Re: [SI-LIST] : Trace Capacitance

Dr. John L. Prince, III (prince@ece.arizona.edu)
Mon, 21 Jun 1999 15:25:17 -0700 (MST)

>I hope someone can explain what is happening with the following capacitance
>measurement. A friend asks if capacitance is directly proportional to
>surface area, why doesn't his measurement scale linearly. On a pcb made
>specifically for trace characteristics measurement, he measures
>1) 10 pf for a 10 cm long trace that is 5 mils (.12 mm) wide
>2) 25 pf for a 10 cm long trace that is 20 mils (.50 mm) wide
>3) Both traces are far apart from each other and from other components.
>4) Both traces are over a solid ground plane.
>5) Equipment is HP 4285A Precision LCR meter set at 1 MHz
>
>Thanks,
>
>Farrokh Mottahedin
>Quantum Corporation
>(408) 324-7934
>farrokh.mottahedin@quantum.com
>
>
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It is likely that there are two things at work here, spreading of the
e-field lines under the trace and lines originating on sidewalls and the
top. The matter of the contribution from the sidewalls was discussed well
by someone else. The spreading of the lines "under" the trace is due to
the fact that the two-d field distribution is only approximated well by a
one-d field distribution (and the associated parallel plate capacitance
equation is only very accurate) for the case where the width-to-separation
ratio for the trace-ground plane combo is in excess of 20 (for a 5% max
error). So, our old, tried and true parallel plate capacitance equation
can badly mislead us even for infinitesimally thin conductors, if the w/h
ratio is not large enough (a factor of 10, my favorite factor, is not good
enough). I had a paper in 1987 in the CPMT Transactions (CHMT in those
days) that showed this, I think.

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