Study of interaction between Cu-Sn and Ni-Sn interfacial reactions by Ni-Sn3.5Ag-Cu sandwich structure

The interaction between Cu-Sn and Ni-Sn interfacial reactions in a soldering system has been studied using a Ni-Sn3.5Ag-Cu sandwich structure. A layer of Cu-Sn intermetallic compound was observed at the interface of the Ni foil after 30 sec of reflowing. Two stages of the Cu-Sn compound growth on the Ni side were observed: (1) in the first minute of reflow, the fast Cu-Sn compound formation was rate-limited by Cu diffusivity in the Cu-Sn compound layer of the opposite Cu side; and (2) after 1 min of reflow, the Cu-Sn compound growth was very sluggish and depended on the Ni diffusion in the Cu-Sn compound of the Ni side. Very little Ni can be detected in the Cu side. This implies that Cu diffused and dissolved in the molten Sn3.5Ag solder much faster than Ni. When the dissolved Cu arrived at the interface of the Ni foil, a Cu-Sn compound layer formed on the Ni interface to prevent the Ni foil from reacting with the solder. The driving force of the dissolved Cu atoms toward the Ni side attributed to the Cu solubility difference across the molten solder, which was established due to the reduction of the Cu solubility near the Ni interface. The reduction of Cu solubility was caused by the presence of dissolved Ni near the Ni interface. Knowing the experimental value of the Cu flux toward the Ni side and assuming the diffusion of Cu atoms in the molten solder following Fick's first law, the diffusivity of Cu is found to be 10^-5 cm²/s.