RE: [SI-LIST] : Parallel Plate Capacitance for Bypass

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From: S. Weir (weirsp@a.crl.com)
Date: Mon Mar 27 2000 - 12:01:18 PST


Ohms???

3-4 mils is quite common in high layer count boards. 4 seems to be no
charge, 3 has a somewhat lower yield, but not bad at the better houses.

Regards,

Steve.
At 02:23 PM 3/27/2000 -0500, you wrote:
>Coupla things.......
>
>You appear to be off by a factor of 2 somewhere. Plane
>capacitance is defined as
>
> Cplane = (0.225*Er*A)/d
>
>for a 2 mil core, with A being 1 square inch, this becomes
>
> Cplane = 0.225*4.1/.002 = roughly 460 ohms
>
>With standard FR4 you can run into a couple of problems
>that you might want to check on. Thin core materials can
>prove to be quite costly and i have heard that during laminating
>that all the resin can be squeezed out creating air pockets.
>Therefore, before going with a standard material, you might
>want to consider the benefit to cost tradeoffs of materials that
>are targetted for this purpose such as Emcap or BC2000.
>I found good information on these materials on the Hadco and
>Merix web sites.
>
>I would also recommend running a frequency simulation on
>your PDS to make sure that you don't have frequency regions
>that have very high impedances. Suffice it to say that bad
>things will occur in your system if this occurs. There are a
>number of good resources (most notably and referenced,
>those by Larry Smith et al.) available on the web to help
>you with your design. You might also want to check out
>Istvan Novaks website as he has a great paper and utility
>for general download. He can be found at:
>
> home.att.net/~istvan.novak/
>
>Best of Luck,
>
>Michael Greim
>
> > -----Original Message-----
> > From: Ron Miller [SMTP:rmiller@Brocade.COM]
> > Sent: Monday, March 27, 2000 1:39 PM
> > To: si-list@silab.eng.sun.com
> > Subject: Re: [SI-LIST] : Parallel Plate Capacitance for Bypass
> >
> > Thanks Steve, I knew it was somewhere around there.
> >
> > So, by extension, 2.2 mil dielectric, available at very little if any
> > extra cost should be about 1000pf/in sq for FR4
> > and eliminatee the need for discrete capacitors below 1000 pf.
> >
> > ron
> >
> > "S. Weir" wrote:
> >
> > Ron,
> >
> > 5 mils should yield about 200pF / in sq, before taking away for
> > vias.
> >
> > Regards,
> >
> >
> > Steve.
> > At 02:30 PM 3/24/2000 -0800, you wrote:
> >
> > Chris
> >
> > Go to the AVX webpage and the American Technical Ceramics
> > web
> > pages. They have S-parameters which show that the smaller
> > profile
> > ceramics are good to above 1 Ghz.
> >
> > Small profile usually means smaller capacitance so you may
> > need
> > a small one of .01 uf and a larger one of about .47 uf.
> >
> > For frequencys 3 Ghz and up use a capacitive layer in your
> > board. Recently I heard that with 5 mil FR4 dielectric you
> > get about
> > 100pf per square inch. So, using a 2 mil layer would give
> > about 7.5 X
> > this value(per square inch).
> >
> > Ron Miller
> >
> >
> >
> >
> > According to specifications, X7R type chip
> > capacitors used for power subsystem bypass seem to become ineffective
> > above 150 to 200 Mhz. For frequencies higher up, you are reliant upon the
> > internal parallel plate capacitance. I have two questions. 1. Is the
> > calculation for the required capacitance that is needed for the parallel
> > plate capacitance the same as that used for a discrete chip capacitor
> > bypass network, or is there a conservation of charge situation where the
> > real answer is Cpp*dVnoise = Cload*dVchange? (where Cpp = parallel plate
> > capacitance, dVnoise = tolerable change in power supply voltage, Cload =
> > sum of load capacitance being switched simultaneously, dVchange = voltage
> > change through driver output switch). 2. If you have a power plane
> > sandwiched between 2 ground planes, does the parallel plate capacitance
> > double for a given area?Thank you,Chris HansenSr. Design EngineerDPT &
> > Adaptec Companies
> >
> > --
> > Ronald B. Miller _\\|//_ Signal Integrity Engineer
> > (408)487-8017 (' 0-0 ')
> > fax(408)487-8017
> > ==========0000-(_)0000===========
> > Brocade Communications Systems, 1901 Guadalupe Parkway, San
> > Jose,
> > CA 95131
> > rmiller@brocade.com, rbmiller@sjm.infi.net
> >
> >
> > --
> > Ronald B. Miller _\\|//_ Signal Integrity Engineer
> > (408)487-8017 (' 0-0 ') fax(408)487-8017
> > ==========0000-(_)0000===========
> > Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA
> > 95131
> > rmiller@brocade.com, rbmiller@sjm.infi.net
> >
>
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