Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 203 of 707

Table V. Pyrophosphate Copper Strike Copper pyrophosphate (Cu2 P2 O7 Potassium pyrophosphate (K4 O7 Potassium nitrate P2 Temperature Current density Agitation Filtration P2 Operating parameters by analysis are: Copper metal Pyrophosphate /Cu ratio O7 ·3H2 ) Concentrated ammonium hydroxide pH O) 25.0-30.0 g/L (3.3-4.0 oz/gal) 95.7-176.0 g/L (12.8-23.4 oz/gal) 1.5-3.0 g/L (0.2-0.4 oz/gal) 0.5-1 ml/L 8.0-8.5 22-30° C (72-86° 0.6-1.5 A/dm2 F) (6.0-15.0 A/ft2 Mechanical and air Continuous 9.0-10.7 g/L (1.2-1.4 oz/gal) 63.0-107.0 g/L (8.4-14.2 oz/gal) 7:1 to 10:1 potassium copper pyrophosphate complex [K6 pyrophosphate (Cu2 P2 O7 ratio of pyrophosphate (P2O7 ) to copper (Cu2+ ·3H2 4— cyanide baths; however, the solutions are relatively nontoxic. The main uses of cop- per pyrophosphate baths have been for electroforming, plating on plastics, and printed circuits. The chemistry involved in copper pyrophosphate plating is the formation of O] from copper Cu(P2 O7 )2 ·6H2 O) and potassium pyrophosphate (K4 P2 O7 ). The ) in the compound is 5.48 to 1. Any pyrophosphate in excess of this ratio is called "free" or "excess" pyrophosphate. Free or excess pyrophosphate is essential for the operation of the bath in providing con- ductivity and anode corrosion. This is done by running a pyrophosphate to cop- per (P2 O7 /Cu) ratio of 7:1 to 8:1 in the plating bath. A strike bath may have a high- er ratio. Potassium pyrophosphate baths are recommended over sodium formulations for better conductivity and higher solubility of the potassium cop- per complex. Anodes for all baths should be high purity copper that is oxide free. Anodes can be copper slabs or copper nuggets in titanium baskets. Anode bags are not rec- ommended. Anode to cathode ratio should be 2:1. Copper pyrophosphate baths tend to build orthophosphate (HPO4 2— phate. Small amounts of orthophosphate are not harmful; however, higher con- centrations in excess of 100.0 g/L (13.3 oz/gal) may cause banded deposits with decreased plating range and conductivity in the standard plating baths. In the printed circuit bath, the orthophosphate concentration should not be allowed to exceed 40.0-60.0 g/L (5.5-8.0 oz/gal) because, beyond this point, there is a decrease in both the throwing power of the solution and ductility of the deposit. Orthophosphate concentration is lowered by dilution or replacement of the bath. The anode and cathode efficiencies of copper pyrophosphate baths are essentially 100%. Maximum agitation is required for the best results. When using air agitation, the volume of air required should be 1 to 1.5 times the surface area to be plated. Ultrasonic agitation may also be used. Strike Copper pyrophosphate plating baths can form immersion coatings, similar to acid 202 ) ) concentration by the hydrolysis of pyrophos-

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