Effects of zinc on the interfacial reactions of tin-indium solder joints with copper

Sn-20.0 wt%In (Sn-20.0In) alloy is a promising base material in Pb-free solders for low-temperature applications. Zn is often used as an additive to Pb-free solders to reduce the extent of undercooling during reflow. Cu is the most commonly used substrate in electronics industry. Interfacial stability at Sn-In-Zn/Cu joints is crucial to reliability of electronic products. In this study, interfacial reactions between Sn-20.0 wt%In-x wt%Zn (Sn-20.0In-xZn) solders and Cu where x = 0.5, 0.7, 1.0, 2.0, 3.0, and 5.0 at 150, 230, and 260 C were experimentally examined. It is found that the reaction phase formation and interfacial morphologies are strongly influenced by Zn concentrations. The reaction phases evolve from the Cu₆Sn₅ phase, CuZn and Cu₅Zn₈ phase, to Cu₅Zn₈ phase with higher Zn doping in the solders. The Cu₅Zn₈ phase acted as a diffusion barrier and suppressed the growth of the Cu₆Sn ₅ phase. The results indicate that 2.0 wt%Zn addition resulted in the gentlest reactions during both soldering and solid-state ageing in Sn-20.0In-xZn/Cu couples.