Dissimilar materials of copper (Cu) to aluminum (Al) with nickel-phosphorus (Ni-P) coatings were joined using resistance spot welding. The Ni-P coatings were electroless plated on the Al surfaces to eliminate the formation of brittle Cu-Al intermetallic compounds (IMCs) at the faying interface between Cu and Al. Three welding schedules with various heat input were employed to produce different interfacial microstructure. The evolution of interfaces in terms of phase constitution, elemental distribution and defects (gaps and voids) was characterized and the formation mechanisms were elucidated. During the welding process, the bonding between Cu and Ni-P forms through solid-state diffusion, while the faster diffusion rate of Cu relative to Ni and P atoms promotes the generation of sub-micron voids. As the heat input increases, gaps at the Cu/Ni-P interface diminish accompanied by increase of sub-micron voids. A moderate schedule helps to remove the gaps and inhibits the void formation. An Al3Ni layer and nanovoids were found around the interface of Ni-P/Al. The increased heat input decreases the grain size of Al3Ni at the interface by eutectic remelting and increases the nanovoids by enhanced nanoscale Kirkendall effect.