Role Of Intermetallic Compound On Thermal Crack Propagation Of Lead Free Solders.

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UNIVERSITI TEKNIKAL MALAYSIA MELAKA

ROLE OF INTERMETALLIC COMPOUND ON THERMAL

CRACK PROPAGATION OF LEAD FREE SOLDERS

This report is submitted in accordance with requirement of the Universiti Teknikal Malaysia Melaka (UTeM) for the Bachelor Degree of Manufacturing Engineering

(Engineering Materials) (Hons.)

WAN MUHAMMAD FAIZ B WAN MOHAMED ARSHAD B051110219

920818-06-5073

FACULTY OF MANUFACTURING ENGINEERING 2015

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DECLARATION

I hereby, declared this report entitled ‘The Role of Intermetallic Compound on Thermal Crack Propagation of Lead Free Solders’is the results of my own research except as cited in references.

Signature : ………

Author’s Name : WAN MUHAMMAD FAIZ B WAN MOHAMED ARSHAD

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APPROVAL

This report is submitted to the Faculty of Manufacturing Engineering of UTeM as a partial fulfillment of the requirement for the degree of Bachelor of Manufacturing Engineering (Engineering Material) (Hons.). The member of the supervisory is as follow:

……….. (PROF. DR. QUMRUL AHSAN)

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ABSTRAK

Kecenderungan perkembangan dalam komponen elektronik dipengaruhi oleh peningkatan prestasi dan kebolehpercayaan pematerian. Penggantian konvensional utama timah (SnPb) pateri kepada pateri tanpa plumbum telah mengubah kebolehpercayaan sesuatu komponen. Pengaloian telah dibangunkan untuk membawa bahan pateri tanpa plumbum untuk mencapai hampir sedekat tahap kebolehpercayaan yang menggunakan pateri konvensional. Walau bagaimanapun, isu kebolehpercayaan adalah pada mekanisme kegagalan pateri tanpa plumbum. Kajian ini akan memberi tumpuan kepada analisis mikrostruktur dan pencirian kelakuan pertumbuhan sebatian antara logam yang di pateri tanpa plumbum dengan pelbagai jenis kemasan permukaan. Bahan pateri yang digunakan adalah SAC 305 yang terdiri daripada timah, perak dan tembaga. Kemasan permukaan termasuk dalam kajian ini terdiri daripada tembaga, timah rendaman, dan tanpa elektrik emas nikel rendaman. Tujuannya adalah untuk menyiasat sebatian antara logam terbentuk dalam pelbagai jenis kemasan permukaan selepas ujian kitaran haba pada 900, 1200 dan 1500 kitaran digunakan dalam suhu 100ºC di mana dapat memberi kesan kepada kebolehpercayaan sambungan pateri. Pertumbuhan sebatian antara logam dalam kemasan permukaan yang berbeza dianalisis dalam kajian ini. Selepas ujian kitaran haba, sampel telah dijadikan keratin rentas, dikisar dan digilap. Kemudian, morfologi sampel itu didedahkan di bawah Mengimbas Mikroskopi Elektron untuk menganalisis pembentukan sebatian antara logam. Pengukuran menunjukkan ImSn menunjukkan ketebalan tertinggi sebatian antara logam berkembang sehingga 11µm. Walaubagaimanapun, sebatian antara logam pada sampel tembaga adalah merupakan yang terendah pada ukuran 4.5µm. Selain itu, kajian ini menunjukkan mengganggu fenomena yang berlaku dalam sampel kemasan permukaan yang berbeza dan mekanisme itu dianalisis.

