A new Ni-Zn under bump metallurgy for Pb-free solder bump flip chip application

Abstract

We developed an Au/Ni-Zn/Ti under bump metallurgy (UBM) for Pb-free solders. Au/Ni-Zn/Ti and Au/Ni/Ti UBM stacks were deposited on SiO2/Si wafer using conventional magnetron sputtering and Sn solder was electroplated on UBM stacks. Then, Sn solder on UBM were reflowed at 260°C for 15 s and aged at 150°C up to 1000 h. The measurement of film stress using a curvature method showed Ni-Zn films having the very low tensile stress could be obtained by controlling the Ar pressure. Ni3Sn4 intermetallic compound (IMC) was formed on both Ni-Zn UBM and Ni UBM after reflow and IMC thickness increased with aging time. Other IMCs besides Ni3Sn4 were not observed after aging. IMC growth of Ni-Zn UBM was slower than that of Ni. UBM consumption rate of Ni-Zn UBM was also lower than that of Ni UBM. These beneficial results were ascribed to the effect of Zn, which played a role of interdiffusion barrier between Sn and Ni. Our results revealed that Ni-Zn layer is a promising UBM for Pb free solders.