Sn–8Zn–3Bi solder paste and Sn–3.2Ag–0.5Cu solder balls were reflowed simultaneously at 240 °C on Cu/Ni/Au metallized ball grid array substrates. The joints without Sn–Zn–Bi addition (only Sn–Ag–Cu) were studied as a control system. Electrical resistance was measured after multiple reflows and aging. The electrical resistance of the joint (R1) consisted of three parts: the solder bulk (Rsolder bulk, upper solder highly beyond the mask), interfacial solder/intermetallic compound (Rsolder/IMC), and the substrate (Rsubstrate). R1 increased with reflows and aging time. Rsolder/IMC, rather than Rsolder bulk and Rsubstrate, seemed to increase with reflows and aging time. The increase of R1 was ascribed to the Rsolder/IMC rises. Rsubstrate was the major contribution to R1. However Rsolder/IMC dominated the increase of R1 with reflows and aging. R1 of Sn–Zn–Bi/Sn–Ag–Cu samples were higher than that of Sn–Ag–Cu samples in various tests.