Effect of Sn grain orientation on growth of Cu-Sn intermetallic compounds during thermomigration in Cu-Sn2.3Ag-Ni microbumps

•Anisotropic diffusion of Cu in Sn grains affects formation of Cu6Sn5 compounds.•Rapid growth of Cu6Sn5 at the cold-end occurs in low α angle Sn grains during TM.•Slow growth rates of Cu6Sn5 at the cold-end in high α angle Sn grains during TM.•Formation of Cu6Sn5 near grain boundaries in solder when Cu fluxes is blocked. The effect of Sn orientation on thermomigration was investigated in Cu/Sn2.3Ag/Ni microbumps with diameter of 20 μm. Results showed that anisotropic diffusion of Cu in Sn grains affected both formation rates and locations of Cu6Sn5 intermetallic compounds (IMCs). Rapid growth of Cu6Sn5 IMCs at the cold-end interface was observed when the c-axis of Sn grains was parallel to the temperature gradient. In addition, diffusion of Cu atoms was blocked by a Sn grain with a high angle between the Sn c-axis and the thermal gradient, resulting in formation of Cu6Sn5 IMCs near the grain boundary inside the solder.