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.