The Role of High-Temperature-Formed Surface Oxide Film in Corrosion Protection of SAC305 Solder

The structural stability of high-temperature-formed oxide films (HTOFs) on SAC305 solder plays a critical role in determining corrosion reliability during long-term thermal exposure, yet the coupled effects of oxide evolution and substrate microstructure changes remain unclear. In this work, SAC305 solder was thermally aged at 150 °C for 10–60 days, and the evolution of the oxide film structure and substrate microstructure was systematically investigated using SEM, XRD, XPS, and electrochemical techniques. The results reveal that HTOF mainly consists of a SnO/SnO2-layered structure with thickness increasing slightly from approximately 16.5 nm to 18 nm, while increasing micro-cracks and Ag3Sn coarsening induced by the Kirkendall effect lead to significant reductions in impedance parameters and corrosion resistance. These findings demonstrate that the degradation of HTOF is governed by the coupled effects of oxide defect accumulation and intermetallic phase coarsening, providing a mechanistic insight into the corrosion failure of SAC305 solder under long-term thermal aging conditions.