覆晶封裝銲錫接點在介面反應，電遷移與熱遷移下之顯微結構變化

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覆晶封裝銲錫接點在介面反應，電遷移與熱遷移

下之顯微結構變化

學生：陳筱芸

指導教授：陳智 博士

國立交通大學 材料科學與工程學系

摘 要

覆晶 (Flip Chip)接合封裝技術中，一個銲錫凸塊往往包含不同的

金屬化墊層(Under Bump Metallization, UBM)結構。而銲錫接點與金

屬墊層的反應則直接會影響到接到的機械性質以及電性。同時隨著無

鉛銲錫的採用，與銲錫反應較為和緩的Ni金屬墊層也逐漸成為關注的

焦點。

本論文第二章，探討了電鍍Ni與無電鍍Ni與銲錫接點的迴銲以及

時效之反應．由於電鍍Ni為結晶結構，而無電鍍Ni雖為非晶質結構，

卻會在反應之過程中形成一層柱狀之Ni

P，而相對的加快了反應之速

度。而由其兩種金屬墊層與錫鉛以及無鉛銲錫之反應，可以明確得到

電鍍Ni確實大大的減緩了介面反應的速率，相對降低界金屬化合物

Spalling的可能性。

同時，隨著可攜式電子產品微小化的趨勢，覆晶封裝銲錫接點也

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須隨之縮小，因此銲錫接點所承載的電流密度逐漸提高，在高電流密

度的影響下，覆晶封裝銲錫接點因電遷移產生可靠度的議題受到重

視。此外，導線所產生的焦耳熱效應嚴重影響銲錫接點內部的溫度分

布，因為溫度差產生的溫度梯度產生熱遷移的破壞，熱遷移的破壞也

越來越受到注目。

後續的三四章，討論電遷移對於銲錫接點的影響。除了利用Kelvin

Four-point Probe監控銲錫接點之電阻在電遷移測試中的變化外，同時

也觀測相對應的微觀結構變化。而為了可以更加準確的預測出銲錫接

點之壽命，利用鋁導線TCR之特性，我們可以成功校正得到銲錫接點

在測試中的真實溫度，有助於正確的估算出電遷移之活化能。不同金

屬墊層材料及設計對於接點壽命的影響，相較於Cu UBM，Ni UBM

則據有較高的抗電遷移以及熱遷移之特性因而有較長的接點壽命。

第五與第六章，針對越來越受到矚目的熱遷移現象做較深入的討

論。與以往觀察到的不同，上方的Cu金屬墊層，除了會因為具有往

冷端移動的特性外，更因為在Sn-based為主的銲錫接點中可沿著c軸進

行非常快速的間隙型擴散，導致在沒有電流通過的和錫接點下中也會

有相當嚴重程度的破壞。反之，Ni金屬並無出現同樣的破壞機制。

同時，另一意外的發現為：在沒有通過電流的銲錫接點上方之鋁

導線也有嚴重的破壞產生。穿透式電子顯微鏡之分析，顯示為擴散阻

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障層之Ti也消失不見，故我們推論不僅僅是Al本身之熱遷移，連同Ti

之熱遷移，才會造成導線之破壞。

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Microstructure Changes Associated with Interfacial

Reactions, Electromigration and Thermomigration

in Flip Chip Solder Joints

Student：Hsiao-Yun Chen

Advisor：Dr. Chih Chen

Department of Materials Science and Engineering

National Chiao Tung University

Abstract

It has been reported that the choice of proper under bump

metallization (UBM) plays an important role in determining the reliability

of solder joints. Ni, compared with Cu, possesses slower reaction rate

with Sn-based solder joint and lower solubility therefore has been studied

recently. Here in chapter 2, we analyze the interfacial reaction between

solder and electroplated Ni and eletroless Ni for the reflow and aging

reaction at the same time with SnAg and SnPb solder joints. Electroless

Ni has much faster reaction rate instead of Electroplated Ni due to the

transformation of amorphous phase to a Ni

P crystalline phase.

