Fracture of Lead-Free Solder Joints for Electronic Applications

FRACTURE OF LEAD-FREE SOLDER JOINTS FOR ELECTRONIC APPLICATIONS:

EFFECTS OF MATERIAL, PROCESSING AND LOADING CONDITIONS

ZHE HUANG

A dissertation submitted in partial fulfillment of the requirements for the degree of

DOCTOR OF PHILOSOPHY

WASHINGTON STATE UNIVERSITY School of Mechanical and Materials Engineering

MAY 2013

To the Faculty of Washington State University:

The members of the Committee appointed to examine the dissertation of ZHE HUANG find it satisfactory and recommend that it be accepted.

______Indranath Dutta, Ph.D., Chair

______David Field, Ph.D.

______Hussein Zbib, Ph.D.

______William Cofer, Ph.D.

ACKNOWLEDGMENTS

I sincerely appreciate my advisor, Indranath Dutta, for his guidance and effects on not only my study and research, but also on my views of life and value in the past four years. He raised me up to more than I can be. I would like to thank my senior and best friend, Praveen Kumar, who used to hold my hands and teach me experiments when I got started as a graduate student; who set an example of a good researcher with his own conduct; who always encourages and helps me when I encounter difficulties. I would also like to thank David Field and William Cofer for their time and intellect toward my research; thank Henry Ruff, Miles Pepper, Kurt

Hutchinson, Gary Held and Robert Lentz for their time and technical support; and thank my dear labmates, Jia Liu, Jake Howarth, Babak Talebanpour and Uttara Sahaym, for their help with my work. Finally, I thank my parents from the bottom of my heart for their emotional support and encouragement during the past 26 years.

This research was supported by National Science Foundation (grant # DMR-0939392),

Semiconductor Research Corporation (contact # 2008-KJ-1855), and Intel Corporation. The collaboration with Intel Corporation, and in particular, the assistance of Dr. Rajen Sidhu with the

ENIG coating of copper substrates, is gratefully acknowledged.

iii FRACTURE OF LEAD-FREE SOLDER JOINTS FOR ELECTRONIC APPLICATIONS:

EFFECTS OF MATERIAL, PROCESSING AND LOADING CONDITIONS

Abstract

by Zhe Huang, Ph.D. Washington State University May 2013

Chair: Indranath Dutta

During service, micro-cracks form inside solder joints making a microelectronic package highly prone to failure during a drop. Hence, the fracture behavior of solder joints under drop conditions at high strain rates and under mixed mode conditions is a critically important design consideration for robust joints. This study reports the effects of (a) loading conditions (strain rate and loading angle), (b) reflow parameters (dwell time and cooling rate), and (c) post-reflow aging on the mixed mode fracture toughness of lead-free solder joints. A methodology based on the calculation of critical energy release rate, GC, using compact mixed-mode (CMM) samples was developed to quantify the fracture toughness of the joints under conditions of adhesive (i.e., interface-related) fracture. In general, the parameters which result in thicker

IMCs layers and harder solder affect GC adversely. The sensitivity of the fracture toughness to all of the aforementioned parameters reduced with an increase in the mode-mixity. A methodology to construct Fracture Mechanism Maps (FMM) for Sn-3.8%Ag-0.7%Cu (SAC387) solder joints has been developed. Such maps are plotted in a space described by two microstructure-dependent parameters, with the abscissa describing the interfacial intermetallic compound (IMC), and the ordinate representing the strain-rate dependent solder yield strength.

iv Line contours of constant fracture toughness values, as well as constant fraction of each of the above mechanisms are indicated on the plots. The plots are generated by experimentally quantifying the dependence of the operative fracture mechanism(s) on the two microstructure-dependent parameters (IMC geometry and solder yield strength) as functions of the strain rate, reflow parameters, and post-reflow aging. Nominally Mode I Fracture

