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May 1999

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Subject:
From:
"Hiteshew, Michael" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum." <[log in to unmask]>, "Hiteshew, Michael" <[log in to unmask]>
Date:
Wed, 12 May 1999 10:29:58 -0400
Content-Type:
text/plain
Parts/Attachments:
text/plain (100 lines)
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 (r) 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 1/2 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 1/4 oz. copper (14.22 ohms) or was etched too
thinly.

 Michael Hiteshew
 Lockheed Martin Launching Systems
 [log in to unmask]
 (410) 682-1259



> ----------
> From:         Ken Patel[SMTP:[log in to unmask]]
>
> This is urgent!!!
> Our procurement spec is calling 100% continuity at maximum 10 ohms. One of
> our fab has 3 mil line on 1/2 ounce copper. Fab house is saying that they
> are measuring it around 13 ohms on 22" line. May be to be safe, he is
> asking
> for deviation which allow him to pass up to 20 ohms.
>
> The questions are:
> (1) Is it OK to accept fabs with above condition?
> (2) Can someone explain me why it's measuring higher than normal - does it
> has to do with the trace width. (I do know for sure that resistance is
> higher with higher length but less with bigger dia. Is it because of
> that?)
> (3) Anything should I know more about the situation.
>
> re,
> ken patel
> ______________________________________________________
> Ken Patel                       Phone:  (408) 490-6804
> 1708 McCarthy Blvd.             Fax:    (408) 490-6859
> Milpitas, CA 95035              Beeper: (888) 769-1808
>
>

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