Fluxless soldering is a desired process for high density solder joint and optical electronic interconnection. There are two species of oxides, one from the copper substrate, and the other from the tin-based solder. To reduce the metallic oxides, hydrogen is always considered. Utilizing hydrogen or forming gas can be a good candidate for the fluxless soldering. So the effectiveness of the reduction processes on the wetting of solder on copper has to be evaluated. The roles of these oxides playing during the process have to be understood that is interesting to scientists and engineers. For these purpose, The wetting behavior of Sn63Pb37 solder on bulk copper in the absence of flux was investigated in a forming gas (5% H2 and 95% Ar) at a series of heating rate. The surface chemistry of the solder and the copper plate was analyzed by X-ray photoelectron spectroscopy (XPS) and the thickness of Cu 2O was estimated. It was found that the wetting temperature increases with the increase of the thickness of Cu2O and heating rate. The wetting temperature versus oxidation time for Cu surface approximately follows a parabolic relation. The measured wetting temperature are found to be close the initial reduction temperature of Cu2O, which implies that once Cu2O is reduced, Sn63Pb37 solder starts to wet copper. An activation energy for wetting was calculated to be 96.4±9 kJ/mol.