2 Type PCB Countersink Holes design , V Countersink Holes and T Countersink Hours .
How to drill PCB Countersink Holes
A countersink is a conical hole cut into a manufactured object, or the cutter used to cut such a hole. A common usage is to allow the head of a countersink bolt or screw, when placed in the hole, to sit flush with or below the surface of the surrounding material.
When you plan to design countersink in your board, below information is required:
Which side of countersink hole be drilled? Like Bottom Layer?
Is the countersink hole plated or non-plated?
What is countersink hole diameter(d2)?
What is through hole diameter(d1)?
What is the depth of countersink hole need to sink(t1)?
What is angle of countersink hole(like 90 degree or other)?
Fr4 PCB Countersink Holes Aluminum PCB Countersink Holes
Introduction
A printed circuit board (PCB) is the foundation of an electronic device, providing the mechanical structure and electrical connections between components. As PCBs have become more complex, certain design techniques have been adopted to improve manufacturability and performance. One of these important features is the countersunk hole.
Countersunk holes serve several crucial functions in PCB design and fabrication. In this article, we will explore what countersunk holes are, why they are used, the different types, how to design and specify them correctly, and some best practices when using countersunk holes in a PCB design.
What is a Countersunk Hole?
A countersunk hole is a type of through-hole in a PCB that has a flared opening on one or both sides of the board. The hole is tapered, with the diameter increasing moving away from the center of the board. This flared opening provides clearance for a fastener head like a screw or bolt when secured to the PCB.
The key elements that characterize a countersunk hole are:
- Chamfer Angle – The angle between the tapered sidewall and the planar PCB surface. Common angles are 80-100°.
- Counterbore Depth – The depth of the chamfer or taper below the top surface of the PCB.
- Hole Diameter – The width of the hole at the center where it passes through the board. Often the same as the fastener shaft diameter.
- Surface Diameter – The width of the flared opening on the top (and sometimes bottom) of the PCB. This clearance allows the fastener head to sit flush with the PCB surface.
The countersink creates a smooth, flattened opening for the fastener head to sit in, allowing it to mount flush to the board surface. This is advantageous both for assembly of the components onto the board and achieving a low profile assembled product.
Why Use Countersunk Holes in PCBs?
There are several reasons why countersunk holes are extremely common in PCB designs:
Consistent Fastener Head Clearance
The tapered opening ensures consistent clearance between the fastener head and pad/trace on the board surface. Without the countersink, any small variations in board thickness, component position, or fastener length would change the clearance above the board surface. This could lead to insufficient clearance.
Flush Fastener Heads
The counterbore depth positions the fastener head flush with the PCB surface (when using flathead screws). This allows tight spacing to other components, heatsinks, and enclosure parts.
Smooth Surface Finish
No fastener heads protruding above the PCB surface gives a smooth, continuous surface ideal for EMI shielding, gaskets, and enclosures.
No Component Interference
If using pan head or other screw heads, the counterbore depth gives adequate clearance between the component and fastener head. This allows locating components near mounting holes.
Consistent Board Support
The edges of the counterbore provide consistent edge support to the PCB surface around the fastener, important for load bearing and vibrational stability.
Improved Manufacturability
A countersunk hole can ease assembly by guiding screws into the tapered opening. The clearance also allows some positioning tolerance.
Using countersunk holes whenever possible provides the most reliable PCB fastening while achieving a low-profile and compact design. The tapered opening ensures a consistent, solid fit for the fastener head.
Types of Countersunk Holes
There are two main types of countersunk holes used in PCBs, characterized according to which side the chamfer appears on.
Front Side Countersink
A front side countersink (sometimes called “countersunk”) has the tapered opening on the top layer or component side of the PCB. This allows fastener heads, such as flat head screws, to sit flush with the top surface where components are mounted.
Front side countersinks are the most common, used with any fastener that mounts components or hardware to the top surface of the board. The counterbore depth positions the fastener flush with the outer board layer.
Back Side Countersink
A back side countersink (sometimes called “counterbored”) has the tapered hole opening on the bottom side of the PCB. The straight hole walls pass through the board thickness with the chamfer on the underside.
Back side countersinks allow fastener heads to sit flush with the bottom surface. This is useful when the underside needs an unobstructed fastener profile, such as securing a PCB to a flat surface. Heat sink plates also benefit from a flat underside.
PCB Countersink Design Considerations
There are several important factors to consider when designing countersunk holes in a PCB. These parameters impact the hole’s manufacturability, component clearances, and fastener fit.
Counterbore Depth
The counterbore depth should match the thickness of the fastener head plus 10-20% clearance. This positions the fastener head flush within the chamfer opening. For example, a 4mm screw head would need ~4.5mm counterbore depth.
Insufficient depth can cause interference with components while excessive depth reduces the board support around the fastener.
Hole Diameter
The drill diameter should match the fastener shaft diameter, generally with 0.1-0.2mm clearance. Too large of a hole reduces friction and torque holding the fastener. Too small of a hole risks cracking the PCB when driving the screw.
Surface Diameter
The surface diameter must provide adequate clearance for the fastener head and any washers. About 0.25-0.5mm clearance beyond the head width is ideal for ease of assembly while minimizing unsupported board area.
