The numbers needed to put a board into an enclosure: hole size, keep-out circle, standoff hex, tap drill, standoff height and screw length.
For an M3 screw, drill the PCB mounting hole 3.2 mm (ISO 273 close fit) or 3.4 mm (normal fit), keep copper and components out of a 7 mm circle, use a 5 or 5.5 mm hex standoff, and tap the enclosure boss with a 2.5 mm drill. The same rules for M2, M2.5 and M4 are in the chart below.
EKINSUN is a custom parts manufacturer that machines the standoffs and enclosures these numbers are for, and assembles complete units around the board, from one piece.
Each row gives the PCB hole, the size of what presses on the board (pan head and washer), the standoff hex and its across-corners size, the keep-out circle that clears both, and the tap drill for a threaded boss in the enclosure.
| Screw | PCB hole, close (ISO 273) | PCB hole, normal | Pan head Ø (ISO 7045) | Washer Ø × thickness (ISO 7089) | Standoff hex AF → across corners | Keep-out Ø, at least | Tap drill in a boss |
|---|---|---|---|---|---|---|---|
| M2 | 2.2 mm | 2.4 mm | 4 mm | 5 × 0.3 mm | 4 → 4.62 mm | 5 mm | 1.6 mm |
| M2.5 | 2.7 mm | 2.9 mm | 5 mm | 6 × 0.5 mm | 5 → 5.77 mm | 6 mm | 2.05 mm |
| M3 | 3.2 mm | 3.4 mm | 5.6 mm | 7 × 0.5 mm | 5 → 5.77 / 5.5 → 6.35 mm | 7 mm | 2.5 mm |
| M4 | 4.3 mm | 4.5 mm | 8 mm | 9 × 0.8 mm | 7 → 8.08 mm | 9 mm | 3.3 mm |
Hole sizes and tap drills: ISO 273 and the clearance hole and tap drill chart. Pan head: ISO 7045, maximum head diameter. Washer: ISO 7089, normal series. Across corners = across flats × 2 / √3. Keep-out = the larger of washer diameter and standoff across-corners.
Have the hole pattern, but not the parts? EKINSUN machines standoffs to any length and enclosures with bosses tapped at your board's hole pattern — 6061 or 7075 aluminium, 304 stainless steel or brass — from 1 piece, quoted in 12 hours. Standard standoff sizes: ask anyway — they may be in stock, and the unit price drops as the quantity rises. Need the whole device built? Box build assembly →
The clearance between screw and PCB hole is the position error the assembly can absorb: when the standoffs are fixed in the enclosure and the screw passes through the board, the hole diameter minus the screw diameter is the total position tolerance, shared between the board's holes and the enclosure's bosses (the fixed-fastener case of ASME Y14.5).
| Screw | Close-fit hole: position budget | Normal-fit hole: position budget |
|---|---|---|
| M2 | Ø 0.2 mm | Ø 0.4 mm |
| M2.5 | Ø 0.2 mm | Ø 0.4 mm |
| M3 | Ø 0.2 mm | Ø 0.4 mm |
| M4 | Ø 0.3 mm | Ø 0.5 mm |
Use the close fit when the board and the enclosure are drawn from one CAD model and made to tight position tolerances. Use the normal fit when they come from different suppliers, when a board revision may move a hole slightly, or when the enclosure is molded rather than machined.
Standoff height is the tallest thing under the board plus the air gap you need. Measure from the board's underside: bottom-side SMD parts, through-hole lead ends and solder joints, and any connector or heat sink that hangs below the board edge. For low-voltage boards the gap only has to stop contact and allow for board flex; for mains voltage, take the clearance and creepage from the safety standard the product is certified to, such as IEC 62368-1.
When the height lands between two catalogue steps, a machined standoff is made to the exact length.
The screw has to be at least board thickness + washer thickness + thread engagement long. A common rule of thumb for engagement is 1 × d in steel, 1.5 × d in brass and 2 × d in aluminium; the standoff's thread must be deeper than the screw reaches.
Common PCB thicknesses are 0.8, 1.0, 1.2, 1.6 and 2.0 mm; 1.6 mm is the default.
The gap between the board edge and the enclosure wall has to cover the board's outline tolerance, the enclosure's tolerance and the position budget of the mounting holes. PCB fabs commonly route outlines to about ±0.2 mm — check your fab's specification — and a molded housing also has draft on its walls, so the gap at the top of a wall is larger than at the bottom. If connectors pass through the wall, their cut-outs set the board position, and the mounting holes take the remaining tolerance.
A plated hole with a pad grounds the board to a metal chassis through the screw and the standoff; a non-plated hole with a copper keep-out isolates it. For an isolated board, use a nylon standoff or a nylon washer, or an anodised aluminium boss, which insulates on its surface. Brass and stainless standoffs conduct.
Standard standoff sizes are cheap from stock; machining pays off when the length, the ends or the enclosure itself is not a catalogue item.
| What you need | In the catalogue? | At EKINSUN |
|---|---|---|
| Standard M3 × 10 hex brass standoff | Stocked by electronics distributors. | Buy it there, or ask us — standard sizes may be in stock, and the unit price drops as the quantity rises. |
| A standoff length between catalogue steps, e.g. 22.6 mm | Fixed length steps only. | Any length from 1 piece, ±0.05 mm: custom standoffs. |
| Different ends, e.g. M3 female / M4 male | Rarely listed. | Machined as one part, from 1 piece. |
| An enclosure with bosses tapped at your board's hole pattern | Not a catalogue item. | Machined from solid in our own CNC factory, from 1 piece: custom enclosures. |
| The whole device: board, enclosure, display, battery, firmware | — | Assembled and tested from 1 unit: box build assembly. |
Related: clearance hole and tap drill chart · custom standoffs · custom enclosures · heatsink enclosures · box build assembly · Deutsch
An M3 PCB mounting hole is 3.2 mm for a close fit (ISO 273) and 3.4 mm for a normal fit; choose 3.4 mm when the board and the enclosure come from different suppliers, because the extra 0.2 mm is position tolerance you can spend.
The keep-out around a PCB mounting hole should be at least the washer diameter: 5 mm for M2, 6 mm for M2.5, 7 mm for M3 and 9 mm for M4 (ISO 7089), which also covers the across-corners size of the usual hex standoff.
A Raspberry Pi uses M2.5 standoffs: its mounting holes are about 2.7 mm on a 58 × 49 mm pattern, so an M2.5 hex standoff with 5 mm across flats fits.
A PCB standoff screw is at least board thickness + washer thickness + thread engagement long: for M3 through a 1.6 mm board with a 0.5 mm washer into a brass standoff (engagement about 1.5 × d = 4.5 mm), that is 6.6 mm, so an M3 × 8 screw, with the standoff threaded at least 6 mm deep.
A plated mounting hole with a pad is used when the board grounds to a metal chassis through the screw and standoff; a non-plated hole with a copper keep-out, and a nylon standoff or washer, is used when the board must stay isolated.
Standoff height is the tallest thing under the board — bottom-side parts, through-hole lead ends, solder joints, connectors that overhang — plus the air gap you need; for mains voltage, take the clearance from the safety standard the product is certified to, such as IEC 62368-1.
Yes, EKINSUN machines standoffs to any length and enclosures with bosses tapped at your board's hole pattern, in 6061 or 7075 aluminium, 304 stainless or brass, from 1 piece, and can assemble the complete device around the board.
Screw size, hole pattern, length, material and quantity are enough to quote; drawings and STEP files by email to [email protected].
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