Sn-3.5Ag無鉛銲料被公認為最具開發潛力的無鉛銲料之ㄧ。但若Ag量超過3.5wt%，則高溫儲存後在銲料與純Cu基板界面處的Ag3Sn會粗大化，同時由Cu6Sn5與Cu3Sn所構成在銲料與純Cu基板間的界面層會變厚，同時此界面層的表面也變粗糙，致使銲點強度、抗熱性與抗潛變性降低。為使高溫熱儲存後銲點內的Ag3Sn變小及壓抑界面層的成長故加入第三元素Sb。但Sb因會提高銲料的熔點致使Sn-Ag-Sb銲料的銲接性變差，為降低銲料熔點，而再添加第四元素Cu。本研究中以熱差掃描熱量測定儀(DSC)來量測Sn-3Ag-1.5Sb-xCu (x=0~1.5wt%)的固、液相線溫度變化，並使用OM、SEM及拉伸試驗來探討應變率與熱儲存前後之微結構對銲點結合強度的影響，以定出適合於工業界使用的Cu添加量界限。
研究結果顯示，Sn-3Ag-1.5Sb-xCu銲料的固相線溫度，在添加的0~1.5wt %Cu範圍內約在492~493K，比未添加Cu的Sn-3Ag-1.5Sb(498K)約降5K，顯示添加Cu後降低銲料固相線溫度的效果良好。剛銲接完成之銲料的硬度與強度會隨著所添加Cu數量的增加而增大，然而在150 熱儲存初期(≦200小時)，硬度與強度值則反之隨熱儲存時間的增加而降低，但在同一熱儲存時間下，當Cu添加量小於1.0wt%時，硬度與強度值則仍維持隨著添加Cu數量的增加而增大，而此時的1.5wt% Cu銲料因界面層增厚及界面層的表面粗糙度較1.0wt% Cu銲料嚴重，而因應力集中效應導致1.5wt% Cu銲料強度降低。當熱儲存時間超過200小時後，硬度、強度的變化則差別不大。所以Sn-3Ag-1.5Sb-1.0Cu銲點的強度為本研究所用Sn-3Ag-1.5Sb-xCu銲料在同一熱儲存條件下最高者。
水冷銲料的硬度，即使經熱儲存後仍比同時效的空冷佳，此乃因水冷組織比空冷的Ag3Sn、Cu6Sn5與環狀β-Sn基地小，以及界面層較薄、表面較平整所致。經150 熱儲存處理過的拉伸試片，在Cu添加量小於1.0wt%、熱儲存小於600小時及應變率小於1.0s-1時的破壞特徵皆呈韌脆混合模式，但在添加1.5Cu的銲點在200小時熱儲存以後的高應變率下(大於1.0s-1)則為劈裂的破壞模式。添加1.0 wt%Cu的試片，因長時間熱儲存下，界面層厚度與界面層表面粗糙度較1.5 wt%Cu小，故時效後銲點的結合強度較大。

Sn-3.5 wt% Ag lead-free solder is recognized as having an excellent potential for soldering applications. However, when the Ag content in the solder exceeds 3.5 wt%, the Ag3Sn intermetallic compound (IMC) formed within the interface between the solder and a Cu substrate following the thermal storage process become large and plate-like. Furthermore, an interfacial layer containing Cu6Sn5 and Cu3Sn particles is formed at the interface between them. Following prolonged storage, the thickness of the interfacial layer increases and its morphology coarsens, and thus the mechanical properties, thermal resistance, and creep resistance of the solder are severely degraded. The addition of small amounts of Sb to the Sn-Ag solder reduces the Ag3Sn grain size, suppresses the growth of the interfacial layer following high temperature storage. However, Sb addition also increases the melting point of the solder, which may potentially damage the solderability of the Sn-Ag-Sb solder. And the melting point of Sn-Ag-Sb solder can be reduced through the addition of a fourth alloying element of Cu. Accordingly, a Differential Scanning Calorimeter (DSC) was used to detect the change in the solidus for different levels of Cu addition. The influences of strain rate, and different thermal storage times on the microstructure and bonding strength of the Sn-Ag-Sb-Cu solder joints in order to find an optimum Sn-Ag-Sb-Cu solder composition for industrial applications.
The experimental results show that the solidus temperature falls about 492~493K with a range of 0~1.5wt% Cu addition. The solidus temperature falls about 5K below that of the Sn-3Ag-1.5Sb solder. Therefore, the effect of Cu addition to the Sn-3Ag-1.5Sb on the solidus temperature was obvious.
The hardness and strength of the as-soldered studied solder increase with increasing Cu addition. During the early stages of thermal storage, the hardness and strength decrease with aging period. For any given storage time, the hardness and strength increase with increasing Cu addition within 1.0wt%Cu addition. The addition of 1.5 wt% Cu to the solder yields a thicker, rougher interfacial layer than 1.0 wt% Cu. The resulting stress concentrations degrade the bonding strength of the Sn-3Ag-1.5Sb-1.5Cu/Cu sample. However, after 200hrs thermal storage, little change in the hardness and strength takes place. And thus for any given aging condition, the Sn-3Ag-1.5Sb-1.0Cu solder has the highest bonding strength of the thermal storage samples.
The same solder with water cooling seems to have the better properties than with air cooling. Following the soldering, the Ag3Sn, Cu6Sn5 particles, and the eutectic area around the β-Sn primary grain was smaller and the surface of the interfacial layer was thinner and more planar with a faster cooling rate than with a lower cooling rate. Therefore, the faster cooling rate can increase the strength of solder joints.
SEM analyses of the aged specimens at 150 following tensile testing have shown that the Sn-3Ag-1.5Sb-0.5Cu and Sn-3Ag-1.5Sb-1.0Cu joints fail as a result of a mixed brittle and ductile failure mode. The thermal aged Sn-3Ag-1.5Sb-1.5Cu joint stressing at a higher strain rate (over 1s-1) fails in a cleavage fracture mode. For prolonged aging condition, the Sn-3Ag-1.5Sb-1.0Cu solder has the highest adhesive strength of the various samples, and the addition of 1.5 wt% Cu to the solder yields a thicker, rougher interfacial layer than 1.0 wt% Cu.

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