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April 2002

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Subject:
From:
Lou Hart <[log in to unmask]>
Reply To:
Date:
Fri, 12 Apr 2002 16:34:20 -0400
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Depending on the length of the trace and the tolerance on trace resistance,
a local trace width or thickness anomaly may or may not affect
acceptability of resistance.  Trace anomalies will affect characteristic
impedance measurements.  Characteristic impedance depends on spacing
between conductors, however they may be configured to achieve that
characteristic impedance, and the intervening dielectric(s), the properties
of which themselves vary with frequency.  I could speculate that someone is
sending a constant current down the line along with an information-carrying
signal.  By the way, I had occasion to talk with a fellow from Carnegie
Mellon University a few weeks ago, who is working on a project to use air
conditioning ducts as transmission lines, to avoid rewiring buildings for
networks.  Lou Hart

-----Original Message-----
From:   Genny Gibbard [SMTP:[log in to unmask]]
Sent:   Friday, April 12, 2002 3:32 PM
To:     [log in to unmask]
Subject:        Re: [TN] Interesting question

I think Valerie had the best answer, so far:

also, and probably most important, trace resistance can
be increased locally by nicks, dishdowns, local reductions
(like to get past a too-close pad), fine fractures in
the copper trace from handling.....

With lighter copper weights becoming standard (1 oz is the standard we spec
for our boards, but I hear that is now heavy, and 1/2 oz is standard),
nicks
and scratches are most likely the biggest concern, or too narrow traces.
Especially if any significant DC power is running thru those traces.  You
might develop hot spots in the board.

-----Original Message-----
From: Don Vischulis [mailto:[log in to unmask]]
Sent: April 12, 2002 9:49 AM
To: [log in to unmask]
Subject: [TN] Interesting question


Techies:

I received the following email today, and it aroused my curiosity.  What
reasons exist for specifying dc track resistance when minimum resistance is
governed by a combination of copper weight and design width, and track
length?  Is this a case of over zealous specification or is there a purpose
behind adding another test requirement to a product with extensive
specifications?

Don Vischulis

Original message:
You may be able to help me, as I would
like to know why - in addition to specifying
impedance - an increasing number of PCB designers
and PCB fabricators are also specifying DC resistance
of PCB tracks?

I have had an increasing number of clients ask
me about series resistance being specified on
PCB tracks, but no one seems to know why..

If you are able to help with some of the reasons
why, I would very much appreciate hearing from you.


Kind regards
Martyn Gaudion
www.polarinstruments.com

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