Abstract
Under bump metallization(UBM) is an important issue in packaging industry, because it directly effects the reliability and the life time of solder joint. To overcome the failure of solder joints by Ni3P in the Ni-P UBM, the Co-bases UBM becomes the substitute of the Ni-P UBM, because of the well diffusion-barrier capability, better reliability and acceptable solderability. On the other hand, the Sn-3.5Ag solder is the one of the replacement for lead-solder. Because the Sn-3.5Ag solder has the better elastic modulus and the lower coefficient of thermal expansion. The addition of Ag in solder can decrease the growth rate of intermetallic compounds (IMCs). For these reasons, the Sn-3.5Ag solder becomes the prefer options in packaging industry. In this study, we systematically investigated the solid-state interfacial reaction between the Sn-3.5Ag and the Co substrate through different aging time at temperature of 150℃, 170℃ and 190℃, respectively. Based on the results from XRD and EDX, CoSn3, Ag3Sn and CoSn4 IMCs are formed in this system. The CoSn3 IMC layer grows thicker with longer reaction time and higher temperature. The kinetic analysis shows that the formation of CoSn3 is mainly control by the interfacial reaction in the beginning. When the thickness of CoSn3 over the critical region, the mechanism changed to diffusion control. The activation energy of CoSn3 is calculated to be 122.47kJ/mole. The diffusivity of Co and Sn in the IMC at 190℃ is 1.60*10-13 cm2/s and 2.11*10-12 cm2/s, respectively. The result of ball push test shows that the shear strength of solder joint decreased after solid-state aging. Most of the solder joints are the ductile fracture. Only 3 of the long-time solid-state aging solder joints showed both ductile fracture and brittle fracture. The coarsening of holes is the main reason to form the brittle fracture.