Evolution of mechanical and electrical properties of tin-lead and lead free solder to copper joint interface

Cu-(Sn37Pb) and Cu-(Sn3.5Ag0.5Cu) solder joints were prepared at the same reflow temperature of 230 °C. The microstructural observation of the solder assemblies in scanning and transmission electron microscopes confirmed the presence of η-Cu₆Sn₅ in case of the former, and Cu₃Sn + η-Cu₆Sn₅ for the latter in the reaction zone. The findings are correlated with the electrical and mechanical properties of the joints. Lead free solder-Cu joint exhibited lower reaction zone thickness and improved electrical conductivity (0.28 × 10⁶Ω^− 1 cm^− 1) and shear strength ∼ 68MPa compared to conventional lead-tin solder-Cu joint. The latter showed electrical conductivity and shear strength of 0.22 × 10⁶Ω^− 1 cm^− 1 and ∼ 55 MPa, respectively. The difference in reaction zone thickness is explained on the basis of melt superheat, with Sn being the primary diffusing species in the intermetallic layer.