The reactive diffusion between Sn–Ag solders and Cu was experimentally examined during soldering and isothermal annealing. Three sorts of solders with compositions of Sn–3.5Ag, Sn–3.5Ag–0.1Ni and Sn–3.5Ag–0.1Co were used for the experiment. Each solder was soldered on a Cu plate at 523K (250°C) for 1–60s in a pure nitrogen gas, and then the solder/Cu diffusion couple was isothermally annealed at 423K (150°C) for 168–1008h. Due to soldering, only Cu 6 Sn 5 is formed at the interface in each diffusion couple. On the other hand, Cu 3 Sn is produced between Cu 6 Sn 5 and Cu owing to the isothermal annealing. The composition of Cu 6 Sn 5 is (Cu 0.8 Ni 0.2 ) 6 Sn 5 and (Cu 0.93 Ni 0.07 ) 6 Sn 5 on the solder and Cu 3 Sn sides, respectively, in the (Sn–3.5Ag–0.1Ni)/Cu diffusion couple, and it is (Cu 0.9 Co 0.1 ) 6 Sn 5 and (Cu 0.99 Co 0.01 ) 6 Sn 5 on the solder and Cu 3 Sn sides, respectively, in the (Sn–3.5Ag–0.1Co)/Cu diffusion couple. Different rate-controlling processes were suggested for the (Sn–3.5Ag)/Cu, (Sn–3.5Ag–0.1Ni)/Cu and (Sn–3.5Ag–0.1Co)/Cu diffusion couples. Finally, thermodynamic models were herein adopted to explore influences of the additives on the thermodynamic interaction of the component elements and the driving force for the growth of intermetallics.