Characteristics of intermetallic compounds (IMCs) can have serious impacts on reliability of solder joint. In the current study, Ni nanoparticles (NPs) have been added to SAC305 solder / Cu substrate interface by flux mixing and their effects on electromigration (EM) under high current density have been investigated. Electromigration (EM) tests were operated under a current density of 1×104 A/cm2, at 80°C with different time variation for samples prepared using 0 wt% and 2 wt% Ni NPs doped flux. It is found that 2 wt% Ni NPs doped flux influenced positively the reliability of SAC305 solder joint under high current density. Ni NPs addition depressed the polarity effect and reduced the voids, cracks and damages in solder joint.