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

摘要

Cu-(Sn37Pb) and Cu-(Sn3.5AgO.5Cu) solder joints were prepared at the same reflow temperature of 230 ℃. The microstructural observation of the solder assemblies in scanning and transmission electron microscopes confirmed the presence of η-Cu_6Sn_5 in case of the former, and Cu_3Sn+ η-Cu_6Sn_5 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 (0.28×10~6Ω~(-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 × 10~6Ω~(-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.