本研究目的在於探討Ag含量(0~3wt.%)對Sn-Ag-Cu無鉛銲料微結構及特性的影響，並探討不同幾何形貌(沙漏型、桶型)之銲點疲勞性質，評估銲料低週疲勞可靠度表現。

實驗試件利用直徑1.12±0.02 mm之錫球，與Cu基板銲接成單點剪切試件，之後再進行IMC觀察以及疲勞性質評估。結果顯示Sn-Ag-Cu三元相銲料的結構，隨著Ag含量的降低，由細小顆粒狀Ag3Sn, Cu6Sn5所組成的網狀組織密度逐漸稀疏，銲料機械強度也隨之下降。Sn-0.7Cu二元相合金由於無Ag添加，亦無網狀共晶組織析出進行散佈強化，導致其抗剪切強度最低。

低週疲勞測試中，銲點疲勞壽命隨著Ag含量提高而增加，原因在於Ag3Sn網狀共晶組織與IMC層相互影響，而形貌不同之桶型與沙漏型銲點疲勞壽命曲線及裂紋成長模式亦不相同。以SEM觀察銲點切面，得知細小裂紋易於初晶β-Sn與析出相交界處起始，主要是因兩相強度不同，於循環受力後容易生成。裂紋成長模式取決於銲點幾何形狀、銲料基地強度與界面層強度三者交互影響，其中沙漏型銲點大部分疲勞裂紋在銲料內部成長；含Ag量較高之桶型銲點疲勞裂紋沿著IMC層以穿晶的方式成長；含Ag量較少之銲點裂紋則非直接穿裂IMC層，由接觸角引入後往銲料內部成長一小段，與分散於IMC層之微裂紋相互連結；Sn-0.7Cu二元合金，裂紋沿界面不遠處往銲料內部成長交會。綜合銲料微結構、銲點受力分析、疲勞壽命曲線與裂紋成長機制分析，研究結果顯示，Ag添加量於1.5~2.0wt.%時已可大幅增強Sn-Cu合金系銲料機械性質。

This research is aimed to investigate the effect of Ag content (0~3wt.%) on the microstructure and the low-cycle fatigue behavior of Sn-xAg-0.7Cu lead-free solder.

Sn-xAg-0.7Cu solder in form of solder balls with 1.12±0.02 mm in diameter were fabricated. Solder balls were reflowed with Cu substrate in the form of single lap shear specimen, and then the low-cycle fatigue test were performed. Solder alloys SAC157, SAC207 show similar microstructure features whilst SAC307 solder alloy displays a very fine microstructure with intermetallic phases dense within the β-Sn matrix. The increase in silver content enhances the mechanical strength of the solder alloy as a result of an increase in volume fraction of Ag3Sn intermetallics.

The fatigue life test reveals that fatigue life curve of the solder joint is absolutely different between barrel type and hourglass type. From SEM observations, it can be found that micro-cracks were first generated along the boundaries between these two phases. As a result of stress concentration effects, solder joints of barrel type would fail with IMC layer fracture mode except Sn-0.7Cu. However, solder joints of hourglass type would fail with solder fracture mode except Sn-3.0Ag-0.7Cu. The cracks generally propagated in a mixed mode and linked up to form large cracks and immediately lead to final failure. Sn-xAg-0.7Cu solder with silver addition by 1.5~2.0wt.% manifest already the better mechanical properties and fatigue behavior than that another Ag addition.

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