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

Fred Balistreri (fred@apsimtech.com)
Fri, 09 Jul 1999 14:10:23 -0700

Dr. Edward P. Sayre wrote:
>
> 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
> >rmiller@brocade.com, rbmiller@sjm.infi.net
> >
> >
> >
> >
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>
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
> | NORTH EAST SYSTEMS ASSOCIATES, INC. |
> | ------------------------------------- |
> | "High Performance Engineering & Design" |
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
> | Dr. Ed Sayre e-mail: esayre@nesa.com |
> | NESA, Inc. http://www.nesa.com/ |
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I'm not sure what people mean when they say its capacitive or inductive.
A via consist of capacitance, inductance and resistance. It is true
that for normal PCB designs a via would have a lower impedance profile
than a 50-80 ohm PCB trace when subjected to a TDR pulse. Because of
the sqrt l/c equation one may conclude that the ratio of capacitance
to inductance is higher on a via than a trace. This may be true in
most designs but is by no means a rule. The physical dimension of the
via and the proximity to gnd/pwr planes will determine this ratio.
As mentioned previously, the size of the cutout in the planes when the
via passes through them also has a large impact on capacitance. There
are 2d solvers that do a respectable job of calculating a via model.
We currently are seeing some designs with microvias. Those type of vias
tend to have a ratio that may lend them to be "inductive" when compared
to PCB traces. So I can't believe any of the arguments I've heard so
far. A measurement is only good for the structure you are measuring.
If the structure changes one cannot apply the same rules without first
obtaining more data points. Its always a good sanity check to see if
the measurement results tend to agree with theory as well. High
frequency measurements of this type are not the easiest to make.

Best Regards,

-- 
Fred Balistreri
fred@apsimtech.com

http://www.apsimtech.com

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