Reliability Evaluation of Under Bump Metallugy (UBM)

in Flip-Chip Interconnects

Yifan GuoSkyworks Solution, Inc.4311 Jamboree Road

Newport Beach, CA 92660

Interconnect Failure Modes in Different Solder Systems

Sn-Pb Solder Sn-Ag Solder

UBM

Solder

INTRODUCTION

Interconnect Failure ModesInterconnect: base metal, UBM, intermetalics and all the interfaces

For a robust and reliable interconnect system, the crack should propagate through the solder, because solder usually has better fatigue resistance than the interconnect.

Crack initiation from flux contamination or local defect such as underfill voids

FM1, FM2

FM3, FM4

FM5

Crack PathsDie

UBM

Intermetalics

Base Metal

Solder

Underfill

Substrate

Passivation

FM1, FM2:- failure in the base metal and UBM

FM3, FM4:- failure in the UBM-IMCand IMC-solder

FM5:- failure in solder

INTRODUCTION

Stress

Prob

abili

ty

UBM StrengthDistribution

UBM StressDistribution

UBM Reliability

Failure Rate

Prob

abili

ty

Stress Free Temperature

150 C - 50 C

UBM StrengthDistribution

Stress

Minimum UBM

Strength

UBM StressDistribution

Process Variations

UBM Reliability

UBM Strength and Stress

UBM SolderDie

Die Pull Thermal Cycle

Force at Break Applied Force fromThermal Mismatch

UBM Strength UBM Stress

Substrate

UBM SolderDie

Substrate

Underfilll

>?

Underfilll

Die Pull

UBM Strength Test

• Tensile stress is applied to the UNB through solder• The stress level in the UBM is the same as in the solder

UBM Solder

Substrate

Die

UBM Solder

Die Pull

Substrate

Die

Minimum UBM Strength

9 7 % Pb / 3 % Sn pr oper t y @RT

6 3 % Sn / 3 7 % Pb pr ope r t y @RT

S t r a i n ( %)

S t r e s s ( MP a )

9 7 % Pb / 3 % S n solder stress level at taffy pull at room temperature

6 3 % Sn / 3 7 % Pb pr ope r t y @1 5 0 C

Eutectic solder stress level at taffy pull at room temperature

0

1 0

2 0

3 0

4 0

5 0

0 1 2 3 4 5 6 7 8 9 1 0

If solder fails when UBM is still intact: UBM Strength > Solder Ultimate Strength— The Minimum UBM Strength is established

Die Pull

UBM Solder

Substrate

Die

Die Pull Equipment & Process

1 2 3 4

• Self centering fixtures

• Cross-head speed of 5.1 mm/minute +/- 0.1

• Failure Modes: Solder failure or UBM failure

100 µm

Fail in Solder Solder Fracture Surface

Failure in Solder Material during Die Pull

( Courtesy of A. De Silva, Motorola Inc. )

( SnCu bumps on electroplated Ni )

30 µm

Failure in Solder Material during Die Pull

• ‘Taffy’ Pull in Solder• Average failure force (Newton) :15.42, SD: 1.36• UBM Fail: 0/168 (6 die x 28 bumps/die)

(Jin-Wook Jang)

Ni UBM

IMC

EutecticSnPb

Failure in UBM during Die Pull

( Courtesy of A. De Silva, Motorola Inc. )

Prob

abili

ty

Stress Free Temperature

150 C - 50 C

UBM StrengthDistribution

Stress

Test 495 bumps with zero failure in UBM (100% failure in solder)--- 99% confidence on 99% reliability

Minimum UBM

Strength

UBM StressDistribution

Process Variations

UBM Reliability

UBM Stress ModelThermal Cycle

-60

-40

-20

0

20

40

60

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

-70-5002050100150UBMstress

T em p. ∆T fromStress-free

T em p.

U nderfillC T E

Strain(% )

SolderC T E

Strain(% )

SolderM echanicalStrain(% )

U B MStress(M pa)

-50 -70 -0.49 -0.14 -0.35 -40

20 0 0 0 0 0

100 80 0.56 0.16 0.40 20

thermaltotalmechanical εεε −=

UBM SolderDie

Underfilll Underfilll

Substrate

( Stress-free temperature = 20 C )

Eutectic Sn/Pb Solder System

Stress (Mpa)

Strain (%)

Solder Strain:

• Stress-free temp. = 20 C• Solder CTE = 20 ppm/C• Underfill CTE = 70 ppm/C• Mechanical strain = 50 ppm/C

0

30

-100 -50 0 50 100 150

37Pb/63Sn 97Pb/3Sn

Temperature (oC)

Interconnect Stress (MPa)Strength of 37Pb/63Sn @ RT

--- Min. UMB Strength if 100% solder failure in die pull test

Interconnect Reliability in Flip-Chip Packages

An interconnect system is reliable if 100% solder failure is achieved in die pull test at room temperature.

