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Plating microwave components can be like many things in life: Simple and easy when things are going well but perplexing and frustrating when they're not. At first glance the processes are not terribly complicated but since poor results are not at all uncommon there must be a devil hidden somewhere in the details. A bit of common sense planning (and a plater dedicated to quality product) can help keep the "devil" under control.
Why do we plate microwave stuff?
One of the main reasons we plate microwave components is that gold is pretty. If you're paying $500 or $5000 for a little chunk of brass or copper shouldn't it at least look like you're getting your money's worth? Gold plating looks good and should keep looking good for years, without showing fingerprints and tarnish that will ruin the cosmetics of the shiniest piece of copper. It's not absolutely clear that normal corrosion on brass or copper components changes the RF loss characteristics by much, but an untarnished gold surface will certainly offer stable performance over time.
Even if copper tarnish doesn't affect RF loss much within a structure it does certainly affect contact resistance. Gold has of course been the "gold standard" for electrical contacts since the discovery of the electron and its contacting value extends beyond edge connectors and switch contacts. Since even the harder gold alloys are still relatively soft metals, gold to gold contact between flanges, components pieces, and covers all tend to be self gasketing, corrosion free, and stable over time. Gold to gold will also provide excellent thermal conductivity for both heat dissipation and cooling in cryogenic applications.
For certain fabrication techniques, particularly wire bonding to a chassis, use of the proper gold plate is essential to the process. Wire bonding prefers a thick, pure, gold plate, and although bonding to gold of 50-100 micro inches thickness is possible a chassis plated with 300 micro inches of pure gold will provide high reliability and make life easier for the bonding technician. High gain amplifiers are much more stable when all the source bonds remain in place, and a little extra spent on gold plate helps keep them there!
Electro-forming parts
