Pick SO when both standoff ends must take screws, SOS when the internal thread must stay sealed from debris, and BSO when the reverse face of the sheet must stay solid and unmarked. That one sentence settles most standoff selections. This guide is for electronics and enclosure engineers stacking PCBs or spacing sub-panels in sheet metal roughly 1 mm thick – it is not for plastic bosses, spacers used in compression only, or thick plates that belong on welded studs. You will leave with the type chosen and the three numbers a supplier needs to quote: thread, panel thickness, and length.
Why the wrong standoff type surfaces late
Standoffs are small, late and annoying. The chain usually runs like this:
- The enclosure engineer treats standoffs as commodity and picks a length from a previous project.
- Nobody records which side of the panel carries the clinch or whether the through-hole may show on the cosmetic face.
- With no type decision on the drawing, purchasing quotes three incompatible variants.
- Assembly discovers screws entering from the wrong side, or the clinch end shows through a painted outer face.
- Panels wait for re-punching while the program holds.
- Decide the type against the panel face that matters – before the hole pattern is cut.
The selection matrix
| Type | Thread side(s) | Reverse face | Typical use |
|---|---|---|---|
| SO | Both ends (male-female, thru) | Clinch end visible | General PCB stacking inside enclosures |
| SOA / SO4 | Thru, metric variants | Clinch end visible | Region-specific thread standards |
| SOS | External + internal, closed end | Clinch end visible | Keep debris out of the internal thread |
| BSO | Internal only (blind) | Solid – unmarked | Cosmetic or gasketed outer faces |
Our SO standoff specification carries the working example: an SO-M3-10ZI needs a minimum sheet of 1.02 mm and a 4.22 mm hole for an M3×0.5 thread – the same kind of three-number check every type requires.
Length, min panel and hole: the three checks
- Condition: board-to-panel gap. Stack-up height decides length; add the mounted component height, not just the PCB thickness.
- Risk: sheet below the minimum thickness. Below the type minimum (e.g. 1.02 mm for SO-M3), the clinch cannot form fully – change type or panel, not the installation force.
- Evidence: hole size from the supplier chart. Oversize holes are the top cause of standoffs that turn under screw torque; undersize holes split sheet coatings.
- Boundary: aluminum vs steel panel. Match standoff material (steel zinc, stainless, aluminium, brass) to the panel to limit galvanic corrosion in humid service.
Validate with samples before the pilot build
Confirm the choice the cheap way: install ten standoffs in a scrap of the actual sheet with the production tool, drive the real screws, then check for rotation, flush seating and reverse-face condition. Ten minutes of shop-floor validation closes every open question in this guide. If your layout is reflow-side mounted instead, ask about our SMTSO surface-mount standoffs and micro types in the same family – see the standoff category overview.
Your next action
Write down: thread (e.g. M3×0.5), panel thickness and material, required length, and which face is cosmetic. Send that line via the quote form or WhatsApp; we confirm type and part number, and free samples ship freight collect so you can run the ten-piece test on your own sheet.