PCBs often require screw holes for fastening; for instance, adding special holes like countersinks and counterbores to the board allows them to be secured in place using different types of screws.
Many PCBs require some form of mounting hardware, hence many PCB designs include non-plated mounting holes. Some customers have specific requirements for PCB drilling to elegantly install the product into an enclosure, such as counterbores and countersinks.
Regarding countersinks and counterbores, they both require different types of specialized tools. There is a distinction between countersinks and counterbores, and incorrect operation can have serious consequences. Therefore, understanding the difference between countersinks and counterbores on a PCB is crucial.
What is a Countersink?
A countersink (denoted by the symbol “⌵”) is commonly used in woodworking with softer materials. It is a conical hole whose angle matches that of the screw, allowing the screw to sit securely and flush with the surface. Countersink angles vary, including 60°, 82°, 90°, 100°, 110°, and 120°, but the most common are 82° and 90°. Below is an example of a countersink:
The widest part of the conical hole (the “Major Diameter” shown in the figure above) is specifically designed to accommodate the screw or bolt head. The angle is very important for these holes as it determines the depth the fastener can sink into – the deeper it sinks, the more secure it is. Besides making the joint stronger, countersinks allow the fastener to sink at a specific angle, reducing the likelihood of it stripping the material.
These holes can be machined using a drill bit with a countersink bit or an end mill. The most important thing is to ensure the hole’s size and shape are suitable for the object being processed.
What is a Counterbore?
A counterbore (denoted by the symbol “⌴”) is also cylindrical but features an enlarged diameter and a flat bottom, helping the fastener sit flush with (or even below, if necessary) the material surface. While this is almost their sole purpose, they are very useful for fasteners like socket head cap screws, allowing them to be flush with the surface. Their walls are perpendicular (90°) to the material surface without any taper, so the hole is straight with a flat bottom.
Drilling these holes is straightforward: first, drill a small pilot hole with a suitable diameter drill bit, then enlarge the hole using a specially designed end mill or counterbore tool. If the counterbore is large enough to accommodate a washer, it can enhance the washer’s clamping force.
Countersink vs. Counterbore: Dimensions
Counterbores are typically not as deep as countersinks, and their edges are straight, not tapered. They also tend to have higher holding power for two main reasons: the force applied by the socket head screw is parallel to the axis, and the force from the screw or bolt is distributed evenly over a larger surface area. Countersinks lack this strength because their tapered sides lead to uneven force distribution. The diagram below gives a better understanding of their differences.
Countersink vs. Counterbore: Shape
A countersink is a conical hole. A counterbore is a cylindrical, flat-bottomed hole designed to accommodate socket head cap screws or hex head screws. In other words, countersinks are tapered, while counterbores are cylindrical.
Countersink vs. Counterbore: Drilling Angle
Depending on the specific application, drill bits of different sizes and angles can be used to create countersinks. Common angles include 120°, 110°, 100°, 90°, 82°, and 60°, but the most commonly used drilling angles are 82° and 90°. For best results, the countersink angle must match the fastener head’s included angle. Counterbores have sides that are parallel to each other, requiring no taper.
Countersink vs. Counterbore: Designation Symbols
The designation symbol for a countersink is “⌵”, while for a counterbore it is “⌴”. These symbols attempt to depict the cross-sectional shape of the different hole types after creation.
Countersink vs. Counterbore: Applications
Countersinks are used for wood and metal screws, while counterbores are primarily for larger fasteners like socket head cap screws. Typically, countersinks require a smaller pilot hole than counterbores, which is why the latter are used for heavy-duty work in construction, machinery, and automotive fields.
Countersink dimensions are usually specified by their diameter (hole width), depth (distance from material surface to the top of the pilot hole), and countersink angle. Counterbore sizes typically range from 3/16 inch to 1 inch, while countersink sizes generally range from 1/16 inch to ½ inch. We always recommend customers refer to standard countersink and counterbore size charts to ensure their part design is compatible with standard tools.
Similarities and Differences Between Countersinks and Counterbores
The process of creating a countersink is called countersinking. Countersinks can have various angles, including standard sizes like 60°, 82°, and 90°, and less common ones like 100°, 110°, and 120°. The most frequently used are 82°, 90°, and 100°.
