Table of Contents
Have you ever attempted to screw a screw into delicate woodwork, only to discover that the screw demands a tapered hole for a flush fit? The same principle applies to PCB design.
When securing a PCB to a workpiece, the heads of standard screws may protrude, creating an uneven surface. This is where countersink holes are used. Such holes enable flat-head screws or rivets to be embedded seamlessly into the board, thereby improving both its appearance and mechanical stability.
As this article proceeds, it covers everything regarding countersink holes in PCB design, definition, types, and applications, manufacturing considerations and comparison of countersink vs counterbore holes.
What is a Countersink Hole?
A countersink hole in a printed circuit board (PCB) serves as a unique hole shape with a conical section at the top surface. When an opening in the top surface of a drilled hole is enlarged into a sloped recess with the same slope as a flat-head screw, this creates a countersink hole.
The screw head is left sticking out on the top of the board by standard holes. The countersunk hole enables the screw head to either be flat or shallow on the board, providing a better fit of the PCB. Countersink holes come in a variety of angles and depths depending on the screw used, as well as the mechanical rules of the assembly.
Types of PCB Countersink Hole
PCB counter sink holes are generally categorized into different types on the basis of two factors, namely angle and shape. The following are some types of PCB counter sink holes.
By Angle
The angle of the countersink determines how the screw will be screwed into the PCB. Most common angles include:
-
82° Countersink hole: Commonly used in the U.S., often used for flathead screws.
-
90° Countersink hole: Commonly used around the world, mostly in Europe and Asia.
-
100° Countersink hole: A wide bearing surface may reduce stress on the PCB.
The angle is chosen depending on the screw standard/requirement and mechanical design.
Based on Depth
Based on depth, PCB countersink holes are further categorized into two, as follows,
-
Through-Hole Countersink: The conical cut is continued until it reaches a hole that has been drilled through the board so that the screw can fully pass through it.
-
Blind Countersink: It is a cut for which the screw fulfills every requirement of the countersink part at the screw head, but does not extend to the other side of the PCB.
Asymmetrical vs. Symmetrical Counter sink
-
Symmetrical Countersink Hole: Perfectly symmetrical with equal angles on both sides of the hole drilled.
-
Asymmetrical Countersink Hole: Asymmetrical Countersink Hole is applied when the screw or part needs an abnormal seating position.
Why Use Countersink Holes?
Countersunk holes offer the following advantages when securing workpieces:
-
Flush Surface: The main advantage of a countersink hole is that screws, especially the flathead type, are flush or slightly indented on the PCB surface. This will avoid protrusion and keep the surface of the board smooth and even.
-
Improved Aesthetics: Countersinking helps to provide a more professional and elegant appearance to electronic assemblies. The PCB gets a clean finish by removing the raised screw heads, thus being highly essential in products that are consumer-facing, since appearance counts.
-
Mechanical Stability: A countersink hole forms a natural point of screw fixation in PCB, so that screws are securely fixed in place. This enhances the mechanical stability of the circuit board, which reduces the risks of loosening with time.
-
Prevents Interference: Space is often restricted in most assemblies, and protruding screws may cause contact with other parts or connectors. Countersink holes can be used to avoid this problem by embedding the screw head, making it smooth in its integration with the other parts.
-
Space Optimization: In small-sized designs, a millimeter of clearance is important. Countersinking allows maximum utilization of available space, and this makes it suitable in cases where the board fits in housing or enclosures that are tight.
How to Manufacture a Countersink Hole
Achieving a perfectly countersunk hole is no easy feat. The main challenge lies in achieving precise positioning and depth control during secondary drilling. The most reliable and widely used process is the two-step drilling method.
- First, a specialized flat-tip or countersink bit with selectable angles (such as 82°, 90°, 100°, 120° or 140°) is used to create the upper countersink groove.
- Next, a standard round drill bit is used to drill the lower through-hole precisely at the centre of the countersink groove. The essence of these two steps lies in using the same positioning coordinate system (typically the L-marker on the PCB workbench).
Ideally, both steps should be completed consecutively in a single setup to avoid centre misalignment caused by minor board expansion/contraction or repositioning. Any slight deviation (e.g. unilateral deviation exceeding 0.1 mm) may result in enlargement of the countersink hole, damaged hole walls or even complete rejection of the board.
Manufacturing Requirements and Considerations
There are a lot of key factors that must be kept in mind while designing and manufacturing the countersink holes in PCB, as countersink holes require utmost precision in alignment. Some of the factors are jotted down:
-
Hole position and orientation: clearly specify the location of the holes and indicate whether the countersunk holes are on the top, bottom or sides of the PCB.
