RE: [SI-LIST] : skin effect/spreading inductance

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From: Gonzo ([email protected])
Date: Thu Aug 31 2000 - 13:49:37 PDT

Sorry for adding yet another email to the well discussed topic. I agree with the statement below. However I have a question, which may be moot or a don't care 2nd order effect, but considering the trapezoidal nature of etched traces at what point does the "spreading inductance" come into play? Since the traces are etched (I suppose using some alkaline anisotropic etchant) would the side walls of the trapezoid have a more smooth finish with fewer asperities in comparison to the top/bottom of the trace? If so does the spreading inductance of the side wall versus a burnished top come into play?
/__\ As a trace got taller, the "smooth" side wall would become longer.

It seems that the spreading inductance that Mr. Larry described may be different for the given surfaces. For really long traces would that difference in spreading inductance come into play once electrons were crowded to the surface?

Just curious,

-----Original Message-----
From: Dunbar, Tony [SMTP:[email protected]]
Sent: Thursday, August 31, 2000 10:48 AM
To: '[email protected]'
Subject: RE: [SI-LIST] : skin effect

 << File: ATT00014.htm >> Muhammad,
Both of your viewpoints (i.e. yours and your bosses) each have merit. But I
think your boss's adds the fine detail correctness. This is because the
current will always try to find the path of least impedance. This path will
tend to be where the coupling is tightest to its return path, typically the
trace's reference plane (or the complementary net of a diff. pair). If we
are considering a trace over/under a reference plane, the higher frequency
current will 'hug' the edge of the trace closest to the plane rather than be
distributed uniformily around the total edge of the conductor.
Therefore, a wider trace will facilitate the distribution of current better
than a taller trace and tend to have a lower resistance per-unit-length.
It's a very fine argument, I believe, within the realms of practical board
trace design because your trace widths are driven by other factors to be a
defined size and you will not be increasing the width to account for skin
effect. Above 100MHz, skin depth of copper begins to drop below 0.25mil so
whether you use 1oz or 1/2oz will not make much difference in the high
frequency band unless you are conducting heavy currents.

-----Original Message-----
From: Muhammad S. Sagarwala [mailto:[email protected]]
Sent: Thursday, August 31, 2000 11:45 AM
To: [email protected]
Subject: [SI-LIST] : skin effect

Hello Si Gurus,
I just had an interesting discussion with my boss on "skin effect on pcb
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...
p.s.: when we talked about pcb traces we were talking about striplines in
Muhammad S. Sagarwala
Design Engineer
Schlumberger SABER
Ph. (408) 586 7065
Fax (408) 586 4668

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