On 27 Jul 99 at 9:07, Hawes, Adam wrote:

>         I feel sure this is probably an issue for a lot of PCB
> designers......
>
>         Recently, I have come across an increasing number of engineers who
> wish to have certain tracks on multilayer PCB's shielded by ground traces
> running adjacently. It seems to be a case of jumping on the bandwagon to me
> because we never used to do this until about 12 months ago.
>
>         I was under the impression that ground tracks only served a useful
> purpose on 1or 2 layer boards and that on a multilayer they were essentially
> a waste of space as most of the capacitive coupling would be straight to the
> ground plane.

Depends on what the circuitry does:

If your shielded traces carry RF signals and/or if you try to build
something with controlled impedance, then the shielded traces act
like a coaxial cable and you should get indeed the same or
better results on a well defined multilayer buildup with a power or
better ground layer.

On the other side, if these shielded traces have to carry very low
analog signals to high impedance inputs (e.g. FET OP Amp inputs),
then these shielding tracks may have the purpose to "catch" leakage
currents from adjacent traces.

It would be necessary to have a deep look in the schematics and
perhaps in the application notes for the components used in this
circuitry.

HTH
Regards
Matthias Mansfeld
-----------------------------------------------
Matthias Mansfeld Elektronik
* Printed Circuit Board Design and Assembly
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