Metal Finishing Guide Book

2013

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Table V. Pyrophosphate Copper Strike Copper pyrophosphate (Cu2P2O7á3H2O) Potassium pyrophosphate (K4P2O7) Potassium nitrate Concentrated ammonium hydroxide pH Temperature Current density 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°F) 0.6-1.5 A/dm2 (6.0-15.0 A/ft2) Agitation Mechanical and air Filtration Continuous Operating parameters by analysis are: Copper metal 9.0-10.7 g/L (1.2-1.4 oz/gal) Pyrophosphate 63.0-107.0 g/L (8.4-14.2 oz/gal) P2O7/Cu ratio 7:1 to 10:1 The chemistry involved in copper pyrophosphate plating is the formation of potassium copper pyrophosphate complex [K6Cu(P2O7)2á6H2O] from copper pyrophosphate (Cu2P2O7á3H2O) and potassium pyrophosphate (K4P2O7). The ratio of pyrophosphate (P2O74—) to copper (Cu2+) 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 conductivity and anode corrosion. This is done by running a pyrophosphate to copper (P2O7/Cu) ratio of 7:1 to 8:1 in the plating bath. A strike bath may have a higher ratio. Potassium pyrophosphate baths are recommended over sodium formulations for better conductivity and higher solubility of the potassium copper 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 recommended. Anode to cathode ratio should be 2:1. Copper pyrophosphate baths tend to build orthophosphate (HPO42—) concentration by the hydrolysis of pyrophosphate. Small amounts of orthophosphate are not harmful; however, higher concentrations 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 copper, on steel and zinc die castings, and cause poor adhesion. A cyanide- or pyrophosphate-copper strike is used for steel, and a cyanide strike for zinc is recommended. The pyrophosphate-copper strike is a diluted version of 274

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