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

Dave Hoover (dhoover@mccurdy.com)
Wed, 7 Jul 1999 17:18:18 -0700

Exposed copper plane edges (PCB trimline) are possible but
generally will not make a good connection or last very long due
to possible corrosion over time. Edgeplated PCB is another option
where the planes are electrically tied to plated edges of the PCB.
Edge Plating gets rather pricey though and is not real easy. You always
have to accomodate for some tabs that don't get plated where the PCB
stays retained in the panel to be trimmed out later. The most
economical approach I see on PCB's is Ground stitching (vias) all
around the outer perimeter of the PCB. Then 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. (Pretty ingenious) Merix has a sketch of it at it's OSP ?
webpage.

Dave Hoover
-----Original Message-----
From: peter baxter [mailto:peter.baxter@tantau.com.au]
Sent: Wednesday, July 07, 1999 04:08 PM
To: si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : Question about Multiple PWBA Ground

David,

I have been looking at doing a similar thing except I want to "BUTT" a
PCB's ground plane (which is on the top layer of a PCB), against the
chassis. It is something I am considering in relationship to reducing
the length of ground/shield connection length from a cable to chassis
ground. While the cables have a shield, they are supplied with plastic
connectors resulting in 10mm (0.4 inch) multiple (20) pigtails because
it is GND/SIG/GND. It could well be a loosing battle, but I am thinking
of what might be possible.

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.

If every product was used in a humidity controlled environment, it would
be fine. But that may not always be the case.

So when I read your message, I immediately thought of my problem.

How do others do it and are they concerned about galvanic problems?

Regards,

Peter Baxter

Spencer, David H wrote:
>
> I am curious about multiple ground points on a PWBA. Assuming a typical 4
> layer PWBA (8x8), with power and ground planes, mounted to a shield via
> metal stand-offs (or screw to pad on the board). The ground plane is
> connected to the metal plate with 4-10 screws (I am trying to keep this as
> generic as possible).
>
> My question is regarding any drawbacks to this configuration. Is there
any
> reason not to use multiple connections to chassis; rather than, the ground
> plane connected back through the LVPS.
> This configuration is becoming common place these days.
>
> I am kind of scouting around for technical reasons for and against this
> configuration
>
> Regards
> David Spencer
>
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