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June 2002

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Subject:
From:
"Ingemar Hernefjord (EMW)" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 26 Jun 2002 16:53:08 +0200
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Interesting, you dare use FR-4 for such voltage (and perhaps high current possibility)? We use Teflon and other materials (that absorb little water) when doing with such levels. Thought wall thickness wasn't most important, but creep distance across.
Regards
Ingemar Hernefjord
Ericsson Microwave Systems

-----Original Message-----
From: Ted J. Casper [mailto:[log in to unmask]]
Sent: den 26 juni 2002 15:13
To: [log in to unmask]
Subject: [TN] Voltage Breakdown of Thick Laminates


I am looking for help in determining how to specify the thickness of a
layer that must handle 7kV.  For discussion purposes lets say the material
is FR4 with a dielectric strength of 750V/mil.  (NOTE: mil=0.001 inches) I
also realize that this dielectric strength number is thickness dependent.
For other types of insulators I have used a square root relationship to
calculate actual voltage breakdown.  For instance:

If I have a piece of Teflon with a dielectric strength of 1000V per mil
measured at 5mil, the breakdown for a 20mil thick piece would be
approximately 1000V/mil * 20mil /[(20mil/5mil)^.5] = 10000V.

Now for my questions:

1) Is there a standard thickness that is used to test dielectric strength
of PCB materials?

2) Does the square root of thickness function apply to PCB material?

3) If not, how would I estimate how much voltage could be applied from one
side to the opposite of a 20mil thick laminate (FR-4)?

I have been reading the postings for voltage breakdown and dielectric
strength that are in these forums.  One posting (actually written in two
parts) by Ralph Hersey was very interesting but cautioned about using thin
single sheet laminates.  Does this caution apply to thicker laminates?

Thank you!

Ted Casper
LECO Corporation
Phone 262-338-2191
e-mail: [log in to unmask]

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