Optimization of mechanical properties of Sn-3.8Ag-0.7Cu alloys by the additions of Bi, In and Ti

Bi, In and Ti were added to Sn-3.8Ag-0.7Cu (SAC387) solder alloy to optimize the mechanical performance. The alloying effects of Bi, In and Ti on the microstructure, thermal and mechanical properties of SAC387 based solder alloys were investigated. The results demonstrate that adding 3.5 wt % of Bi could refine the microstructure, optimize the thermal properties, and improve the tensile strength. Meanwhile, the ductility of the solder alloys reduced evidently. Adding 2.8 wt % of In into SAC387-3.5 wt %Bi alloy could increase both the strength and ductility, which is attributed to the beneficial effect of In addition, as adding In could improve the solubility of Bi in the beta-Sn matrix. Meanwhile, the melting point was reduced, and the wettability improved with the addition of In. Introducing amounts of Ti into SAC387-3.5 wt % Bi-2.8 wt % In alloy could further increase the strength. However, the ductility was significantly reduced when 0.8 wt % of Ti was added due to the formation of the coarse Ti2Sn3 phase. The undercooling was remarkably reduced with the addition of Ti. The nanoindentation tests demonstrate that the hardness increased mainly due to the hardening effect of the Bi addition. Among all the samples prepared, alloy SAC387-3.5 wt % Bi exhibited the highest creep resistance at the ambient temperature. Further adding In and Ti into SAC387-3.5 wt % Bi alloys reduced the creep resistance of the solder alloys. The mechanism associated with the different mechanical responses is also discussed in this study.