Mirror-Bright Silver Plating from a Cyanide-Free Bath

ilver has good electric conductivity and solderability and possesses S excellent electric characteristics as a contact and is thus widely used for plating electronic industrial parts. A cyanide plating bath is most often used, but the cyanide compounds have strong toxicity and a large amount of the cost is rxpkd for securing safe working conditions and waste treatment. Because of this, many cyanide-free baths have been proposed and reported, such as the silver chloride-sodium thiosulfate system,' the silver nitrate-tartaric acid system? and the silver iodide system added with gelatin: but none of them is in practical use. The authors found that a plated coat having performance comparable to that of the cyanide silver plating bath can be obtained from a silver methane sulfonate-potassium iodide bath to which N-(3-hydroxyl1 -butylidene)-p-sulfanilic acid (HBPSA) is added and has succeeded for industrialization; however, the plated coat obtained from this bath bas poor brightness. The authors have carried out research and development work on a cyanide-free bath capable of providing a bright silver plate.G It was determined that a bright coating can be obtained from silver succinimide.7,8 Jayaknshnan and Natarajan9 reported an alkaline silver plating using succinimide, but they gave no information for the stabilization of the bath. The authors found that the bath can be stabilized by adding boric acid and that mirror-bright silver plating can he obtained by adding polyethyleneimine (PEI) as a brightener. In the development work reported in this article the influences of adding PEI on the appearance of the plate coat, current efficiency, surface morphology, cathodic polarization characteristics, and the like were examined. The solderability, contact electric resistance, and hardness obtained were compared with those obtained from a conventional cyanide bath.