The difference in the types of intermetallic compound formed between the cathode and anode of an Sn-Ag-Cu solder joint under current stressing

An investigation of microstructural evolution with various current densities in a lead-free Cu/SnAgCu/Au/Cu solder system was conducted in this study. Current stressing induced migration of Cu toward the anode and resulted in the formation of Cu6Sn5 at the interface. The consumption rates of Cu were calculated to be 2.24 × 10−7 μm/s and 5.17 × 10−7 μm/s at 1.0 × 103 A/cm2 and 2.0 × 103 A/cm2, respectively, while the growth rates of Cu6Sn5 were 6.33 × 10−7 μm/s and 7.72 × 10−7 μm/s. The atomic fluxes of Cu were found to be 2.50 × 1012 atom/cm2 s and 5.88 × 1012 atom/cm2 s at the above-mentioned current densities. The diffusivities of Cu in Cu6Sn5 were 2.02 × 10−11 cm2/s and 2.38 × 10−11 cm2/s under 1.0 × 103 A/cm2 and 2.0 × 103 A/cm2 of current stressing. Current stressing effectively enhances the migration of Cu in Cu6Sn5 and results in a 1000-fold increase of magnitude in diffusivity compared to thermal aging. (Cu1−x,Aux)6Sn5 compound was formed near the anode after a long period of current stressing.