W
Winfield Hill
- Jan 1, 1970
- 0
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.
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.