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The carry-on current density in the solder joint needs to be increased

to 10

A/cm

or

higher due to the shrinking of portable devices. Under

such high current density, the electromigration and the companied

thermomigrtion in the solder joints becomes a serious reliability issue. To

predict a more accurate mean time to failure, the Black’s equation needs

to modify by calibrating the true test temperature and therefore the

eletromigration activation energy, which both these two can have great

influence on prediction. By utilizing Kelvin four-point probes, a criteria

of 20% resistance increase is established and the microstructure change

has been analyzed as well. With the temperature calibration by using Al

TCR effect, up to 10% discrepancy in activation energy can be reached.

For thermomigration, unexpected void formation was observed in

powered and unpowered bumps. Besides Sn thermomigration itself, we

proposed a model of thermomigration of Cu-Sn IMC to explain. The fast

interstitial diffusion of Cu inside Sn matrix combined with the tendency

of thermal migrate to cold end of Cu, damage appeared at those bumps

even with current stressing. For further investigation, void formation

inside Al trace and the disappearance of Ti layer was found surprisingly.

According to literatures, Al has the tendency to cold end, and so is Ti,

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which possesses a much larger heat of transport. More details will be

discussed later in chapter 5 and 6.

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Acknowledgements

Graduate student life is no doubt a long lonely process. However, beyond my imagination and expectation, I have a wonderful trip in this journey and experienced life more. When I look back at events through the past 6 years, even forced to be stopped for one year because of unexpectedly car accident, another year of studying aboard, still, I deeply appreciate for all the people around me.

I would like to begin by recognizing my Ph.D. advisor Prof. Chih Chen for his endless guidance and support to me in my dissertation and in my graduate life as well. He always knows when to direct me to the right way and when just to let me be. My thanks also go to Prof. King-Ning Tu for being the greatest support to me when I was in UCLA. Life is never easy and you show me how to be strong and face it with courage and patient. Special thanks go to Prof. Jenq-Gong Duh, Prof. Tsung-Eong Hsieh, Prof. Robert Kao and Dr. Annie Huang for serving as my committee members and giving me great comments to my research. Thanks to everyone in C.C. group from the very beginning till now, it is my honor to have you guys around me and thank you all for being supportive these days. Thanks to those in NCTU badminton team who ever stood by me and were there for me when I need. Especially coach Liao who took me as his own daughter and teach me unselfishly. I would not be that strong if I never join the team and I am really proud of being part of the team ever.

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Last, but not the least, I would like to thank my parents and my brother, and my very best friends, Yi-Ting Wang and Hsin-Yin Chu for their unconditional and endless support, affirmation and encouragement. Thanks to everyone who joins my journey to become a doctor. I am grateful of who I am, what I am and where I am.

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Table of contents

Abstract (in Chinese)

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Abstract (in English)

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Aknowlodgement

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Table of contents

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List of Tables

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Figure Captions

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Chapter 1 Overview of Interfacial Reaction, Electromigration

and Thermomigration

1.1 Flip-chip Technology

1.2 Introduction of Interfacial Reaction

1.3 Introduction of Electromigration

1.3.1 Failure sites and flux divergence………... 5

1.3.2 Current crowding effect………... 7

1.3.3 Joule heating effect………... 9

1.3.4 Mean-time to failure………... 10

1.4 Introduction of Thermomigration

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1.4.2 Thermomigration accompanying electromigration in flip chip solder

joint………... 13

1.4.3 Thermomigration in composite SnPb flip chip solder joints……… 14

1.4.4 Thermomigration in Pb-free flip chip solder joints………... 14

1.5 Summary

2.6 References

Chapter 2 Kinetic study of the Intermetallic Compound

Formation of Eutectic SnAg an Eutectic SnPb solder with

Electroplated Ni Metallization in Flip-chip Solder Joints

2.1 Introduction

2.2 Experimental Procedures

2.3 Results and Discussions

2.3.2 NiSnP phase and Kirkendall void formation………... 38

2.3.3 IMC Growth kinetics………... 41

2.3.4 Ni consumption rate………... 47

2.4 Summary

2.5 References

Chapter 3 Measurement of electromigration activation energy in

eutectic SnPb and SnAg flip-chip solder joints with Cu and Ni

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under-bump metallization

3.1 Introduction

3.2 Experimental Procedures

3.3 Results and Discussion

3.3.2 TCR effect measurement and Joule Heating effect………... 75

3.3.3 Activation energy fitting………... 77

3.4 Summary

3.5 References

Chapter 4 Effect of under-bump-metallization structure on

electromigration life time of Sn-Ag solder joints

4.1 Introduction

4.2 Experimental Procedures

4.3 Results and Discussions

4.3.2 Joule heating effect………... 97

4.3.3 Simulation results………... 98

4.3.4 Electromigration activation energy measurement………... 99

4.4 Summary

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Chapter 5 Failure induced by thermomigration of Cu-Sn

