PCBs often require screws for mounting, and sometimes the screw heads need to be flush with or slightly recessed into the PCB surface. Therefore, special holes, such as counterbore and countersink, are added to the board, allowing different types of screws to secure them properly.

Flat head socket cap screw

Socket cap screw

Counterbore and countersink serve the same purpose: to allow the heads of screws or other fasteners to be flush with or slightly below the surface of the material. The difference between them lies in the drilling angle of the larger hole. A countersink has a larger, conical hole drilled at a specific angle, while a counterbore is drilled vertically into the material with a flat bottom left between the counterbore and the narrower inner hole.

What are Counterbore and Countersink?

A counterbore is defined as a cylindrical hole with a flat bottom, typically enlarged into another concentric hole. This type of hole is usually used when threaded fasteners (such as hexagon socket head screws) need to be recessed below or flush with the surface of the workpiece.

A countersink is a conical hole drilled into a PCB laminate. This type of hole is typically used to make the heads of counterbore rivets, screws, or bolts flush with or slightly below the surface of the surrounding material.

What Parameters are Required for Making Counterbore and Countersink Holes?

      1. Counterbore

        Pilot hole diameter(D1);

        Counterbore diameter(D2);

        Depth the counterbore is to be drilled(H);

        Hole attribute: PTH or NPTH;

        Which side is the counterbore hole on( Top side or Bottom side).

      2. Countersink

        Pilot hole diameter(D1);

        Countersink diameter(D2);

        Depth the countersink is to be drilled(H) or Countersink angle;

        Hole attribute: PTH or NPTH;

        Which side is the countersink hole on( Top side or Bottom side).

Symbols for Counterbore and Countersink

      1. The symbol for counterbore is '⌴'.

      2. The symbol for countersink is '⌵'.

How to Represent Counterbore and Countersink on Engineering Drawings？

      1. On engineering drawings, counterbore can be represented as shown in the following illustration:

         A pilot hole is a small-diameter hole, typically the first hole to be drilled.

         The counterbore example above is used for an M20 hexagon socket bolt with normal clearance.

      2. On engineering drawings, countersink can be represented as shown in the following illustration, with the bottom sectional view indicating the details referred to by the top dimensions.