Hi Ken,

I agree with Michael.  We have seen a few cases of the copper weight being
incorrect but the most likely the cause is the average conductor widths are
narrow.  Unfortunately, it is common for the average conductor widths to be
1 to 2 mils narrower than specified in the Gerber files.  Check with your
supplier to see if they used artwork (etch) compensation.

Whatever the cause, your circuit requirements will determine whether or not
the boards are acceptable - check with your circuit designers.

If you have any questions feel free to contact me off line.

Best regards,

Tim Estes
Conductor Analysis Technologies Inc.
Phone: 505-797-0100
Fax: 505-797-1605
Email: [log in to unmask]
Web: http://biz.swcp.com/cat


-----Original Message-----
From:   TechNet [mailto:[log in to unmask]] On Behalf Of Hiteshew, Michael
Sent:   Wednesday, May 12, 1999 8:30 AM
To:     [log in to unmask]
Subject:        Re: [TN] Fab: What if resistance is higher than 10 ohms on 100%
continuity test?

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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