| Lead and Pb compounds have been cited by the Environmental Protection Agency (EPA) as one of the top 17 chemicals posing the greatest threat to human beings and the environment, which indicate that developing viable alternative lead-free solders for electronic assemblies and other soft soldering is a trend. Through more than 10 years study, Sn-Ag, Sn-Zn et al are summarized as properly substitutes of Sn-Pb alloy. In all Sn-based solders, Sn-Zn is a low cost , closed melting point solder and has excellent mechanical properties, however the bad wetting and oxidation behavior which caused by active Zn element need be resolved before applying.Many elements such as In, Bi, Al are added to Sn-Zn alloy, but the improvement of wetting is still limited. Through previous experiments' results discover, the Zn atom will transform into Cu-Zn compound when the alloy add Cu element that can reduce the oxidation of Zn in the surface of solder. So in this text, the microstructure , melting point and mechanical properties are investigated by alloying Cu element and the interfacial microstructures and wetting property of the new SZ-xCu solders with Cu substrate are also investigated. For improving the wetting property and decreasing the melting point, 3wt%Bi and (0~1)wt% Ni are added to (Sn-9Zn)-2Cu solder. The wetting reaction between (Sn-9Zn)-3Cu and under bump metallic(UBM) Ni and the effect of 170℃ thermal aging on the joint are investigated for studying the reliability of solder joint. All the results show that:1,The addition of Cu can decrease the oxidation of Zn in liquid surface, which actually lessenthe surface tension and acquire the smaller angles and elevate the melting point too. The tensile strength start increase and decrease later as the Cu contents increase and the variety of shear strength is caused by the change between IMC and solder. When the Cu content between 0-8%, the needle Zn phase change into Cu-Zn compound and the contents exceed 8%, Cu6Sn5 begin to appear, when the Cu content at 0~2%Cu ,Cu5Zn8 compounds is formed at the SZ-xCu /Cu interface and both Cu6Sn5 and Cu5Zn8 compounds is formed at 2% ~ 6%, nearly single scallop Cu6Sn5 layer was detected as the addition is beyond 8%.2,The addition of 3% Bi can decrease the melting point and Ni adding to SZC-Bi alloys willrefine the microstructures and at 1%Ni, the rod-shape Cu5Zn8 phase changed into square-shape (Cu, Ni)5Zn8 phase. Alloying with Bi,Ni, SZC solder can improve the wettabilityand the wetting angles decrease half from that of Sn-9Zn by using RMA flux, when Ni content increase, The additives have little effect on the composition of interfacial IMC and the reaction layer decrease from 4um to 3um in SZC-Bi-Ni alloy for the formation of (Cu, Ni)-Zn compound, which resulted less Zn atoms diffusing to the interface. 3,(Sn-9Zn)-3Cu wet well on Ni substrate and acquire 67° even using RMA flux; When thermal aging at 170℃, The plane MC Ni5Zn21 will not decompose after 1000h and the IMC thickness with time is parabola relation before 500h,however after 1000h the thickness do not change which is caused by no more Zn atom diffusing into interface.
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