M3 screw drill diameter

[sean87]

Hi all,

I need to put pads for mounting my PCB. I have chosen M3 screws because I have plenty of them My question is, what should be the size of pad and its drill diameter?

Thanks.

[mikeselectricstuff]

Depends how accurate are the hole positions are in what you are mounting it to.
You want accurate size on one side and oversize on the other to allow for tolerance.
3.5mm would be a generally normal size for an M3 mount, but M3.5 on both pieces may get too sloppy, so you'd normally do 3.2mm on one and 3.5 on the other. 
If it's just a general purpose mount hole with no specifiiic 'other bit', use 3.5mm

I'd normally do the pad as 6mm, to correspoind with the typical screw head size, to avoid any tracks ending up under the screw.

[Rufus]

I'd normally do the pad as 6mm, to correspoind with the typical screw head size, to avoid any tracks ending up under the screw.

6mm is a bit small. Typical pan head M3 screw is about 5.7mm but you need to add twice the PCB hole slack.

If you use a conductive washer (and I often use stainless crinkle washers on PCBs) the washer might be 6.3mm with 3.3mm hole so you need 6.3mm + twice the washer hole slack + twice the PCB hole slack.

[Randall W. Lott]

I recently started using LP Wizard to calculate hole and pad sizes.

• Hole size to yield the "IPC Nominal Fit" for an M3 screw is 3.2mm.
• Pad size for a pan head screw is 6.5mm.
• Pad size for a flat washer is 7.7mm.
• Satellite vias can be used to grip the screw head.

Make sure to have no soldermask on the pad or satellite vias.

[JuKu]

Add 10mm clearance region, so you can use a nut driver without cutting tracks or dismounting resistors.

[vk6zgo]

Great to see some commonsense ideas from people designing PCBs.
Too often,I have seen mounting holes so close to a track that you have to put nylon or fibre washers under the screw heads to prevent shorts.

[Spawn]

There is a calculation formula for this but since I am lazy I use tables like this one here, if you need for more size just click here

[westfw]

>> ... Pad size for a flat washer is 7.7mm.
>> ... Add 10mm clearance region, so you can use a nut driver

Screw holes are depressing.  What sort of mounting options are there than don't consume more space than a multi-gflop CPU ?  (and wait: why use a nut driver on a screw?)

[stefanh]

I recently just updated my PCB library with the following mounting hole footprints according to following blog which has been mentioned in the forums elsewhere.

http://blogs.mentor.com/tom-hausherr/blog/2011/03/18/pcb-design-perfection-starts-in-the-cad-library-part-13/

[poorchava]

Depends on how your manufacturer handles plated hole diameters. One group specifies that hole diameter will shrink by 0.1 or 0.15mm after metallization process, which means that you need to design holes 0.1 or 0.15 larger in diameter than mechanical dimension of whatever you want to put in that hole. Other group enlarges holes on their own based on their knowledge of their own process. This has the upside of getting holes very close to spec. On the other hand though, enlarging holes without your knowledge may lead to bad consequences (particularily broken traces/shorts if you run traces close to holes).

Anyway, i generally aim at finished hole diameter of 3.2mm and this has always worked for me.

[FJV]

For though holes.

Fine ø3.2mm

Medium ø3.4mm

Coarse ø3.6mm

Source :Roloff / Matek Machine-onderdelen Tabellen

[russdx]

Very helpful thread!

[Jeroen3]

Find the DIN spec of the M3 parts you have here: https://www.rvspaleis.nl/bouten (dutch)
They have an excellent website.

[HwAoRrDk]

Make sure to have no soldermask on the pad or satellite vias.

I've been wondering why to exclude solder mask around screw mounting holes. Many boards I see do this, but many don't. Have never been able to find a decent explanation of why not to.

[bitslice]

Make sure to have no soldermask on the pad or satellite vias.

I've been wondering why to exclude solder mask around screw mounting holes. Many boards I see do this, but many don't. Have never been able to find a decent explanation of why not to.

Normally to link the groundplane of the PCA to the case via the screw.

[HwAoRrDk]

Oh no, I understand why for that case, where the hole is plated or has surrounding vias. I mean when there is no electrical connection, and the hole is unplated.

[helius]

The thinking could be that soldermask is slippery, so the washers or nuts will not be as reliable. leaving a square hole in the mask also makes the mounting points easier to see. (The same way test points are typically circular pads surrounded by square holes in the mask.)

[nctnico]

Make sure to have no soldermask on the pad or satellite vias.

I've been wondering why to exclude solder mask around screw mounting holes. Many boards I see do this, but many don't. Have never been able to find a decent explanation of why not to.

IMHO it just looks nicer and clearly identifies the mounting point. During PCB design a hole in the solder mask also clearly marks the keep-out area.

[HwAoRrDk]

Hmm, it hadn't occurred to me that people do it simply for visual distinction. Certainly makes it very clear at a glance where the mounting holes are.

[chris_leyson]

You have to tolerance things up for mounting holes, screw diameter is not an issue it's center offset, how accurately can you place your centers ? PCB and CNC manufacturers can get good accuracy but you have to add tolerances.

I wouldn't trust drill size tables, they're compiled or written in a different context. Nowadays tolerances are much better.

[nctnico]

You have to tolerance things up for mounting holes, screw diameter is not an issue it's center offset, how accurately can you place your centers ? PCB and CNC manufacturers can get good accuracy but you have to add tolerances.

True but 0.2mm extra (3.2mm) is already a lot. If you go to 3.5mm the PCB has a lot of room to move so connectors may not line up so nicely in the casing. My default is 3.2mm and this usually is fine even for (automatic) pressed in studs. Some customers explicitely ask for wider holes though.