Re: [SI-LIST] : Decoupling caps selection

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From: Michael Nudelman (mnudelman@tellium.com)
Date: Mon Oct 30 2000 - 12:28:43 PST


I saw it....once. It's not that frequent. And, well - one has to be carefull who
he is choosing as an assembly house.
Mike.

Stefan Ludwig wrote:

> Hi,
>
> thanks for this interesting thread.
>
> What's the word on manufacturability of 0508 (short-wide) vs. 0805 caps
> (long-narrow). I heard a horror story from 10 years ago about solder splashes
> causing shorts between the terminals. Is this still an issue in today's
> manufacturing processes?
>
> Thanks,
>
> Stefan Ludwig
> SiByte, Inc.
>
> Thomas Jackson wrote:
>
> > The "Impedance vs Frequency" data that the capacitor manufacturers give is
> > just that. It shows at what frequencies their parts cease being capacitive
> > and start becoming inductive. Note that for most common electrolytic
> > capacitors, that is in the ones to tens of MHz range even though current
> > clock rates go into the 100s of MHz and frequency content of the edge rates
> > go into the GHz.
> >
> > This is why short-wide capacitor packages like 0508 are more desirable than
> > the common narrow-tall packages like 0805. The short-wide capacitors stay
> > capacitive up to about twice the frequency as the comparable narrow-tall
> > equivalent.
> >
> > The idea is to minimize inductance over the frequencies of interest (up to
> > the edge rate "knee" frequency). One can also minimize the inductance by
> > minimizing the current loop area from the power plane to the capacitor and
> > back to the ground plane. Again, one can get about a factor of 2
> > improvement by the correct layout.
> >
> > Edge rates are, primarily, functions of the devices being used. While the
> > semiconductor manufacturers, generally, don't want to add it to their specs
> > and, therefore, test for it, general edge rate information is available in
> > the SI literature for common logic families and it is buried in device IBIS
> > models such that simulators can use it. I have not had any luck calculating
> > edge rates from IBIS models by hand.
> >
> > I hope this helps.
> >
> > Thomas L. Jackson, P.E.
> > Staff Product Development Engineer
> > Network Access Development
> > Systems Solutions Group
> > FUJITSU MICROELECTRONICS, INC.
> > 3545 North First Street
> > San Jose, CA 95134-1804
> > telephone: (408) 922-9574
> > facsimile: (408) 922-9618
> > http://www.fujitsumicro.com
> >
> > -----Original Message-----
> > From: Dilpreet Saini [mailto:DSAINI@altera.com]
> > Sent: Friday, October 27, 2000 3:15 PM
> > To: SI List (E-mail)
> > Subject: [SI-LIST] : Decoupling caps selection
> >
> > Hi all,
> > I was wondering if the selection of the decoupling capacitor of a
> > circuit is MAINLY governed by the edge rates of the switching signals or by
> > the frequency of their switching. I believe the answer is later cause all
> > the documentation that I have seen on the dcoupling cap selection so far do
> > not have any mention of the edge rates. Most of the capacitor manufacturer
> > have "Impedance Vs Frequency" charts in their specs/catalogs. I am assuming
> > that the Frequency that they are referring to is the frequency of operation
> > of the circuit. Any ideas/comments are greatly appreciated.
> >
> > Dilpreet
> >
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