A Comprehensive Guide to Countersunk Holes in PCB Design

When designing printed circuit boards (PCBs), engineers often need to create holes in the board to mount components or attach connectors. Two common hole types are countersunk and counterbored. While they may seem similar at first glance, there are important differences between countersunk and counterbore holes that impact their use in PCBs. Both terms are commonly used in CNC machining. Normally countersunk is a cone-shaped hole and the counterbore is a cylindrical flat-bottomed hole.

This article will examine the key differences between countersunk and counterbore holes and discuss optimal uses for each in PCB design. Countersunk has different angles such as  60°, 82°, and 90°. Meanwhile the counterbore comes with sides parallel to each other without tapering.

In this article, we will delve into countersunk holes, exploring their drilling process, applications, and key design considerations.

What is a Countersunk Hole?

A countersunk hole is more complex compared to its counterpart because of its tedious drilling process. A countersunk 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 countersunk 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. It is represented as ⌵. countersunk can be made at 6 different angles 60°, 82°, 90°, 100°, 110°, or 120°, commonly used angles are 82° and 90°.

Why are Countersunk Holes Important?

Countersunk holes go well beyond a design decision. They will greatly affect the lifetime and performance of your PCB:

1. Improved Stability: By allowing fasteners to sit flush with the PCB, countersunk holes enhance mechanical stability, making them ideal for devices prone to vibration or movement, such as smartphones or automotive electronics.

2. Sleek Appearance: For products where aesthetics matter, countersunk screws provide a clean, professional finish by sitting below the surface, perfect for open-frame gadgets or wearables.

3. Space Optimization: In high-density PCB designs, countersunk holes save space by preventing fastener protrusion, allowing more room for components and routing on compact boards.

4. Electrical and Thermal Benefits: In some cases, flathead screws in countersunk holes can serve as grounding points or help dissipate heat, improving both electrical and thermal efficiency.

How to Drill a Countersunk Hole?

Usually, these types of holes are made by automatic drilling machines in our manufacturing house. But to drill the holes manually, these are some steps that can be followed:

1) Tools Selection: Arrange all the required tools and materials such as drill, countersunk bit, and safety gears. With a pencil mark the center of the hole where the countersunk is made. Select the countersunk bit according to the angle and size, Commonly used angles are 82° and 90°. Put countersunk bit drill chuck or drill press.

2) Drill Depth: Set drill depth or use depth stop on the drill press to control the countersunk. The objective is to make holes that help the screw head to sit flush with the surface of the working point.

3) Drilling Process: Set the drill press on the marked middle point and start drilling. Lower countersunk bit in workingpiece. Use pressure to drill to make a smooth and countersunk hole. When the countersunk hole is made, strip drilling and check the depth and angle of the countersunk. Make sure that screw heat fits completely into the countersunk without protruding over the workingpiece’s surface. If there are any rough edges or burrs about countersunk holes, then sandpaper can be used to clean the surface.

Design Considerations of Countersunk Type Holes:

Designing for countersunk holes calls for some consideration in several aspects:

Choice of screws: The kind of screw you intend to use will define the suitable countersunk depth and angle.

PCB thickness: The thickness of your PCBs will affect the depth of the countersunk hole also known as counterbore depth.

Component clearance: Ensure the countersunk hole allows enough space to prevent conflicts with surrounding components.

Manufacturing tolerance: To guarantee a satisfactory fit, give thought to the PCB and the screw tolerance assembly.

Chamfer angle: The hole wall and board surface angle is usually 90-100°. Larger angles over 100° increase head clearance but decrease edge support. Below 90° angles are possible but harder to make.

Application and Use Case Scenario of Countersunk Hole:

● Mounting flush screws on the outer surfaces of PCBs.

● Accommodating low-profile bolt heads that need to sit flush with the board surface.

● Providing a smooth aerodynamic surface on exposed external PCB layers.

● Mounting PCBs to thin aluminum panels or covers.

Which Hole Type is More Suitable in PCB Design?

Let's dive into their usage in the PCB. However, both types of holes are mainly used in wooden and metal surfaces. By understanding the key differences and use cases of both counterbore and countersunk holes the best suitable decision can be made. 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 to the manufacturing process at the risk of causing damage to the board.

Conclusion

Both countersunk and counterbore holes use different types of screws in distinct types of materials. However, both work in a similar manner in a PCB. A countersunk makes a cone-shaped hole, whereas a counterbore makes a cylindrical shape hole. This article briefly explains the definition, similarities, differences, and drilling processes of countersunk holes. Also, this article shows how and when to use these drill bits in a PCB according to their usage and application. For more information about the hole type visit our recent guide on countersink and counterbore type of holes in a PCB design.