PCB Mounting Holes: Types, Sizes, and Placement Rules

Introduction

Whether you’re a PCB designer, hardware engineer, or hobbyist, choosing the right mounting holes is crucial for a safe design. In this article, you will learn standard PCB mounting hole types and sizes as well as clear placement rules to ensure proper fit, mechanical stability, and seamless assembly.

What are PCB Mounting Holes?

PCB mounting holes are typically located at the corners or edges of a board and are used for mechanical fastening. They keep the PCB stable and properly aligned in its device enclosure.

These important features provide physical stability and electrical grounding. When you use screws to mount your PCB through the holes, you prevent damage from vibration or movement. Additionally, grounding the PCB mounting holes connects the board to a metal chassis, reducing electromagnetic interference and improving safety. Mechanical holes in a PCB do not carry data as signal vias do. They are designed exclusively for matching with the internal alignment of your device’s support brackets or standoffs.

Common Types of PCB Mounting Holes

PCB housing holes fall under the category of plated (PTH) & non-plated (NPTH), depending on the electrical conduction. The type you choose will depend on whether you need the mounting point to be an electrical ground or just a mechanical anchor.

Plated Through-Holes (PTH)

A PTH mounting hole PCB has a copper feature inside the hole, which is used for grounded PCB mounting holes. At 2.4 GHz, they are useful for EMI and similar high-frequency applications. Connecting the PCB mounting hole to ground stabilizes the entire system at a specific reference potential.

Non-Plated Through-Holes (NPTH)

A PCB that utilizes an NPTH mounting hole has no conductive plating and is used for physical fastening only. These unplated mounting holes PCB design choices are economical and easier to manufacture for low-complexity projects. They are perfect for electrical insulation of PCB mounting holes where there needs to be no electric connection between the screw and circuit.

When you select plated vs. non-plated PCB mounting holes, you are making a decision between functional integration and manufacturing simplicity. The PCB designs for mounting PTH holes optimize the internal layers’ electrical connection for safeguarding purposes. Conversely, an NPTH mounting hole PCB is isolated and often used for easy mechanical support. Selecting PCB mounting hole design guidelines is a choice that you make based on your project’s fabrication cost.

Key Specifications for PCB Mounting Holes

The mounting holes of a PCB must be designed considering some physical and electrical requirements so that the PCB can fit in its enclosure. In this way, these specifications will affect the integrity of your structure and the assembly of your final product.

Correctly planning mechanical holes in PCB prevents failure of PCB and manufacturing delays. The size of the PCB mounting hole must balance with the PCB size, not to compromise the overall strength and other functional goals. Adhering to the guidelines for PCB mounting hole design ensures that the board stays fixed during the operational life of the PCB.

Size

The correct size of the PCB mounting hardware depends on the hole diameter and depth. When you choose a clearance hole size for PCB mounting, it should match the screw and the thickness of the PCB. If the size is not selected correctly, it can weaken the structure of the board or prevent the insertion of fasteners required for it.

Tolerance

Having a precise tolerance for the PCB mounting hole is critical to the stability and consistency of your assembly. Ensuring that these variations are kept to a minimum locks the board in the housing with no undue leverage or play. For applications requiring high precision, controlling these dimensions closely avoids assembly problems and improves the quality of the mechanical fit.

Plating

The plating uniformity and thickness of the PTH mounting hole PCB designs dictate conductivity and mechanical robustness. After assessing the need for corrosion resistance or the requirement of a grounding material, a common material like copper, gold, or nickel is selected. PCB mounting holes that are grounded and properly plated will maintain their electrical connection through time.

Layout

The placement of PCB mounting holes should distribute load to provide support without hindering component placement and routing. The layout of the circuit must ensure an optimum balance between mechanical fixation and electrical requirements. Consider the distance between the PCB’s edge and the mounting hole. If this distance is small, during the screw tightening process, the board could crack.

Hole Styles

Typically, standard designs have round holes for PCB mounting. However, more specialized applications might have unique square shapes or slots. Using the right style for your mounting hardware will help keep your assembly process as efficient as possible.

Countersunk Holes

Flat-head screws can sit flush with the board surface, creating a smooth exterior, thanks to the conical openings. They are particularly useful in slim consumer electronics where clearance at PCB mounting holes is limited.

Tooling Holes

Tooling holes refer to small, unplated mounting holes used in PCB design strictly for alignment during the manufacturing and assembly processes. Standard holes are rated around 1 mm to 1.5 mm and are created far from active circuit areas.

PCB Mounting Holes Size Chart

PCB mounting holes are the “screws” that hold your board inside its box. Unlike via holes, which act like “tiny wires” to move electricity, these holes keep your board still.

