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December 2001

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Subject:
From:
"Hiteshew, Michael" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 13 Dec 2001 14:30:58 -0500
Content-Type:
text/plain
Parts/Attachments:
text/plain (100 lines)
Gaetan:

   Here's something I worked out some time ago:


        Resistance of a Trace
        ---------------------

        A) Resistivity (r) of copper @ 20 deg c = 10.37 [(circular mils *
ohms)
        divided by feet]

        B) The units of resistivity (r) are: circular mils, ohms, feet


        if   : 1 circular mil = 0.78540 square mils

        then :

        Resistivity (r) of copper = 10.37 * 0.78540 [(square mils * ohms)
        divided by feet]

                                  = 8.15 [(square mils * ohms) divided by
feet]

        also if   : 1 foot = 12 inches (and 1 divided by 12 = .083333...)

        then :

        Resistivity (r) of copper = 8.15 * .083 [(square mils * ohms)
        divided by inches]

                                  = 0.679 [(square mils * ohms) per inch]


        The formula for resistance of a material is:

        R = r(l/a)

        where
        R = resistance in ohms
        r = resistivity
        l = length in inches
        a = cross sectional area of conductor (trace) in square mils

        So:

        If an internal trace on a circuit board is .010 inch wide, is one
inch
        long and was etched on 1 oz/sqft (.0014 inch thick) copper clad
laminate,
        then its' resistance is :

        a = 10 * 1.4 = 14 sq_mils
        l = 1

        R = .679 sq_mil-ohms per inch ( 1 / 14 sq_mils )
          = .679 sq_mil-ohms per inch ( .071 in/sq_mils )
          = .049 ohms


I realize this is English units, so you'll need to adjust, obviously, to
metric.

Michael Hiteshew
Lockheed Martin NE&SS Marine Systems
410-682-1259


-----Original Message-----
From: Gaetan Fournier [mailto:[log in to unmask]]
Sent: Thursday, December 13, 2001 12:27 PM
To: [log in to unmask]
Subject: [TN] Trace resistance


Does anybody knows a good link or freeware for DC resistive calculation
in
function with the thickness, width and length of a PCB track. I already
know
about "trace with calculator"
http://www.aracnet.com/cgi-usr/gpatrick/trace.pl but this is not exactly
what I'm looking for, current and temperature rises aren't the issues
for my
application. It's more a question of DC equivalent resistive for a coil
made
with a long PCB trace.
Thanks
GF

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