本研究目的在於探討Ag含量(0~3 Wt.%)對Sn-xAg-Cu無鉛銲料微結構及特性的影響，並探討在常溫下使用改良後圓角桶狀銲點之疲勞性質，評估銲料低週疲勞之可靠度表現。

實驗試件利用直徑1.11±0.02 mm之錫球，與銅片銲接成單點剪切試件後，再進行銲點組織結構與IMC觀察，並評估其疲勞性質。在Sn-Ag-Cu銲料微結構方面，銲點是由Ag3Sn、Cu6Sn5與β-Sn所組成之網狀共晶組織。隨著Ag含量的增加，Ag3Sn析出愈多愈越密集，析出強化效果提升銲料之強度。而在Sn-0.7Cu二元合金中，因無Ag3Sn析出強化，故其為本實驗所使用之抗剪強度最低的合金成分。

低週疲勞測試中，因網狀共晶組織與IMC層相互影響，銲點疲勞壽命曲線及裂紋成長模式會有不同之趨勢。以OM與SEM觀察不同疲勞循環負載下之銲點的裂紋成長變化，發現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，此外銲點形貌亦有影響，圓角桶狀銲點的疲勞裂紋往往從圓角處起始，而Ag含量較高之銲點易貫穿IMC層的方式成長；Ag含量減少時，銲點之疲勞裂紋則是往銲料內部成長，且會破壞於IMC層附近；Sn-0.7Cu二元合金，裂紋則是往銲料內部成長。

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 aims at investigating the effect of the silver content (0~3wt.%) on the microstructure of Sn-xAg-0.7Cu lead-free solder. The Low-cycle fatigue properties of the improved solder joints were also evaluated.

Sn-xAg-0.7Cu solder balls with 1.12±0.02 mm in diameter were fabricated. Solder balls were re-flowed with Cu substrate in the form of single lap shear specimen, and then the Low-cycle fatigue test were performed. Experimental results show that microstructure of Sn-xAg-0.7Cu alloys consists of particle-like Ag3Sn intermetallic compounds, together with the copper riched rod-like Cu6Sn5 within β-Sn matrix for all alloys. The increase of silver content enhances the mechanical strength of the solder alloy as a result of an increase in volume fraction of Ag3Sn intermetallic. However, the Sn-0.7Cu solder has the worst strength.

Solder joints have different kinds of fatigue crack tendency and fatigue life curve under the low-cycle fatigue test owing to different kinds of microstructure. From the OM and SEM observations, it can be found that fatigue cracks grows from the sharp corner of the barrel type solder joint. As the result, the failure type of the Sn-3.0Ag-0.7Cu solder joints is IMC facture mode. However the other solders are solder fracture mode.

Sn-xAg-0.7Cu solder with 1.5~2.0wt.% silver addition has enhanced the mechanical properties and the fatigue behavior significantly.

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