and Ni-Sn intermetallic compounds during

electromigration in Pb-free SnAg solder joints

5.1 Introduction

5.2 Experimental Procedures

5.3 Results and Discussions

5.3.2 Temperature distribution measurement... 123

5.3.3 Theoretical calculation... 124

5.3.4 Thermomigration Model... 126

5.4 Summary

5.5 References

Chapter 6 Void formation in Al interconnects induced by Ti

thermomigration and Al-Cu reaction in flip-chip solder joints

6.1 Introduction

6.2 Experimental Procedures

6.3 Results and Discussions

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6.3.3 Simulation results... 146

6.4 Summary

6.5 References

Chapter 7 Conclusion and Future Work

7.1 Conclusion

7.2

Appendix Effect of Polyethylene Glycol Additives on

Pulse Electroplating of SnAg Solder

A.1 Introdiction

A.2 Experimental Procedures

A.3 Results and Discussions

A.3.1 Composition characterization 165

A.3.2 Electrochemical Analysis 168

A.3.3 Deposit microstructure and phase identification 169

A.4 Summary

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List of Tables

Table 1.1 Melting temperature, diffusivity and diffusion mechanism for Cu, Al, Pb and SnPn solder.

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Table 1-2 Experimental values of for some pure metals. 21

Table 2-1. Average thickness of Ni3Sn4 for various reflow conditions in this study (SnAg system).

52 Table 2-2. Average thickness of Ni3Sn4 for various reflow conditions in this

study (SnPb system).

53 Table 2-3. Average thickness of Ni3Sn4 for various reflow times for

bumped-die samples reflowed at 210°C.

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Table 3-1 Calibrated temperature and the average failure time of SnAg/Cu, SnPb/Cu and SnAg/CuNi solder bumps under three testing hot plate temperatures.

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Table 4-1 Material Properties used in our simulation. 103

Table 4-2. Electromigration lifetime statistics of SnAg/CuNi solder joints with reanalyzed bump temperature by using Kelvin probes under 7.9103 A/cm2 current stressing.

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Table 4-3. Electromigration lifetime statistics of SnAg/Cu solder joints with reanalyzed bump temperature by using Kelvin probes under 7.0103 A/cm2 current stressing.

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Table A-1. Composition of SnAg electroplating baths (mol dm-3) in our study. 174 Table A-2. Composition analysis of the SnAg film from various plating baths

with the process window listed for eutectic Sn-3.5wt% Ag. Also listed is the resistivity of the as-deposited film.

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List of Figures

Figure Figure 1-1 (a) Tilt-view of SEM image of arrays of solder bumps on silicon die. (b) A flip-chip solder joint to connect the chip side and the module side. (c) The chip is placed upside down (flip chip), and all the joints are formed simultaneously between chip and substrate by reflow.

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Figure 1-2 (a) Blech’s pioneering electromigration sample, showing an aluminum strip deposited on a conducting TiN layer. (b) SEM images of the morphology of a Cu strip tested for 99 hrs at 350°C with current density of 5 105 A/cm2.

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Figure 1-3 Schematic diagram of two-dimensional conductor film with grain boundary and intersection.

24 Figure 1-4 (a) Unique line to bump geometry of a flip chip solder bump joining in a interconnect line on the chip side (top) and a conducting trace on the board side (bottom) and (b) two-dimensional (2D) simulation of current distribution in a solder joint.

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Figure 1-5 (a) SEM images of a sequence of void formation and propagation in a flip-chip eutectic SnPb solder bump stressed at 125°C at 2.25 × 104 A/cm2 for 40 hrs. (b) SEM image of void formation in flip-chip 95.5Sn-4.0Ag-0.5Cu solder bump at 146 °C at 3.67 × 103 A/cm2.

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Figure 1-6 Plot of Carbon content (logarithmic scale) vs. 1/T for sectioned alpha iron specimen.

