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

polus@lexmark.com
Wed, 7 Jul 1999 18:32:35 -0400

I tend to use a 0 ohm resister between my PCB ground and any screw holes. That
way I have the option of connecting grounds or not connecting them. In my EMC
evaluations, I have had several instances where either having a ground
connection or blowing a connection aids in reducing radiated emissions from my
system. The board is one thing, but when you screw the board to the frame you
enter a whole new world of return loops, etc, for high frequency stuff.

The metal plate serving as an image plane is a good thing. Just be sure you
don't have any large gaps in the metal plate. I have experienced gaps that act
as very nice slot antennas if just in the right spot.

I also recommend grounding screws near connectors, since logic ground of the
connectors is not always enough to reduce cable emissions or stuff getting out
onto the cables.

Therefore, the four corners of the card might be a good guideline, but you must
check each application's topography and make adjustments.

Harold Polus
Lexmark International, SI Card Design, EMI Test and Evaluation

David.Spencer%usa.xerox.com@interlock.lexmark.com on 07/07/99 01:41:12 PM

Please respond to si-list%silab.eng.sun.com@interlock.lexmark.com

To: si-list%silab.eng.sun.com@interlock.lexmark.com
cc:
Subject: [SI-LIST] : Question about Multiple PWBA Ground

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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