pcb material voltage breakdown
FR4 material standards show an electric strength (breakdown voltage) around 40 kV/mm, which is quite a lot, I think. Tracking resistance is another question, it may be particularly a problem with high frequency high voltage gradients at the surface, but this common to most organic isolators. Protective coating of high voltage circuits can reduce the danger of partial discharges slowly destroying the isolators surface.
A lot of high voltage circuits have been manufactured on FR4, I think. With fast pulses, the geometry has to be designed in a way, that field strengths are kept in a safe range also with transients and at conductor edges. Verifying, that the PCB is free from partial discharges during operation may be regarded as an indicator to my opinion.
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