摘要
To achieve Moore’s Law in semiconductor industry, more Si chips are stacked within a single package. However, the thermal warpage caused by a mismatch of coefficient of thermal expansion between Si chips and polymer substrates is critical. To mitigate this issue, lower soldering temperature has attracted more attention. This study investigates the behavior of Cu/SAC305-Sn-58Bi composite solder joints reflowed at 165 ℃, focusing on the migration of the SAC305/Sn-58Bi boundary during thermal aging. After 4 weeks of aging at 90 ℃, the SAC305 region thickness decreased from 73.55 μm (as-reflowed) to 51.8 μm, with the migration driven by the solid-state diffusion of Bi. This led to phase redistribution, formation of smaller Bi grains, and increased Ag3Sn and Cu6Sn5 intermetallic compounds (IMCs), reducing Bi grain aggregation and enhancing thermal aging resistance. Shear-strength testing revealed a 7.3 MPa decrease in the composite solder (CS) joint, indicating good mechanical stability. Finite element analysis (FEA) simulations showed lower stress concentrations near the SAC305/Sn-58Bi interface and higher stresses at the Sn-58Bi/Cu interface, consistent with fracture surface analysis, which identified failure on the Sn-58Bi side. These results confirm that the SAC305/Sn-58Bi composite solder system outperforms pure Sn-58Bi in terms of thermal stability and aging resistance, offering significant potential for advanced electronic packaging applications. © The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2025.