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

The formation of interfacial intermetallic compound (IMC) and the mechanical properties of 95Pb-5Sn, 90Pb-10Sn and 85Pb-13Sn-2Cu (in wt.%) flip-chip solder joint with the Ti/Cu/Cu under bump metallization (UBM) after single and three times reflows were investigated. After reflow soldering, Cu
₃
Sn IMC formed on the Cu under bump metallurgy (UBM) for all of the solder alloys. The thickness of the Cu
₃
Sn IMC layers increased with increasing reflow times. The average IMC grain size of 85Pb-13Sn-2Cu/Cu was larger than those of 90Pb-10Sn/Cu and 95Pb-5Sn/Cu after same times reflows. Ball shear tests were carried out to evaluate the UBM solderability and the bonding quality of solder bumps after single reflow and three times reflows. The shear strength of 85Pb-13Sn-2Cu/Cu is higher than that of 90Pb-10Sn/Cu and 95Pb-5Sn/Cu after both one and three times reflows. The shear curves show that the solder joints occured significant plastic deformation and behaves a “ductile” fracture for all of the three solders.