27 Figure 1-7 The formation of voids on the chip side and accumulation of solder on the substrate side for the solder bump with (a) downward flow and (b) upward electron flow.

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Figure 1-8 (a) A cross section of a composite 97Pb3Sn and 37Pb63Sn flip-chip solder joint. (b) Scanning electron microscopy (SEM) image of the cross section. The darker region at the bottom is the eutectic SnPb. The brighter region is the 97Pb3Sn phase.

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Figure 1-9 (a) Schematic diagram depicting 24 bumps on the periphery of a Si chip. Each bump has its original microstructure, as shown in Figure 4(b), before EM stressing. EM was conducted at 1.6 × 104 A/cm2 at 150°C through only four pairs of bumps on the chip’s periphery: pairs 6/7, 10/11, 14/15, and 18/19. (b) TM affected all the un-powered solder joints: The darker eutectic phase moved to the hot Si side.

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Figure 1-10 SEM images of cross-sectioned bump with markers before and after TM test at 1.01× 104 A /cm2 and 100°C. (a) Before TM, and (b) After 800 h of the TM test. The markers moved toward the substrate end.

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Figure 1-11 (a) Cross-sectional SEM images representing the microstructure for an un-powered bump before a thermomigration test. (b) Temperature distribution measured by an infrared microscope when the neighboring bumps were stressed by 0.55A at 150°C. The built-in thermal gradient was 1143°C/cm across the solder bump. (c) After the TM test for 60 hrs. The Cu UBM was dissolved.

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Figure 2-1 (a) Schematic structure for sample used in this study. EP-Ni was adopted on the chip side and EL-Ni was fabricated on the substrate side; (b) SnPn solder with EP-Ni on the chip side and EL-Ni on the substrate side. (c) Schematic illustrations for the three types of bump-die sample used in this study: 5 μm-Cu/3 μm-Ni.

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Figure 2-2 Morphology of Ni3Sn4 IMC in SnAg solder with plated Ni and the electroless Ni(P) UBM system. (a) Whole bump; (b) Cross-section view at the plated Ni interface (chip side); (c) Cross-section view at the electroless Ni(P) interface (board side).

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Figure 2-3 Cross-sectional SEM images for the SnAg solder bumps: (a) Whole bump; (b) magnified image for the interfacial structure in 5 μm-Cu/3 μm-Ni sample; (c) the interfacial microstructure after 10 times reflow process.

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Figure 2-4 (a) IMC thickness versus with reflow times in 5 μm-Cu/3 μ m-Ni sample; (b) IMC consumption thickness versus with reflow times.

58 Figure 2-5 Cross-sectional SEM images showing morphology of Ni3Sn4 IMC at the interface of the SnAg solder with the EP-Ni and EL-Ni reflowed at 240°C. (a) for 5 min; (b) for 10 min; (c) for 20 min.

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Figure 2-6 Cross-sectional SEM images showing morphology of Ni3Sn4 IMCs in Sn-Pb solder with EP-Ni UBM and EL-Ni(P) metallization (a) for whole bump, (b) enlarged view of EP-Ni on the chip side, and (c) enlarged view of EL-Ni on the substrate side.

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Figure 2-7 Cross-sectional SEM images showing morphology of Ni3Sn4 IMCs at interface of Sn-Pb solder and EP-Ni UBM reflowed at 210°C for (a) 5 min, (b) 10 min, (c) 1 hr, and (d) 9 hrs.

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Figure 2-8 Cross-sectional SEM images showing morphology of Ni3Sn4 IMCs at the interface of Sn-Pb solder and EL-Ni UBM reflowed at 210°C for (a) 5 min, (b) 10 min, (c) 1 hr, and (d) 9 hrs.

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in SnAg sample reflowed at 250°C for 5 min. (b) Kirkendall voids formed in samples of SnAg solder with Ni-P UBM reflow at 230°C for 5min.

Figure 2-10 (a) Enlarged BEI SEM image on the substrate side showing the IMCs, NiSnP, Ni3P, and the remaining Ni(P) layer after being reflowed at 210°C for 1 hr, and (b) numerous voids were observed in the Ni3P layer, indicated by arrows.

