A study of the thin film on the surface of Sn–3.5Ag/Sn–3.5Ag–2.0Cu lead-free alloy

The ternary lead-free solder alloy Sn–Ag–Cu has been considered as a potential alternative to lead–tin alloys, but oxidation of the solders often occurs at the injection-ball, even during reflow. This study investigates the characteristics of the solidification thin film of both the Sn–3.5Ag alloy and the Sn–3.5Ag–2.0Cu alloy to discuss and clarify the mechanism acting on the oxidative thin film. The results indicate that the solidification surface thin film of the Sn–3.5Ag–(2.0Cu) is mostly composed of SnO phase and SnO2 phase. The concentration of Sn4+ is higher than that of Sn2+ in the film. The concentration of Sn2+ and O increases at sites near the solidified thin film. On the surface thin film, increasing the Cu content can decrease the diffusion distance of Sn2+ and repress the growth of SnOx phase. The thickness of the oxidative thin film of Sn–3.5Ag–2.0Cu alloy was 1060 ± 85 nm and part of the oxidative thin film was tetragonal SnO2.