Recently, preventing environmental pollutions, lead-free(Pb-free) solders are about to replace tin-lead(Sn-Pb) eutectic solders. Sn-Ag-Cu alloys are leading candidates for lead free solders. However Sn-Ag-Cu alloys have some problems for manufacture. Among those, most critical problem is that Sn-Ag-Cu alloys are oxidized easily after aging at high temperature(150℃). To prevent oxidation problem of Sn-Ag-Cu alloys, Sn-Ag-Cu-Ge alloys were developed, but the mechanism of thermal oxidation has not studied yet. So we report the surface oxidation mechanism of lead-free solders in this paper. This mechanism has been investigated after high temperature storage using X-ray photoelectron spectroscopy. It was founded that, in Sn-Ag-Cu-Ge alloy system, the solder surface is surrounded by GeO₂, and then the surface of metallic solder is protected by this thin GeO₂. Based on this result, we have performed the package level reliability test using solder ball of Sn-Ag-Cu-Ge alloy, and compared with normal Sn-Ag-Cu alloy.

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