Hi Nicholas,

From the equation (R=ρL/A) there can only be 2 explanations:

1. You are using the wrong value for ρ, and/or
2. You are measuring a conductor that is smaller in cross-sectional area
than you expected.

This assumes you know the length of the conductor and have measured the
resistance correctly.

So, let's start with the first possibility.  You probably used a value for
ρ of 1.72 micro-ohm-cm.  This is for pure copper, no impurities or voids.
In a previous life at Sandia National Laboratories, I found that vendor and
plated copper is closer to 2.0 micro-ohm-cm.

On to possibility two.  A = HW where H is the height and W is the width of
the conductor.  If you are measuring an innerlayer conductor, the actual
thickness is probably thinner than you expected.  The reason for thinner
copper is that the laminate suppliers typically provide the copper at the
low end of the specification (less copper-more profit).  Additionally, the
copper thickness is reduced by the PCB manufacturer (ie. the cleaning and
oxide processes).  If the conductor is on an outer layer then all bets are
off...the thickness is dependent on plating.

Now for the width (W) of the conductor.  The width that PCB manufacturers
typically measure is the base (bottom) of the conductor.  This is not the
cross-sectional width you are using.  Thus, the average conductor width is
actually smaller than you expected.

To summarize:  ρ may be larger than expected, H may be smaller than
expected, and W may be smaller than expected.  In your case, these resulted
in a 20% increase in resistance.

Hope this helps.

Best regards,

Tim Estes
Conductor Analysis Technologies, Inc.
Phone: 505-797-0100
Email: [log in to unmask]
Web: http://biz.swcp.com/cat

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Nicholas LAI
Sent: Tuesday, October 05, 1999 7:37 PM
To: [log in to unmask]
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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