The effects of Cu alloying on the microstructure and mechanical properties of Zn-25Sn-xCu (x = 0–1.0 wt%) high temperature Pb-free solders

The effects of Cu alloying on the microstructure and mechanical properties of Zn-25Sn-xCu (x = 0–1.0 wt%) high temperature Pb-free solders were investigated in the present study. The 1 wt% Cu alloying enhanced the ultimate tensile strength and the micro-hardness of the primary η-Zn phase by 23.1% and 144.0% increments, respectively. These mechanical property variations were attributed to the combined effects of grain refining strengthening of the primary η-Zn phases, precipitation strengthening of the fine ε-CuZn5 compounds in the η-Zn matrix (Cu content greater than 0.3 wt%), and solid-solution strengthening of the Cu inclusion in the η-Zn matrix. The granular two-phase (η-Zn + ε-CuZn5) microstructure, composed of refined η-Zn matrix and fine ε-CuZn5 precipitations, contributed to superior mechanical strength of the Zn-25Sn-xCu high temperature Pb-free solders.