Ni-based under-bump metallization (UBM) for flip-chip application is widely used in today's electronics packaging. In this study, electroplated Ni UBM with different thickness was used to evaluate the interfacial reaction during multiple reflow between Ni/Cu UBM and eutectic Sn-Pb solders in the 63Sn-37Pb/Ni/Cu/Ti/Si3N4/Si multilayer structure. During the first cycle of reflow, Cu atoms diffused through electroplated Ni and formed the intermetallic compound (IMC) (Ni1-x,Cux)3Sn4. After more than three times of reflow, Cu atoms further diffused through the boundaries of (Ni1-x,Cux)3Sn4 IMC and reacted with Ni and Sn to form another IMC of (Cu1-y,Niy)6Sn5. After detailed quantitative analysis by electron probe microanalysis, the values of y were evaluated to remain around 0.4; however, the values of x varied from 0.02 to 0.35. The elemental distribution of IMC in the interface of the joint assembly could be correlated to the Ni-Cu-Sn ternary equilibrium. In addition, the mechanism of (Cu1-y,Niy)6Sn5 formation was also probed.