Mechanical properties of Sn–Zn lead-free solder alloys based on the microstructure array

The aim of this study is to develop a comparative experimental study interrelating mechanical properties, solidification thermal parameters and microstructure characteristics of a hypoeutectic Sn–4 wt.% Zn, a hypereutectic Sn–12 wt.% Zn and a eutectic Sn–9 wt.% Zn solder alloys. A water-cooled vertical upward unidirectional solidification system was used to obtain the samples. It was found that a more homogeneous distribution of the eutectic mixture, which occurs for smaller dendritic spacings in hypoeutectic and hypereutectic alloys, increases the ultimate tensile strength. The resulting microstructure of the eutectic Sn-9 wt.% Zn alloy has induced higher mechanical strength than those of the Sn–4 wt.% Zn and Sn–12 wt.% Zn alloys. It was found that the eutectic alloy experiences a microstructural transition from globular-to-needle-like Zn-rich morphologies which depend on the solidification growth rate. It is also shown that a globular-like Zn-rich morphology provides higher ultimate tensile strength than a needle-like Zn-rich eutectic morphology.