RE: [SI-LIST] : Another capacitor question

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From: Dan Swanson (dswanson@bartleyrf.com)
Date: Thu Jan 27 2000 - 04:38:53 PST


Dave,

Yes there is a difference, at least at GHz type
frequencies. American Technical Ceramics has
published some stuff on this.

http://www.atceramics.com/technotes.htm

Their parts are marked so you can tell what
the plate orientation is. Cost might be prohibitive
compared to what you buy now.

Dan

Dan Swanson EMAIL: d.swanson@ieee.org
Bartley R.F. Systems TEL: (978) 241-1091
38 Water Street FAX: (978) 388-7077
Amesbury, MA 01913

-----Original Message-----
From: David Instone [SMTP:david_instone@uk.xyratex.com]
Sent: Thursday, January 27, 2000 4:51 AM
To: si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : Another capacitor question

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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