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Suitable hole for 2.9mm selftapping screw?

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MrEd

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Does anyone know the correct hole diameter for a self tapping 2.9mm screw and for 3.5mm selftapping screw for mounting in a 2.5mm thick circuitboard.
Same as for metal?

Cheers
Eddie
 

If the board is glass epoxy (i.e. FR4 or similar material), self tapping screws are not recommended. Glass epoxy boards are brittle, and the screw will simply eat the sides of the hole away after a time. You could put a dab of epoxy on the screw to permanently bond it in place - but then what's the point of using self tapping screws. Normally on FR4, or similar boards, you would use a machine screw with a self broaching nut on the opposite side of the board (such as PEM nut products: http://www.pennfast.com/fastening_products/prod_desc.php3?cat=Fasteners+for+PC+Boards ).

If the board is bonded paper or soft phenolic, you can use the same hole size recommended for metal.
 

Thanks for the advice, the board is 2.5mm thick and I will give it a try. But I don't know the correct hole size for metal either.

Cheers
Eddie
 

According to "Machinery's Handbook" 24th Edition, page 1479: Hole size in Phenol Formaldehyde for 2.9mmx1 self-threading screw is 2.64mm; the hole size for 3.5mmx1.3 screw is 3.05mm.

If the hole is in die cast zinc or aluminum: 2.9mm screw => 2.49mm hole in 2.77mm thick material; 3.5mm screw => 3.5mm hole in 3.18mm thick material.

You'll have to use an educated guess to figure out what the approximate hole size might be for FR4, since self tapping screws aren't recommended in that material.
 

    M

    MrEd

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Great, now I know the "official" hole sizes. I'm going to get some sample board pieces from the circuit board manufacturer to check if I need to adjust the hole diameter for FR4 before I make them. I'll post my conclusions.

Cheers
Eddie
 

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