The oxidation of several kinds of solder bumps was investigated using sequential electrochemical reduction analysis (SERA) and Transmission Electron Microscopy analysis. Also, the bondability of these solder bumps was evaluated using a newly developed method, which we call the “Bump Fusion Test.” For SAC solder, a very thin amorphous SnO layer was formed on the top surface of the bumps, and a mixture of crystalline SnO and SnO₂ was found after long-term aging. High Ag content of the solder bumps suppresses their surface oxidation. The result of the “Bump Fusion Test” was consistent with that of the SERA analysis and we are able to evaluate the influence of the thickness of the surface oxide layer on the bondability of solder bumps.