[SI-LIST] : Re: [SI-LIST]: Parallel Plate Capacitance for Bypass (air breakdown)

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From: Charles R. Patton (crpatton@pe.net)
Date: Fri Mar 31 2000 - 15:23:56 PST


Chris Padilla wrote in part:

> .... Doing some
> quick unit manipulations, I get that air's strength is about 75 V/mil.
> So 5x that is about 375 V/mil. FR4 is at least 10 times that and most
> likely more....

I would like to make a few comments with regards to air breakdown.
Pasha=
l=92s law dominates the low voltage breakdown and basically says you
can=92=
t breakdown lower than about 375 volts and that would be at
about at 1/2 mill separation. The voltage would start to go up again as
=
the separation decreases and increases . Pashal=92s curve hits the
minim=
um voltage where the product of the separation in cm and
the gas pressure in mm is about equal to one. So with close separations
=
the voltage climbs again to near 1 KV. (These rules change drastically
i=
f the ambient is not nitrogen, but instead some other gas
such as helium.) Several times in past lives I used close, uncovered
(by=
 solder mask) traces to form low cost spark gaps to protect I/O lines
int=
o IC=92s. With 8 mill separations, the breakdowns were in
the 750 volt range. That having been said, be aware that continuous
volt=
ages have other problems -- mainly that the CO2 and moisture in the air
f=
orm carbonic acid on the surface of the board, which
under the high voltage stress, will lead to leakage paths and worst on
th=
e board. This, in part, is why leakage path standards for PCB =91s are
s=
o much large than just breakdown would indicate.

Charles R. Patton, Owner
Synergy Co.
=93Creative, Cost Effective, EMI/ESD solutions=94
21490 Camino Arriba
Murrieta, CA 92562
Phone: 909-698-9657
Fax: 909-698-0224
Email: charles.r.patton@ieee.org

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