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ABSTRACT

The tendency of development in electronic components is influenced by performance enhancement and the reliability of the soldering. The replacement of conventional tin lead (SnPb) solder to lead free solder have changed the reliability of the component. Alloying constituents have been developed for lead free solder materials to achieve almost as close a reliability level as using conventional solders. However, the issue of reliability is on the failure mechanisms in lead free solder interconnection. This study focused on the microstructural analysis and characterization of the intermetallic compounds growth in the lead free solder with different types of surface finish. The solder material used is SAC 305 which composed of tin, silver and copper. The surface finishes included in this research were bare copper, immersion tin, and electroless nickel immersion gold. The aim was to investigate the intermetallic compound growth in different types of surface finish after 900, 1200 and 1500 cycles of thermal cycling test is applied at 100ºC of cyclic temperature in which affecting the solder joint reliability. After thermal cycling test, the sample were cross sectioned, grinded and polished. Then, the morphology of the sample was reveal under Scanning Electron Microscopy to analyze the formation of intermetallic compounds. The measure shown that immersion tin surface finish produced the highest thickness of intermetallic compounds growth up to 11µm. However, intermetallic compound in copper surface finish shows the lower thickness about 4.5µm. Besides, this research show voiding phenomena happened in the samples of different surface finish and its mechanism is analyzed.

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DEDICATION

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ACKNOWLEDGEMENT

Firstly, I would like to express my gratefulness to Allah S.W.T for giving me strength and wisdom to do my Final Year Project. I also would like to express my gratitude to the following individuals for providing me the inspiration and help.

I would like to express my warm gratitude to my FYP supervisor, Professor Dr. Qumrul Ahsan for his willingness to accept and giving me chances to complete my final year project. In addition, I would like to thanks to Redring Soldering Sdn. Bhd. and Electronic Packaging and Research Society as an industrial provider for solder materials samples.

Furthermore, thanks to my parents for giving me moral supports and also financial support. Their undivided loves and pray gives me strength to conduct my final project. Thanks also to all my brothers and sisters for all their encouragement.

Besides that, I would like to appreciate my sincere thankful to all friends and my teammates under the same supervision of Prof Dr. Qumrul Ahsan for their help and kindness toward myself. Absolutely, I cannot survive to continue and complete all my task without their great supervision and concerned.

Last but not least, I wish to acknowledge to all persons who give supporting, advice, and assistance that are directly or indirectly involved to the success internship. Thank you so much.

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Abstrak i

Abstract ii

Dedication iii

Acknowledgement iv

Table of Content v

List of Table ix

List of Figure x

List of Abbreviations xiv

CHAPTER 1: INTRODUCTION

1.1 Background 1

1.2 Problem Statement 3

1.3 Objectives 3

1.4 Scope of Study 4

1.5 Project Outline 4

CHAPTER 2: LITERATURE REVIEW

2.1. Soldering 5

2.2. Solder Materials 6

2.2.1. SAC Alloys 6

2.3. Surface Finish 7

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2.4.1. Reliability of Solder Joint 9

2.5. Solder Intermetallic Compound 10

2.5.1. Formation of Intermetallic Compound 11

2.5.2. Types of Intermetallic Compound 12

2.5.2.1. Cu-Sn Intermetallic 13

2.5.2.2. Ag-Sn Intermetallic 14

2.5.2.3. Ni-Sn Intermetallic 15

2.5.3. Growth of IMC 16

2.6. Failure Modes of Lead Free Solders 18

2.6.1. Voids 18

2.6.2. Crack of IMC 19

2.6.3. Thermal Fatigue 21

2.6.4. Creep 23

2.7. Summary of Literature Review 23

CHAPTER 3: METHODOLOGY

3.1. Methodology of Flow Chart 25

3.2. Material Selection 26

3.3. Material Conformation 26

3.4. Thermal Cycling Test 27

3.4.1 Sample Preparation 27

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vii

3.5. Sample Preparation 29

3.5.1. Cross-Sectioning 29

3.6. Microscopic Observation 29

3.7. Morphology Analysis 30

3.7.1. Scanning Electron Microscopy (SEM) 30

CHAPTER 4: RESULTS & DISCUSSION

4.1. Optical Microscopy 31

4.1.1 900 Thermal Cycles 32

4.1.2 1200 Thermal Cycles 34

4.1.3 1500 Thermal Cycles 36

4.2. Scanning Electron Microscopy (SEM) 38 4.2.1 Copper Surface Finish 38 4.2.2 Immersion Tin Surface Finish 40 4.2.3 Electroless Nickel Immersion Gold Surface Finish 42

