HV breakdown is a current topic here as well. In a basic double sided
application that has a continuos 7KVAC with frequencies up to 30 MHz, is
there any advantage to using the more expensive polyimide laminate in place
of FR4?
Is there any truth to the rumor that polyimide is more 'arc tolerant'
because of it's higher Tg?
Norm Dill
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----------
From: TechNet-request
To: technet
Subject: Re: DES:FAB:ASSY: Breakdown
Date: Wednesday, August 07, 1996 8:46AM
Dom Bragge asked a question on high voltage design rules: 12 mil space 1500
V
First the caveats: The voltage you quote is definitely in the "marginal"
area.
I believe that it violate IEC guidelines. I would be less concerned if the
voltage represented a fault condition or was applied for a short time. A
continuous application of 1500 V is very risky.
That said, let me address your question. There are at least four possible
failure modes. Ranked in order of risk they are the following: dust
buildup,
surface flash-over, dielectric failure, and air breakdown. Let me comment
on
the risk from each of these
Dust build up is the most serious problem. I assume the voltage is DC (if
it
is AC there are other problems). The fields associated with a DC voltage
attract dust. (Think of the high voltage supply in a TV set). Over time
the
dust will build up and span conductors. Under high humidity conditions,
dust
absorbs moisture and creates a leakage path that can carbonize leading to
failure. We have seen this failure under field conditions. The only
solution
is to this is to maximize the separation of uncoated features and to assure
there are absolutely no pinholes in the soldermask. Pinholes are the real
risk so a double soldermask coating should be used. A conformal coating of
the
assembled board would also be a good idea.
Surface flash-over is the second serious risk. Any dielectric surface
provides a breakdown path for high voltage. The breakdown follows the
surface, so a smooth surface is riskier than a rough surface (Think of the
shape of the insulators they use on high voltage transmission lines).
Unfortunately soldermask is very smooth and 1500 volts can arc across gaps
up to 30 mils wide. This phenomena is initiated at any "triple point" where
air, dielectric and metal meet. The solution is to assure that the
conductor
is totally encapsulated. Any soldermask pinhole is a source of an arc that
will flash to the nearest uncoated land. Therefore you must assure pinhole
free soldermask.
I have little information on the long term reliability of either FR-4 or
soldermask under high voltage conditions. Voltage is an acceleration factor
in dielectric failure, but my sense is that most modern materials provide
margin and 1200 volts is not a problem. I have seen application where
standing voltages of 300 to 500 volts gave no long term problem. I would be
willing to gamble that you could get away with 1500 volts. However there is
a
risk of long term dielectric failure, particularly if the temperature or
humidity is high.
Air breakdown should not be a problem. The Paschen curve says that the
maximum
spark gap for 1500 volts is about 8.5 mils. If you assure a 12 mil space
air
gap breakdown will not be an issue.
To summarize. The two biggest issues with 1500 V are dust buildup and
surface
flash-over. The solution to both problems is to absolutely exclude
soldermask
pinholes. (The electric field easily finds even the most minute hole on a
line
edge). If you can exclude pinholes and guarantee a 12 mil space I think you
may a reasonable shot at a good system life.
Good Luck
Bob Holmes
Lucent Technologies
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------------- Begin Original Message -------------
Using 1.6mm FR4 with LPISM, we need information on how to prevent
breakdown between tracks on the outside layer separated by 12thou,
when subjected to 1500V continuously. What sort of coatings/processes
aid this?
--
Regards, (A4fonHPUX9.0onHP715,710) JNA Telecommunications
Limited ___ _____ 16 Smith Street
Dom Bragge (VK2YAK) __ / / |/ / _ | Chatswood NSW 2067
PWB Designer, (R&Ddiv) / // / / __ | AUSTRALIA
Tel: (+61 2) 9935 5792 \___/_/|_/_/ |_| email [log in to unmask]
Fax: (+61 2) 9417 3862 http://www.jna.com.au
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