What are the challenges in selecting a conductive coating for HF applications?

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First off, maybe this should have gone in the Material Engineering thread, but since it is so common in the EE field...well, I justified it.

I have an aluminum housing which gets plated with Ag over Ni. Once received, the housing goes through the following process:
Cleaning (alcohol solution)
220 degree temp cycle for 2 hours.
Send out for RF pins to be installed and leak tested. (Consists of heating, soldering, cleaning, leak testing with Helium)

After the parts return from leak test, they have a brownish/redish film on the housing. Any suggestions as to what this might be? Bad plating? Bad raw material? Dirty cleaning solution?

Any help would be great...thanks.
 
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Have you followed the process through each step? This is what I would do.
 
Yeah, as far as I could.

I spoke with someone with some plating experience, and he said that it is sort of common when silver oxidizes slightly to get a brownish color. The advantage, apparently, to using silver is that when it does oxidize it does not effect the conductivity of the material.

I'm just not sure why it wasn't noticed when it came back from the plater...

If someone knows otherwise, please let me know.

Thanks,
 
One potential problem is if a "dirty" cleaning solution is used to clean the parts. Kind of sounds like a double negative huh? Anyways, we cleaned the parts with a dirty solution and they put the housing on a hot plate, and sure enough it left some discoloration.

Not sure if this is in fact the problem, but found it an interesting data point.
 
silver does tarnish easily. iirc, sulphur compounds make it worse. there's not a whole lot you can do about it unless you want to coat it to keep it shiny.
 
Hello everybody!

For HF applications, one needs a conductive coating which has no magnetic permeability, as this would radically worsen skin effect and associated losses. This is why nickel can't be used bare, and silver is acceptable though is has drawbacks.

In fact, few materials are good here. Solderability precludes aluminium and chromium, corrosion precludes many metals, and so on. Nickel is a good choice only in LF. For HF, you have silver (with limited corrosion resistance as you've observed), gold (excellent choice, but accelerated underlying aluminium corrosion if scratched) or palladium (used for SMD capacitors, not cheaper than gold). As a common drawback, they dissolve in liquid tin solder.