Perimeter frame earth track on PCBs

On quite a few industrial grade PCBs which are mounted in plastic enclosures, I saw these frame earth tracks going a long the perimeter of the PCB. They typically are on both sides of the PCBs, form a loop, top and bottom track are connected with plenty of vias and are connected to the protective earth terminal of the enclosure. The circuit ground (GND) is typically connected via a 1MOhm/10nF to this track.

The purpose seems to be an EMI shield.

Is this a useful thing and recommended to do on a 2-layer PCB?

As the track forms a loop I am concerned that it could be EMI wise counter productive.

Ricky

Reply to
H. M.
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That perimeter track should really be for screwing onto the metal enclosure or connecting to film/foil paper etc etc that surrounds the product.

Pure [lastic enclosure seems odd. Sure its not coated on the inside?

Reply to
jc

Yes, it is supposed to be an EMI "guard track", not all that uncommon. You have to ensure that the guard track does not carry any return current though. It is also commonly used to limit field fringing on multi-layer power planes, with the guard track going around the edge of the plane.

On a double sided board in a plastic case, the EMI effectiveness of an outer guard track is somewhat limited, but it certainly can't hurt. BTW, it should go as close around the circuit as possible, not around the outside of the board. If you are looking at reducing EMI on a double sided board, this is the last resort, there are many other things you can do layout and circuit wise that will have a much greater impact.

Not unless it's left floating.

Dave :)

Reply to
David L. Jones

Thank's for the explanation and your help.

Ricky.

Reply to
H. M.

see below

or it has current flowing in it.

If you intend to use it as a "field stopper" to prevent EM leakage at the edges, it typically needs a lot of vias (governed, of course, by the highest frequencies of interest). In this case it is connected to the relevant power plane (usually 0V). It can use a lot less space than the other "trick" to stop edge leakage, which is having the 0V plane extend out something like 20x the plane-conductor separation.

Its pretty common to see this sort of EMI "trick" in 2-sided PCBs, most of which have an appalling layout to begin with, and where it is of dubious efficacy, if not downright detrimental. Without a solid 0V plane, edge leakage will generally be a tiny fraction of the total emissions.

Cheers Terry

Reply to
Terry Given

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