Metal Finishing Guide Book

2012-2013

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plating processes, procedures & solutions COPPER PLATING BY ROMUALDAS ���RON��� BARAUSKAS ROHM & HAAS ELECTRONIC MATERIALS L.L.C., FREEPORT, N.Y.; www.rohmhaas.com COPPER CYANIDE BATHS Copper cyanide plating, with its accompanying health hazard and waste disposal problems (also shared with other cyanide plating baths), is still essential in many plating operations as a strike and, to a decreasing extent, for thick deposits. In dealing with the chemistry of the copper cyanide bath, a distinction must be made between total cyanide and free cyanide. Cuprous cyanide must be complexed with either potassium or sodium cyanide to form soluble copper compounds in aqueous solutions. The major complexed form is considered to be either potassium copper cyanide, K2Cu(CN)3, or sodium copper cyanide, Na2Cu(Cn)3. The sum of that required for the complexation of copper cyanide plus the amount of cyanide required for the proper functioning of the bath (free cyanide) is the total cyanide. The total cyanide required by weight is given in the following equation: Total potassium cyanide = (Copper cyanide required �� 1.45) + free potassium cyanide required Total sodium cyanide = (Copper cyanide required �� 1.1) + free sodium cyanide required As an example: A plating bath needs 2.0 g/L of copper cyanide and 0.5 g/L of free potassium cyanide. How much potassium cyanide is required for the bath? Total potassium cyanide = (2.0 �� 1.45) + 0.5 = 3.4 g/L The plating bath formulations provided here are suitable for the majority of uses in cyanide copper plating and are easy to control. These formulations can be modified by the plater after reading the operating parameters and notes on maintenance and control. It is recommended, whenever possible, that the potassium formulations be used for extended plating range and a greater tolerance to deviation from recommended operating parameters. Anodes for all baths should be high purity copper that is oxide free. They can be bagged copper slabs or bagged steel baskets containing copper nuggets. Plain steel anodes may be mixed with the copper to control copper cyanide content if copper content in the plating bath tends to increase with use. Anode/cathode ratio should be 1:1 to 2:1. General Purpose Strike The general purpose strike (see Table I) is used for improved adhesion, activation of passive substrates, or as an insurance step in the cleaning cycle. When used for zinc die castings, the hydroxide concentration should be kept at a maximum of 3.75 g/L (0.5 oz/gal). Deposits are usually in the range of 0.5-2.0 ��m (0.02-0.08 mil) in thickness. 313

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