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.