Interface microstructure and properties of nickel/copper foil reflow soldering and laser soldering

Under the background of replacing aluminum copper direct welding with nickel foil "bridge", heterogeneous connection of nickel copper foil is carried out using reflow soldering and semiconductor laser soldering. A comparative analysis was conducted on the formation mechanism of joint morphology, weld seam and interface microstructure, and their mechanical properties were evaluated. The research results showed that the weld defects were not detected for two welded joints. The microstructure of laser soldering welds was composed of Sn solid solution and Cu-Sn intermetallic compounds (IMCs) with equiaxed crystal morphology, while microstructure of reflow soldering welds was composed of Sn solid solution. The Cu-Sn and Ni-Sn interfaces of the two joints showed different morphologies, and the thickness of the Cu-Sn interface layer was greater than that of Ni-Sn interface. The microstructure at the Cu-Sn interface during reflow soldering was scallop-shaped, consisting of Cu solid solution →Cu₃Sn IMCs→ Cu₆Sn₅ IMCs. For laser soldering joints, the Cu-Sn interface consisted of various Cu-Sn IMCs. At the Ni-Sn interface, a continuous distribution of (Cu, Ni)₆Sn₅ IMCs was presented in the reflow soldering welds, while the short rod-shaped (Cu, Ni)₃Sn₄ and strip shaped (Cu, Ni)₆Sn₅ was showed in the laser soldering welds. The maximum shear force values of the two joints can reach over 320 N, far higher than actual production requirements. Based on this research results, it further demonstrated that laser soldering is a feasible technology to replace reflow soldering for the nickel/copper foil connection.