Nicholas
There are a couple of possibilities:
1) Are you doing a four point resistance measurement (separate current and
separate voltage points)?  You can get "contact resistance" during a 2-point
measurement that will distort the value.
2) The quality of copper varies.  With the caveat that I'm not a PWB shop
guy and do not know the quality of copper in plating, copper layers will
have higher than theoretical values because of granular structure of the
copper layer.  Also, there may be variations in layer thickness.

To All:
Did I forget some of the other basic answers?
Carey
-----Original Message-----
From: Nicholas LAI <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Tuesday, October 05, 1999 7:41 PM
Subject: [TN] Resistance of trace


Hello all,

Is there anyone can answer the following question.

When we calculate the resistance value of an trace on a PCB Board.

We use R=ρl/A  where l is the line of trace, A is the coss-section area of
the
trace and ρis the presistivity of copper.

After we measure of the board, the actual resistance value is larger than
the
Theorical value around 20% . Why the different is so larger. Do I use the
wong
formula?
Can anyone help on me?

Best Regard,
Nicholas Lai
10/6/99

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