, 25]. Therefore, the initial free volume fraction can be estimated from themoisture weight gain test using Csat measurement data extrapolated to 100% RH.This is a lower-bound estimation since the moisture will be usually in the mixtureof water and vapor in free volumes. The free volume fraction is usually between 1and 5% for typical packaging materials [26].

1.6 Hygroscopic Swelling Measurement

There are a number of experimental techniques for the characterization of hygro-scopic swelling of polymers [27–29]. However, relatively few studies took thetime effect into consideration even though polymers exhibit viscoelastic behaviorunder hygrothermal aging. To address the phenomenon of hydroscopic swelling, asilicon/underfill/FR-4 assembly was built and tested [30]. The purpose of the studywas to investigate the hygroscopic swelling behavior of the underfill material ina flip-chip assembly-like configuration. An integrated multi-functional micro-moiréinterferometry system was developed [30], by combining moiré interferometry (MI)technique with thermoelectric heating and cooling technique (for thermal cycling)and a humidity environment system (for hygrothermal aging), to investigate thehygroscopic swelling of the underfill. Specimen grating with a frequency of 1,200lines/mm was replicated onto the surface of the assembly at room temperature. Theassembly was then put into the miniaturized moisture chamber with the hygrother-mal loading conditioned at 85◦C/85% RH for 168 h. In order to eliminate thermaleffect and investigate the aging effect, a separate thermal aging test without mois-ture was carried out with the same test configuration and the moiré fringe patternswere acquired at the same time intervals. The detailed experimental procedure andresults are presented in Chapter 6.

The normal strain and the shear strain obtained from the fringe measurement areplotted in Fig. 1.11. The strain components increased significantly at the beginning.

‘blue’ line - εx in thermal aging

‘red’ line - εx in hygrothermal aging

‘black’ line – pure swelling induced

(a) (b)

εx

‘blue’ line - γxy in thermal aging

‘red’ line - γxy in hygrothermal aging

‘black’ line – pure swelling induced γxy

Fig. 1.11 Swelling-induced strains by using superposition method: (a) normal strain εx vs. time(b) shear strain γxy vs. time

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