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

From: rachild.chen ([email protected])
Date: Thu Mar 30 2000 - 19:43:45 PST

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-----

>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
>> >
>> > >Date: Mon, 27 Mar 2000 10:39:10 -0800
>> > >From: Ron Miller <[email protected]>
>> > >To: [email protected]un.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:
>> > >>
>> >
>> > **** To unsubscribe from si-list or si-list-digest: send e-mail to
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>>
>> --
>> Ronald B. Miller _\\|//_ Signal Integrity Engineer
>> (408)487-8017 (' 0-0 ') fax(408)487-8017
>> ==========0000-(_)0000===========
95131
>> [email protected], [email protected]
>>
>>
>
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