4.3 Failure Mechanisms 45

4.3.1 Copper Surface Finish 45

4.3.2 Immersion Tin Surface Finish 46

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viii

CHAPTER 5: CONCLUSION & RECOMMENDATIONS

5.1 Conclusion 49

5.2 Recommendation 50

REFERENCES

51 APPENDICES

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LIST OF TABLES

3.1 Testing parameter of thermal cycling test 28 4.1 The average thickness of IMC growth of copper surface finish 39

at different thermal cycles.

4.2 The average thickness of IMC growth of ImSn surface finish 41 at different thermal cycles.

4.3 The average thickness of IMC growth of ENIG surface finish 43 at different thermal cycles.

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LIST OF FIGURES

2.1 Schematic illustration of the cross-section of the solder joint 8

2.2 Comparison of thermal fatigue reliability 9

2.3 Definition of the parameters to characterize the IMC microstructure 10 2.4 Microscopic view of the formation of intermetallic compounds 11

2.5 Cu substrate and solder alloy interface 13

2.6 Typical SEM images of Sn–3Ag–0.5Cu solder joints with Cu 13 2.7 SEM images of the Sn3.0Ag0.5Cu/Cu interfaces 14

2.8 SnNi intermetallic formed 15

2.9 Microscopic view of the growth of intermetallic compounds 16

2.10 Growth of intermetallic compounds 17

2.11 A SEM micrograph with measurement marks 18

2.12 SEM view of crack in the IMC layer 19

2.13 Several modes of crack initiation and propagation 20 2.14 Solder joint fatigue failure (without aging) 21 2.15 Solder joint fatigue failure (after cycles of aging) 22

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3.1 Flow Chart of Methodology 25

3.2 The sample of SAC 305 test board 26

3.3 The samples of different surface finish material. 26

3.4 The image of cutting process of sample test board. 27

3.5 Thermal Cycle Chamber 28

3.6 SEM machine 30 4.1 Optical microscopy of sample of copper surface finish that underwent 32

900 thermal cycles under magnification of (a) 50x and (b) 20x 4.2 Optical microscopy of sample of ImSn surface finish that underwent 33

900 thermal cycles under magnification of (a) 50x and (b) 50x 4.3 Optical microscopy of sample of ENIG surface finish that underwent 33

900 thermal cycles under magnification of (a) 20x and (b) 50x 4.4 Optical microscopy of sample of copper surface finish that underwent 34

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xii 4.5 Optical microscopy of sample of ImSn surface finish that underwent 35

1200 thermal cycles under magnification of (a) 20x and (b) 50x

4.6 Optical microscopy of sample of ENIG surface finish that underwent 35 1200 thermal cycles under magnification of (a) 10x and (b) 20x

4.7 Optical microscopy of sample of Cu surface finish that underwent 36 1500 thermal cycles under magnification of (a) 10x and (b) 50x

4.8 Optical microscopy of sample of ImSn surface finish that underwent 36 1500 thermal cycles under magnification of (a) 20x and (b) 50x.

4.9 Optical microscopy of sample of ENIG surface finish that underwent 37 1500 thermal cycles under magnification of (a) 20x and (b) 50x.

4.10 SEM image of intermetallic growth of Cu surface finish which include 39 (a) 900, (b) 1200 and (c) 1500 thermal cycles.

4.11 SEM image of intermetallic growth of ImSn surface finish which include 41 (a) 900, (b) 1200 and (c) 1500 thermal cycles.

4.12 SEM image of intermetallic growth of ENIG surface finish which include 42 (a) 900 and (b) 1500 thermal cycles.

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xiii 4.13 Comparison of intermetallic compounds growth of different surface finish 43

at different thermal cycles.

