Solute-vacancy complexes govern Cu diffusion anisotropy in β-Sn: Insights from first-principles and experiments

This study combines first-principles density functional theory calculations with experimental electromigration testing to explore the atomic scale mechanisms behind the phenomenon that adding Bi, In, Sb, and/or Ga solutes to third-generation lead-free solders enhances the thermal shock/cycling resistance of solder joints but reduces their electromigration resistance. Density functional theory results reveal that solutes such as Bi, Sb, In, and/or Ga in β-Sn form stable solute-vacancy complexes, which significantly reduce the energy barriers for Cu interstitial diffusion, thereby accelerating electromigration degradation. Notably, Sb exhibits the lowest anisotropy in Cu diffusion energy barriers, suggesting its potential to mitigate orientation-dependent failure susceptibility. Experimental validation via high-current stressing (8.15 ×10³ A/cm²) demonstrates accelerated Cu₆Sn₅ intermetallic compounds growth in SAC305–1Bi and SAC305–1Sb joints, exhibiting 10–13 % higher intermetallic compounds area fractions than baseline SAC305 after 100 h. These findings highlight the role of solute-vacancy interactions in governing mass transport mechanisms in β-Sn and provide insights for solder alloy design strategies that balance thermomechanical fatigue resistance with electromigration reliability.