近年來，為了提高電子封裝（electronic package）的密度，球柵陣列封裝（ball grid array）中的焊接點持續地微縮，介金屬化合物（intermetallic compounds）之機械性質將逐漸主導焊接點的可靠度。其中，焊料合金的選擇與底層凸塊金屬（under bump metallization, UBM）的材料選擇是影響焊點微結構與介金屬化合物的重要議題。因此，本研究著重於探討於微焊點中銀含量之調變與銅鋅底層凸塊金屬對於機械性質與可靠度之影響。
首先，為了瞭解Sn-Ag-Cu（SAC）焊料中銀含量對於微焊點機械性質的影響，對直徑為90 μm的Sn-3.0Ag-0.5Cu/Cu（wt.%）與Sn-4.0Ag-0.5Cu/Cu微焊點進行低速剪切測試（low speed shear test）並觀察其微結構與破裂形貌，並與直徑為250 μm的焊點進行比較。研究發現90 μm的SAC305/Cu與250 μm的SAC405/Cu中，有無片狀Ag3Sn的生成對於剪切強度的影響並不大；然而對於90 μm SAC405/Cu微焊點，片狀Ag3Sn生成於焊點內時，剪切強度有顯著的上升。
此外，直徑90 μm的SAC305/Cu-30Zn與SAC405/Cu-30Zn微焊點經過迴焊後，與SAC305/Cu及SAC405/Cu相比，除了剪切強度提升外，微焊點的韌性也有大幅提升，並有效地抑制片狀Ag3Sn生成。從微結構分析發現SAC305/Cu與SAC405/Cu中，細小Ag3Sn顆粒均勻散布在焊料中；然而細小Ag3Sn於SAC305/Cu-30Zn與SAC405/Cu-30Zn微焊點中，其分布密度較高且呈現網狀分布，並有明顯的錫樹枝晶（Sn dendrite）結構，這種結構使焊料有更高的機械強度與更好的延展性，使焊點韌性大幅提升。

In recent years, in order to enhance the density of electronic packaging, the ball grid array (BGA) solder joints have been continuously miniaturized, thus the mechanical properties of intermetallic compounds (IMC) will gradually dominate the reliability of the solder joints. The choice of solder alloy and under bump metallization (UBM) layer are the critical issues which significantly affect the microstructure of solder joint. This study focuses on the effect of Ag content in solder alloy and Cu-Zn substrate on microstructure and mechanical properties of micro-BGA solder joints.
First, to investigate the influence of Ag content in Sn-Ag-Cu (SAC) solder on the mechanical properties of micro-BGA joints, low speed shear test was conducted on Sn-3.0Ag-0.5Cu/Cu (wt.%) and Sn-4.0Ag-0.5Cu/Cu micro-BGA joints with a diameter of 90 μm, and compare the results with SAC405/Cu solder joints with a diameter of 250 μm. Their microstructure and fracture morphology were also observed. It was found that the formation of plate-like Ag3Sn did not affect the shear strength in 90 μm SAC305/Cu and 250 μm SAC405/Cu. However, for 90 μm SAC405/Cu micro-BGA joints, a significant increase in shear strength was measured when huge plate-like Ag3Sn was observed on the fracture surface.
In addition, both the shear strength and the peak energy of SAC/Cu-30Zn were higher than those of SAC/Cu with a diameter of 90 μm. Besides, the plate-like Ag3Sn was effectively inhibited by using Cu-30Zn to replace Cu substrate. By microstructure observation, for SAC305/Cu and SAC405/Cu, the fine Ag3Sn particles were uniformly dispersed in the solder matrix. While the network-type distribution of Ag3Sn particles and the β-Sn dendrite were observed in SAC305/Cu-30Zn and SAC405/Cu-30Zn micro-BGA joints. Additionally, the occupied area of Ag3Sn particles in SAC/Cu-30Zn was larger than SAC/Cu. The network-type structure enables the micro-BGA joints to have higher shear strength and better ductility, which greatly improves the toughness of micro-BGA joints.

Content----------------------------------------------------------I
List of Tables---------------------------------------------------III
Figure Captions--------------------------------------------------IV

Chapter I. Introduction
1.1 Background-----------------------------------------------1
1.2 Motivations and Goals in This Study----------------------2

Chapter II. Literature Review
2.1 Electronic Package---------------------------------------4
2.2 Solder Bump----------------------------------------------6
2.2.1 Pb-free solder-------------------------------------------7
2.3 Under Bump Metallization---------------------------------8
2.4 Metallurgical Reactions in Sn-Ag-Cu/Cu Solder Joints-----10
2.4.1 Interfacial reactions in Sn-Ag-Cu/Cu solder joints-------11
2.4.2 Solidification process of off-eutectic SAC solder alloy--12
2.4.3 Large undercooling of beta-Sn nucleation-----------------13
2.4.4 Plate-like Ag3Sn formation mechanism---------------------15
2.5 Reliability Issues in Small Size Solder Joints-----------16
2.6 Zn addition to Sn-based Solder Joint---------------------18
2.6.1 Minor Zn doping in solder alloys-------------------------18
2.6.2 Effects of Zn addition in Cu-based UBM-------------------20

Chapter III. Experimental procedures
3.1 Sample Preparation---------------------------------------38
3.2 Microstructure Evaluation and Fracture Surface Observation-------------------------------------------------------------------39
3.3 Low-Speed Shear Test-------------------------------------39
3.4 Thermal Analysis-----------------------------------------40

Chapter IV. Results and Discussion
4.1 Microstructural and Shear Strength Variation of Sn-Ag-Cu/Cu Micro-BGA Joints with Different Ag Contents----------------------42
4.1.1 Microstructural observation------------------------------42
4.1.2 Results of shear testing for SAC305/Cu and SAC405/Cu micro-BGA joints-------------------------------------------------------43
4.1.3 The influence of plate-like Ag3Sn on shear strength of micro-BGA joints-------------------------------------------------44
4.1.4 The comparison of plate-like Ag3Sn effect between solder joints with different diameters----------------------------------46
4.2 Improving the Shear Strength of Sn-Ag-Cu/Cu-Zn Micro-BGA Joints via Microstructural Modifying-----------------------------54
4.2.1 Shear testing and fracture analysis----------------------54
4.2.2 Inhibiting plate-like Ag3Sn IMC formation----------------56
4.2.3 Microstructural observation of SAC/Cu-30Zn micro-BGA joints------------------------------------------------------------------57
4.2.4 Mechanism for the improvement of shear reliability in SAC/Cu-30Zn micro-BGA joints-------------------------------------60

Chapter V. Conclusion--------------------------------------------68
References-------------------------------------------------------71

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