Jeff,

The calculation tool I have lists copper at 1.62E-08 ohm meter which is in
the same ball park you used.  One interesting note is that "normal" rigid
substrates use ED copper and flex often uses RA copper as the base.  Sounds
as if some confirmation is needed for the resistivity of copper in this type
of design.  I agree with you regarding the other items not considered in the
calculation. Line tolerance, allowance for nicks and roughness, and the etch
factor for the circuit will affect the end result.  The best etching in the
world doesn't produce a vertical side wall, and the side wall is usually a
curved or complex surface.


Just a couple more cents in the pot.

Don Vischulis

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Jeff Seeger
Sent: Thursday, November 21, 2002 1:49 PM
To: [log in to unmask]
Subject: Re: [TN] Track Resistance


> snip <
> particular tracks have to meet certain resistance.
>
> eg.     Track width - 1.27mm (50 tho)
>         Track Length - 68.9mm
>         Copper Thickness - 105um
>         Resistivity of copper 0.0000000177 ohm.metre
>         Required resistance - less than 9 milli-ohms

        I took the above into a PCB tool I have, and saw an expected
        value of 8.67 mOhms - if everything were just perfect you make
        it.  A case of reality not measuring up to theory, I fear, and
        will likely lead to an argument on conductivity of Cu.  I lack
        the time to "hit the books" but did a little with the tool and
        found a startling range of answers for this question.

        The default value in this tool for base copper agrees with your
        value above (I'm assuming it's RA), but the value for "plated
        copper foil" is only 57% of this conductivity.  I did find one
        anecdotal tidbit claiming ED Cu at 80% the conductivity of RA.

        Here are the data points I was able to extract for your trace
        at the thicknesses you've mentioned:

        Case              conductivity          thick       R

        RA (base copper)   595900 mho/cm      0.075 MM  12.1389  mOhm
                                              0.105 MM   8.67067 mOhm
                                              0.120 MM   7.58684 mOhm
                                              0.141 MM   6.45688 mOhm

        ED (anecdotal 80%) 467720 mho/cm      0.075 MM   15.4657 mOhm
                                              0.105 MM   11.0469 mOhm
                                              0.120 MM    9.6660 mOhm
                                              0.141 MM    8.2264 mOhm

        "Plated" (default) 343000 mho/cm      0.075 MM   21.0892 mOhm
                                              0.105 MM   15.0637 mOhm
                                              0.120 MM   13.1808 mOhm
                                              0.141 MM   11.2177 mOhm

        Unfortunately this tool won't allow me to mix Cu types (as
        in RA base with additive ED).  I assume this would behave as
        parallel resistors at DC.  Also this tool makes no provision
        for nicks, dish down, or other realities of life.

        So it appears "Your Mileage May Vary" considerably.  I'll let
        you draw your own conclusions.

        Good luck on this one!
--

      Jeff Seeger                         Applied CAD Knowledge Inc
      Chief Technical Officer                  Tyngsboro, MA  01879
      jseeger "at" appliedcad "dot" com                978 649 9800
            PCB Design and Implementation Services since 1983

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