Most PCB mounting holes are simple cylinders drilled through the full board thickness. Some products, however, need a screw head to sit below or level with the PCB surface. That requirement leads to two related machining features: the counterbore and the countersink.
They may look similar on a mechanical drawing, but they accept different fasteners and require different fabrication data. This guide explains the difference, the dimensions your board house needs, and the design checks that prevent broken laminates, exposed copper, and hardware that does not fit.
Counterbore vs. Countersink: The Short Answer#
A counterbore is a larger cylindrical recess with a flat bottom. It is normally used with a socket-head cap screw, fillister-head screw, or other fastener that has a flat bearing surface beneath the head.
A countersink is a conical recess. It is used with a flat-head screw whose underside matches the specified cone angle. When the dimensions are correct, the screw head can sit flush with the PCB surface.
| Feature | Counterbore | Countersink |
|---|---|---|
| Recess shape | Cylindrical, flat bottom | Conical |
| Typical fastener | Socket-head or flat-bearing screw | Flat-head screw |
| Key dimensions | Hole diameter, recess diameter, recess depth | Hole diameter, major diameter or depth, included angle |
| Main risk | Too little laminate left below the recess | Angle or depth mismatch leaves the head proud or over-seated |
What Is a PCB Counterbore?#
A counterbore begins with a smaller through-hole and adds a larger coaxial recess from one face of the board. The cutting tool produces a vertical wall and a nominally flat shoulder. The screw head rests on that shoulder instead of on the outer PCB surface.
Counterbores are useful when the enclosure has limited clearance above the board or when a designer wants the hardware head captured in a controlled recess. They can also help locate certain spacers or mechanical inserts, but the recess should never be treated as a precision bearing feature unless its tolerances have been agreed with the fabricator.
The drawing should identify:
- the finished diameter of the central mounting hole;
- the counterbore diameter;
- the counterbore depth from a named board side;
- whether the central hole is plated or non-plated;
- diameter and depth tolerances;
- the number and exact locations of the features.
What Is a PCB Countersink?#
A countersink widens the mouth of a mounting hole into a cone. The cone receives the tapered underside of a flat-head screw. The included angle is part of the design; common hardware families use different angles, so the screw specification must be selected before the PCB feature is defined.
The fabricator normally needs the finished through-hole diameter, the major countersink diameter or depth, the included angle, the board side, plating status, and applicable tolerances. Giving only a note such as “countersink for M3 screw” is risky because screw standards, head dimensions, finishes, and desired seating can differ.
A countersink that is too shallow leaves the screw head above the surface. One that is too deep can reduce the bearing area, cut through outer copper clearance, or leave too little laminate around the hole.
Why These Features Are More Difficult in a PCB#
A PCB is a layered composite, not a homogeneous metal plate. Copper foils, glass-reinforced resin, plated holes, solder mask, and internal planes all interact with the machining operation. Removing material after lamination can expose glass fibers, interrupt copper, and create local stress.
Board thickness is the first constraint. The recess depth must leave enough structurally useful laminate beneath it. Thin boards may not support the intended screw head at all. If the mounting load is significant, the complete joint—including washers, standoffs, enclosure bosses, torque, and expected vibration—should be reviewed mechanically.
Copper clearance is equally important. All copper layers, not just the outer layer, need clearance from the maximum machined envelope plus registration and routing tolerances. A recess that looks clear in a top-layer view can still cut into an internal ground plane.
Plated or Non-Plated?#
Mechanical mounting holes are often specified as non-plated through holes (NPTH), especially when the screw should remain electrically isolated. A plated mounting hole may be intentional when the chassis connection is part of the grounding strategy, but the electrical and mechanical requirements must be documented.
Do not assume that the counterbored or countersunk surface itself will be plated. These recesses are commonly secondary machining operations, and process order affects which surfaces receive copper or finish. If conductivity, exposed copper protection, or chassis bonding depends on the feature, confirm the achievable construction with the manufacturer rather than relying on a generic drill symbol.
For a broader explanation of standard mechanical holes, see our guide to PCB mounting holes.
How to Dimension Counterholes Correctly#
Start with the actual fastener drawing. Use the maximum head diameter, maximum head height, and specified underside geometry rather than a nominal screw name alone. Decide whether the head must be fully recessed, exactly flush, or only prevented from interfering with a neighboring part.
Then create a dedicated fabrication note or controlled mechanical drawing. A complete callout should include:
- feature type: counterbore or countersink;
- finished through-hole diameter and tolerance;
- recess major diameter and tolerance;
- depth for a counterbore, or included angle and depth/major diameter for a countersink;
- machining side: top, bottom, or both;
- PTH or NPTH designation;
- quantity and reference locations;
- remaining thickness or other critical mechanical limit, if applicable.
Modern fabrication outputs such as ODB++ and IPC-2581 can communicate richer hole attributes, and some PCB design systems support counterhole definitions directly. Even then, a human-readable drawing remains useful for resolving side, depth, and acceptance criteria.
PCB Counterbore and Countersink DFM Checklist#
- Confirm capability early. Counterholes are special mechanical operations and may affect price and lead time.
- Check remaining laminate. Do not leave an unreasonably thin web below a counterbore.
- Clear every copper layer. Base the keepout on the maximum recess diameter plus fabrication tolerance.
- Keep away from the board edge. Insufficient material can crack during machining or screw tightening.
- Review nearby traces and components. Include the real screw head, driver access, washer, and standoff in the mechanical model.
- Control screw torque. FR-4 can crush or delaminate under excessive localized load.
- Identify the machined side. A mirrored counterhole is not interchangeable with the intended feature.
- Avoid ambiguous files. Do not encode the recess only as overlapping drill hits.
When Should You Avoid a Counterhole?#
If the board is thin, heavily loaded, or frequently serviced, a counterbore or countersink may be the wrong solution. A spacer, enclosure recess, shoulder washer, captive insert, or different screw-head style can shift the mechanical load away from the laminate.
A redesign is also sensible when the recess would violate copper clearance, approach a routed edge, or require a depth tolerance that the PCB process cannot hold economically. The lowest-risk mounting joint is usually the one that uses standard holes and lets the enclosure provide precision and strength.
Final Design Advice#
Choose the fastener first, then design the PCB feature around its controlled dimensions. Use a counterbore for a flat-bearing screw head and a countersink for a tapered flat-head screw. Specify the complete geometry, side, plating status, and tolerances, and ask the manufacturer to review the remaining thickness and copper clearances before production.
If your design includes unusual holes, edge machining, or tight enclosure constraints, send the PCB files and mechanical drawing together when requesting a manufacturing review and quote.
PCB Counterbore and Countersink FAQ#
Can a countersink be used with a socket-head cap screw?
Normally no. A socket-head cap screw has a flat bearing surface and is better matched to a counterbore. A countersink is intended for a tapered flat-head screw.
Can counterholes be plated?
The central hole may be plated or non-plated, but the recess surface depends on the manufacturing sequence and supplier capability. Specify the electrical requirement and confirm it with the fabricator.
Should counterholes appear in the drill file?
They should be represented in supported fabrication data and clearly described in a drawing or fabrication note. Never rely on an ambiguous graphic layer alone.
What countersink angle should I use?
Use the included angle defined by the selected screw standard and actual fastener drawing. Do not assume that every flat-head screw uses the same angle.
Technical References#
For software-supported counterhole attributes and the required dimensions, see Altium's counterbore and countersink documentation. General rigid-board design requirements are covered by the IPC-2220 family of PCB design standards.