We fabricated Cu core Sn–Ag solder balls by plating pure Sn and Ag on Cu balls and clarified that Sn/Ag plating began to melt at a rather low temperature, the eutectic temperature of Sn–Ag–Cu. This early melting at the eutectic temperature was ascribed to the diffusion of Cu and Ag into the Sn plating during the heating process. We investigated the solderability of the BGA joint with the Ni/Au coated Cu pad to compare it with that of the commercial Sn–Ag and Sn–Ag–Cu balls. After reflow soldering, we observed a eutectic microstructure composed of β-Sn, Ag₃Sn, and Cu₆Sn₅ phases in the solder, and a η^′-(Au, Cu, Ni)₆Sn₅ reaction layer was formed at the interface between the solder and the Cu pad. The BGA joint using Cu core solder balls could prevent the degradation of joint strength during aging at 423 K because of the slower growth rate of the η^′-(Au, Cu, Ni)₆Sn₅ reaction layer formed at the solder-pad interface. Furthermore, we were able to fabricate Cu-cored, multicomponent Sn–Ag–Bi balls by sequentially coating binary Sn–Ag and Sn–Bi solders onto Cu balls. The coated balls also exhibited almost the same melting and soldering behaviors as those of the previously alloyed Sn–2Ag–0.75Cu–3Bi solders.