You are probably suffering from a variation of the Debye-Falkenhagen
(DF) effect. Epoxy contains a small amount of hygroscopic ionic species,
notably sodium chloride, as a result of prepolymerisation reaction with
epichlorohydrin. As a result, moisture absorption is inevitable. You
have an AC voltage of 3500 V peak or 7000 V peak-to-peak at 500 kHz. You
therefore have changes of voltage gradient of 23 kV/mm in as short a
time as 1 µs (one half-cycle). As the humid ionic species are polar,
they constantly try to orient themselves to this changing electric
field. This requires energy, which has to come from the field. A current
therefore flows in phase with the voltage (as opposed to the quadrature
current due to the capacitive effect at 90°). Because this latter
current is out of phase, it produces little heating effect (the product
of V x I is zero, assuming a lossless dielectric). However, the DF
current is in phase and therefore causes heat dissipation, which could
be sufficient locally to create the conditions for breakdown.

Brian

Bill Higinbotham wrote:

> 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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