Electronics > PCB/EDA/CAD

Screw Mounting Holes - How far from board edge?

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HLA-27b:
Some boards, especially those with rounded corners have very little between the hole and the edge of the board, making it easy to break.
Others have too much space and waste board space.
How far should a mounting hole (say M3) be from the board edge for good mechanical sturdiness while not wasting board space?

Is there an established standard or rule of thumb?

This board for example has at least 3,4mm of material between the board edge and the hole.
http://www.elpa.it/eng/rd77gb.html


Arduino on the other hand seems to have too little, note the two holes on the right.

ejeffrey:
I typically use 1 screw diameter clearance.  So for an M3 screw I would have a 3 mm gap between the edge of the hole and the board.  The head size is around 6 mm, so you would have 1.5 mm clearance from the screw head to the edge of the board.  I would suggest as a minimum that the entire head fit on the board, so for an M3 screw, you would want at least 1.5 mm.

HLA-27b:
Thank you.

If there are many heavy parts on the board like switchmode transformers, relays etc. how do engineers know that it won't break loose under vibration? Isn't there an established norm like ISO or DIN that deals with that?

Bored@Work:

--- Quote from: HAL-42b on January 26, 2012, 04:52:20 pm ---Isn't there an established norm like ISO or DIN that deals with that?

--- End quote ---

There are many established standards. The standards that apply depend on the product, application and the risks to protect against. E.g. earthquake tests and standards are different from vibration standards for equipment in trains. Which are in turn different from automotive standards. Military standards are of course different from industrial standards. Spaceflight is yet another different game. And there are of course national standards that differ between countries.

HLA-27b:

--- Quote from: BoredAtWork on January 26, 2012, 05:36:06 pm ---There are many established standards. The standards that apply depend on the product, application and the risks to protect against. E.g. earthquake tests and standards are different from vibration standards for equipment in trains. Which are in turn different from automotive standards. Military standards are of course different from industrial standards. Spaceflight is yet another different game. And there are of course national standards that differ between countries.

--- End quote ---

Yes I've been browsing military and aerospace standards for months now but we on earth simply do not have their requirements nor their money. I should look more into automotive and railroad standards.

I'm looking for non proprietary things that are already widely established, like metric threads, DIN rails, 19" racks etc. It seems like the industry has gone to great lengths to prevent this from happening to PCBs. Since they can't copyright functional attributes they have played with hole locations on many PC boards so as to be able to claim non-functional attribute and thus copyright the damn hole locations!

At least the Arduino guys should have known better. With their quirky hole locations they have made it impossible to design for their boards. You either have to design specifically for Arduino or you have to exclude it altogether. Hence my search for established patterns for open source.

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