Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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plating procedures ELECTROLESS (AUTOCATALYTIC) PLATING BY JAMES R. HENRY WEAR-COTE INTERNATIONAL, ROCK ISLAND, ILL.; Electroless plating refers to the autocatalytic or chemical reduction of aqueous metal ions plated to a base substrate. The process differs from immersion plat- ing in that deposition of the metal is autocatalytic or continuous. THE ELECTROLESS BATH Components of the electroless bath include an aqueous solution of metal ions, reducing agent(s), complexing agent(s), and bath stabilizer(s) operating in a specific metal ion concentration, temperature, and pH range. Unlike conventional electroplating, no electrical current is required for depo- sition. The electroless bath provides a deposit that follows all contours of the sub- strate exactly, without building up at the edges and corners. A sharp edge receives the same thickness of deposit as does a blind hole. The base substrate being plated must be catalytic in nature. A properly prepared workpiece provides a catalyzed surface and, once introduced into the electroless solution, a uniform deposition begins. Minute amounts of the electroless met- al (i.e., nickel, copper, etc.) itself will catalyze the reaction, so the deposition is auto- catalytic after the original surfaces are coated. Electroless deposition then con- tinues, provided that the metal ion and reducing agent are replenished. If air or evolved gas, however, are trapped in a blind hole or downward facing cavity, this will prevent electroless deposition in these areas. In electroless plating, metal ions are reduced to metal by the action of chem- ical reducing agents, which are simply electron donors. The metal ions are elec- tron acceptors, which react with electron donors. The catalyst is the workpiece or metallic surface, which accelerates the electroless chemical reaction allowing oxidation of the reducing agent. During electroless nickel deposition, byproducts of the reduction, orthophosphite or borate and hydrogen ions, as well as dissolved metals from the substrate, accumulate in the solution. These can affect the per- formance of the plating bath. As nickel is reduced, orthophosphite ions (HPO3 2— ) accumulate in the solution and at some point interfere with the reac- tion. As the concentration of orthophosphite increases, there is usually a small decrease in the deposition rate and a small increase in the phosphorus content of the deposit. Ultimately, the accumulation of orthophosphite in the plating solu- tion results in the precipitation of nickel phosphite, causing rough deposits and/or spontaneous decomposition. The metal ion and reducer concentration must be monitored and controlled closely in order to maintain proper ratios, as well as the overall chemical balance of the plating bath. The electroless plating deposition rate is controlled by tem- perature, pH, and metal ion/reducer concentration. Each of the particular plat- ing reactions has optimum ranges at which the bath should be operated (Table I). A complexing agent(s), also known as a chelator, acts as a buffer to help con- 359

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