# Re: [SI-LIST] : DC resistance of a trace

From: H. Selfridge ([email protected])
Date: Fri May 25 2001 - 13:03:44 PDT

Dr. Howard Johnson's book, HIGH SPEED DIGITAL DESIGN, gives a formula for
DC resistance:

R (ohms per inch) = 0.65866 E-6 /( W (width of line in inches) x T
(thickness of line in inches))

Thickness of a PCB trace is generally specified as copper weight in
ounces. Internal traces are frequently done in 0.5 oz copper which would
be 0.000675 inches thick.

Assuming you have good clean probes that are suitable for low resistance
measurement, and that you are getting good contact from the probe to the
circuit trace - resistance as high as you are seeing would suggest perhaps
an over-etched trace that is either overly thin or overly narrow. Another
possibility would be a crack or contaminated plating at a pad or via hole.

Regards - Harry

At 10:30 AM 5/25/01 -0700, you wrote:
>Hello,
>
>I am looking for a formula and an explanation or definition of the
>variables to calculate the DC resistance of a trace. Maybe a chart or
>cheat-sheet? If possible, I would love any information or related links.
>I think actual DC resistance of a trace is somewhere in the milliohms/inch
>depending on trace thickness and copper weight. What might cause a 4"
>trace to measure 14-17 ohms with a DMM?
>
>Thank you for your time and help.
>
>Cheers!
>Drew
>
>
>
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