References

1. G.O. Mallory and J.B. Hajdu, Ed., Electroless Plating: Fundamentals and Applications, American Electroplaters and Surface Finishers Society, 1990

2. G. Stremsdoerfer, H. Perrot, J.R. Martin, and P. Clechet, Proc. Electrochem. Soc., Vol 88 (No. 12), 1988, p 61

3. L. D'Asaro and Y. Okinaka, U.S. Patent 4,162,337, 1979

4. M.F. El-Shazly, U.S. Patent 4,374,876, 1983 Process Description

As mentioned earlier, the deposition of gold without an external source of current can occur by a simple displacement reaction between two metals:

A typical immersion plating process is the displacement reaction between a less noble metal (nickel) and a more noble metal (gold) in an immersion gold bath:

The immersion gold process is relatively slow, and it is limited by the decreasing availability of the nickel substrate as the thickness of deposited gold increases. A maximum gold bath thickness of 0.25 pm can be obtained (Fig. 2).

Fig. 2 Thickness of gold as a function of time for an immersion electroless gold plating bath

In order to achieve thicker gold deposits, the use of a true electroless gold bath is required. Such baths are based on a reducing agent that provides the electrons necessary for the reduction of gold ions absorbed on a gold substrate. The catalytic effect of metallic gold promotes the process. This deposition reaction is called an autocatalytic process:

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