A computer-aided design (CAD) tool is used by PCB manufacturers to ensure accurate board specifications. For example, the size of an M3 PCB mounting hole is ideal and reliable for mid-size electronics in general. Always consider the tolerance of PCB mounting holes in the design to prevent the board from cracking or the screws from becoming loose.

PCB Mounting Holes vs Via Holes on a PCB

PCB-mounted holes mostly assist in the mechanical fixation of the PCB. Similarly, the via holes of the PCB will help in the electrical communication of certain circuits. Knowing the difference is significant for the structural integrity of your design and its electrical performance, too.

Most lead or pad holes and component hole openings look similar; they both seem like drilled holes. However, they serve very different purposes according to the PCB mounting hole design guidelines. Overall, the mounting holes are larger and positioned to accommodate the stress caused by the sizes of PCB mounting hardware. On the other hand, via holes are optimized for signals and are mostly much smaller to save board space. Mixing up the two can result in serious assembly mistakes or electrical shorts, for example, with grounded PCB mounting holes versus signal vias.

How & Why Connect Your Mounting Holes to Ground?

The mounting holes of the PCB that are grounded indeed connect the internal ground planes of your circuit with the chassis to eliminate electrical noise. When these holes are connected to ground, the discharge path will be provided, thus improving the overall EMC of your device.

Connecting the mounting holes to ground will mainly reduce EMI and provide a stable reference potential. Grounding PCB mounting holes turns the metal case or enclosure into a large metallic shield, effectively shielding sensitive devices from external signals. In order to achieve this, you have to use a PTH mounting hole PCB design where the metalized plating connects to the ground plane. This is often achieved by placing vias around the hole, which allows proper functioning even with multi-layer connections and also allows electrical return current and heat to escape.

Mounting Holes with Via Stitching

In high-performance design, it is common to see vias surrounding PCB mounting hole structures as a means to hold layers together. To make sure that mechanical and electrical stress is evenly distributed across the internal copper of the PCB, these small vias are placed in the PCB mounting hole pad.

• Stress Distribution: The vias help distribute the stress from the screws for PCB mounting holes to a larger area. This stops the copper pads from lifting or the board from cracking due to the physical tension of the fastener.
• Electrical Grounding: When you use vias around a PCB mounting hole, you create a low-impedance electrical connection between the top, bottom, and layer ground planes. The grounding provides a common ground potential throughout the assembly so as to reduce noise in high-frequency circuits.
• Thermal Management: The thermal management of PCB mounting holes with thermal relief and vias can act as a heatsink. A direct thermal path is created, which pulls heat away from the circuit and transfers it to the mounting hardware or the metal chassis.

Best Practices for PCB Hole Placement

Strategically placing holes in your PCB for mounting can keep a board secure and resistant to short circuits. By following the guidelines for PCB mounting hole design, you can avoid traces from cracking. It also aids in preventing damage to the device during vibrations or heavy use.

• Corner Placement: Position your PCB mounting holes at the corners of your board for symmetric mechanical support and stability. When putting holes in a standard-size 100 mm x 50 mm board, the distance must be at least 5 mm from the edge to ensure good strength.
• Clearance from Components: Ensure clearance from components: the PCB mounting hole keep out area shall be at least 3 mm to 5 mm from any trace or components nearby. This buffer prevents damage to the circuit while using screws for PCB mounting holes or installation tools.
• Symmetry: Spread your holes out symmetrically on the board surface to avoid uneven mechanical stress. A balanced layout keeps the PCB from bending or breaking if an external load is applied or if it expands.
• Enclosure Alignment: Always try to align the coordinates of your PCB mounting holes with the 3D model of the enclosure. To ensure that the PCB standoff holes match your housing perfectly, you should check alignment against the design files before sending the board to fabrication.
• Avoid High-Density Areas: Mechanical holes in PCB should not interfere with high-speed signals and dense components. The likelihood of preventing short circuits at the mounting holes of the PCB protects sensitive signals against a short circuit.

Choosing the Right PCB Mounting Hardware

Selecting suitable hardware guarantees that your board remains secure, as the size of the mounting hole on the PCB corresponds to the board’s thickness. Choosing fasteners for PCB mounting holes comes down to balancing strength and assembly. For high-vibration systems, you can use metal screws. For simple gadgets, use tool-less clips.

Screws and Standoffs

Fastening the PCB using screws is the most common way to do this. Screws are usually used with standoffs, which raise the PCB above the surface. A standard 1.6 mm board can be used for the hole M3 PCB mounting size with 5 mm to 10 mm standoffs. Perfect setup for applications that need easy removal. Metal hardware offers the most reliable hold in vibration situations.

