Countersinks, Counterbores and PCB Fasteners
Why Holes Are Shaped
A plain hole accepts a screw, but the screw head sits proud of the surface. Where the board is mounted against a chassis, where a head must not touch a neighbouring assembly, or where the surface has to be flat, the hole is shaped to receive the head. A countersink is a conical recess that accepts a flat-head screw and lets it sit flush. A counterbore is a cylindrical recess that accepts a socket-head screw and lets it sit below the surface. Both are mechanical features machined after plating, and both have their own tolerances, tooling and failure modes.
Countersink
A countersink is specified by the head angle, almost always 82 or 90 degrees, by the diameter at the surface and by the depth. The depth is the controlling dimension, because the screw has to sit flush without the head bottoming on the surface and without the countersink cutting into the barrel of the hole. Because the cone is machined, its depth tolerance is a function of the machine and the tool wear, and a countersink that is cut slightly too deep will breach the pad or thin the material around the hole. Where the countersink is on a plated hole, the machining operation removes plating from the cone, so the edge of the countersink becomes exposed laminate unless the finish is applied afterwards.
Counterbore
A counterbore has a flat floor and a cylindrical wall, and it is specified by its diameter and its depth. It is more forgiving than a countersink in one respect, since the floor gives a defined surface for the screw head to bear on, and less forgiving in another, because the floor has to be flat and at the right depth. A counterbore that is too shallow leaves the head proud; too deep and the remaining material around the main hole becomes thin. Where a counterbore is used in a thick board, the remaining wall between the counterbore and the plated barrel has to be sufficient to carry the clamping load, which is a real mechanical constraint rather than a cosmetic one.
Depth Tolerance and the Remaining Wall
Both features are defined by the material left rather than the material removed. The specification should state the depth from the surface plus the minimum remaining thickness beneath the feature, and it should account for the board thickness tolerance, because the machine references the surface and the board thickness varies. On a thin board the arithmetic is unforgiving: a 1.0 mm board with a 0.6 mm deep counterbore leaves very little, and the region under the head becomes the weakest part of the mounting. Where the board is thin, a stiffener or a washer is often the better answer than a deep recess.
Material and Copper Interactions
Machining a recess removes the laminate and any copper in its path, so the design has to keep the feature clear of traces and planes. On a board with an internal ground plane, a deep counterbore can expose the plane, which is a short-circuit risk if a metal screw is used, and the plane has to be kept away by a clearance that reflects the machining tolerance. The same applies to a countersink on a plated hole: the cone will cut into the plane if the plane is too close. Where the mounting must also provide a ground connection, the design is normally inverted: a plated hole is used, the plane is connected to it deliberately, and the fastener is specified with a conductive finish.
Fasteners and Clearances
The hole diameter has to clear the screw shank plus a tolerance, and the clearance has to account for the positional tolerance of the mating parts. Where the board is one of several stacked parts, the clearance should be generous enough that the assembly can be put together without forcing, and the tolerance stack across all parts should be calculated rather than assumed. Where the fastener will see vibration, a locking element, a washer or a thread-forming screw into a metal boss is used, and the board then has to tolerate the clamping force without crushing the laminate around the hole, which is another reason to keep the remaining wall thick.
Design Rules and the Drawing
Specify the feature as a separate note on the mechanical drawing with its diameter, its depth and its tolerance, and state whether the feature is machined before or after the surface finish. Keep the feature clear of copper, of the board edge and of any component that will be placed nearby. Check the remaining material under the feature against the clamping load, and check that the countersink or counterbore does not intersect a plane. Where several mounting holes are used, ensure they are dimensioned from the same datum so that the assembly can be aligned on the mating part.
FAQ
What is the difference between a countersink and a counterbore? A countersink is a conical recess for a flat-head screw; a counterbore is a cylindrical recess with a flat floor for a socket-head screw.
What angle should a countersink be? It has to match the screw head, most commonly 82 or 90 degrees.
Why is the remaining wall important? Because the clamping load passes through the material under the head, and a thin section under a recessed feature is where a board cracks or crushes.
Does the recess remove plating? Yes, so the finish sequence has to be considered, and a feature that must remain conductive has to be plated afterwards.
Can a recessed hole be near a plane? Only if the plane is kept clear by the machining tolerance, otherwise a metal fastener creates a short.
Conclusion
Countersinks and counterbores are mechanical features with mechanical tolerances, so they should be specified by the material remaining and the load they carry rather than by the depth alone. Keep them clear of copper and of the edge, check the remaining wall, decide the finish sequence and dimension them from a common datum. Mechanical features of this kind are described alongside PCB capabilities, the machining steps belong to PCB manufacturing, and the hole and clearance rules are part of PCB design and layout. Boards with mechanical features of this kind are assembled through PCB assembly in 2026.
