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

From: Ritchey Lee ([email protected])
Date: Mon Apr 10 2000 - 09:13:04 PDT

FR-4 class materials have a breakdown voltage rating in excess of 1000 Vo=
lts per
mil of thickness. This would say that your 5 mil dielectric will withsta=
nd 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-----
> =B7=A2=BC=FE=C8=CB: Jerry Johnson <[email protected]>
> =CA=D5=BC=FE=C8=CB: [email protected] <[email protected]=
m>
> =C8=D5=C6=DA: 2000=C4=EA3=D4=C230=C8=D5 9:55
> =D6=F7=CC=E2: 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 =3D 1033
> >pF/square inch.
> >
> >I commented that capacitance wasn't an inverse square relationship wit=
h
> >distance
> >between plates. I calculated more like [5/2.2] * 200 =3D 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 discre=
te
> >>
> >> 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 in=
ch
> >>
> >> radius, which for FR4(velocity 170 ps/inch) or 85 ps.
> >>
> >> The frequency which has a quarterwavelength of 85 ps is 1/340ps=3D~3=
Ghz
> >> 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 b=
e
> >> sharing
> >> the same capacitance and the board has dimensional limits so we must
> >> add capacitances. Scaling up in frequency, the effective capacitan=
ce
> >> 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 =3D 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:
> >> > >>
> >> >
> >> > **** To unsubscribe from si-list or si-list-digest: send e-mail to
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BE
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> >>
> >> --
> >> Ronald B. Miller _\\|//_ Signal Integrity Engineer
> >> (408)487-8017 (' 0-0 ') fax(408)487-8017
> >> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D0000-(_)0000=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D
> >> Brocade Communications Systems, 1901 Guadalupe Parkway, San Jose, CA
> 95131
> >> [email protected], [email protected]
> >>
> >>
> >
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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
>   Additional Information:
>   Version   2.1
>   Last Name Chen
>   First NameRachild
>   Label WorkShenzhen 518057 China
>   Revision  20000331T034345Z
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