Back in the last century I was the responsible engineer on the redesigned
solid-state power amplifier for the Phoenix Missile. We were really pushing the
state of the art with the design. Operating at X-Band the final amplifier of the
three-stage PA used 16 Impatt diodes in a cavity combiner. The diodes had to be
well-matched for a number of parameters, which really drove our vendors crazy.
We decided to provide each of them with a Hughes-designed waveguide fixture and
the test equipment to operate it using our test software (which I wrote). One of
my guys wrote this up in Microwave and RF Magazine, August 1987. We built the
first copy of the fixture in our engineering machine shop and plated the brass
body with plenty of gold over a nickel flash.
Then we sent the prints out to an outside vendor to make several copies to send
to our vendors. When we tested them we couldn't make power. Try to not get
ahead of me here... after agonizing tear down and physical measurements we
couldn't find anything wrong. I finally asked someone, "How thick is the
gold?" There was the answer. When you send a drawing to a plating shop that
only says, "Gold over nickel barrier, total thickness not to exceed... you get
back lots of nickel and one atom of gold. We stripped the plating and did it
over in our shop and everything worked.
Wes N7WS
On 10/1/2023 5:46 PM, Jim Lux wrote:
More than one engineer has been bitten by the nickel flash under the gold plating on microwave circuits - similar to
the discussion in the article. Yeah, silver plating is usually done to make it look pretty - and that
isn’t often optimum for RF properties. There are plating processes that are intended for RF, but it’s
specialized, and often with a much thicker plating than used for “pretty”. You see it when silver
plating aluminum (or carbon fiber) waveguides, for instance.
One other aspect of silver plating is that it can form whiskers - I’ve not heard about it as
much as tin whiskers, but that’s probably because silver plating is pretty rare these days -
if you want solderability, it’s gold, usually ENIG (gold over nickel).
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