Effects of Sn grain size on intermetallic compounds formation in 5 µm diameter Cu/Sn pillar bumps

Uniform 5 µm diameter Cu/Sn micro-bump array has been fabricated by multilayer electrodeposition, and the effect of Sn grain orientation and size on Cu 6 Sn 5 and Cu 3 Sn growth have been studied through FIB–SEM and electron backscatter diffraction. The solder layer in Ф5 µm Cu/matte Sn micro-bump we fabricated contains only four to five columnar Sn grains with no obvious preferred orientation and there is no significant difference in the growth of Cu 6 Sn 5 in different oriented Sn grains. However, the Sn grain size has much more effect on intermetallic compound (IMC) growth. At 150 °C, the diffusion coefficient of Cu/bright-Sn system(~ 200 nm Sn) is calculated to be 13.09 × 10 −17 m 2 s −1 , nearly three times that of Cu/matte–Sn system (2–5 µm Sn), about 4.51 × 10 −17 m 2 s −1 . Also, a simple model of boundary diffusion controlled IMC growth is proposed to explore the mechanism of grain size effect. Large Sn grain size will slow down the interfacial reaction in micro-bump due to low grain boundary density and large bulk diffusion distance.