Al₂O₃ nanofibers reinforced Sn58Bi/SAC 305 hybrid joints for low temperature ball grid array bonding

In the artificial intelligence (AI) era, adopting low-temperature soldering technology serves the dual purpose of meeting carbon-neutral demands and mitigating warpage issues in Package-on-Package assemblies. In this study, Al₂O₃ nanofibers (NF) was dispersed in Sn58Bi solder paste and employed for Sn58Bi/SAC 305 hybrid bonding. The investigation explored the relationship between the Al₂O₃ NF content and aspects including microstructure, shear strength, and reliability of hybrid joints in as-reflow and after 100°C thermal aging for 144, 256 and 500 h durations. Adding 0.1 wt% Al₂O₃ NF maximized the spreading ratio of Sn58Bi solder paste to 79.9%. During the reflow process of hybrid joints, Al₂O₃ NF hindered the diffusion at the Sn58Bi/SAC 305 junction, leading to a 10% decrease in % Bi diffusion. Al₂O₃ NF refined the Sn58Bi microstructure and suppressed the (Cu, Ni)₆Sn₅ IMC growth in as-reflow and after thermal aging. 0.05 wt% Al₂O₃ NF lowered the diffusion coefficient from 1.25 × 10⁻¹² m²/s to 6.21 × 10⁻¹³ m²/s after 500 h of thermal aging. In the as-reflow condition, the average value of shear strength increased linearly with NF content reaching a maximum of 38.5 MPa for the 0.15 wt%. Meanwhile, higher NF contents increased the agglomeration tendency in some joints leading to an early fracture. Hybrid joints with 0.15 wt% NF maintained the highest shear strength of 33.6 MPa after 500 h of thermal aging, equivalent the shear strength of monolithic joints in as-reflow state.