Abstract
Sn–Ag–Cu lead-free solders have recently been applied generally in electrical packaging, but a discoloring of the solders often occurs at the injection-ball or the reflow processes. This study investigates the solidification surface characteristics of both the Sn–3.5Ag alloy and the Sn–3.5Ag–2.0Cu alloy to discuss and clarify the differences in composition between the surface and the thin film of subsurface. The results indicate that the solidification surface films of the Sn–3.5Ag–(2.0Cu) are mostly composed of SnO phase and SnO2 phase. The concentration of Sn4+ is higher than that of Sn2+ on the film. And the binding energy of O atoms increases at sites near the solidification surface. On the solidification surface thin film, the Ag content of the Sn–3.5Ag–2.0Cu specimen is higher than the Sn–3.5Ag specimen. In addition, there is few Cu on the solidification surface film of the Sn–3.5Ag–2.0Cu specimen, and adding Cu into Sn–3.5Ag alloy not only can repress the growth of the SnOx phase but also reducing the degree of discoloring.