4.14 SEM image of defects in copper surface finish which include (a) 1200 and 45 (b) 1500 thermal cycles.

4.15 SEM image of defects in ImSn surface finish which include (a) 1200 and 46 (b) 1500 thermal cycles.

4.16 SEM image of defects in ENIG surface finish which include (a) 1200 and 47 (b) 1500 thermal cycles.

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LIST OF ABBREVIATIONS, SYMBOLS AND

NOMENCLATURE

Ag - Silver

Au - Gold

BGA - Ball Grid Array Cu - Copper

ENIG - Electroless Nickel Immersion Gold IMC - Intermetallic Compound

ImSn - Immersion Tin

Ni - Nickel

Pb - Lead

PCB - Printed Circuit Board

SAC - Tin, Silver, Copper (Sn, Ag, Cu) SEM - Scanning Electron Microscopy

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CHAPTER 1

INTRODUCTION

This chapter discusses about the introduction of this project and gives a basic overview to the purpose and scope of this research.

1.1 Background

Recent years, lead free solder alloy has the trend due to widely applied in the electronic interconnections. The past uses of tin lead (SnPb) resulted in a measurable increase of lead in the environment. Consequently along with the market forces, world environmental legislation had seek for initiatives that is driving the advantageous of lead free solders. However, as a consequence of changing the conventional solder alloy resulted in a significant changes to the soldering process. Basically, the attention is on the high melting temperatures associated with lead free solder alloy. Hence, due to changes of these process, the concerns is on the reliability of the solder joint interconnection. Furthermore, solder interconnection act an importantly in electronics packaging due to its role to serve both as electrical components interconnections in chip board, and as a mechanical platform for components.

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2 The tendency of development in electronic components is influenced by performance enhancement and the reliability of the soldering. However, a number of issues regarding the reliability of solder joint interconnection is focusing to the interfaces between molten solder and substrate surface. This is due to the microstructure formed due to reaction rate between solder-substrate interfaces. There are several factor that influenced the reliability of soldering. However, the researchers shown that the intermetallic compound (IMC) layer formed between the solder and the substrate as the most important factor that influenced the solder joint reliability.

Even though it is proved that the formation of an intermetallic compound layer provides a good bonding between the solder and substrate, however, IMC have its main disadvantage which is typically be the most brittle part in the solder interconnection. Hence, as the result, the solder joints become even smaller in size, the naturality of IMC and its impact on solder joint reliability becoming the most concern part with the introduction of new lead-free solder alloy systems (Salam, 2005). However, the reliability of solder joint not only concerning on the intermetallic compound formed. The service reliability of the solder joint is determined by many factors including the solder microstructure and morphology, creep deformation, thermo-mechanically induced recrystallization and thermal mismatch regarding to the differences in coefficient of thermal expansion or combinations thereof (Grossman & Zardini, 2011).

This project is an industrial driven research provided and spearheaded by Electronic Packaging Research Society (EPRS) for final year project. In this project, the crack propagation and intermetallic compound (IMC) formation were investigated using the solder material provided by Redring Solder (M) Sdn. Bhd. Solder includes SAC305 which soldered with three types of surface finish test board that were copper, immersion tin (ImSn) and electroless nickel immersion gold (ENIG) on copper substrate.

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1.2 Problem Statement

The study of the different types of intermetallic compound (IMC) formed in different type of solder joints is important as to investigate the crack propagation behavior due to the issue on reliability of solder joint interconnection and the performance of electronic appliances. The intermetallic compound is the actual bond formed in soldering from the diffusion of solder alloy and substrate. In addition, consequence from the brittle property of intermetallic compound effected the solder interconnection. This is caused by a strong stress concentration during the thermal cycles. Hence, the cracks were found to initiate and propagate near the intermetallic compound layers. The crack propagation is analyzed to understanding the role of the different IMC on the morphology of the solder joints after crack propagated. Furthermore, this project will provide an understanding of the failure mode mechanism of lead free solder particularly on the IMC layer morphology and the role of the IMC formation and growth rate on the thermal crack propagation of lead-free solder.

1.3 Objectives

1. To measure the growth of intermetallic compounds at the solder and the substrate interfaces under different thermal cyclic conditions.

