Saturday, May 3, 2008

Copper Pyrophosphate Plating

The copper pyrophaspate plating bath offers a number of desirable features. It has good throwing power, is noncorrosive, and nontoxic. Because of these advantages, copper pyrophosphate solutions are widely utilized. The most common used is in the fabrication of through hole printed wiring boards by the electronics industry. Because of its importance and the special problems associated with it, this process is described in detail later in this material.

The production of electroforms for objects such as waveguides, point-spray masks, helical antennae, heat exchangers, and molds for making toys is another common use of this solution. Copper pyrophaspate solutions are also used to plate steel and aluminum parts, plastics having a conductive surface, and zinc die castings, before bright nickel and chromium plating. Miscellaneous applications include use as a lubricant for wire deep-drawing operations, as a stopoff on the steel for selective hardening operations such as nitriding and carburizing, in roll plating, and in minimizing hydrogen embrittlement.

The copper pyrophaspate plating bath contains copper pyrophaspate (Cu2P2O7.3H2O), either potassium pyrophaspate (K4P2O7) or the corresponding sodium salt (Na4P2O7), nitrate and ammonia (NH3).
The phyrophiosphate salts react in aqueous solution to form alkaline complex anions, the major complex Cu(P2O7)26- and also (CuP2O7)2-. The solution is of intermediate stability. Between pH value of 7 and 11, the stability of the complex anion is evideneed by the slow but sure hydrolysis of (P2O7)4- to (PO4)3-. At pH value above 11, Cu(OH)2 precipitates, at pH value noticeably below 7, either CuH2P2O7 or Cu2P2O7 precipitates. Acidification below pH 7 convert the (P2O7)4- to either (H2P2O7)2- or (HPO4)2- and thus destroys the complex anion.

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