Effect of pulse plating parameters on electrical contact behaviour of nickel coatings

Pulse plated nickel coatings on copper and copper alloys were investigated in view of their use as underlayer in Au/Ni/Cu or as external protective coatings. Deposition parameters were varied in order to determine their influence on the microstructure and consequently on the electrical contact properties of the electrodeposited Ni films. Roughness and hardness measurements as well as SEM studies were performed on these samples. Smooth and dense coatings were obtained for high pause-to-pulse ratio, 25-75 Hz pulse-plating frequencies and low current density. The contact resistances R_c of the as-deposited samples in approach and insert mode were measured versus load F_c in the range 0.1-10 N. It was found that initial contact resistance follows the power law R_c=KF_c ^-n where n is hardness and coating topography dependent. Moreover, aging samples in daily cyclic atmosphere shows an increase in contact resistance during the test, but ohmic behaviour prevails as verified by the current-voltage characteristics. Finally, the mechanical behaviour investigated by a fretting study shows that the pulse reverse condition improves contact resistance stability