Screws for electronics: M2.5, M3 and everything else

Sooner or later it happens to everyone: the board is ready, so is the enclosure, and the right screw is missing. For a few weeks now our catalogue has included Bossard fasteners. Bossard is a Swiss company that started out in 1831 as a hardware shop in Zug and today specialises in screws and fastening systems: more than two thousand items, from metric screws to screws for plastics, nuts and washers, almost all in packs of 100 or 200. Below is how to find your way through the codes without going mad.

The sizes most used in electronics

M3×10 means a 3 mm metric thread and a length of 10 mm. For boards, two sizes cover almost every case:

• M2.5: the screw for the mounting holes of the Raspberry Pi (2.7 mm holes) and of many compact boards;
• M3: fits the holes of the Arduino Uno (3.2 mm) and of most modules, standoffs and enclosures.

The smaller M2 and M1.6 are for displays and miniature modules; the larger M4, M5 and M6 for brackets, profiles and cabinets. On cheese head and button head screws the length is measured from under the head, on countersunk screws it is the overall length.

Head and drive

• Socket head cap screw (DIN 912, ISO 4762): the classic Allen screw, it tightens well and takes up little room at the side.
• Button head (ISO 7380): low and rounded, it looks good on panels and lids.
• Countersunk flat head (DIN 965 cross recess, DIN 7991 hex socket, ISO 14581 Torx): sits flush with the surface, and needs a countersunk hole.
• Cheese head or pan head, slotted or cross recess (DIN 84 and DIN 85 slotted, DIN 7985 cross recess): the traditional screw, driven with a screwdriver.

The drive tells you which tool you need: Phillips (PH) or Pozidriv (PZ) cross recess, Torx (TX), hex socket (with the key size, for example HEX 2 mm), slot. With Torx and hex sockets the tool is less likely to slip out of the recess: you tighten better and damage the head and the bit less.

Material: not just a question of price

• Zinc-plated steel: cheap, perfectly fine inside a dry enclosure.
• A2 stainless steel: does not rust, for outdoor use and damp places; A4, with molybdenum, copes better with salt air and aggressive environments.
• Black oxide steel: black, for a more finished look; it gives little protection against rust, so keep it indoors.
• Brass: does not rust and conducts well, also used for earth connections.
• Polyamide (nylon): insulates, so it makes no contact with tracks and components if the screw touches the circuit. Nylon screws, nuts and washers are the safest way to mount a board; tighten them gently, because a plastic thread strips easily.

Nuts and washers

The hex nut (DIN 934, ISO 4032) is the standard one; the thin nut (DIN 439) is for tight spaces; the nylon insert lock nut (DIN 985) resists vibration, useful on motors, fans and robots. The flat washer (DIN 125) spreads the load and protects the surface. The spring washer (DIN 127) is the classic split lock washer: still widely used, but where there is strong vibration a lock nut is the safer choice.

Screwing into plastic

In plastic enclosures and 3D prints there is not always room for a nut. Screws for plastics have a thread designed to cut its own way into the material without splitting it: all you need is a hole slightly smaller than the screw diameter. Don't confuse them with self-tapping screws for sheet metal, which have a different thread and are meant for thin metal.

If you would rather start with a kit

For the workbench, an assorted box with screws, nuts and standoffs in the most common sizes is the sensible choice.

You will find everything in Metric Screws, Self-Tapping & Plastic Screws, Nuts and Washers.

Can't find the size?

The Bossard catalogue is huge and not every size is on display. If you need a particular screw, send us the code or a photo with a ruler next to it and we will look for it.