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May 1999

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Subject:
From:
"Mcmaster, Michael" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum." <[log in to unmask]>, "Mcmaster, Michael" <[log in to unmask]>
Date:
Wed, 12 May 1999 11:01:43 -0700
Content-Type:
text/plain
Parts/Attachments:
text/plain (201 lines)
I don't think that 13 ohms is unrealistic for this trace.  First, of
all, a three mil trace refers to the bottom trace width and most traces
have a trapezoidal profile with a narrower top.  Also half ounce foil is
almost never 0.7 mils thick, especially in a finished board where the IL
have been through a microetch or two..

If the copper is 0.55 mils thick and the trace is 3.0 mils wide on the
bottom, 2.5 on the top (2.75 average) the resistance is 9.87 ohms.   A
0.25 mil reduction in the trace width increases the resistance to 10.86
ohms.  Still not 13, but close enough that if you add via, OL trace and
contact resistance, you're in the ballpark.

> ----------
> From:         Timothy A. Estes[SMTP:[log in to unmask]]
> Reply To:     TechNet E-Mail Forum.;Timothy A. Estes
> Sent:         Wednesday, May 12, 1999 9:42 AM
> To:   [log in to unmask]
> Subject:      Re: [TN] Fab: What if resistance is higher than 10 ohms
> on 100%             continuity test?
>
> 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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