Re: [SI-LIST] : Another capacitor question

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From: Ray Anderson (raymonda@radium.eng.sun.com)
Date: Wed Jan 26 2000 - 13:28:50 PST


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
> Datex-Ohmeda
> Louisville, CO

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