[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.
l=92s law dominates the low voltage breakdown and basically says you
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
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
f the ambient is not nitrogen, but instead some other gas
such as helium.) Several times in past lives I used close, uncovered
 solder mask) traces to form low cost spark gaps to protect I/O lines
o IC=92s. With 8 mill separations, the breakdowns were in
the 750 volt range. That having been said, be aware that continuous
ages have other problems -- mainly that the CO2 and moisture in the air
orm carbonic acid on the surface of the board, which
under the high voltage stress, will lead to leakage paths and worst on
e board. This, in part, is why leakage path standards for PCB =91s are
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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