I am looking for information on causes and design guidelines for using
thin FR4 between DC voltage potentials. I have a couple of data points
that FR4 can have punch through and arcing at voltages far below the
dielectric breakdown specified for the materials.

1.) 48 Vdc punched through 4-5 mils of FR4 within 6 months continual
usage
2.) 35 Vdc punched through 0.4 mils of FR4 in about 630-650 hours
3.) FR4 type was not specified, but these are credible data points.

This (in part) appears to be dielectric erosion exacerbated by
Electro-migration of the copper traces/planes. More information would
be great.

I am concerned by the data points because I am looking at ~125 Vdc on 4
mil thick FR4 and the product is expected to last 24x7x365.25x10
(613620 hours) minimum.

If any one could help, there doesn't seem to be a lot of information
available. Thanks.