This is a re-post from a May 99 thread. Search for 10 ohms in the archive if
you need further info.
Jeffrey A. McGlaughlin
PCB Designer
Battelle Memorial Institute
Columbus Ohio
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Ken,
Here's how to calculate the theoretical (read ideal) resistance of a copper
trace.
        The formula for resistance of a material is:
        R = r(l/a)
        where
        R = resistance in ohms
        r = resistivity
(Resistivity ® of copper at 20 degrees C = 8.15 * 0.083 [(square mils *
ohms)
                divided by inches] = 0.679 [(square mils * ohms) per inch])
                l = length in inches
                a = cross sectional area of conductor (trace) in square mils
        So:
                If an internal trace on a circuit board is 0.003 inch wide,
is 22
inches
                long and was etched on ½ oz/sqft (0.0007 inch thick) copper
clad
laminate,
                then its' resistance is :
        a = 3 * 0.7 = 2.1 sq_mils
        l = 22

                R = 0.679 sq_mil-ohms per inch ( 22 in/ 2.1 sq_mils )
                = 0.679 sq_mil-ohms per inch ( 10.48 in/sq_mils )
                = 7.11 ohms
Keep in mind that all test equipment will add some internal resistance of
its own, but this is usually in the 25-50 milli-ohm (0.025 - 0.050 ohm)
range. If they're getting values in the 13 ohm range (twice as high) then
something has increased the ideal resistance by a factor of two. This means
(obviously) that something in that formula has changed by a factor of two. I
don't think the trace was etched to be twice as long. Therefore, it was
either etched in ¼ oz. copper (14.22 ohms) or was etched too thinly.
Michael Hiteshew
Lockheed Martin Launching Systems
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(410) 682-1259


        -----Original Message-----
        From:   mdsn500 [SMTP:[log in to unmask]]
        Sent:   Monday, 19 July, 1999 15:15
        To:     [log in to unmask]
        Subject:        [TN] DC Resistance

        Does anyone have a formula equating copper foil size, trace wirdth
and dc
        resistance values(not impedance)?- Thanks Mike hawkins

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