本研究主旨在探討電子構裝用Sn-3.5Ag-0.5Cu無鉛銲錫受階梯狀負荷的潛變行為以及求出一套適合此潛變行為的壽命評估模式。潛變實驗採固定溫度下受二階段應力以及固定應力下受二階段溫度兩種方式進行，並利用掃描式電子顯微鏡(SEM)來觀察此無鉛銲錫之潛變破壞特徵。 實驗結果顯示，此款無鉛銲錫在受兩階段應力或溫度的潛變狀況下，由於負荷大小的次序不同，的確會對材料造成不同程度的損害累積。在本實驗中，第一階段負荷條件切換至第二階段負荷條件時，由高到低的負荷次序會比由低到高的負荷次序在第二階段對材料產生更大的損害。此外，受階梯狀負荷時，對於由高到低的負荷次序，其在第二階段的潛變速率會高於受同負荷條件的單一負荷之潛變速率。然而，在由低到高的負荷次序下，第二階段的潛變速率相較於所對應的單一負荷潛變速率，並無明顯的不同。常用的線性損壞模式無法提供此款無鉛銲錫材料準確的壽命預測，亦無法描述不同負荷次序對潛變壽命所造成的不同影響。本文所提出一修正型非線性潛變損害累積模式，對此種材料在受二階階梯狀負荷下可提供合理的壽命預測，也正確的描述負荷次序對材料損害及壽命產生的效應，即是，壽命比率值的總合對在低至高的負荷次序下，會大於由高至低的負荷次序。 由SEM觀察破斷後的試棒得知，在破斷的表面可以發現相當數量的延性小孔洞。此外，在此款無鉛銲錫中可發現許多微小孔洞以及微裂縫會形成在相界以及介金屬與錫基的界面上，應為潛變損害的起始處。 The purpose of this study is to investigate the creep behavior and develop a suitable creep damage model for a lead-free Sn-3.5Ag-0.5Cu solder alloy subjected to variable-step creep loading. Creep tests were conducted under two-step loading with various combinations of stress and/or temperature. Fractography analysis with scanning electron microscopy (SEM) was conducted to determine the creep fracture mechanism for the given solder. Experimental results showed existence of sequence effects on cumulative creep damage for the given ternary alloy under two-step loading or temperature. A high-low sequence is more damaging than a low-high sequence for all given two-step loading/temperature conditions. Furthermore, the creep rate in the second step of a high-low sequence was greater than the corresponding one under single-step loading while no significant difference in creep rate was observed in the second step of a low-high sequence compared with the corresponding one under single-step loading. The linear damage rule could not be applied to predict lifetime and describe sequence effect for the given lead-free solder under varying-step creep loading. A nonlinear cumulative creep damage model was proposed and made reasonably good predictions of creep lifetime for all the given two-step creep testing conditions. This model also predicts the sequence effects that a low-high step loading would produce a larger sum of lifetime fraction than a high-low one. SEM observations indicated there existed microvoids on the creep fracture surfaces. Creep microvoids and microcracks were found to nucleate at phase boundaries and interfaces between intermetallic compound and Sn matrix for the given Sn-3.5Ag-0.5Cu solder alloy.
