Effects of Ga addition on the microstructure and mechanical performance of Sn-58Bi based alloys and the solder joints

In the research, 0.5 wt. % Ga were added in Sn-58Bi solder to investigate the effect of Ga addition on the microstructure and mechanical performance of Sn-58Bi based solders and corresponding solder joints. Elemental analysis of Ga-bearing bulk solder confirms the existence of solid solution Ga in β-Sn matrix and network α-Ga phase. The result of tensile test and post-test fracture surface analysis indicates that Ga addition improves the ultimate tensile strength, however, it depletes the plasticity by the precipitation of α-Ga phase with low melting point. With 0.5 wt. % Ga addition, the eutectic structure was much refined and the interfacial product changes from traditional scallop-like Cu₆Sn₅ layer to smooth Cu₉Ga₄ IMCs. In addition, the shear strength of Sn-58Bi solder joint was effectively improved by Ga addition due to the refinement strengthening of solder matrix and the improvement of interfacial IMC morphology. The shear fracture path changes from transgranular fracture in Cu₆Sn₅ layer to interfacial fracture between solder and Cu₉Ga₄ IMC layer.