本研究主旨在探討Sn-3.5Ag與Sn-3.5Ag-0.5Cu兩款電子構裝用無鉛銲錫之機械性質及於低週疲勞行為，並與過去在工業上較常使用的Sn-37Pb銲錫做比較，以了解未來運用無鉛銲錫替代傳統含鉛銲錫之可行性。此外，亦利用掃描式電子顯微鏡(SEM)觀察表面裂縫與疲勞破斷面，以了解此兩款無鉛銲錫之疲勞破裂機制。
實驗結果顯示，在10-2、10-3與10-4 s-1三種不同應變速率的拉伸測試下，基本上Sn-3.5Ag-0.5Cu合金有最高的抗拉強度，Sn-3.5Ag合金次之，Sn-37Pb合金最低，且三種銲錫的抗拉強度皆會隨應變速率增加而提昇。而在延伸率與真實破裂延性方面，可以發現Sn-37Pb合金在兩較慢應變速率下，會展現出超塑性之特性，但對於兩款無鉛銲錫而言，延伸率與真實破裂延性反而會隨著應變速率下降而減少，主要是由於潛變機制引入所造成。從應變比為-1、應變速率為10-3 s-1的低週疲勞實驗得知，基本上無鉛銲錫比傳統銲錫有較高的疲勞阻抗，其中Sn-3.5Ag-0.5Cu合金的疲勞強度比Sn-3.5Ag合金高，主要是與其具有較高強度與抗潛變特性相關。Coffin-Manson關係式，對上述三種銲錫之低週疲勞行為皆有不錯的描述性，而利用延性修正的Coffin-Manson關係式，對此兩款無鉛銲錫的疲勞壽命評估，也有不錯的整合性。
由SEM觀察得知，細小的表面裂縫容易於富錫樹枝狀結構與共晶組織兩相交界處起始，主要是因兩相的強度不同，於循環受力後容易產生晶界差階而衍生應力集中所造成。而裂縫的成長與相互連結過程，主要沿著富錫樹枝狀結構與共晶組織交界處，伴隨穿過共晶組織相之穿沿晶混合模式進行，一旦連結成大裂縫並向試棒內部成長，便會導致試棒快速斷裂而失效。

The purpose of this study is to investigate the mechanical properties and low-cycle fatigue (LCF) behaviors of Sn-3.5Ag and Sn-3.5Ag-0.5Cu lead-free solders. These properties were compared with those of conventional Sn-37Pb solder to evaluate the feasibility of using lead-free solders to replace the Pb-contained solders in the future. Fractography analyses with scanning electronic microscopy (SEM) were conducted to determine fatigue fracture mechanisms of these two lead-free solders.
Experimental results show that the tensile strength was increased with increasing strain rate from 10-4 to 10-2 s-1 for all three types of solder alloys. Sn-3.5Ag-0.5Cu alloy had the highest tensile strength followed by Sn-3.5Ag alloy, and then the Sn-37Pb alloy. Due to the influence of a creep mechanism during tensile deformation processes, the elongation and true fracture ductility of lead-free solders were decreased with a decrease in strain rate. However, the Sn-37Pb alloy exhibited superplastic behavior when tested at lower strain rates. Under a strain ratio of R=-1 and a strain rate of 10-3 s-1, it could be found that the lead-free solders usually had better LCF resistance than the traditional Sn-37Pb solder. Because of its greater strength and creep resistance, Sn-3.5Ag-0.5Cu alloy exhibited longer fatigue life than Sn-3.5Ag. The LCF behavior of all solders generally followed the Coffin-Manson relationship. The fatigue life was dominated by the true fracture ductility and could be well described by a ductility-modified Coffin-Manson relationship.
From SEM observations, it can be found that, due to the difference of strength between the dendrite and eutectic phases, steps were generated along the boundaries between these two phases. These steps caused formation of surface microcracks as a result of stress concentration effects. The surface microcracks generally propagated in a mixed mode (intergranularly and transgranularly) and linked up to form large cracks. The intergranular path was along the dendrite boundaries and the transgranular path was through the eutectic phases. The large cracks would eventually grow into the specimen body and lead to final failure.