Re: [SI-LIST] : skin effect

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From: Doug McKean (dmckean@corp.auspex.com)
Date: Thu Aug 31 2000 - 13:13:06 PDT


> "Muhammad S. Sagarwala" wrote:
>
> Hello Si Gurus,
>
> I just had an interesting discussion with my boss on "skin effect on pcb traces".
> I was of the opinion that increasing the trace thickness from 1/2 oz. to 1oz. would help reduce the
> skin effect but according to him skin effect does not reduce significantly with the increase in trace thickness.
> He was of the opinion ( and also had some data to back him up) that skin effect is more dependent on the
> width of the trace.
>
> I always thought that if one increases the overall perimeter of the pcb trace - regardless of whether it is done by
> increasing the width or increasing the thickness - the skin effect would reduce. I would appreciate if somebody
> could come up with a better explanation...
>
> Muhammad
>
> p.s.: when we talked about pcb traces we were talking about striplines in particular...

Wow! I guess I may be repeating info, but here goes.

Well, excuse my stickler for terms, but it seems
you and your boss are mixing terms. There's
"skin effect" and "skin depth". The former related
to the latter.

Here's the scoop.

At a particular frequency, the "skin depth" won't change
no matter what you do. Skin effect obviously being the
depth at which most of the high frequency current resides.
The mechanism for its cause is related to the Lorentz
force as best I can determine.

At a particular frequency, the "skin effect" will change
depending upon what you do. Skin effect is the change in
resistance due to conducting high frequency current. This
happens since more and more current resides near the surface
at higher frequencies and thus less cross sectional area
is available for the current.

The only way to change this available cross sectional area is
by what someone has already mentioned - affect the perimeter.
One of the most dramatic ways to do this is by changing width.
It should change by adjusting height but since the current
distribution in the reference plane (as someone again has
already pointed out) is not given greater area, the distribution
within the trace will become skewed.

Forgive my ASCII art but here goes. Exaggerated of course.
Dots inside indicate surface currents.

        +---------+ +---------+
        |. .| |. .|
        | .| |. |
        |. .| |. .|
        | .| |. |
        |. .| |. .|
        +---------+ +---------+

So, if you change the perimeter of the conductor:
 - yes, the "skin depth" will not change,
 - and yes, the "skin effect" will change.

Regards, Doug McKean

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