Tuesday, May 20, 2008

Maintenance and Control (Pyrophosphate Copper)

Equipment

The equipment, a tank, heater, and filter, is similar to that used for bright nickel plating. A steel tank coated with rubber, Koroseal, PVC, or a plastisol is suitable, although rubber linings should be leached before use with dilute solution of potassium hydroxide to remove all alkali soluble matter which might contaminate the plating solution. For heating the solution, either steam heating coils or electric immersion heaters made of stainless steel or Duriron may be used.

Continuous filtration will help prevent rough deposits. The filter should be constructed of stainless steel, rubber, or PVC-lined steel or cast iron. OFHC anodes are best; however, electrolytic sheet and rolled electrolytic copper anodes are also satisfactory. Cast anode of good purity can also be used but they should be free of impurities such as lead, nickel, silver, and tin because these tend to promote deposit roughness. Anode bags are not usually used; they are not needed, and can restrict circulation of the plating solution around the anode and cause polarization. Anodes can be left in the bath when it is not being used since they do not dissolve in the absence of applied current.

Purification
Cyanide, oil, lead, zinc, decomposition products of the organic additives, and excess orthophosphate are the impurities most often encountered in a copper pyrophosphate plating bath.

Cyanide, oil and lead cause streaked, dull deposits and also lower the effective current density range. Precautionary method will prevent these materials from entering the bath. Thorough rinsing when copper cyanide striking is used will prevent cyanide contamination; a clean air line of lead coils, lining, fittings, or filters will prevent lead contamination.

Cyanide (which can be detected by a copper benzidine test) can be removed by treatment with either 1.25 ml/l of 30% H2O2 or with 0.1 to 0.2 g/l potassium permanganate followed by treatment with activated carbon. The tendency toward streaked, nonuniform pyrophosphate deposits due to residual traces of cyanide left on the surface after copper cyanide striking can be minimize by brief cathodic treatment in a solution of potassium pyrophosphate before it is placed in the plating bath. Oil can be removed by treating the bath with 2.5 to 6 g of activated carbon per liter of bath at 55oC for 4 to 8 hrs. Low current density electrolysis will remove lead.

It has been recommended however, a high P2O7/Cu ratio (preferably 7.5:1) be maintained under this circumstance so as to allow some excess pyrophosphate for complexing with the contaminant. Excessive additive decomposition products can caused stressed, brittle deposits. Control of addition agents plus occasional bath purification will minimize buildup of these products. An activated carbon alone is not sufficient purification. To ensure removal of the decomposition products, it is best to treat the solution.