Effect of Sn grain structure on electromigration reliability of Pb-free solders

The anisotropic properties of the Sn crystal play an important role in the electromigration (EM) reliability of Sn-based Pb-free solder interconnects in flip chip packages. This study investigates the effect of the Sn grain structure on the EM lifetime and statistics of Pb-free solder bumps. The electron backscattering diffraction (EBSD) technique was applied in this study to characterize the Sn grain size and the grain orientation of the Pb-free solders. Results of failure analyses on Sn2.5Ag solders after EM tests showed that the Sn structure is important in controlling in the kinetics of the intermetallic compound (IMC) growth and void formation under EM. Further microstructural analysis revealed that the grain sizes and orientations of Sn2.5Ag solders with multiple solder reflows were statistically different from those with a single solder reflow and resulted in an improved EM lifetime and statistics. This demonstrates that the EM reliability of Pb-free solders can be improved by optimization of the Sn grain structure.