Effect of Multiple Reflow Cycles and Isothermal Aging on Shear Strength in SAC305/Sn58Bi/Cu-OSP Hybrid Solder Joint

This study investigates the effects of multiple reflow cycles and isothermal aging on the shear strength and microstructure of SAC305/Sn58Bi/Cu-OSP hybrid solder joints. The joints combine SAC305 solder spheres and Sn58Bi solder paste reflowed on Cu-OSP substrates. Microstructural analysis revealed two distinct regions: a Bi-rich phase at the bottom and an Ag-rich phase at the top. Reflow cycles and isothermal aging at 85 °C, 125 °C, and 150 °C for up to 120 hours were evaluated for their impact on mechanical performance and reliability. Shear tests at 100 mm/s and 2000 mm/s showed that hybrid joints outperformed conventional SAC305/Cu-OSP joints, even after multiple reflow cycles. Aging at 85 °C and 125 °C had minimal impact on shear strength, indicating good stability under moderate conditions. However, aging at 150 °C, above the melting point of Sn58Bi (138 °C), caused a significant decrease in strength. These findings highlight the potential of hybrid solder joints for improved mechanical performance and thermal stability, offering advantages over conventional solder joints for advanced electronic packaging.