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Figure 2-11 (a) Plot of the Ni3Sn4 thickness against the nature log of reflow time for both EP-Ni and EL-Ni metallizations; (b)Arrhenius plot for both EP-Ni and EL-Ni metallizations. The activation energies were determined to be 25 KJ/mol and 38 KJ/mol for the IMC growth on EP-Ni and EL-Ni, respectively.

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Figure 2-12 Top view SEM images of the interfacial microstructure of the SnAg solder at the EP-Ni side (a) after 5min reflow, (b) after 10 min reflow.

66 Figure 2-13 (a) The fitting curve of the time exponent for parameter n value of the IMC thickening process. (b) Arrhenius plot for metallization of both EP-Ni and EL-Ni using the data at 1 hr of reflow duration. The activation energies were determined to be 51 kJ/mol and 48 kJ/mol for the IMC growth on EP-Ni and EL-Ni, respectively. (b) Arrhenius plot for the results published by Kim et al.[9], which is for eutectic Sn-Pb solder on bulk Ni. Data from reflow durations of both 5 min and 10 min were used.

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Figure 2-14 Consumption rates of Ni layers as a function of reflow time at 240°C for eutectic Sn-Pb on EP-Ni, EL-Ni, and Ni foil.

68 Figure 2-15 Cross-sectional SEM images showing morphology of Ni3Sn4 IMCs at the interface of the Sn-Pb solder and the EP-Ni UBM reflowed at 210°C for (a) 5 min, (b) 10 min, (c) 1 hr, and (d) 9 hrs.

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Figure 3-1 (a) Cross-sectional schematic of the layout for Kelvin bump probes. The Al trace connected all four solder bumps together. Crosssectional schematic for the solder bumps with a (b) Cu UBM and (c) Cu/Ni UBM.

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Figure 3-2 Backscattered SEM images for solder bumps before current stressing. (a) SnAg bump with a Cu UBM, (b) SnPb bump with a Cu UBM, and (c) SnAg bump with Cu–Ni UBM.

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Figure 3-3 SEM images of the SnAg bumps with Cu UBMs stressed by a downward current of 0.8 A at (a) 135°C, (b) 150°C, and (c) 165°C. The bump resistance increased by 20% or more.

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Figure 3-4 Backscattered SEM images of SnPb bumps with Cu UBMs subjected to 0.8 A downward current stressing at (a) 135 °C, (b) 150°C, and

xviii (c) 165°C.

Figure 3-5 Cross-sectional SEM images of the failed SnAg bumps with Cu–Ni UBMs subjected to downward current stressing of 0.9 A at (a) 135°C, (b) 150°C, and (c) 165°C.

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Figure 3-6 (a) Temperature distributions in the central Al trace and the Al pad measured by the IR microscope when powered by 0.8 A at 100°C. Bumps B2 and B3 located directly below the left Al pad and right pad respectively. (b) The measured Al-pad and Al-trace temperatures under various applied currents.

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Figure 3-7 Plot of the measured resistance of the central Al trace against oven temperature for the three samples. The TCR coefficients can be obtained from the slopes of these curves.

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Figure 3-8 Plots of MTTF against 10 -3/T for the (a) eutectic SnAg bumps with Cu UBMs, (b) eutectic SnPb bumps with Cu UBMs, and (c) eutectic SnAg solder with Cu–Ni UBMs.

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Figure 4-1 Cross-sectional schematic of the layout design. The Al trace connected all the four solder bumps together.(a) electromigration setup, (b) SnAg solder with CuNi UBM, and (c) SnAg solder with Cu UBM.

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Figure 4-2 The corresponding cross-sectional SEM images for solder bump before current stressing. (a) SnAg solder with CuNi UBM (b) SnAg solder with Cu UBM.

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Figure 4-3 The corresponding Bump resistance for the stressing circuit as a function of stressing time up to failure which represented as the resistance of Bump 3 increased 20 % when powered by 0.9 A (7.9104 A/cm2) at 150 °C. (a) Cu/Ni UBM system, (b) Cu UBM ystem.

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Figure 4-4 The corresponding cross-sectional SEM image for solder bump. (a) SnAg/CuNi after being stressed by 7.9104 A/cm2 at 150 °C current stressing for 160 hrs till the resistance of Bump 3 increasing 20%. (b) SnAg/Cu after being stressed by 7.9104 A/cm2 at 150 °C current stressing for 44 hrs till the resistance of Bump 3 increasing 20%.

