# Re: [SI-LIST] : 回复: [SI-LIST] : Parallel Plate Capacitance for Bypass

From: e ([email protected])
Date: Wed Apr 12 2000 - 02:12:36 PDT

Lee,

This is pretty useful info. Do you know where I can get other info on FR-4 and other board dialectric materials compiled together in one reference?

TIA
E

Ritchey Lee wrote:

> FR-4 class materials have a breakdown voltage rating in excess of 1000 Volts per
> mil of thickness. This would say that your 5 mil dielectric will withstand at
> least 5000 Volts.
>
> Lee
>
> rachild.chen wrote:
>
> > Who can tell me how much the break voltage of 5 mils FR4 is?We have do a
> > test.It will be break under 1500V.
> > 5 mils is the littlest thickness of FR4.How much is it to support a
> > 1500V break voltage?
> >
> > Regards,
> >
> > Rachild
> >
> > -----Original Message-----
> > 发件人: Jerry Johnson <[email protected]>
> > 收件人: [email protected] <[email protected]>
> > 日期: 2000年3月30日 9:55
> > 主题: Re: [SI-LIST] : Parallel Plate Capacitance for Bypass
> >
> > >Ron,
> > >
> > >Could be we're missing each other somehow.
> > >
> > >What I saw was Steve say that 5 mils of FR4 would yield ~200 pF/square
> > >inch.
> > >
> > >Then I thought you said "So, by extension, 2.2 mil dielectric. . .
> > >should be
> > >about 1000pf/in sq for FR4 . . ."
> > >
> > >That looked to me like you had taken [(5^2)/(2.2)^2] * 200 = 1033
> > >pF/square inch.
> > >
> > >I commented that capacitance wasn't an inverse square relationship with
> > >distance
> > >between plates. I calculated more like [5/2.2] * 200 = 455 pF/square
> > >inch.
> > >
> > >Perhaps I completely misunderstood your reasoning. If so, I'm sorry.
> > >
> > >Jerry Johnson
> > >
> > >Ron Miller wrote:
> > >>
> > >> Jerry
> > >>
> > >> maybe I am missing something or maybe we are not communicating
> > >> well. The capacitance I am talking about would be an IC or a discrete
> > >>
> > >> device connected to power and ground. The capacitance would be
> > >> somewhat limited by the quarterwave distance from that pin for any
> > >> frequency of intrest, because above that distance the delay is too
> > >> great for the capacitance to be of any effect.
> > >>
> > >> So, the 1 inch concern would roughly describe a circle with a 1/2 inch
> > >>
> > >> radius, which for FR4(velocity 170 ps/inch) or 85 ps.
> > >>
> > >> The frequency which has a quarterwavelength of 85 ps is 1/340ps=~3Ghz
> > >> At 3 Ghz and below we get 200 pf per square inch, or about .27 ohms.
> > >>
> > >> Scaling from 3 Ghz, down in frequency we get higher Xc, but the
> > >> effective
> > >> capacitive area goes up. The limits are that several devices will be
> > >> sharing
> > >> the same capacitance and the board has dimensional limits so we must
> > >> add capacitances. Scaling up in frequency, the effective capacitance
> > >> goes
> > >> down but so does the Xc.
> > >>
> > >> TRACES across a plane is an entirely different matter, and PLANE
> > >> capacitance for the entire board is again a different matter.
> > >>
> > >> BOTTOM LINE:
> > >> Capacitors below 200 pf in value are of little value for bypassing
> > >> if you have 5 mil dielectric spacing power to ground.
> > >>
> > >> Ron Miller
> > >>
> > >>
> > >> Gerald Johnson wrote:
> > >>
> > >> > Ron,
> > >> >
> > >> > Capacitance of planes is NOT a function of inverse square of
> > >> > distance.
> > >> > See thread in this list about a week ago. Well covered there.
> > >> >
> > >> > Valid to use inverse square distance for point source, not for
> > >> > planes.
> > >> >
> > >> > Result, capacitance would be about (5/2.2)*200 = 454.6 pF if the
> > >> > 100 pF per in^2 at 5 mils is correct.
> > >> >
> > >> > Jerry Johnson
> > >> > Teradyne Inc.
> > >> >
> > >> > >Date: Mon, 27 Mar 2000 10:39:10 -0800
> > >> > >From: Ron Miller <[email protected]>
> > >> > >To: [email protected]
> > >> > >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:
> > >> > >>
> > >> >
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> > >> --
> > >> 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
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> > >>
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> >
> > ------------------------------------------------------------------------
> >
> > Chen Rachild <[email protected]>
> > Huawei Tech. Ltd
> > SI Depart.
> >
> > Chen Rachild
> > Huawei Tech. Ltd <[email protected]>
> > SI Depart.
> > Shenzhen
> > 518057
> > China
> > Version 2.1
> > Last Name Chen
> > First NameRachild
> > Label WorkShenzhen 518057 China
> > Revision 20000331T034345Z
>
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