High-temperature aging time-induced composition and thickness evolution in the native oxides film on Sn solder substrate

The presence of a native oxides film on Pb-free Sn solder joint surface is inevitable and its structure could be influenced by high-temperature aging due to heat release of electronic device itself during application. In this work, the effect of high-temperature aging time at 90 °C on composition and thickness evolution of a native oxides film, formed on pure Sn solder substrate after 24 h exposure to 90% RH atmosphere at 25 °C, has been characterized by AR-XPS and EIS. Results indicate that the outer layer of the as-obtained native oxide film consists of more Sn(OH)4 and less SnO2, and the inner layer consists of more SnO and less Sn(OH)2. The aging process initially accelerates both the dehydration/oxidation of hydroxides/stannous oxides in the film and the fresh Sn substrate to form SnO2, contributing to an increasing thickness and improved corrosion resistance of the film. However, an extended aging time deteriorates the structure of the film with more cracks and lowered corrosion resistance. Moreover, comparison finds that, without any assumption on models of resistivity distribution through the oxides film, the film thickness determination by Cole–Cole representation of the EIS results fits well with that by AR-XPS analyses.