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I'd like to mount a heat sink on a Sharp S202S02 SSR. Here's a drawing the indicates that the back surface of the device might be connected to AC:

enter image description here

In the "Design Guide" section of the datasheet it is recommended to add insulation between the device and the heat sink. That's quite clear. However, I still don't know if a metal screw used to mount the heat sink could still lead to a short between the back surface and the heat sink.

Does mounting a heat sink with a metal screw cause a short between AC and the heat sink, even if I add sheet insulation between the device and the heat sink?

I can of course use a nylon (or other plastic) screw, but it will get as warm as the SSR and degrade over time. Another option is to use a smaller screw and add a nylon sleeve, although the SSR's mounting hole is made for an M3 screw.

Here are photos of the real device. The first one shows the front, including the small area that is common to pin 1:

enter image description here

And here's the heat sink side. No metal to be seen:

enter image description here

It's all plastic, except those small rectangles at the front.

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This package appears to have insulation around the mounting hole built in, so a metal M3 bolt should be acceptable without a separate bush. To confirm this, look at an actual part to verify that there is a sleeve of insulation around the bolt hole on the heat sink side; the data sheet drawings do not explicitly show that. However while they explicitly recommend an insulation sheet, they make no mention of a bush, so I believe they are designed for use without one.

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  • \$\begingroup\$ It's a bit confusing. The whole outside is epoxy, including the bolt hole's wall and the heat sink side. Only the small rectangular area at the tip of the "Common to pin 1" arrows is metal (and indeed zero resistance to pin 1). If I didn't have the datasheet, I wouldn't think that insulation is needed at all. \$\endgroup\$ – Christoph Feb 19 '13 at 8:15
  • \$\begingroup\$ It's that small rectangular area that needs insulation from the heatsink (unless you are happy to float the heatsink at high voltage : not recommended! So a thin insulating sheet (and thermal grease, if the sheet requires it) is all you need. \$\endgroup\$ – Brian Drummond Feb 19 '13 at 8:50
  • \$\begingroup\$ That small area is not on the heat sink side, though. I'll make a photo or two later (but I'll add insulation anyway, don't worry). \$\endgroup\$ – Christoph Feb 19 '13 at 9:05
  • \$\begingroup\$ @Christoph There are insulating shoulder washers for TO-220 which are used to insulate screws. If the hole fits M3 screw, perhaps one such washer could be used to insulate the screw as well. Search for TO-220 insulation kit and you'll probably find numerous examples. \$\endgroup\$ – AndrejaKo Feb 19 '13 at 22:03
  • \$\begingroup\$ I found some, but they wouldn't fit in the gap between an M3 screw and a 3,2 mm hole. \$\endgroup\$ – Christoph Feb 20 '13 at 15:25

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