The PCB assembly process is a complex one from start to finish, full of intricacies and complexities to bring your product to life. One of the many intricacies throughout the process is the drilling of holes throughout the PCB board to allow screws to be fixed with the surface of the board, such as counterbore and countersink holes.

What is a counterbore hole?

A counterbore hole is likely the first screw you will need to drill into your PCB board. A counterbore hole is a cylindrical, flat-bottomed hole. These holes are predominantly drilled into the surface of the boards for screw caps to be fixed within, or to sit flush under the surface of the board. 

If you’re using the counterboring method it means you’re using a method of drilling that creates an even surface on the inside wall of the pipe end. In order to create this cylindrical, flat-bottomed hole, special drills are used that have two different cutting diameters, one to make the hole for the fastener body, and one larger drill to make the head for the fastener, such as a screw cap. Counterbore holes tend to be easier to drill due to their simplicity of being a straight, vertical hole.

From a manufacturing point of view, counterbore holes are fairly straight forward as there is relatively little you need to know and consider before drilling – you simply need to know the depth you need to drill and the dimensions of the top and body of the screw. This does, however, mean that counterbore holes are only suitable for standard screw cap fixes; there is little flexibility in the sizing or variation. 

What is a countersink hole?

A countersink hole is more complex compared to its counterpart. A countersink hole has a canonical shape, which matches the shape of a screw meaning that any screw cap attached to it will sit slightly below the surface of the board. The depth of the hole can differ, depending on whether the screw should be visible on the top of the board, or driven deeper in order to cover the top and hide its appearance. 

The term countersink can also be used to describe the cutter that is making the hole into the board which allows for a countersunk screw to sit below the surface of the board when placed.

The process of drilling a countersink hole is typically more complex than that of a counterbore due to the angle and precision needed to achieve this accurately when drilling.

What are the key differences between counterbore vs countersink?

Whilst counterbore and countersink holes serve the same purpose, to allow a screw to sit flush within the board, they have a few key differences in their capabilities and how they’re created. 

1. Counterbore holes are more square in shape which allows for the addition of washers before fixing, whereas the conical shape of countersink holes matches the angled shape on the lower head of flat sized screws. The overall shape of countersink holes are also bigger than that of counterbore. Although the terms are used interchangeably, the main difference lies in the shape of the bottom of the hole. Countersinking creates a conical hole matching the angled shape on the underside of a flat-head screw.

2. Counterbore holes tend to be deeper than countersink to allow the screw to lie flush beneath the surface of the board.

3. The angle at which they are drilled differ greatly. A counterbore is typically drilled straight down with no angle needed, whereas the depth of a countersink hole will be determined by the angle, the major diameter and a greater range of applications and angles used. Standard angle sizes include 60, 82 and 90 degrees. 

4. Countersinking is often stronger because when you drive a screw without countersinking it can easily be overtightened which can cause the wood to split of become damaged

Which is better for PCBs?

Understanding the key differences between counterbore and countersink holes is imperative in knowing when one should be used over the other in the PCB assembly process.

Which method you use often depends on the material you’re drilling into. As mentioned, if your material is wood, countersinking is often preferred in order to avoid damage to the wood from over tightened screws. 

During the PCB assembly process, the majority of manufacturers will use the counterbore method over countersinking, in order to avoid any unnecessary damage to the PCB, as countersinking requires an angle to be used and additional depth from the drill. Countersinking also requires more equipment and resources which can add time onto the manufacturing process at the risk of causing damage to the board.

Using counterboring can create a connection by using a ground or earth terminal, which is literally a physical connection to the earth and acts as a safe return point to deplete surplus current. 

If you require a more secure mounting of your PCB, counterbore holes are also the best method. These holes are usually large enough to accommodate socket fixtures, hence the screws from them will create a more secure attachment.