40

-50

-40

-30

-20

-10

20

10

50

Strength of 97Pb/3Sn @ RT

--- Min. UMB Strength if 100% solder failure in die pull test

UBM Reliability in Lead Free Solder Systems:

SnAgSnCu

SnAgCu

Opening and cover of Teflon-coated moldfor reflow processing of solder

Solder tape from Indium Corporation used for specimens

4mm8mm

28mm

Solder Sample Geometry

Sample Preparation forLead-Free Solder Material Property Tests

Material Properties for Three Lead-Free Solders

SnAgCu

0

10

20

3040

50

60

70

0 1 2 3 4 5 6

Strain [%]

Stre

ngth

[MPa

]

SnAg

0

10

20

30

40

50

60

70

0 1 2 3 4 5 6

Strain [%]

Stre

ngth

[MPa

]

SnCu

0

10

20

30

40

50

60

70

0 1 2 3 4 5 6

Strain [%]

Stre

ngth

[MPa

] -40C -25C 25C 100C 180C

Temperature-dependent stress-strainproperties of Sn-Ag, Sn-Ag-Cu, and Sn-Cu

0

10

20

30

40

50

60

0.0 0.5 1.0 1.5 2.0

(Sn/Ag/Cu)

180 C

25 C

-25 C

-40 C

100 CUBM Stress

Stress (Mpa)

Strain (%)

UBM Stress

0

10

20

30

40

50

60

(Sn/Ag)

180 C

25 C

-25 C

-40 C

100 C

0.0 0.5 1.0 1.5 2.0

Stress (Mpa)

Strain (%)

0

10

20

30

40

50

60

0 0.5 1 1.5 2

180 C

25 C

-25 C

-40 C

100 C

(Sn/Cu)

Stress (Mpa)

Strain (%)

UBM Stress

UBM Stresses in Three Lead-Free Solder Systemsin Thermal Cycles

( Stress free temperature = -50 oC )

-10

0

10

20

30

40

-100 -50 0 50 100 150 200

SnAgSnAgCu SnCu

Temperature (C)

Stress (Mpa)

Ultimate Strength of SnCu

Ultimate Strength of SnAg

Ultimate Strength of SnAgCu

UBM Stresses in Three Lead-Free Solder Systems( Stress free temperature = -50 oC )

-10

0

10

20

30

40

-100 -50 0 50 100 150 200

SnAgSnAgCu SnCu

Temperature (C)

Interconnect Stress (Mpa)

Strength of SnAg @ RT (Mixed failure mode)

Strength of SnAgCu @ RT (Mixed failure mode)

INERCONNECT RELIABILITY— in Three Lead-Free Solder Systems

Strength of SnCu @ RT= Min. UBM Strength

100% solder failure was only achieved in the SnCu systemIn the die pull test at room temperature

FEM Model on UBM StressValidation of the Simplified Stress Modle

Die

Substrate

Underfill

Solder bumpUBM

FEM Validation on the Stress Model

-60

-50

-40

-30

-20

-10

0

10

20

30

-1 -0.5 0.5 1 1.5

FEM Model with gap

FEM Modelwith adhesion

Theoretical Model

UBM Stress (MPa)

Solder Strain (%)

( FEM Using FR-4 Substrate Property )

FEM Validation on the Stress Model( FEM Using Ceramic Substrate Property )

-60

-50

-40

-30

-20

-10

10

20

30

-1 -0.50

0.5 1 1.5

UBM Stress (MPa)

Solder Strain (%)

FEM Model with gap

FEM Modelwith adhesion

Theoretical Model

CONCLUSIONS ( I )

• A methodology was established in evaluating UBMreliability when different solder systems were used

• UBM Strength (minimum strength) was determined from die pull test

• UBM Stress (stress distribution) was determined form stress model

• By comparing the minimum Strength and theStress distribution, UBM reliability was determined

• Solder material has strong impact on the interconnect reliability.

• An interconnect system is reliable if 100% solder failure is achieved in die pull test at room temperature.

• This methodology can be used in reliability evaluations and predictions of different bumping technologies with different solder systems.

CONCLUSIONS ( II )

Acknowledgement

• Betty Yueng• Shun-Meen Kuo• Jang-Kai Lim• Lei Mecardo• Ananda De Silva