SnCuNi is one of the most common ternary intermetallic compounds formed in the Sn-based solder joint, and its formation and properties can be greatly influenced by the amount of Ni. Ni can participate in the interfacial reaction and diffuse into the intermetallic compound layer from either the solder or from the pad. In this research, comparative studies of different SnCuNi intermetallic compounds were conducted using two kinds of SnCuNi solders with organic solderability preservatives pad finish and a SnCu solder with electroless nickel/immersion gold pad finish. In the former case, Ni can only diffuse into the intermetallic compound from the solder matrix, while in the latter the Ni is only from the metallization layer on the Cu base. Scanning electron microscopy and transmission electron microscopy were employed to inspect the morphologies and interfacial microstructures of the intermetallic compounds. The thermal aging test was conducted to investigate their growth behavior under elevated temperature conditions. Mechanical strength after different aging hours was also evaluated via high speed ball pull test.