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Figure 4-5 The corresponding cross-sectional SEM image for solder bump. (a) SnAg/CuNi after being stressed by 7.9104 A/cm2 at 150 °C current stressing for 160 hrs till the resistance of Bump 3 increasing 20%. (b) SnAg/Cu after being stressed by 7.9104 A/cm2 at 150 °C current stressing for 44 hrs till the resistance of Bump 3 increasing 20%.

109

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function of stressing time up to failure which represented as the resistance of Bump 3 increased 100 % when powered by 0.9 A (7.9104 A/cm2) at 150 °C in Cu/Ni UBM system.

Figure 4-7 The corresponding cross-sectional SEM image for solder bump. (a) SnAg/CuNi after being stressed by 7.9104 A/cm2 at 150 °C current stressing till Bump 3 totally failed. (b) SnAg/Cu after being stressed by 7.9104 A/cm2 at 150 °C current stressing till the Bump 3 totally failed.

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Figure 4-8 The corresponding Bump resistance for the stressing circuit as a function of stressing time up to totally failure of Bump 3 when powered by 0.9 A (7.9104 A/cm2) at 150 °C in Cu/Ni UBM system.

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Figure 4-9 IR images showing the temperature distribution and the hot spot point near the entrance point the SnAg bump at (a) CuNi system; (b) Cu system.

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Figure 4-10 (a) Cross-sectional view of current density distribution in solder joint for CuNi UBM system; (b) corresponding cross-sectional view for current density distribution for Cu UBM system.

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Figure 4-11 Plots of MTTF against k -3 10

, (a) eutectic SnAg solder joints with Cu/Ni UBM, and (b) eutectic SnAg solder joints with Cu UBM.

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Figure 5-1 Schematic diagrams for (a) a SnAg solder joint with a 5-mm Cu UBM, (b) a SnAg solder joint with a 5-mm Cu/3-mm Ni UBM, (c) cross-sectional view of the test layout. The electron flows are indicated by the arrows.

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Figure 5-2 Back-scattered SEM images for solder bumps before current stressing. (a) SnAg bump with a Cu UBM. (b) SnAg bump with a Cu/Ni UBM.

131

Figure 5-3 Cross-sectional SEM images showing the microstructures of the four bumps after the current stressing of 0.55 A through N3 and N4 at 150°C for 76 hrs. (a) Bump 1, (b) Bump 2 with a resistance increase of 200%, (c) Bump 3 with a resistance increase of 300%, (d) Bump 4. Voids formed in the chip side in all the four bumps.

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Figure 5-4 Cross-sectional SEM images representing the microstructure for the un-powered bump 1 in Cu UBM system before and after current stressing in bump 2 and 3 at 0.55A at 150°C for 60 hrs. (a) before, (b) after.

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Figure 5-5 Cross-sectional BEI images showing the microstructures of another set of bumps after the current stressing at 0.55 A through N3 and N4

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at 150°C for 82 hrs. (a) Bump 1, (b) Bump 2 with a resistance increase of 100%, (c) Bump 3 with a resistance increase of 350%, (d) Bump 4. Consumption of Cu UBM and spalling of Cu-Sn IMCs were observed in Bumps 1, 2 and 4.

Figure 5-6 Microstructures at the interface of the chip and the solder after the current stressing of 0.55 A through Bumps 2 and 3 at 150°C for 82 hrs. (a) SEM image for Bump 1, (b) Ion image for Bump 1, (c) SEM image for Bump 4, (d) ion image for Bump 4.

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Figure 5-7 Enlarged cross-sectional SEM images of the chip-solder interface for (a) the bump before aging, and (b) the same bump after the aging at 165°C for 90 hrs. The Cu-Sn IMCs grew thicker, but no migration of Cu-Sn IMCs was observed.

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Figure 5-8 Cross-sectional SEM images of the solder joints with Cu/Ni UBMs after the current stressing at 0.55 A through N2 and N3 at 150°C for 180 hrs. (a) Bump 1, no current; (b) Bump 2, with an upward electron flow; (c) Bump 3, with a downward electron flow; (d) Bump 4, no current. Only electromigration damages were observed in the chip side of Bump 3.