Zero degrees (vertical/perpendicular) is the only angle for counterbores, but it works just as effectively. Special attention is needed when working on wooden surfaces: if countersinking isn’t done initially and screws are forced in, it can compromise the wood’s stability and strength, resulting in an unsightly finish.
If the wood fibers aren’t fully compromised, they might crack and cause dents. Drilling on wooden surfaces is easier because the hole doesn’t require such precise angles and accuracy. Furthermore, drilling only requires a screw with a flat bottom, a socket head cap screw that fits the surface, or potentially a washer placed in the hole.
Counterbores are typically used for a single purpose, whereas countersinks, due to their different angles, serve various purposes. Counterbores usually resemble an enlarged version of a small coaxial hole, while countersinks are a tapered version of a small coaxial hole.
If you don’t need the fixture head, use a counterbore, or tighten the bolt from the HDI surface you are working on. On the other hand, due to the different angles, countersinks come in different styles. When using countersinks, the depth can be the same for all screws.
Countersink and Counterbore Comparison Table
| Feature | Counterbore | Countersink |
|---|---|---|
| Shape | Cylindrical | Conical |
| Purpose | Allows flat-head screw to sit flush with surface | Completely hides screw head for absolute flushness |
| Suitable Screws | Socket Head Cap Screws, Hex Head Screws | Flat Head Screws, Countersunk Screws |
| Machining Method | Using an end mill or drill bit | Using a countersink cutter of a specific angle |
| Commonality in PCBs | Less Common | Very Common |
| Structural Impact | Larger (removes more material) | Smaller |
| Colloquial Names | Sometimes mistakenly called “countersink”, but strictly should be “counterbore” | Often directly referred to as “countersink” |
Countersink and Counterbore PCB Applications
Generally, PCBs are secured in place using screws. If you need more aesthetically pleasing fixtures and safer installation, you can use countersinks or counterbores.
The drilling process is usually done automatically by automated equipment. When drilling countersinks for flat-head screws, this hole provides a good foundation for the screw to overlap and embed perfectly into the hole.
What are countersinks and counterbores used for?
Countersinks on PCBs create space for neat installation and are the best choice for tight-fit applications in small devices. Furthermore, PCB countersinks are very common in mobile phones and other small wearable devices.
Counterbores provide a cylindrical fit for screws. Their flat bottom allows for the use of socket head cap screws after machining. Because socket head cap screws have a hexagonal head, they need to be securely mounted on the PCB. They can be used with washers and in other electronic devices requiring tight, secure fits.
Special Notes in PCB Design and Manufacturing
- Terminology Mix-up: In the PCB industry, because countersinks are far more common than counterbores, people often habitually use “countersink” to refer to both types, or simply call them “counterbore/countersink holes”. However, in strict mechanical design terms, they are different.
- Machining Process:
- Countersink: Typically done on the PCB CNC drilling machine. After drilling the through-hole, a specific angle countersink drill bit is used to machine the hole on the top or bottom side of the board. This is an additional step that increases cost.
- Counterbore: Machining counterbores in PCBs is also usually done by milling, but depth and diameter must be carefully controlled.
- Design Considerations:
- Non-Conductive Area: Whether countersink or counterbore, the area where the screw head contacts must be free of solder mask and typically should also avoid any traces to prevent short circuits. This area is indicated in the Gerber files by a non-copper ring larger than the screw head.
- Board Thickness: The depth of the countersink/counterbore must not be too deep, otherwise it will weaken the PCB’s structural strength or even drill through inner layer traces. The depth is usually 0.2mm – 0.5mm, depending on board thickness and screw specifications.
- Standard Libraries: The default options provided in the footprint libraries of many PCB design software (like Altium Designer, KiCad) for mounting holes are usually for countersinks.
Summary
- If you want to use flat-head screws and have the screw face flush with the PCB surface, you should choose a Countersink. This is the most common choice in PCB design.
- If you want to use socket head or hex head screws and wish to completely hide the screw head without it protruding, making the mounting surface absolutely flat, you should choose a Counterbore. This is a special requirement in PCBs.
When communicating with PCB manufacturers, it’s best to provide detailed mechanical drawings indicating the hole type (countersink or counterbore), angle, and dimensions for the major and minor diameters to ensure accurate processing.
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