Effect of Cu on interfacial reaction in high-lead solder bumps

Abstract

The formation of interfacial intermetallic compound (IMC) and the mechanical properties of 95Pb-5Sn, 90Pb-IOSn and 85Pb-13Sn-2Cu (in wt.%) flip-chip solder joint with the Ti/Cu/Cu under bump metallization (UBM) after single and three times retlows were investigated. After retlow soldering, CU3Sn IMC formed on the Cu under bump metallurgy (UBM) for all of the solder alloys. The thickness of the CU3Sn IMC layers increased with increasing retlow times. The average IMC grain size of 85Pb-13Sn-2Cu/Cu was larger than those of 90PbIOSn/Cu and 95Pb-5Sn/Cu after same times retlows. Ball shear tests were carried out to evaluate the UBM solderability and the bonding quality of solder bumps after single retlow and three times retlows. The shear strength of 85Pb-13Sn-2Cu/Cu is higher than that of 90Pb-lOSn/Cu and 95Pb-5Sn/Cu after both one and three times retlows. The shear curves show that the solder joints occured significant plastic deformation and behaves a "ductile" fracture for all of the three solders.