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

Uniform 5 mu m diameter Cu/Sn micro-bump array has been fabricated by multilayer electrodeposition, and the effect of Sn grain orientation and size on Cu6Sn5 and Cu3Sn growth have been studied through FIB-SEM and electron backscatter diffraction. The solder layer in ?5 mu m 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 Cu6Sn5 in different oriented Sn grains. However, the Sn grain size has much more effect on intermetallic compound (IMC) growth. At 150 degrees C, the diffusion coefficient of Cu/bright-Sn system(200nm Sn) is calculated to be 13.09x10(-17)m(2)s(-1), nearly three times that of Cu/matte-Sn system (2-5 mu m Sn), about 4.51x10(-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.