Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 240 of 707

electroplating solutions SILVER PLATING BY ALAN BLAIR USFILTER ELECTRODE PRODUCTS, UNION, N.J.; It is not surprising that silver was one of the first metals to be deposited by elec- troplating during the early development of this manufacturing technique in the mid-19th century. Decorative application of a silver finish on hollowware and flat- ware fabricated from less-expensive metals was immediately a great commercial success. The formulation of a typical, decorative silver-plating solution in use today is remarkably similar to that patented by the Elkington brothers in 1840. Despite the environmental, health, and safety issues associated with cyanide salts, cyanide-based silver-plating solutions offer the most consistent deposit quality at the lowest cost. This is particularly true for decorative applications. Although commercially viable, noncyanide processes have recently been made available to electroplaters. Electroplated silver has many applications beyond decorative finishing. Its use on electronic components and assemblies has increased significantly during the past two decades. Recent application of silver to waveguides used in cellular telecommunications systems has added to its established use in packaging of inte- grated circuits. CYANIDE SYSTEMS A typical, traditional silver-plating solution suitable for rack work would be as fol- lows: Silver as KAg(CN)2 Potassium cyanide (free) Temperature Current density 15-40 g/L 12-120 g/L Potassium carbonate (min) 15 g/L 20-30° C 0.5-4.0 A/dm2 (2.0-5.5 oz/gal) (1.6-16 oz/gal) (2 oz/gal) (70-85° F) (5-40 A/ft2 ) Barrel plating usually results in much greater drag-out losses and lower current density during operation so lower metal concentrations are desirable. A typical for- mula would be: Silver as KAg(CN)2 Potassium cyanide (free) Potassium carbonate (min) Temperature Current density 5-20 g/L 25-75 g/L 15 g/L 15-25° C 0.1-0.7 A/dm2 (0.7-2.5 oz/gal) (3.3-10.0 oz/gal) (2 oz/gal) (60-80° F) (1.0-7.5 A/ft2 ) The formulas above will produce dull, chalk-white deposits that are very soft (<100 Knoop). Additions of grain refiners or brighteners will modify deposits, caus- ing them to become lustrous to fully bright. Examples of these additives are certain organic compounds, which usually contain sulfur in their molecule, and complexed forms of a group V or VI element such as selenium, bismuth, or antimony. Deposits become harder as brightness increases; for fully bright deposits the usual hardness range will be between 100 239

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