Perimeter frame earth track on PCBs

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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

Re: Perimeter frame earth track on PCBs



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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?



Re: Perimeter frame earth track on PCBs


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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.

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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.

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Not unless it's left floating.

Dave :)


Re: Perimeter frame earth track on PCBs


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Thank's for the explanation and your help.

Ricky.

Re: Perimeter frame earth track on PCBs


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see below


or it has current flowing in it.

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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

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