Capillary-driven solder climbing on Au-finished SMT components with vertical micro-grooved surface during reflow

As the demand for high-performance and compact surface mount technology (SMT) products continues to grow, soldering remains a critical interconnection method, with Au surface finishes widely employed to improve product quality. To enhance the reliability of advanced semiconductor packaging, it is essential to understand the underlying causes of failure during manufacturing processes. In this study, a solder climbing phenomenon was observed in Au-finished SMT products during reflow soldering. The climbing height was approximately 50 μm for Ni-finished samples, whereas it increased to around 400 μm for Au-finished samples. For Au-finished samples, the solder climbed to about 50% of the component height, resulting in a weak joint due to solder depletion. The weakened joint properties were evaluated through mechanical strength testing and intermetallic compound analysis. Factors contributing to solder climbing were systematically investigated, including surface finish, reflow conditions, pad geometry, and anisotropic surface texture. The solder spreading behavior under each condition was experimentally examined through in-situ monitoring using an SMT scope. As the primary failure mechanism, a vertical micro-groove texture was identified on the Au-finished samples exhibiting pronounced solder climbing. This anisotropic texture, in combination with the Au finish, significantly enhances solder wettability. The surface grooves act as capillary pathways, promoting capillary-driven solder climbing along the groove direction. These findings provide mechanistic insight into solder climbing in Au-finished SMT assemblies and contribute to improved joint reliability in advanced packaging processes.