To add to what has been said, Also look at where the shield will connect
to the ground of the chassis.  You want to make this as short as possible.

For example if you terminate the shield to a mounting hole on the pwb you
would want that hole to be the closest to where the "module" chassis ground
is.  Basically what you are doing is putting a metal box around the circuit and
connecting it to chassis ground.  This also helps with ESD hits.

John Laur
Allen-Bradley

> From [log in to unmask] Wed Oct 23 10:34:03 1996
> Resent-Date: Wed, 23 Oct 1996 10:21:35 -0700
> Old-Return-Path: <[log in to unmask]>
> From: "Walker, Don" <[log in to unmask]>
> To: [log in to unmask]
> Subject: Re[2]: Internal and external ground planes
> Resent-From: [log in to unmask]
> 
> 
> 
> This is a good idea.  Especially if you have high speed signals running
> near the board edge.  The only problem I see is where you say to "...tie
> the external planes and the internal ground plane together..." I think
> you probably meant to say "...tie the external planes and the internal
> GUARD RINGS together..."  This prevents any possibility of the shield
> becoming a part of the return path.
> 
> Don Walker
> [log in to unmask]
> 
> >    to run a guard ring of ground around the periphery of the board on all
> >    layers and tie the external planes and the internal ground plane
> >    together through closely spaced (.25 inch) vias only at the guard
> >    ring.
> 
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