Effect of Copper Oxide Layer on Solder Wetting Temperature under a Reduced Atmosphere

The wetting behavior of SnPb solder on copper surface in absence of flux was investigated in a forming gas atmosphere (5% H₂ and 95% Ar). X-ray photoelectron spectroscopy (XPS) analysis indicated that Cu₂O oxide layer, 2.0-4.2 nm thick, was formed after exposing the milled and polished copper surface in air from 5 min to 5 hours. The wetting temperature did not change much after copper was exposed to air for more than 3 hours. Wetting took place when molten solder was in contact with reduced copper surfaces, the reaction between pure copper and tin followed afterward. The initial temperature of solder wetting on copper was close to the reduction temperature of Cu₂O and increased with the thickness of the surface oxide.