Ni-based under-bump metallization (UBM) has attracted wide attention due to its low reaction rate with Sn, compared with Cu and Cu alloy. In this study, the interfacial reactions between eutectic Sn-3.5Ag solder and Ni-based UBM, including electroplated Ni (EP-Ni) and electroless Ni (EL-Ni) are investigated. Morphology and growth kinetics of Ni 3Sn 4 intermetallic compounds are studied at different reflow temperatures and durations. The growth rate and the growth activation energy of Ni 3Sn 4 were measured for the two sets of samples. The activation energies are measured to be 25 kJ/mol and 38 kJ/mol for the Ni 3Sn 4 growth on EP-Ni and EL-Ni, respectively. The Ni 3Sn 4 on EP-Ni UBMs shows a slower growth rate and the Ni 3Sn 4/solder interface is void free even after 20-min reflow at 240 °C. On the other hand, the interface of Ni 3Sn 4/EL-Ni has a lot of microvoids after reflowing at 240 °C for 20 min.