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Common through-hole power semiconductor devices, like TO-220 and TO-247, can be mounted to a heatsink by running a screw through the hole in the device. However, on TO-220 the hole is often electrically connected to one pins of the device. TO-247 often isolates the hole from the device backplate, but this isolation is not guaranteed, and often fails below 1500VDC. I frequently need 3kV withstands.

We often use clips to hold the device to the sink, which can help the problem, but makes construction more problematic, and may still have insulation problems. Shoulder washers are also used occasionally, but again, that makes construction more complex. Life would be simpler if we could use insulated screws. The only ones I'm aware of are nylon, and good luck getting any torque out of those!

Are there any screws (M3, M4, #6, #8, or similar) which are made of an insulating material, but which still have sufficient mechanical strength to hold a device to a heatsink? Especially in high-vibration environments?

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    \$\begingroup\$ I build a fair few PCB's with TO-220's screwed to a heat-sink with nylon screws, these go into vehicles that are used heavily off-road (noise, vibration, heat, impacts) and have not seen one fail yet. Unless you have seen otherwise, I would question the requirement for more clamping than these can provide. \$\endgroup\$ – John U Jun 7 '13 at 14:07
  • \$\begingroup\$ Interesting! Do you use any particular washers? \$\endgroup\$ – Stephen Collings Jun 10 '13 at 14:34
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    \$\begingroup\$ Nope, standard nylon screws + nuts. Boards are conformal coated (aerosol) which helps stop stuff moving / unscrewing but it's hardly set in stone. \$\endgroup\$ – John U Jun 11 '13 at 9:57
  • \$\begingroup\$ There are adhesive tapes, too. Acrylic heat sink adhesive tape. \$\endgroup\$ – mkeith May 11 '18 at 0:26
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To insulate transistors from a mounting plate, I typically use metal screws with an insulating pad as I describe in my answer here: how to electrically isolate a PCB from a heat sink.

But if you really want to use non-conductive screws, have a look at fiberglass reinforced plastic ones such as Isoplast. I haven't used them personally and they don't make any specific claims as to shear strength, but they look a lot stronger than nylon.

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You can get isolating bushes in mounting kits: Infineon app note

So you use a metal screw, but a plastic collar goes around it, isolating the screw from the device.

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There appear to be a few materials on the market for threaded fasteners that could indeed have advantageous properties over nylon in some applications:

  • FR4 (the stuff tough circuit boards are made of).

  • Polyetherimide (ULTEM)

  • Polyetheretherketone (PEEK)

  • Various fibre-reinforced composite materials, based on various plastics (eg Polyarylamide, Polyurethane)

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