Solution Composition and Operating Conditions

With the changes that take place in the plating tank when a modulated periodic reverse pulse is impressed on the electrolyte, changes in the other operating conditions or even in the formulation may be required. Generally speaking, better results are obtained with simple, rather than sophisticated, formulations (Ref 3). Typical solutions used in pulse plating are given in Table 1.

Table 1 Typical solutions used in pulsed-current plating

Constituent or condition

Amount or value

Watts nickel solution for reel-to-reel plating

Nickel sulfate, g/L (oz/gal)

650 (87)

Boric acid, g/L (oz/gal)

50 (7)

Temperature, °C (°F)

60 (140)

pH

3-4

Anodes(a)

Platinized niobium (insoluble)

Organic additives

None

Pure gold

Potassium citrate, g/L (oz/gal)

150 (20)

Citric acid, g/L (oz/gal)

15 (2)

Potassium phosphate, g/L (oz/gal)

26 (3)

Boric acid, g/L (oz/gal)

72 (10)

Gold metal, g/L (oz/gal)

8.2 (1)

Temperature, °C (°F)

60 (140)

pH

3.5-4.0

Anodes

Platinized titanium

Hard gold

Citric acid, g/L (oz/gal)

65 (9)

Potassium citrate, g/L (oz/gal)

50 (7)

Cobalt, g/L (oz/gal)(b)

0.5-0.6(0.07-0.08)

Gold, g/L (oz/gal)

8.2 (1)

pH

3.8-4.0

Temperature, °C (°F)

32-38 (90-100)

(a) When using soluble nickel anodes with reversing pulse modes, the use of an anode activator such as chloride is not required because the reversing current keeps the anode active and soluble.

(b) The higher voltage of pulse plating relative to continuous dc plating favors the deposition of the alloying agent. The operator should analyze the deposits to determine if the amount of cobalt in the solution should be adjusted. In most cases, the amount of available cobalt (or other alloying agent) should be reduced (from the amount used with continuous current) to obtain the desired properties.

Additives. The polarization imposed by the power pattern on the bath reduces, or even eliminates, the need for some addition agents. In many cases, additives can actually inhibit the effectiveness of the pulsed-current pattern. For example, large-molecule additives do not respond as they do under conventional power; in a high-frequency pulse field, their molecular size is a disadvantage. Small-molecule organics or inorganics will generally function well as additives. In many cases, the use of brighteners can be reduced as much as 90% without diminishing the brightness of the deposit because of the improved grain structures. If brightener levels are not reduced, longer pulses--i.e., lower frequencies and/or higher duty cycles--may be required (Ref 3).

Electrolyte conductivity must be maintained at a high level to allow the peak pulse current to be completely effective. If the conductivity is not high enough, an excess in voltage will be required to attain the desired peak current. Such peaks are power-inefficient and less effective.

Anode-to-cathode ratios for pulse plating are rarely the same as those for conventional power applications. Generally speaking, in acid or alkaline nonchelating formulations, the anode area should be reduced. In cyanide or other chelating formulations, the reverse is generally the case, and a greater anode area is required.

Temperature and agitation conditions for conventional processes may also have to be altered for modulated power pattern plating. Unfortunately, no general rule applies; each application has its own requirements, and optimum conditions must be established on a case-by-case basis.

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