Robust effects of Bi doping on microstructure development and mechanical properties of hypoeutectic Sn–6.5Zn solder alloy

Abstract

Sn–9Zn lead-free solders possess remarkable potential as candidates for low temperature electronic applications. In this study, it is proposed to utilize the advantages of Bi alloying and reducing the amount of Zn to improve oxidation resistance and reliability of eutectic Sn–9Zn solder. The results indicate that hypoeutectic Sn–6.5Zn alloy composed of β-Sn-rich phase and α-Zn fiber. 1.0 wt Bi addition caused a strong inhibition of α-Zn fiber inside the alloy matrix. Moreover, the fiber spacing of α-Zn phase increased and its diameter decreased. With increasing Bi content to 3.0 wt, small bright of Bi particles are observed. The addition of Bi could effectively reduce the onset and eutectic temperatures, while the amount of undercooling <1.0 °C was recorded. The tensile strength of Bi-containing solders was enhanced to about 180 %, although the ductility was slightly decreased. The higher strength was contributed by solid solution effect and precipitations hardening of Bi atoms or particles, which can remarkably modify the microstructure, blocks the dislocation motion and increases the tensile strength of Bi-containing solders.