Re: [SI-LIST] : Question about Multiple PWBA Ground

Jeff Seeger (jseeger@appliedcad.com)
Wed, 07 Jul 1999 22:01:35 -0400

Dave Hoover wrote:
>
> snip <
> do the UFO pad approach
> on the screw tooling holes. That's the one where the main hole is a
> NPT and it's surrounded by a large metal pad that had PTH vias in
> it. The hole stays clear of solder at the wave yet still makes contact
> to the grounds.

Hey Dave!

This is a good approach as it solves the temptation of
putting a PTH under compression, which is a no-no (subject
to broken connection).

> As currents flow, and I have two dissimilar metals (PCB: either copper,
> tin-lead plate or nickel plate, Chassis: zinc treated/passivated sheet
> steel) I have all the ingredients for galvanic corrosion and reduction
> in effectiveness. The zinc is the killer but that seems to be the way
> all PCs are made.
>
This is absolutely an issue and may introduce a resistance
right where you're not expecting one.

Perhaps this is acceptable in PCs as in my experience they
don't stay assembled for too terribly long ;)

I'm no chemist but I believe tooth washers (in stainless?)
are the answer to this...

Cheers,

-- 
 
      Jeff Seeger                         Applied CAD Knowledge Inc
      Chief Technical Officer                  Tyngsboro, MA  01879
      jseeger "at" appliedcad "dot" com                978 649 9800

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