The effects of Au and Ni plating on pulsed laser welding of thin sheets in the lap-joint configuration

In electronic products and devices, base metals such as Al, brass, steels, Cu, Kovar and Ni are often plated with materials such as Ni, or Ni and then Au to enhance the corrosion and oxidation resistance or to improve the decorative appeal of the substrates. In this study, laser seam welds were performed on 200 μm thick sheets Al, Ni, Kovar and cold-rolled, plain carbon steel (CRS) in the lap-joint configuration using a Lumonics JK702H Nd:YAG pulsed laser welder. Uncoated, Ni and Au/Ni plated sheet was used. Tensile shear tests were performed and weld dimensions and weld microstructures were examined. Except for the Al specimens, the coatings did not affect the minimum beam intensity necessary for joining at the sheet interface. Once the weld pool penetrated across the interface into the second sheet, there was a very rapid increase in joint strength to a maximum strength value that was always less than the base metal strength. In almost all cases, failure occurred by shear in the heat affected zone (HAZ) at the fusion boundary. The Au/Ni plated Ni, Kovar and CRS specimens exhibited a Au/Ni braze at the sheet interface adjacent to the fusion boundary due to melting of the Au/Ni plating layers. In most cases, the shear strength of the joints was not affected by the plating; however, the presence of the Au/Ni braze caused a shift of the failure location away from the fusion boundary and into the HAZ or base metal.