-
Clearly Specify Angle and Diameter: PCB designers must accurately define the diameter and angle of the countersink hole in the PCB design file, which will be sent to the PCB fabricator. Key parameters include:
-
Through-hole diameter: define the final size of the main hole, ensuring it matches the screw shaft diameter for a proper fit.
-
Countersink diameter: This should be slightly larger than the screw head diameter to allow clearance. For instance, a 6-millimetre screw head might need a 6.5-millimetre countersink to prevent it from getting stuck.
-
Countersink angle: provide the desired angle, typically 82° or 90°, to match standard flat-head screws.
-
Countersink depth: If precise depth is required, specify the depth numerically.
-
-
Plated or unplated countersunk holes: In many cases, PCB mounting holes are non-plated through holes (NPTH), meaning the copper plating does not extend throughout the entire hole. Countersunk holes can be treated similarly, but plating may be required in certain situations. For example, if a screw is intended to form an electrical connection, such as grounding the PCB to the chassis, plating is necessary to ensure the screw can make contact with the copper.
-
Tolerance: It is recommended to ensure that screws fit properly in the countersink hole by maintaining precise tolerances to avoid any mismatch. The standard countersink hole depth tolerance is ±0.05 mm, with an angle tolerance of ±1°.
-
PCB Thickness: The thickness of the PCB is an important countersink hole manufacturing consideration that PCB designers must address while designing countersink holes in PCBs. If the PCB is not thick enough and the designer uses a countersink hole in that PCB, then this weakens the fabricated PCB.
-
Material Consideration: Drilling a countersink hole in a fabricated PCB imposes stress in certain areas in the PCB or even leads to a stress crack in the PCB. For this reason, FR-4 material is used in the fabrication processes of PCBs. If any other special material or laminates are used in PCB fabrication, then such a PCB may require additional care.Additionally, to protect exposed materials and ensure the longevity of the PCB, manufacturers may apply coatings or surface treatments to prevent oxidation. When machining countersunk holes in extremely thin circuit boards, designers must ensure that there is sufficient material beneath the hole to avoid compromising the PCB’s structural integrity.
- Communication with PCB Manufacturers: It is important to communicate clearly with PCB fabricators regarding each and every detail of countersink, along with PCB requirements, to ensure optimal manufacturability.
- A clear requirement example: Holes A and B: Drilled 3.00 mm ±0.05, countersunk 6.00 mm ±0.1 to the top surface at 90°. Unplated.
Countersink vs Counterbore Hole
The terms countersink and counterbore are often confused, but they are different.
|
Feature
|
Countersink Hole
|
Counterbore Hole
|
|
Shape
|
Conical recess
|
Cylindrical recess
|
|
Screw Type
|
Flathead screws
|
Socket head or roundhead screws
|
|
Surface Result
|
Screw head sits flush with or below the PCB surface
|
Screw head sits flat but not flush (recessed in a cylindrical cavity)
|
|
Applications
|
Slim designs, aesthetics, and flush mounting
|
Heavy-duty assemblies requiring deeper seating
|
Conclusion
Countersink holes could appear to be a minor consideration when it comes to PCB design, yet they can have huge implications in functionality, durability, and appearance. These features are essential to the needs of space-constrained applications, or when design accuracy is vital (such as flush mounting or mechanical stability).
ELEPCB can assist you in your next PCB project when you are unsure of whether you should use countersink holes or not. Contact us and have the required design and manufacturing of a professional PCB that perfectly suits you.
FAQ
A1: Depending on the screw standard and application, the most frequent countersink angles are 82°, 90°, and 100°.
A2: Yes, they may be unilateral or bilateral, depending on whether the screws are going to be attached to one side or both sides.
A3: Countersink holes are conical, intended to receive flathead screws, whereas counterbore holes are cylindrical, intended to receive socket head screws.
A4: The IPC-2221/IPC-2222 standards cover general PCB design requirements. While they do not contain extensive sections on countersinks, they do clearly document mechanical characteristics and provide recommended tolerances.
A5: PCB manufacturing files require additional information about countersunk holes. You should perform one or more of the following actions:
Include a manufacturing drawing that clearly marks the countersunk holes and specifies the required countersink diameter and angle (e.g. ‘Top countersink 5.5 mm x 90°, Hole #10’). Using the ⌵ or “V” countersink symbol to abbreviate countersunk holes is common practice.
If your CAD software supports it, generate a separate drilling file for countersunk holes (sometimes called a ‘counterdrill’ file). This file will contain the coordinates and tool dimensions for the countersink drill. Communicate explicitly with the manufacturer.
A6: They incur certain machining expenses and complexity, but when ordered appropriately, the payoffs exceed the incremental cost in assemblies of precision.
CONTACT US
English 
Italian 
Hungarian