2. To characterize the role of intermetallic compound on the crack initiation and propagation behavior at different thermal cyclic conditions by scanning electron microscopy.

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1.4 Scope of Study

The scope of study is to examine the mechanism of crack propagation caused by thermal mismatch on the solder intermetallic for different type of surface finishes on copper substrate. The solder material involved is SAC 305, when three types of lead free surface finish include copper (Cu), immersion tin (ImSn), and Electroless Nickel Immersion Gold (ENIG) on copper substrate. This study focused on the morphology of the solder intermetallic corresponding with different duration of thermal cycles. Then, the solder intermetallic formed in the solder are characterized and analyzed. The failure mechanism in the solder interconnection is explained in this project by comparing the microstructure of as-received samples with the samples after thermal cycling.

1.5 Project Outline

Chapter 1 includes the basic introduction of this project and emphasizes about the reasons and objectives of conducting this research. Then, Chapter 2 consists of the literature review which explains about the critical points of current knowledge related to the project. Followed by Chapter 3, the methodology to establish this project and procedure to conduct test and experiment is explained. Next, Chapter 4 displays all the observation and result from experiments. Furthermore, discussion were made and explained through this chapter. Lastly, Chapter 5 concluded the results of this research and recommendations for further study on this research.

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CHAPTER 2

LITERATURE REVIEW

This chapter gives an overview on previous research work in various areas which are relevant to this project.

2.1 Soldering

In the electronic materials trending world, soldering plays an important role in the assembly and interconnection of the silicon die or chip. The soldering process has been a fundamental aspect in the realization of all electronic products since the commencement of the electronic age, and it is anticipated that it will remain the primary assembly and interconnection technology for some time to come (Kotadia et al., 2014). Soldering uses molten filler metal to wet the surfaces of a joint, and to form metallurgical bonds between two metal parts. The melting temperature of the filler metal is lower than 450ºC. The soldering process can be depicted and can be roughly divided into three stages: spreading, base metal dissolution, and formation of an intermetallic compound layer (Lee, 2011).

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2.2 Solder Materials

Lead (Pb) based solders have been the cornerstone technology of electronic interconnections for many decades. However, with legislation in the European Union and elsewhere having moved to restrict the use of Pb, it is imperative that new Pb-free solders are developed which can meet the long established benchmarks set by leaded solders and improve on the current generation of Pb-free solders such as SAC105 and SAC305 (Kotadia et al., 2014). It was reported that lead-free solder joints had fine and stable microstructures due to the formation of small-dispersed particles and thus had higher shear strength than SnPb solders (Arulvanan et al., 2006).

2.2.1 SAC Alloys

Today, SnAgCu (SAC) soldering alloys are becoming one of the most favored lead-free soldering alloys. SAC alloys have been studied extensively in terms of material properties, mechanical behavior, solder-substrate interactions, thermomechanical fatigue life analysis, etc. (Mayyas et al., 2013).

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2.3 Surface Finish

Based on the view of Siewiorek et al., (2001) surface finish of the substrate can be a critical factor that changed the quality of the solder joint interconnection. Solder intermetallic compound formed as a critical interface between the electronic component and the substrate, as a metallurgical process and evidence of chemical bonding happened. The main functions of this surface finish are to protect the exposed copper circuitry from oxidation and a solderable surface which enhanced wetting. In the other words, Jasbir has emphasized the statement of Siewiorek by stated that the surface finish not only can provide good solderable surface but minimize the copper dissolution during assembly operation so that the growth of intermetallic can be controlled. In addition, the growth of intermetallic has to be controlled in order to prevent the embrittlement from happened in the solder joint (Bath, 2007).

Surface finish can be described as a thin coating on the outmost layer of PCB which known as Solder Mask Over Bare Copper (SMOBC). It will dissolve into solder paste during reflow or wave soldering process. There is a number of surface finishes that commonly implemented in industries. However in this project, the surface finishes involved are copper substrate (Cu), Immersion Tin (ImSn), and Electroless Nickel Immersion Gold (ENIG).

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