After experiencing a PCB high-voltage meltdown, I am driven to ask for advice from experienced folks here on s.e.d. s/n 19 of my new +/-1100-volt amplifier suffered a severe insulation breakdown between two BNC output-connector-mounting holes and pads, spaced only 0.10-inch apart. A carbon path is now clearly visible on the bottom of the PCB. The HV breakdown is so severe that after being exposed to 500V, 3mA pumped into the output can't raise the voltage more than 100 volts (implying a 33k short).
It's possible the carbon pathway first began forming in an uncleaned solder-flux region between the two pads. The carbonization eats well into the interior of the PCB, obscuring evidence of the initial path.
The guilty pc-mounting BNC connector (not my choice, BTW), with its two holes and pads, is not being used. The high-voltage hole is needed for an output wire. The guilty ground hole 0.1" next to it was also used, but the ground connection can be made elsewhere.
My solution for now is to completely drill out the dangerous ground hole, leaving only surface conduction to a ground plane 0.1" away. (The rest of the board has healthy clearances for the up to 2.5kV voltages seen, but sadly I didn't give the PCB design of the output-connector region, with its 1.1kV potential, the same attention.)
I wonder if we can count on about 0.1" of PCB insulation to maintain a low-leakage (over 500M) over time. Perhaps we should spray or coat this region of the PCB with a conformal-coating sealant, after a careful cleaning.
We have made 160 of these rather complex amplifiers, and now we're seeking advice for a robust, but we hope not too painful, solution to this problem.