Optimisation of Thiourea Concentration in a Decorative Copper Plating Acid Bath Based on Methanesulfonic Electrolyte

Abstract

The role of thiourea as an organic additive in the nucleation and growth mechanism was studied for copper deposition and its application in the decorative electroplating and fashion accessory industries. The bath was designed to reduce the environmental and ecological impacts using methanesulfonic acid as electrolyte as an alternative to alkaline cyanide baths. We evaluated the nucleation and growth mechanism of copper exploiting voltametric and chronoamperometric measurements with a brightener concentration ranging from 0 to 90 ppm. We used the Scharifker–Hills model to estimate the type of nucleation mechanism after progressive addition of thiourea. Scanning electron microscope was employed for surface analysis and morphological characterisation of the nuclei. We verified that progressive nucleation is a key step in the obtainment of a shiny and homogeneous copper film, but an excess of thiourea could cause parasitic adsorption reactions on the surface of the substrate. X-ray fluorescence spectroscopy was used for the thickness determination of the copper deposits and the electrodeposition efficiency correlated to thiourea concentration. Finally, the optimal concentration of thiourea was assessed to be 60 ppm for the used formulation of copper plating.