Jeff ,
For 1oz copper one sheet of prepreg .0025" is not enough.
During lamination of the multilayer the prepreg flows around the
conductors. So you have less than .0025" between the conductors
of the inner layers. In this case you can also have shortcuts.
Two prepregs are recommended.
The dielectric breakdown voltage of FR4 is about 790Volts per mil.
Nominal value regarding MIL-S-13949H is 750V/mil.
Simon
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