Change aspects of microstructure and mechanical behavior of Bi and Zn-doped Sn–0.5Cu solders for microelectronic applications

Sn–Cu alloys were recommended as a promising substitute for traditional Sn–Pb alloy in wave soldering applications. In the present study, the change of microstructure, thermal and mechanical behavior associated with alloying of Bi and Zn into Sn–0.5Cu solder have been investigated. DSC analysis reveals that large kinetic undercooling arising from the rapid solidification condition is the vital mechanism to realize the eutectic lamellar structures for hypoeutectic Sn–Cu solder. Unlike Bi addition, Zn doping is very effective in reducing undercooling and the onset temperature, although the pasty range is slightly increased. Correlations between thermal analysis, microstructure, and mechanical behavior were established. One conspicuous feature of Zn addition is that Zn promotes the nucleation of fine β-Sn grains, which in turn increases the ductility, but the yield strength, ultimate tensile strength and Young’s modulus are decreased. These properties emerge the plastic energy dissipation ability of solder joints at room temperature. In contrast, the higher mechanical strength of Bi-containing solder is contributed by the solid solution effects and precipitations hardening mechanism of Bi.