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Figure 5-9 Temperature distributions in the four bumps with Cu UBMs. (a) Bump 1, no current; (b) Bump 2, with upward electron flow of 0.55 A; (c) temperature profile along the white line in (a); (d) temperature profile along the white line in (b); (e) Bump 3, with downward electron flow of 0.55 A; (f) Bump 4, no current; (g) temperature profile along the white line in (e); (h) temperature profile along the white line in (f). The temperature gradients are labeled on the bumps.

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Figure 5-10 Temperature distributions in the four bumps with Cu/Ni UBMs stressed at 0.55 A through N3 and N4 at 100°C. (a) Bump 1, no current, and a temperature gradient of 857°C/cm; (b) Bump 2, with upward electron flow of 0.55 A, and a temperature gradient of 1286°C/cm; (c) Bump 3, with a downward current flow of 0.55 A, and a temperature gradient of 1429°C /cm; (d) Bump 4, no current, and a temperature gradient of 857°C /cm.

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Figure 5-11 Schematic diagrams of the possible atomic fluxes of Cu- and Sn-caused electromigration and thermomigration. (a) Atomic fluxes in Bumps 1 and 4, which have no currents passing through: only thermomigration takes place. (b) Diffusion fluxes of Cu and Sn in Bump 2 with an upward electron flow. (c) Atomic fluxes of Cu and Sn in Bump 3 with a downward electron flow.

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Figure 6-1 (a) Cross-sectional schematic of the layout design. An Al trace connected all the four solder bumps together. (b) Cross-sectional SEM image showing that the microstructure for the solder bump used in this study with Cu UBM. (c) Cross-sectional SEM image with Cu/Ni UBM.

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Figure 6-2 Cross-sectional TEM image of Bump before current stressing. 151 Figure 6-3 (a) Cross-sectional SEM image for Bump 4, showing the microstructures at the interface of the chip and the solder after the current stressing of 0.55A through Bumps 2 and 3 at 150°C for 82 hrs, (b) FIB Ion image for Bump 4 on the same area of (a).

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Figure 6-4 (a) Cross-sectional SEM image for Bump 1, showing the microstructures at the interface of the chip and the solder after the current stressing of 0.55 A through Bumps 2 and 3 at 150°C for 82 hrs, (b) FIB Ion image for Bump 4 on the same area of (a).

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Figure 6-5 (a) Cross-sectional SEM image for Bump 1, showing the microstructures at the interface of the chip and the solder after the current stressing of 0.55 A through Bumps 2 and 3 at 150°C for 165 hrs, (b) FIB Ion image for Bump 1 on the same area of (a). (c) Bump 4 after current stressing. (d) FIB Ion image for Bump 4 on the same area of (a).

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Figure 6-6 Cross-sectional TEM image of Bump 4 after the current stressing of 0.55 A through Bumps 2 and 3 at 150°C for 82 h.

155 Figure 6-7 Cross-sectional TEM image of the Al/Cu interface for the Bump 4 after the current stressing of 0.55 A through Bumps 2 and 3 at 150°C for 82 hrs.

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Figure 6-8 (a) Schematic structure of Bump 4 for simulation. (b) Temperature distribution along line 1 in Bump 4.

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Figure A-1 Characterizations of the as-deposited SnAg film from pulse electroplating of STD1 formulation; (a) current profile for the electrodeposition process, (b) SEM micrograph of typical surface morphology, and the EDX analysis of film composition: Sn: 96.57 wt.%; Ag: 3.63 wt.%, and (c) DSC curve with endothermic peak recorded at 221.8°C.

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Figure A-2 Relationship between current density at positive polarity and the resulting Ag concentration in the alloy film for various baths.

177 Figure A-3 Current–potential curves for several baths with PEG as dditive. STD1 is included for comparison.

178 Figure A-4 SEM images of surface morphology of the as-deposited SnAg alloy films for (a) STD1, (b) STD5, (C) STD2, (d) Bath9, and (e) Bath10.

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EDX data confirmed their near-eutectic composition.

Figure A-5 XRD data for eutectic Sn-3.5wt.%Ag film showing coexistence of -Sn and -Ag3Sn.

180 Figure A-6 DSC curve of heat flow for occurrence of solidus temperature of as-deposited SnAg film from different plating baths.