Screw Mounting Holes - How far from board edge?

[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]

Isn't there an established norm like ISO or DIN that deals with that?

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]

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.

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.

[Zad]

Normally I would say 0.5 to 1 diameter clearance, with further internal supports as needed. If you find that your panel is flexing unpredictably and causing stresses in the PCB, consider using a plastic skeleton under the PCB. This can be as simple as a sheet of acrylic (Perspex), with spaces routed out for ventilation if needed.

In the case of the Arduino, I would say - do many break? If no, then good enough is good enough. If yes - then stop being so rough with a piece of equipment designed around cheapness.

[HLA-27b]

In the case of the Arduino, I would say - do many break? If no, then good enough is good enough. If yes - then stop being so rough with a piece of equipment designed around cheapness.

The problem is that I am not designing a PCB but a module (vagueness intentional) to accept open source projects. With every project having its mounting holes spread randomly I simply can't accommodate all of them. So I am searching for established patterns and precedence.

[ndictu]

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.

Standards for medical equipment (if there are any, which I assume there must be) should be pretty robust while not being ridiculous.

[JuKu]

I don't know about any standards other than company standards, but I have opinions:). The top board is clearly an industrial board: Sturdy mechanical mounting, room for a nut, washer and nut driver. For EMI, the mounting holes are grounded, which is the right thing to do.

The bottom board is, in this regard, a badly layed hobbyist board: Too little board material around the holes; if you flex the enclosure, the board will break. On top hole there is good change of short circuit to the pad and no change to mount a header and still use the hole. On bottom right hole good change to cut the A5 wire when mounting and it doesn't look like there is room for a nut driver around bottom left hole. Also, there is no direct connection from the ground plane (if there is one in the first place) to the chassis. That makes a radio transmitter. That's ok an a hobbyist bench top as long as you are not a ham operator, but don't try to sell your creation.

[SeanB]

I will add to the discussion that placing the board and attaching it should not cause stress to be applied to components. I see many boards with a clip in fastening that require you to press on components to clip it in. As well arrange components around the hole to reduce the flexing stress they experience. IC's should not be close to them, or any multilead components, and definitely no BGA devices.

[sonicj]

ATX motherboards are a good design example. Various pressures exerted on the board with mounting holes/supports placed to accommodate.
-sj

[ndictu]

ATX motherboards are a good design example. Various pressures exerted on the board with mounting holes/supports placed to accommodate.
-sj

It's always fun, when building computers, to have one or two screws in your hand and can't find the remaining holes There are quite a lot of them, all over the place. Best part - screwing the one hidden below that enormous heatsink.

[sonicj]

Best part - screwing the one hidden below that enormous heatsink.

skewdriver

[ndictu]

skewdriver

Probably wouldn't fit. I have Noctua NH-U12P, which isn't even that big compared to some modern ones.

(Not my picture, random google search)

I have something like this and it works:

[HLA-27b]

Thanks for the suggestions guys. I am looking into computer mainboard form factors. Not surprisingly many are based on the imperial system which is...meh, I shall look into those which are not.

[ndictu]

Thanks for the suggestions guys. I am looking into computer mainboard form factors. Not surprisingly many are based on the imperial system which is...meh, I shall look into those which are not.

I wish it would die, but that won't happen in a thousand fortnights ...