A copper pyrophosphate plating bath may be operated over a relatively wide range of operating conditions. The optimum range, similar to that given by Counch and Stareck that is shown in table below:
Optimum Operating Conditions
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P2O7/Cu ratio 7.0:1 - 8.0:1
pH 8.0 - 8.8
Temperature,oC 50 - 60
Cathode current density, A/dm2 1 - 8
Anode and cathode efficiency, % 100
Anode/cathode ratio 1:1 - 2:1
Agitation (m3/min (m-2) of surface) 1 - 1.5
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The copper pyrophosphate bath is analyzed for both copper and pyrophosphate and it is therefore convenient to describe the bath in terms of the ratio between these constituents. For optimum plating, the weight ratio of pyrophosphate to copper should be kept in the range 7.1 to 8.1. Ratios of 8.5 : 1 or higher promote formation of orthophosphate and thereby decrease the bright plating range. Operation with ratio of less than 7:1 tends to produce a rough-surface and renders the bath unstable.
PH
Optimum solution pH, determined using either pH meter or pH papers, is in the range of 8 to 9. As discussed earlier, pH outside this range results in hydrolysis of pyrophosphate to ortho-phosphate or the formation of precipitates. Also if the pH is too high, both anode corrosion and operating current density range decrease.
Pyrophosphate bath will operate acceptably even at pH value as high as 9.3 provided the copper concentration exceeds 26 g/l. When a bath is operated with this high pH, the tendency to be roughness can be eliminated by increasing the P2O7/Cu ratio.
In practice, a pH of 7 to 9 is easily maintained because at this pH both anode and cathode efficiencies are virtually 100 %, there is no chemical breakdown of the constituents, and the solution is highly buffered. When adjustments must be made, pyrophosporic acid is used for lowering pH and potassium hydroxide for raising it.
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