What's the Difference: Countersink VS Counterbore Holes in PCBs?

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 countersink and counterbore holes that impact their use in PCBs. Both terms are commonly used in CNC machining. Normally countersink is a cone-shaped hole and the counterbore is a cylindrical flat-bottomed hole.

This article will examine the key differences between countersink and counterbore holes and discuss optimal uses for each in PCB design. Countersink has different angles such as  60°, 82°, and 90°. While the counterbore comes with sides parallel to each other without tapering. So let us start with Counterbore vs Countersink.

Difference between a Countersink and Counterbore Hole:

What is a Countersink Hole?

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

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. It also offers a clean finished look since fasteners can be hidden. It is represented as ⌴.

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

Design Considerations When Selecting a Hole Type

When planning a PCB layout, engineers should weigh the following factors when choosing between countersunk vs. counterbored holes:

1) Available Board Space: Countersinks require more area due to their tapered shape.

2) PCB Layer Location: Countersinks optimize external layers, while counterbores are better for internal layers.

3) Shear Strength Needed: Counterbores provide stronger screw joints than countersunk holes.

4) Environmental Exposure: Countersunk holes improve waterproofing for external PCBs.

5) Aesthetic Priorities: Countersunk holes provide a smoother surface finish.

Considering these criteria early in the design process will help ensure the selected hole type provides the right functionality for the application. Once the hole type is selected, proper drilling and finishing steps are needed to achieve a functional, reliable PCB.

Drilling and Finishing Countersink v/s Counterbore Holes:

How to Drill a Countersink 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, countersink bit and safety gears. With a pencil mark the center of the hole where the countersink is made. Select the countersink bit according to the angle and size, Commonly used angles are 82° and 90°. Put countersink bit drill chuck of drill or drill press.

2) Drill Depth: Set drill depth or use depth stop on the drill press to control the countersink. 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 countersink bit in workpiece. Use pressure to drill to make a smooth and countersink hole. When the countersink hole is made, strip drilling and check the depth and angle of the countersink. Make sure that screw heat fits completely into the countersink without protruding over the workpiece’s surface. If there are any rough edges or burrs about countersink holes, then sandpaper can be used to clean the surface.

How to Drill a Counterbore Hole?

1) Tools Selection:

There is a need for a drill press or handheld drill, a counterbore bit, and some safety instruments like goggles and ear protection. The size of the counterbore bit must be larger than the head of the bolt or screw. Counterbore will make a flat-bottomed hole with a larger mount for handling screws or fasteners.

2) Sizing and Drill Bits: Put a small drill bit in the drill chuck, then tighten the chuck to secure the drill bit in position. Set drill press for controlling the depth of the hole. It makes sure that the hole will be at an accurate depth and not highly deep.

3) Drilling Process: Set the drill bit on the marked middle point and start the drill slowly. The drill bit makes a pilot hole in the working component. This hole is the instruction of the counterbore bit. Decreased the counterbore bit in the pilot hole and started drilling slowly. Use moderate pressure on the drill to make sure it has a smooth counterbore.

Key Differences Between Countersink and Counterbore Holes:

While both hole types allow hardware to sit flush on the board surface, there are some important differences that impact their use:

1) Hole Shape: The primary difference is the hole shape. Countersinks are conical while counterbores have straight cylindrical walls. This impacts how flush the fastener sits against the board.

2) Top Opening Size: Relatedly, the size of the top opening varies between the two holes. The top of a countersink tapers gently from the full hole width. A counterbore has a wider concentric cut on top that matches the fastener diameter.

3) Vertical Walls: Countersinks have angled side walls while counterbores have vertical walls. Vertical walls provide more shear strength for fasteners.

4) Bottom Opening Width: The bottom hole opening of a counterbore is wider than a comparably-sized countersink. This provides more design flexibility when selecting fastener sizes.

5) Material Removal: More material is removed from the PCB to create a countersink. Counterbores conserve PCB material and strength.

Countersinking is a simpler process than counterboring and needs less complicated instruments. It is a faster process than counterboring since it uses fewer steps. On the other hand countersink fasteners are not as strong as counterbored since the load is distributed over a small area. It is not best for hard materials since the process can result in material cracking.

Application and Use Case Scenarios:

Common Uses for Countersink Holes:

● 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.

Common Uses for Counterbored Holes:

● Mounting machine screws and bolts that need high shear strength.

● Allowing washers or spacers to sit flush with the PCB surface.

● Accommodating larger fastener heads with minimal material removal.

● Providing maximum strength for heavily-loaded screw connections.

● Joining internal PCB layers together securely while minimizing surface imperfections.

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 countersink 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 countersink and counterbore holes use different types of screws in distinct types of materials. However, both work in a similar manner in a PCB. A countersink 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 both countersink and counterbore holes. Also, this article shows how and when to use these drill bits in a PCB according to their usage and application.

Countersinks provide a flush exterior surface while counterbores maximize shear strength, especially on internal layers. With the right drilling and finishing processes, both hole types can be implemented successfully on printed circuit boards. Considering the key differences outlined in this article will lead to robust and reliable PCB hole designs.

Frequently Asked Questions

Q: When should I choose a countersunk hole over a counterbored hole?  

Opt for a countersunk hole when a smooth, flush surface is needed for aesthetic reasons, aerodynamic efficiency, or weather protection. Use a counterbored hole when high shear strength is essential or when accommodating larger fastener heads on internal layers.

Q: Do countersunk holes weaken the PCB more than counterbored holes?

Yes, countersunk holes tend to reduce strength more because they remove more material with their angled taper, whereas counterbored holes preserve more of the material.

Q: Is it possible to drill both a countersink and a counterbore in the same hole?

While it's feasible to create a combination hole, it requires extra drilling steps and precise measurements. Generally, it's easier to stick to one type of hole.

Q: What is the best method to finish a countersunk hole?  

After drilling, use a countersink abrasive cone sized for the hole. Apply gentle pressure to deburr and improve the fit of the fastener. Be cautious not to over-finish, as it may enlarge the hole.

Q: How are the dimensions for counterbores and countersinks specified?

For counterbores, both the minor hole diameter and the larger counterbore diameter are specified. For countersinks, the major and minor diameters are listed, and sometimes the taper angle is included as well.