Re: [SI-LIST] : Another capacitor question

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From: Ron Miller (rmiller@Brocade.COM)
Date: Thu Jan 27 2000 - 09:57:56 PST


Hi Dave

ATC has been recommending this moounting for their capacitors for
25 years for microwave use and has published s-parameters to prove
the superiority. However, their caps are nearly rectuangular cube which
is easy to rotate. Some of the flat caps may be difficult to rotate.

Ron miller

david_instone@uk.xyratex.com wrote:

> So is there anything to be gained, or lost, by mounting the capacitor so
> its 'plates' are perpendicular to the PCB instead of parallel to it.
> That way each plate gets equal connection inductance.
> (Also, if there is a difference, and the section is almost square how
> do we tell which way its layered.)
>
> Dave Instone
>
> Ray Anderson wrote:
> >
> > Our group has looked at this issue and performed some Ansoft Maxwell simulations
> > on a typical ceramic capacitor structure. We found that as the capacitor
> > structure becomes taller the current tends to crowd towards the bottom of
> > the capacitor.
> >
> > So yes, your theory matches what we've seen.
> >
> > I'll check with the guy who made the measurements for an answer to
> > your points 2 and 3.
> >
> > -Ray
> >
> > >
> > > I read a short article by Howard Johnson not too long ago that got me
> > > thinking about the current density cross section in surface mount
> > > capacitors. As we shrink the pc board layer thickness from component layer
> > > to reference layer, the height dimension of the capacitor starts to look
> > > very significant. I would expect that a .020" to .040" thick cap over a
> > > 0.005 epoxy glass layer would show significant variation over it's vertical
> > > profile, but I have no tools for modeling this.
> > >
> > > So I have questions for this group:
> > >
> > > - Has anyone ever tried to model this and am I totally out of my mind?
> > >
> > > - Is the effect significant enough to warrant finding a shorter but wider
> > > capacitor for a given value?
> > >
> > > - Does the mutual inductance of the metalization layers in the capacitor
> > > come into play, or can it be treated as a uniform dielectric block for
> > > modeling.
> > >
> > > This is probably all very silly and trivial, but I'm really interested in
> > > the answer. Thanks!
> > >
> > > Brent DeWitt
>
> --
> Regards
>
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--
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017
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