Tuesday, July 8, 2008

Effects of Impurities (Acid Copper)

Acid copper baths are more tolerant of ionic impurities than many other plating solutions. Many metallic ions introduced regularly by carryover with the work, by dissolution of impurities in the anode, or by dissolution of the basis metal (for example, iron, nickel, or zinc) can be expected to accumulate in the bath, because conditions are usually not satisfied for effecting co deposition of such metallic impurities (nickel, cobalt zinc, iron) with copper. For example, less than 2 ppm nickel was codeposited with copper in a sulfate bath containing 15 g/l nickel.

Nickel and iron reduce the conductivity of the sulfate bath to the same degree as an equivalent increase in copper. The deposition potentials of arsenic and antimony are apparently near that of copper in the sulfate bath, since codeposition was reported to occur. Arsenic and antimony in concentration of 10 to 80 g/l and 0.02 to 0.1 g/l, respectively, embrittled deposits and roughness surfaces. Additions of gelatin or tannin, however, effectively inhibited codeposition of these impurities and prevented roughness and embrittlement caused by them. Antimony readily codeposits with copper, but only small amounts of arsenic could be detected. Bismuth, like arsenic and antimony, caused granular deposits. Antimony and bismuth are believed to form insoluble complex compound with arsenate.

Small concentration of alkali metal and alkaline earth salts were found to smooth copper deposits. Tin salts were also reported to smooth deposits and were, at one time, purposely added to the sulfate, and silver partly. If silver is a contaminant, a small amount will be codeposited with copper. A high current density favors the codeposition of silver. Silver chloride caused pitting in semibright copper plate.

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