Given normal circuit geometry and construction, 5 ohms is getting close
to the upper limit for a hard short in a board.  I believe a 5 mil wide
trace on 0.5 oz copper will have a resistivity of about 0.6 ohms/inch at
room temperature.  Resistive heating will cause some increase but can't
account for 10Mohms by itself.  The only way to get a resistance that
high is through contact resistance.  I suspect that you've got some
metallic contamination in between the two nets.  Resistive heating
results in thermal expansion and drives up the contact resistance.

Happy hunting!

> ----------
> From:         Robert McGarry[SMTP:[log in to unmask]]
> Reply To:     [log in to unmask]
> Sent:         Tuesday, April 06, 1999 10:07 AM
> To:   Mcmaster, Michael
> Subject:      RE: [TN] test-phenom
>
> > What are the resistive values initially and after stabilizing).
> >
>
> Initially around 5 ohms the after about 7 seconds it will stabalize
> around 10 Megohms
>
>
> > > ----------
> > > From:     Robert McGarry[SMTP:[log in to unmask]]
> > > Reply To:         [log in to unmask]
> > > Sent:     Tuesday, April 06, 1999 9:39 AM
> > > To:       [log in to unmask]
> > > Subject:  [TN] test-phenom
> > >
> > > Hello Tech-Net,
> > >  I'm working with a bare PCB which is a 10 layer board  and has
> > > various signal and plane layers internally and solder mask and
> > > trace layers externally. I'm experiencing a strange phenomena
> > > during electrical test. The initial test fails as a short but when
> I
> > > put
> > > an ohm-meter to the points, the resistance slowly increases then
> > > becomes stable as if it were a capacitor charging up. when I
> > > reverse the leads it discharges and then slowly increases
> > > resistance.
> > > Has anyone ever seen this occur or possibly have an explanation
> > > or idea as to what might be happening.
> > > Thanks,
> > >
> > > Robert P. McGarry jr
> > > Technical Sales Analyst
> > > Proto Circuit Inc.
> > > 7 Ascot Pkwy.
> > > Cuyahoga Falls, OH. 44223
> > > 330-572-3400 fx. 330-572-3434
> > > [log in to unmask]
> > > http://www.protocircuit.com
> > >
> > > "QUALITY MANUFACTURED PRINTED CIRCUIT BOARDS"
> > >
> > > ################################################################
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>
>
> Robert P. McGarry jr
> Technical Sales Analyst
> Proto Circuit Inc.
> 7 Ascot Pkwy.
> Cuyahoga Falls, OH. 44223
> 330-572-3400 fx. 330-572-3434
> [log in to unmask]
> http://www.protocircuit.com
>
> "QUALITY MANUFACTURED PRINTED CIRCUIT BOARDS"
>

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