Effects of Co and Ni addition on reactive diffusion between Sn–3.5Ag solder and Cu during soldering and annealing

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 523 K (250 °C) for 1–60 s in a pure nitrogen gas, and then the solder/Cu diffusion couple was isothermally annealed at 423 K (150 °C) for 168–1008 h. Due to soldering, only Cu₆Sn₅ is formed at the interface in each diffusion couple. On the other hand, Cu₃Sn is produced between Cu₆Sn₅ and Cu owing to the isothermal annealing. The composition of Cu₆Sn₅ is (Cu_0.8Ni_0.2)₆Sn₅ and (Cu_0.93Ni_0.07)₆Sn₅ on the solder and Cu₃Sn sides, respectively, in the (Sn–3.5Ag–0.1Ni)/Cu diffusion couple, and it is (Cu_0.9Co_0.1)₆Sn₅ and (Cu_0.99Co_0.01)₆Sn₅ on the solder and Cu₃Sn 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.