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

Mike Degerstrom (degerstrom.michael@mayo.edu)
Fri, 9 Jul 1999 13:09:16 -0500

Ron makes a good point in that vias are more inductive than capacitive.
I would suppose that if you had a very weak and/or slow driver one
could treat the vias as lumped capacitance. Other examples where
vias could be treated as capacitors is when they are used for test
points and do not carry any real current. We studied many MCM
technologies a few years ago and the vias were almost exclusively
capacitive when hit with a 20ps risetime TDR. Even a MCM-L
technology had capacitive vias and today's advanced pcb board
are not that much different from a four year old MCM-L process.
So I think that one would have to consider the technology and that
vias within the same technology can different capacitive/inductive
ratios depending on how many layers the signal tranversed.

Mike

On Jul 9, 9:50am, Ron Miller wrote:
> Subject: Re: [SI-LIST] : Via Capacitances ...
> 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===========
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>-- End of excerpt from Ron Miller

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