The characteristics of interfacial intermetallic compounds (IMCs) can have serious impact on the reliability of solder joints. In this study, Ni nanoparticles (NPs) were added to SAC305 solder/Cu substrate interface by flux mixing. The effects of Ni NP addition on electromigration (EM) under high current density were investigated. EM tests with a maximum duration of 1128 h were conducted under a current density of 1 x 10(4) A/cm(2), at 80 +/- A 3 A degrees C on the solders prepared using 0 and 2 wt% Ni NPs-doped flux. At 2 wt% Ni NPs addition to flux, the growth rate of interfacial IMC at the anode side decreased by more than five times. No significant change in electrical resistance was observed in 2 wt% samples for up to 1128 h. Ni NP addition at the solder/substrate through flux mixing is, therefore, expected to lead to more reliable solder joints.