在電子封裝技術中，焊錫為其一重要製程可以提供晶片和基板間的電子通訊連接以及機械力學支持。為了環境與健康考量因素，含鉛焊錫逐漸被無鉛銲錫所取代，如錫銀或是錫銀銅等無鉛銲錫材料。在積體電路或是印刷電路板中，銅因為其良好的導電性被廣泛運用為晶片的金屬墊片和金屬黏接版。在熱壓封裝過程中，銅和原本存在焊錫內的銀皆會和錫反應生成介金屬化合物，如Cu6Sn5和Ag3Sn。而在下一個電子封裝世代，三維立體封裝被認為是一個重要技術可以增進封裝品質以及效能，但也由於其晶片堆疊效應會產生大量的焦耳熱，為了排散這些廢熱勢必會在焊錫內建立一溫度梯度，而溫度梯度可能會導致無鉛微米焊錫熱遷移破壞。熱遷移導致介金屬化合物極化效應已在許多不同焊錫界面反應系統被探討，熱遷移主要導致的破壞為冷端介金屬化合物快速成長和熱端金屬快速溶解，兩者皆為重要的可靠度議題且會降低焊錫的電性和機械性質。本研究中，我們藉由Ag3Sn當作屏障層來抑制在溫度梯度下熱遷導致的銅原子流量，為了瞭解Ag3Sn介層阻擋熱遷銅原子流量的效用，我們使用兩組不同三明治結構來比較，一是銅/銀/錫銀焊錫/銅，另一是銅/錫銀焊錫/銅，並為模擬電子產品使用情況，利用熱端和散熱端建立約7000 ℃/cm的溫度梯度。結果顯示多了Ag3Sn介層的試片，其冷端的介金屬化合物成長速率較沒有Ag3Sn介層的緩慢，熱遷移導致的銅流量於有Ag3Sn介層的試片被有效抑制，其大小約為沒有Ag3Sn的試片的三分之一，而且在銅/銀/錫銀焊錫/銅中幾乎沒有發現介金屬化合物或是金屬墊片的溶解現象，這些都表示Ag3Sn介層可以在熱遷移測試中有效抑制銅原子從熱端遷移至冷端。另外，我們發現在有鍍銀的試片中，Ag3Sn可能會影響錫晶粒的晶粒方向，使錫晶粒的c軸較垂直於溫度梯度；並且認為Ag3Sn介層影響熱遷導致的銅流量程度會隨著錫的不同晶粒方向而改變，當錫晶粒的c軸較平行於溫度梯度的情況下，Ag3Sn抑制熱遷導致的銅流量比例較大。

In the electronic packaging technique, soldering is an essential process to provide electrical interconnection and mechanical support between the chips and the substrates. Due to the environmental and health concerns, SnPb solders have recently been replaced by lead-free solder materials such as Sn3.5Ag and Sn3.0Ag0.5Cu. Cu is widely used as the under bump metallurgy (UBM) on the chip and the bonding pad in the integrated circuit (IC) substrate or printed circuit board (PCB) since its naïve good conductivity. During thermal compressing bonding, both Cu and Ag which is originally in solder matrix would react with Sn to form Cu6Sn5 and Ag3Sn intermetallic compound (IMC) respectively. Furthermore, for the next electronic packaging generation, three-dimensional integrated circuit (3DIC) is the most promising technic to improve the quality and performance. However, due to chips stacking effect, the joule heating becomes a big concern. To remove the heat, a temperature gradient must be established across the solder joint, which leads to thermomigration-induced failure in Pb-free microbumps. The polarity effect of thermomigration on IMC has been investigated in various solder interfacial systems. The major failures that induced by thermomigration are the high growth rate of IMCs at the cold end and the fast dissolution of metallization at the hot end. Both of them are the serious reliability issues and may degrade the electrical and mechanical properties of solder. In this study, we utilized Ag3Sn as a barrier layer to suppress the thermomigration-induced Cu flux (JCu,TM) under a temperature gradient. To understand the effect of Ag3Sn layer, two different kinds of sandwich structures of samples were introduced, Cu/Ag/Sn3.5Ag/Cu and Cu/Sn3.5Ag/Cu. A heat sink and a heat source were employed to establish a temperature gradient of about 7000 °C/cm which was regarded as the using condition for electronic devices. The results show that the growth rate of IMCs at cold end in Cu/Ag/Sn3.5Ag/Cu was slower than that in Cu/Sn3.5Ag/Cu. Thermomigration-induced Cu fluxes were also inhibited effectively in the specimens with Ag deposited, and the calculated value of JCu,TM was about one-third times as that in the specimen without Ag. Besides, the dissolution of IMCs and UBM at hot end was barely seen in Cu/Ag/Sn3.5Ag/Cu. Those indicates that Ag3Sn layer can be a diffusion barrier to inhibit the Cu flux from hot end to cold end effectively under thermomigration test. In addition, we found that Ag3Sn layer may have affect the crystallographic orientation of Sn grains, leading to the c-axis of Sn grain be more vertical to the direction of JCu,TM for the samples with Ag deposited. The proportion for the amount of suppressing JTM due to Ag3Sn interlayer decreased with increasing angleα.

