Rate of electroless plating

The rate of electroless plating can be measured by several different methods, including nonelectrochemical techniques, such as those based on weight gain determination, electrical resistance measurement, and optical transmission measurement. The latter methods have been adopted to continuously determine plating rates for the purpose of process control. Electrochemical methods described below have also proven to be useful for automatic control of various electroless processes. 

Ohno and Haruyama [16] have shown that the instantaneous deposition rate of electroless plating is inversely proportional to the polarization resistance of complete plating solutions. The following generalized derivation of this relation was developed by the above authors. The partial anodic and cathodic current densities and are first written in the following generalized form ... 

The coulostatic method was applied to in situ measurement of the polarization resistance i p (Eq. (25)). Thus, the rate of electroless plating was determined by Suzuki et al. [21-23]. Two advantages of this technique have been cited by the inventors (1) measured values are not influenced by the solution resistance, and (2) measurements can be finished very rapidly, within a few tens of milliseconds. The principle is briefly explained below. 

Thus, / t can be calculated by subtracting an impedance value measured at a very high frequency from a value measured at a very low frequency. Ohno et al. [20] applied the above principle to the measurement of the rate of electroless copper deposition and demonstrated that i , is inversely proportional to the plating rate determined by weight gain measurement. This is exemplified by the relationship between polarization resistance Rp and the plating rate experimental results are shown in Fig. 10. 

Suzuki et al. [22] demonstrated that this method may be used to measure the plating rate of electroless nickel and copper plating processes. An example of a potential decay curve is shown in Fig. 14. The authors also demonstrated the validity of Eq. (25) and determined the K values of various plating systems. 

Fig. 32. Effect of copper and nickel contaminants on the plating rate of electroless gold.

Formaldehyde oxidation rate in electroless plating solution is also higher compared to Cu(II)-free formaldehyde solution. In this case, a nonadditive could be equally explained by the higher catalytic activity of freshly deposited copper metal. 

Electrodeposition of Metals. Citric acid and its salts are used as sequestrants to control deposition rates in both electroplating and electroless plating of metals (153—171). The addition of citric acid to an electroless nickel plating bath results in a smooth, hard, nonporous metal finish. 

Electroless plating rates ate affected by the rate of reduction of the dissolved reducing agent and the dissolved metal ion which diffuse to the catalytic surface of the object being plated. When an initial continuous metal film is deposited, the whole surface is at one potential determined by the mixed potential of the system (17). The current density is the same everywhere on the surface as long as flow and diffusion are unrestricted so the metal... 

The advantages of electroless nickel over hard chromium include safety of use, ease of waste treatment, plating rates of as much as 40 p.m/h, low porosity films, and the ability to uniformly coat any geometric shape without burning or using special anodes. Increased chemical safety is another... 

Fig. 10. Time dependence of corrosion rate of Co8P and Co films, electrolessly plated and sputter-deposited onto NiP substrates, in a droplet of DI water. The results were obtained by a repeated application of the polarization resistance technique with the potential scanned at 1 mV/sec in a potential range 15 mV above and below the corrosion potential [125]. (Reprinted by permission of The Electrochemical Society). 

DSA-02 oxide-coated Ti anode (DSA = Dimensionally Stable Anode) is placed, thus creating a large cathode volume. The effluent solution flows perpendicularly through the electrodes with a typical flow rate of 0.5 dm3 s-1. The flowthrough metal electrodes have an active area approximately 15 times their geometric area. The cell allows air sparging to increase the mass-transfer. The current efficiency is about 40% when the inlet concentration of the metal ions is 150 to 1500 ppm and the concentration at the out-let is about 50 ppm. The cell is currently used for the treatment of recirculated wash-waters from acid copper, copper cyanide, zinc cyanide, zinc chloride, cadmium sulphate, cadmium cyanide and precious metal plating and washwaters from electroless copper deposition. Since the foam metal electrodes are relatively expensive the electrodes... 

Fig. 6.14. EflFect of nickel concentration on the electroless plating rate.

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