Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 258 of 707

electroplating solutions TIN-NICKEL ALLOY PLATING BY S. K. JALOTA CSL INC., SANTA CLARA, CALIF.; An alloy of tin and nickel can be electroplated directly on copper, copper alloys, nickel, and undercoats of copper and nickel on ferrous and zinc diecast sub- strates. The tin-nickel alloy is a single-phase metastable compound of ~65% tin and ~35% nickel by weight, which corresponds to an equiatomic ratio of one part tin and one part nickel. The tin-nickel alloy is nonmagnetic and nonallergic to skin. It finds application in replacing chromium in decorative and light engineering industries and in printed circuit boards as an etch resist. The nonallergic property allows it to replace electroplated nickel on fasteners for garments. Electroplating is accomplished mainly from aqueous solutions containing stannous (tin) chlo- ride, nickel chloride, and ammonium bifluoride maintained at 2 to 5 pH and ~55 to 70° C (~130-160° F) with or without proprietary additives. Electroplating may also be accomplished from a pyrophosphate bath containing glycine at near neutral 7.5 to 8.5 pH. More work remains to be carried out before this bath can be commercialized. The tin-nickel alloy electroplating is covered under ASTM spec- ification B 605-95a. PROPERTIES AND APPLICATIONS Color and Tarnish Resistance Electroplated tin-nickel has an attractive faint pinkish color. It is resistant to tar- nish under both indoor and mild outdoor conditions. Replacement Coating for Chromium The hardness of tin-nickel alloy lies between those of nickel and chromium (Table I), and as such it offers an alternative to electroplated chromium for both decorative and protective applications such as in hardware, automotive, electrical, and electronics accessories. Tin-Nickel Alloy Replaces Nickel Plating on Garment Fasteners Nickel plating on fasteners is known to be allergic to skin. Tin-nickel alloy is non- allergic. Fasteners for garments have been successfully produced with tin-nickel plating inexpensively. Frictional and Contact Resistance The tin-nickel alloy has the ability to retain oil on its surface. This property per- mits its use in automotive breaking systems, heavy-duty switch gears, mixing valves, and numerous applications involving frictional parts. Application in Printed Circuit Boards Tin-nickel alloy finds the following applications in through-hole plating of the printed circuit boards: as an etch resist and as a partial substitute for gold. Table I. Hardness of Nickel, Tin-Nickel, and Chromium in Vickers Diamond Pyramid Numbers Plated layer Nickel Hardness, VHN 100-175 Tin-nickel 600-700 Chromium 900-1,000 257

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