Chamfer Angle
The angle between the hole wall and board surface is typically 90-100°. Larger angles over 100° provide more head clearance but less edge support and weaker laminate strength near the hole. Angles below 90° are possible but harder to manufacture.
Hole Pattern
Follow the fastener manufacturer’s recommended pattern for hole spacing relative to board edges. This avoids splitting the board or having it bend/bow under fastening force.
Plated vs Non-Plated
Countersunk holes are usually non-plated since only the top and bottom board surfaces require copper. Plated holes add unnecessary cost unless used as thru-hole vias. Non-plated holes can be plugged after drilling.
Copper Clearance
All copper on the chamfer surface must be cleared from the tapered hole. Any copper remnants create an uneven fastener interface. Remove copper at least 0.2mm beyond the surface diameter.
Annular Rings
Avoid locating pads or traces near a countersink opening. The drilled hole will not have complete annular rings, risking via breakout and limiting solder masked openings.
By carefully controlling these design factors, countersunk holes provide reliable, consistent clearance for fastener heads securing a PCB.
Specifying Countersunk Holes
Countersunk holes require several specifications when documenting the PCB layout. Different CAD tools use varying terminology and procedures for defining countersinks. However, the key parameters to communicate are:
- Hole diameter
- Counterbore depth
- Surface diameter
- Which side is countersunk
- Chamfer angle (sometimes assumed as 90°)
For example, a backside countersunk hole for an M3 screw with 4mm head would be specified as:
- Hole diameter: 3.1mm
- Counterbore depth: 4.6mm
- Surface diameter: 8mm
- Side: Back
- Angle: 90°
Providing these details in manufacturing drawings or CAD models ensures the PCB fabricator drills the holes properly for robust fastening.
Design Best Practices
Here are some best practices to consider when using countersunk holes in a PCB design:
- Minimize the number of mixed hole types when possible for simpler fabrication.
- Orient all screws/fasteners in one direction for easier assembly.
- Cluster groups of countersunk holes together, avoiding single isolated holes.
- Place holes in less critical board areas that are not dense with routing.
- Allow adequate spacing around holes for laminate to absorb loads without cracking.
- Specify tight diameter and depth tolerances for quality standards.
- Remove all copper from chamfer area and provide annular ring clearance (>0.2mm)
Carefully following these guidelines will result in a countersunk hole design that maximizes the benefits of flush fastening while minimizing manufacturing risks.
Applications and Examples
Countersunk holes are used in nearly all types of PCB designs where mechanical fastening is required. Here are some examples:
Component Mounting
PCB standoffs use front side countersinks to allow tight spacing between boards in a stacked assembly.
Heat Sink/Cooling Plates
Processors often mount heat sinks using countersunk holes for smooth contact surface and thermal transfer.
Shielding Cans
EMI shielding cans fasten to boards with countersunk holes to eliminate gaps in conductive enclosure.
Case/Enclosure Mounting
Nearly all types of electronic enclosures use countersunk screw holes to achieve a tight fit and smooth exterior surface.
Keyed Interconnects
Some board-to-board connectors orient and key boards using countersunk holes and guide pins.
There are almost limitless examples of the versatility of countersunk holes for securing PCBs in final products across all industries and applications.
Conclusion
In summary, countersunk holes are a critical PCB design feature that enables secure, reliable mechanical assembly. The tapered hole profile allows fastener heads to sit flush with the board surface, ensuring proper clearance, orientation, and maximum surface contact.
By understanding the different types of countersinks, key design parameters, and manufacturability considerations, PCB designers can effectively leverage countersunk holes in their projects. Following the best practices outlined above will result in a robust mechanical interface for any electronic device.
The countersink is a simple yet powerful concept that facilitates cost-effective, streamlined production of complex PCB assemblies, making it an essential tool for any designer working on rigid boards.
Frequently Asked Questions
What are the key benefits of using countersunk holes in PCBs?
The main benefits of countersunk holes are:
- Consistent fastener head clearance from components and board surface
- Flush mounting of fastener heads for smooth exterior finish
- Avoidance of component interference near mounting locations
- Even support and load distribution around fasteners
- Easier component assembly and alignment to board
Can plated-through holes be countersunk?
Yes, plated-through holes (PTHs) can be designed as countersunk holes. However, it does require more complex fabrication involving masked jet plating. Countersunk PTHs are generally more expensive and only used when electrically connecting between both sides is required.
What is the downside of excessive counterbore depth?
Too much counterbore depth leaves a thinner ring of board laminate material around the top of the hole. This can crack under fastening strain. Typically only 10-20% clearance beyond the fastener head thickness is recommended.
When should back side countersunk holes be used vs front side?
Back side countersinks allow flush fastener heads on the bottom board surface. This is advantageous when the underside requires smooth, unobstructed contact, such as for heat sinks. Front side countersinks are used in most applications to avoid component interference.
Can clearance holes be used instead of countersunk holes?
Clearance holes (straight through holes) can replace countersinks in some cases, but do not provide as reliable or consistent a fastening surface. Clearance holes may also require washers to prevent fastener heads contacting the board surface.
