Effect of Sn orientation on Cu diffusion for Pb-free solders under a temperature gradient

Packaging technology is transitioning toward the three-dimensional integrated circuit (3D IC), a larger temperature gradient is expected to be established when dissipating the heat from the surface. Furthermore, when the size of the solder bump shrinks into few micrometers, one solder bump consists of only few grains; thus how the grain orientation of Sn affects the reliability of the solder joint is an emerging issue in the analysis of reliability for packaging technology. This study investigates the effect of Sn grain orientation on Cu atomic diffusion and Cu-Sn interfacial reaction in Pb-fee solders under the presence of temperature gradient with a structure of Cu/Sn3.5Ag/Cu. The experimental results and theoretical derivation showed that the distinct microstructural evolutions after thermomigration were strongly affected by grain orientation of Sn. When the c-axis of Sn was parallel to the direction of temperature gradient, large thermomigration flux was induced and Cu atoms were driven to migrate from hot end to cold end. The migration resulted in a prominent asymmetrical feature in microstructure; serious dissolution of intermetallic compound (IMC) and severe consumption of Cu substrate occurred at hot end whereas accumulation and growth of IMC were observed at cold end. On the other hand, when c-axis of Sn was perpendicular to temperature gradient, thermomigration was strongly mitigated due to a small induced flux. A near symmetrical feature in microstructure was found. Neither dissolution of hot-end IMC nor abnormal growth of cold-end IMC was observed.