Re: [SI-LIST] : Via Capacitances ...

Dr. Edward P. Sayre (esayre@nesa.com)
Fri, 09 Jul 1999 14:48:55 -0400

Folks:

It may not be convenient and may stress your personnel or financial
assests, but this is why you make SI measurements to confirm your results.
Almost anybody who has made the measurement knows that vias for whatever
reason turn out to be capacitive. If one checks with Dr. Johnson's book,
you will find that vias are modeled by both capacitors and inductorance.
By both measurement and SPICE simulation, NESA has shown this to be true.
(See papers on our web site.)

(The next sentence should be read in the context that I was one of the
original students involved in the development of the Method of Moments
field simulators and know a lot about old and new simulation software.)
There is ample evidence to show that many field simulator derived SPICE
models (especially those from 2-D simulators) are often unreliable with
respect to the component values but usually have the correct topology.
That is to say, if you make a measurement and then adjust the SPICE
component values and re-simulating one or twice, you can get satisfactory
comparisons with the measurements.

Otherwise, there is no way to verify the correctness of field solver
derived SPICE models for interconnect physical features like vias or
connectors.

ed sayre
============
At 09:50 AM 7/9/99 -0700, you wrote:
>Doug
>
>I modeled a via some time back with ground planes and
>clearance holes through it. I had intended to use the capacitance
>to make up for an inductance and needed 300 ff.
>
>I used an OEA field solver Metal and to my amazement there was
>hardly any capacitance. I surmised that this was due to the small
>area of the ground planes in tha horizontal direction. Since I needed
>capacitance I added some big fat ground vias all around the signal
>via, and got the 300 ff.
>
>Also, HP ADS which is the cadillac of RF simulators models vias
>as inductor, as does Touchstone before EEsof got swallowed up by
>HP.
>
>Ron
>
>Douglas McKean wrote:
>
>> Could someone give rough estimate
>> of via capacitance?
>>
>> I'm thinking the orientation of the traces
>> connected to the via have a major impact.
>>
>> For instance, an .062 board with a via
>> from one side to the other, different
>> capacitances would be had with the two
>> following constructions ...
>>
>> I. Top Trace
>> -------------
>> |via
>> -------------
>> Bottom Trace
>>
>> II. Top Trace
>> -------------
>> |via
>> -------------
>> Bottom Trace
>>
>> Ideas? Comments?
>>
>> Regards, Doug McKean
>>
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>--
>Ronald B. Miller _\\|//_ Signal Integrity Engineer
>(408)487-8017 (' 0-0 ') fax(408)487-8017
> ==========0000-(_)0000===========
>Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA 95131
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>
>
>
>
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