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Durability of different finishes for PC board contacts

2006

Hello,

I am a junior design engineer. I have drawn a board with some pads on it which will be exposed to a rotary movement switch to make contact.
The product need to pass a 40k scratch test of the rotary movement switch on the pads without electrical contact failure.

Since I can't afford to make boards of different finishing to carry out the test, I need to choose the most reliable finishing first.

Among the below finishing,

i) Immersion Tin
ii) Immersion gold
iii) flash gold

Questions,
1) Which finishing is more durable against scratch test? The rotary movement switch will scratch/turn on the pads for 40k time and the pads must still have proper electrical contact with the switch.
2) Which finishing is most suitable, in order to avoid oxidation issue which will eventually cause electrical contact failure?
3) The switch is made of stainless steel. In long run, which finishing is best to avoid chemical reaction (if it happen) between metal and the finishing that might cause electrical contact problem?

P.S. I will be glad if anyone can send me a presentation detailing each flow in the complete PCB fabrication process.

Thank you,

CY

Tang Chi Yung

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2006

I am not an expert on PWB design but I don't think any of those alternatives will work. I think you need electroless nickel or, if absolute minimum electrical resistance is a requirement, acid hard gold.

You can get a copy of "Understanding Surface Finishing" from www.nasf.org for just a few dollars. Another good one although impossible to find is the booklet by F. A. Lowenheim "Guide to the Selection and Use of Electro-plated and Related Finishes". It was sponsored by ASTM, so you might be able to get it from them, although I doubt it :-(

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey