What would a ballpark insulation voltage (HI-POT) rating be for a TO-220 package (with a "hot" tab) mounted to a grounded heat-sink with SIL pad and shoulder washer?

I am not seeing much documentation about this. I want to replace a TO-220 "FP" insulated package with a non-insulated package + SIL pad + shoulder washer if possible.

I found this paper, and at the bottom of the tenth page it reads, "The breakdown voltage [of a SIL pad] is 3500-4500 Vac". It references a Bergquist brand Sil-Pad® 400 product datasheet.

Perhaps the electrical insulation resistance of the pad itself is 4 kV, but I highly doubt the whole assembly would be rated for that. When compressed, even if grease were used to fill the voids around the screw, and it is kept meticulously clean.

The TO-220 device in question is a 16 A, 1200 V rated SCR, being supplied half-wave 408 V AC (~600 V DC pulses).

  • 1
    \$\begingroup\$ To increase rating on the assembly you can use an aluminum oxide insulator and TO-247 package. \$\endgroup\$
    – Jeroen3
    Apr 25, 2019 at 5:44
  • \$\begingroup\$ Just a note, on an unrelated repair unit I have finally seen one SIL pad with a hole in it, where arcing had occurred through the pad. Must've been either a defect in the pad, physical contaminant during assembly, and/or one very strong over-voltage event. Rare, but it can happen. \$\endgroup\$
    – rdtsc
    Nov 9, 2021 at 17:33

1 Answer 1


SP400-.007" is good for upto 5kV Hipot.

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Farnell Ref


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