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August 2004

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Subject:
From:
Ahne Oosterhof <[log in to unmask]>
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Date:
Tue, 24 Aug 2004 12:14:05 -0700
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FR4 material is not homogenous, it (more or less) consists of epoxy, glass
fibers, gas pockets and contaminants. That means that between copper layers
it looks like a number of different capacitors in series. Capacitors with
different dielectric constants resulting in different fields across each.
And that means one of these capacitors may be the beginning of the
breakdown.
Also, I expect that the 740V/mil is for DC, while you are applying AC. I
would start with applying a variable DC voltage to one conductor and connect
the other one through a resistor to ground. Look across that resistor with a
scope and see when corona starts to occur as you increase the DC voltage
(noise on the scope trace). Once you have corona, breakdown will follow,
either quickly or after a longer time after the ozone eats away the epoxy.
But at least the test will tell you whether the 750V/mil is real.
To do the same with AC you will need to either filter the 500kHz out, or use
a microphone that is sensitive well beyond our hearing and "listen" for
corona (feed the microphone signal to the scope). Once upon a time HP made
an HF converter that did exactly that, it listened to high frequency noise
and down-converted it so you could hear it. I did use it to solve your kind
of problem.

Sorry for not being able to give you rules to live and design by.

Ahne.



-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bill Higinbotham
Sent: Tuesday, August 24, 2004 9:25
To: [log in to unmask]
Subject: [TN] 500 KHz breakdown of FR4

We are using a multilayer FR4 board with 12 mils between layers and getting
breakdown between traces on different layers.  The voltage is about 3500 V
peak.  THis board should be good for 740 V/mil x 12 or 9 KV.  I wonder if it
is excessive leakage due to moisture absorption?

Bill

Bill Higinbotham
RFL Electronics Inc.
E-mail : [log in to unmask]
Voice: 973.334.3100 Ext. 386
Fax: 973.334.3863

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