Mechanism Maps (loading angle φ = 0o) are developed for SAC387/Cu joints w/ and w/o surface finish, Electroless nickel immersion gold (ENIG); equi-mixed mode loading conditions (loading angle φ = 45o) is also presented for SAC387/Cu joints (w/o ENIG). The maps allow rapid assessment of the operative fracture mechanism(s) as well as fracture toughness values for a given loading condition (strain rate and loading angle) and joint microstructure without conducting actual tests, and may serve as both predictive and microstructure-design tools.

v TABLE OF CONTENTS

Page

ACKNOWLEDGMENTS ...... iii

ABSTRACT...... iv

TABLE OF CONTENTS...... vi

LIST OF TABLES...... ix

LIST OF FIGURES ...... x

I. INTRODUCTION ...... 1

1.1. BACKGROUND ...... 1

1.2. THREE GROUPS OF PARAMETERS AFFECTING FRACTURE BEHAVIORS

OF SOLDER JOINTS...... 3

1.2.1. Dwell Time ...... 4

1.2.2. Cooling Rate ...... 5

1.2.3. Aging...... 5

1.2.4. Strain rate and loading angle...... 8

1.3. ROLE OF SURFACE FINISH...... 9

1.4. METHODOLOGY TO CALCULATE FRACTURE TOUGHNESS OF SOLDER

JOINTS ...... 10

1.5. FRACTURE MECHANISM MAP...... 13

II. OBJECTIVE ...... 15

III. EXPERIMENTAL PROCEDURE...... 17

vi IV. A GENERAL METHODOLOGY TO CALCULATE FRACTURE TOUGHNESS OF

ADHESIVE JOINTS ...... 23

4.1. STRESS STATE IN CMM SAMPLES...... 23

4.2. FEM MODELING DETAILS ...... 25

4.3. SCHEME FOR K AND GC CALCULATION AND DETERMINATION OF

GEOMETRY FACTORS ...... 28

4.3.1. CTOD Extrapolation Method ...... 30

4.3.2. J-Integral method...... 34

4.4. CALCULATION OF CRITICAL CRACK LENGTH ac...... 37

4.5. EXPERIMENTAL ILLUSTRATION...... 40

4.6. CONCLUSIONS...... 41

V. RESULTS AND DISCUSSION...... 43

5.1. FRACTURE BEHAVIOUS OF SAC387/Cu JOINTS...... 43

5.1.1. Effect of Processing Conditions on Solder Joint Microstructure...... 43

5.1.2. Effect of Test Conditions and Process Parameters on Fracture...... 45

5.1.3. Effect of Aging on Fracture ...... 50

5.1.4. A Discussion on Mixed Mode Interfacial Fracture Toughness ...... 53

5.1.5. A Discussion on Meandering Crack ...... 55

5.1.6. Conclusion ...... 56

5.2. DEVELOPMENT OF FRACTURE MECHANISM MAP FOR SAC387/Cu

JOINTS ...... 58

5.2.1. Strain rate-compensated solder yield strength and

roughness-compensated IMC thickness...... 59

vii 5.2.2. Mode I Fracture Mechanism Map (FMM)...... 63

5.2.3. Effect of loading mode-mixity and Fracture Mechanism Map for loading

angle (φ) 45o...... 66

5.2.4. Conclusions...... 72

5.3. EFFECTS OF TESTING, REFLOW AND AGING PARAMETERS ON

Sn-3.8Ag-0.7Cu/ENIG-plated Cu JOINTS ...... 74

5.3.1. Effects of Strain Rate and Mode Mixity...... 75

5.3.2. Effects of Reflow Parameters ...... 78

5.3.3. Effects of Aging...... 80

5.3.4. Optimization ...... 81

5.3.5. Modification of Definition of teff ...... 84

5.3.6. Mode I FMMs for SAC387/Cu and SAC387/ENIG Joints ...... 90

5.3.7. Conclusions...... 93

LIST OF REFERENCES...... 95

viii LIST OF TABLES

Table 1: Various material constants for the model...... 26

Table 2: Dependence of σYS on dwell time, cooling rate and aging condition ...... 61

Table 3: Summary of some of the critical parameters of samples shown in Figure 47...... 87

ix LIST OF FIGURES

Figure 1: Schem