摘要…………………………………………………………………………………......i
Abstract…………………………………………………………………………………iii
致謝………………………………………………………………………………..........v
Table of content………………………………………………………………………...vii
Table of caption page…………………………………………………………………....x
Figure of caption page………………………………………………………………….xii
1. Introduction……………………………………………………………………………1
2. Literature review………………………………………………………………………4
2.1 Thermomigration……………………………………………………………….....4
2.1.1. Theory of thermomigration……………………………………………………..4
2.1.2 Thermomigration in Pb-free solders…………………………………………….5
2.2 Effect of Ag addition in lead free solder…………………………………………..9
2.3 The inherent property of β-Sn grain_annisotropy………………………………..14
2.4 Issue of serrated dissolution for packaging technology………………………….20
2.5 Motivation………………………………………………………………………..25
3. Experimental procedure……………………………………………………………...26
3.1 Sample preparation………………………………………………………………26
3.2 Thermomigration test set up and characterization……………………………….27
3.3 Isothermal test……………………………………………………………………28
3.4 Finite element ANSYS simulation……………………………………………….28
4. Results………………………………………………………………………………..32
4.1 Simulation of temperature gradients……………………………………………..32
4.2 Microstructural evolution of Cu/Ag/Sn3.5Ag/Cu……………………………….35
4.2.1 Initial microstructure of as-fabricated solder layer.………………………..35
4.2.2 Microstructural evolution of solder layer after thermomigration test……..39
4.2.3 Microstructural evolution of solder layer after isothermal aging test……..47
4.2.4 Crystallographic orientation of Sn grain in Cu/Ag/Sn3.5Ag/Cu………….51
4.3 Microstructural evolution of Cu/Sn3.5Ag/Cu…………………………………...54
4.3.1 Initial microstructure of as-fabricated solder layer………………………..54
4.3.2 Microstructural evolution of solder layer after thermomigration test……..56
4.3.3 Microstructural evolution of solder layer after isothermal aging test……..62
4.3.4 Crystallographic orientation of Sn grain in Cu/Sn3.5Ag/Cu……………...66
5. Discussion…………………………………………………………………………...69
5.1 Comparison between isothermal test and thermomigarion test in Cu/Ag/Sn3.5Ag/Cu…………………………………………………………….69
5.2 Comparison between isothermal test and thermomigarion test in
Cu/Sn3.5Ag/Cu………………………………………………………………...75
5.3 Effect of Ag3Sn layer……………………………………………………………..82
5.3.1 Suppression of thermomigration-induced Cu flux………………………....82
5.3.2 Crystallographic orientation of Sn grain…………………………………...85
5.4 Effect of Sn grain orientation…………………………………………………….87
5.4.1 Suppression of thermomigration-induced Cu flux…………………………87
5.4.2. Serrated dissolution in Cu/Sn3.5Ag/Cu…………………………………...93
6. Conclusions…………………………………………………………………………...95
7. Future works………………………………………………………………………….97
8. References………………………………………………………………………….....98

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