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.
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