Re: [SI-LIST] : Another capacitor question

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From: Ray Anderson (raymonda@radium.eng.sun.com)
Date: Thu Jan 27 2000 - 10:33:32 PST


There is a definite change in electrical characteristics that is a function
of whether the part's internal plates are parallel to or perpendicular to the
board's ground plane.

I believe ATC has some curves or charts in their catalog that quantify the effect.
The ATC parts are very nearly cubical and thus can be mounted in either orientation.
You need to refer to the laser markings on the part to identify which way is which.

Typical ceramic SMD capacitors are more rectangular and thus don't lend themselves
very well to mounting in the rotated position.

This effect was very noticeable in designs I have done that involved RF and microwave
filters and amplifier matching networks. I don't think the effect is of large enough
magnitude to really make or break an SI decoupling scheme. However, they may be
applications in the SI world where it does make a difference. Caveat Emptor.....

If I remember correctly the main effect was in a bit of extra stray capacitance
from the plates to ground, though I suppose a minor change in inductance due the
current trying to crowd towards the return path may play have played into it as
well.

-Ray Anderson

>
> 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

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