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?
Best Answer
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.