A study of electrodeposition and electroless ternary alloys

Abstract

Electroplating and electroless plating processes are widely used in the surface-finishing industry. Nickel plating had been used in many surface treatment processes in the past 30 years. One of the most important applications is acting as a diffusion barrier in printed circuit board and gold plated ornaments to prevent the interdiffusion of the top layer of plated gold and the base metal. However, human perspiration can penetrate through the cracks of the gold overplate of ornaments and corrodes the nickel underneath. The leached nickel product can irritate the human skin. On 30 June 1994, the Council of European Union has adopted the directive 76/769/EEC that listed nickel compounds as one of the regulated chemicals. Ornament manufacturers seek for replacements for nickel, the replacement material is palladium; but the rapid rise in the cost of palladium has deterred the use of this metal as a barrier. Cobalt and its alloys can form a good barrier and are less toxic in terms of skin toxicity but the corrosion resistance is worse than nickel. The main objective of this project is to develop ternary electroplated cobalt alloys and electroless plated cobalt alloys as a barrier having good corrosion resistant properties. We also determined the chemical and physical properties of the coating. The formulation for the plating solution and the plating conditions were optimized. The Cobalt-Tungsten-Phosphorus (Co-W-P) and Cobalt-Tungsten (Co-W) alloys were successfully developed for use in electroless-plating and electroplating processes respectively. The formulation for producing electroless plated Co-W-P alloy with 8.0 wt. % W and 4.0 wt. % P were optimized. An Co-W electroplating bath was also developed and W content can be controlled in the range of 20-35wt. % W while P content is less than 1 wt. %. The interdiffusion coefficients of copper / barrier were determined. Micro profiling and Boltzmann-Matano methods were used to obtain the interdiffusion coefficients. It was found that the interdiffusion coefficients of cobalt and its alloys are lower than nickel alloys. The soldering property of the plated cobalt alloys is similar to nickel coating. These indicated the potential of cobalt alloys for replacing nickel and it alloys.