I’m considering using non-plated counterbored mounting holes on a low-cost 2-layer FR4 board (62mil thickness) board with 120 mil drill / 187 mil counterbore OD / 25mil counterbore depth
How common is this feature on commercial boards and what depth tolerance should I reasonably expect and specify for the counterbore?
I want to keep this cost-effective, so if counterboring adds significant complexity or cost compared to standard through-holes, I’ll explore other options.
Non-plated counterboring holes are widely supported by many PCB fabricators. For callouts in your fabrication drawings, specify the counterbore diameter and which side gets the counterbore. Most fab shops can achieve ±5 mils tolerance on the counterbore diameter.
Don’t make the counterbore so deep that it breaks through to the other side. Leave some material thickness so there’s a straight section of the hole remaining on the back. This prevents the back side from getting burrs or rough edges during manufacturing. In your case with 62 mil thick board and 25 mil counterbore depth, you’ll have 37 mils remaining, plenty of material to avoid this issue.
Counterbores are generally supported, but the part most people overlook is how loosely some fabs control the depth unless it’s called out as critical. For a 25 mil depth, many offshore shops will only guarantee something in the ±3–5 mil range, so it’s worth confirming their actual capability before locking the spec in.
Where this affects cost is not the feature itself but whether the fab treats it as a special machining step. If they already run counterbores in their process, the cost impact is small; if not, it can trigger a setup fee. For low-cost builds, it’s often cheaper to relax the depth tolerance or use a mechanical spacer if you need very precise standoff height.
One thing to keep in mind is that FR4 thickness itself has a wide tolerance often around ±10% for standard material. So even if the fabricator holds the counterbore depth accurately, your actual standoff height can still shift by ~±6 mil simply due to core thickness variation.
If the assembly height needs to be tightly controlled, you may need to specify a tighter laminate thickness tolerance or measure and sort boards after fab. For typical mounting applications, the combined tolerance is usually acceptable as long as the mechanical design accounts for it.