本研究主旨在探討0 度(純拉伸負載模式)、45 度(混合負載模式)以及90 度(純剪力負載模式)三種不同受力模式對Sn-3.5Ag-0.5Cu 無鉛銲錫接點試片低週疲勞行為的影響。並進一步和文獻中塊材型式的試片在相同條件下所測得的實驗結果做比較。此外，亦利用掃描式電子顯微鏡(SEM)觀察表面裂縫與破斷面，以了解此款無鉛銲錫接點之疲勞破裂機制。
實驗結果顯示，在各種位移振幅、負載模式的試驗條件組合中，von Mises 等效應變可以有效的將三種不同負載模式下的低週疲勞壽命做有效的統合。在本實驗中所使用臨界平面觀念為主的KBM 參數以及使用von Mises 等效應變之Basquin-Coffin-Manson 方程式都能針對在不同負載模式下的低週疲勞行為做出很好的整合性描述。將接點試片在0 度受力模式下和塊材在相同情形下作等效應變-壽命曲線分析比較，發現在低應變振幅下接點有比較好的疲勞壽命，但在高應變振幅下則得到相反的結果。銲點試片和塊材試片所顯示的應力振幅下降曲線趨勢非常相近。
由SEM 觀察得知，微小的裂縫起始於銅片和錫球之間介面處的某些微孔洞所在位置。微裂縫以及微孔洞的相互連結是導致疲勞裂縫生成與成長的主因。而在不同負載模式下，試片斷裂的位置都位於銅片和錫球間的介面處，這是因為該介面為試片抵抗疲勞損害最弱的地方。

The purpose of this study is to investigate the LCF properties of solder joints made of a promising lead-free solder alloy, Sn-3.5Ag-0.5Cu. Displacement-controlled LCF tests were conducted on the solder joint specimens under various loading conditions, including purely axial loading, purely shear loading and mixed-mode loading. The effect of multiaxial loading on the fatigue life and fracture mode was characterized. In addition, the difference in the uniaxial LCF life between solder joints and bulk solders was discussed by making a comparison with a previous study.
Experimental results showed the von Mises equivalent strain was a superior parameter to the maximum normal strain and the maximum shear strain in correlating the LCF life of solder joints under various loading modes. Several fatigue models were applied to describe the LCF lives of the given solder joint specimens subjected to different modes of loading. Among the applied approaches, KBM parameter and the von Mises equivalent strain provided the best unified correlation with the fatigue life of solder joint at all given loading modes through a single power law or a double law relationship.
Although a similar trend of load drop curve to that of the bulk solder was found for the solder joint, the LCF behavior under purely axial loading between the solder joint and bulk solder was different. Based on the fractography analysis results, the loading mode did not have a significant influence on the cracking path of the solder joint, as the fatigue fracture always occurred at the interface between the solder and copper pad for all the given loading modes.

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