Snap-In Clips

With the snap-in clips, there’s no need for any tool to mount your device. It simply snaps into pre-drilled mechanical holes in the PCB. Attention, consumer electronics devices have a very small weight and are very easy to install. These clips are perfect for high-volume production, which promotes on-speed assembly. However, they are not as effective on a heavy board.

Adhesive Pads

A PCB can be fixed with adhesive pads for lightweight and temporary applications, where PCB mounting hole placement design is not required. In conditions with a lot of vibrations, prolonged use is not recommended, as these may detach. The adhesive may weaken over time in the heat. They are better suited to rapid prototyping.

Rail Systems

DIN rail clips and brackets are used in industries to fix PCBs on the rail. This is used in control systems frequently for aligning various boards in a rack. It enables organized and easily accessible cable management during maintenance. The board can be exactly aligned with industrial cabinet standards.

Critical Design Rules for Mounting Holes

The mechanical and electrical requirements of PCB mounting holes must be well-defined to ensure functionality and manufacturability. By following standardized designs for mounting holes on a PCB, you will avoid assembly issues such as board cracking, mechanical misalignment, or electrical interference.

• Standardize Hole Sizes: Use standard hole sizes, such as 3.2 mm for M3 PCB mounting hole size. Standard hole sizes reduce the cost of production and offer ease of production.
• Maintain Edge Clearance: Ensure that the clearance from the edge of the PCB to the mounting hole is not less than 1.25 x the diameter of the hole to prevent cracking of the substrate during assembly.
• Define Keepout Areas: Establish a PCB mounting hole keepout area to ensure that screw heads or washers do not accidentally crush nearby components or traces.
• Select Plating Carefully: Choose your plating with care. PTH mounting hole PCB can ground through the mounting hole, while NPTH mounting hole PCB can provide isolated mechanical support. Make your choice based on your EMI.
• Verify Tolerance: To ensure the sizes of your PCB mounting hardware fit well, always specify a PCB mounting hole tolerance that allows for drill bit wear and plating thickness.

Design Tips for PCB Outline and Mounting Holes

To ensure stable PCB performance and smooth assembly, you should pay close attention to the board outline and mounting hole placement early in the layout process. This helps prevent mechanical stress, assembly issues, and production delays. Here are some practical design recommendations aimed at improving manufacturability and ease of assembly.

• Account for Board Thickness: Make sure the sizes of your PCB mounting hardware match the board thickness, which is normally in the 0.8 mm to 2.4 mm range.
• Reinforce Large Boards: For large PCBs, or boards over 200mm, it is best to add mechanical holes every 100mm along the center for the substrate not to flex.
• Use Design Software Features: Take advantage of design software features for an Altium PCB mounting hole or KiCad to mount your hole to define plated or non-plated correctly.
• Test Fitment: Try out your design to make sure that the clearance and alignment of the mounting hole on the PCB match your enclosure.

Common Mistakes to Avoid in PCB Outline and Mounting Hole Design

Even expert engineers can miss small details that lead to big delays or broken boards. Follow these simple rules to keep your printed circuit board mounting hole standards high and avoid common mistakes:

1. Ignoring Manufacturer Guidelines: If a PCB mounting hole tolerance is not respected, the boards will not be able to fit the housing of their intended use.
2. Overlooking Stress Points: If the holes for mounting the PCB are placed at less than 2mm from the edge of the board, the material gets weakened, and assembling with screws generally leads to cracks.
3. Forgetting Grounding Needs: Not utilizing a grounded PCB mounting hole in a high-frequency layout can result in EMI problems and signal integrity issues.
4. Inconsistent Outline Layers: Make sure your outline is on the appropriate mechanical layer. If it is assigned to the wrong layer, it won’t be manufactured.

FAQs

What Is the Most Common PCB Mounting Hole Diameter?

The standard diameter for PCB mounting holes is 3.2 mm, which allows for a clearance fit with M3 screws. This is the common size used throughout the industry for standard medium-sized boards, such as Arduino and many consumer electronics. It offers good mechanical strength with little wasted space.

Is A 2.5 Mm Mounting Hole Common?

A 2.5 mm mounting hole is quite uncommon in general electronics, but it is the exact standard used for all Raspberry Pi boards to fit M2.5 screws. Most industrial and hobbyist hardware centers around the 3.2 mm size. You will mostly find the 2.5 mm or 2.75 mm M2.5 PCB mounting hole in miniaturized or specific single-board computer ecosystems.

Should Mounting Holes Be Plated or Non-Plated?

Depending on whether you need to ground it electrically or make manufacturing simpler, the hole could be plated or non-plated. PTH mounting holes are preferred when you need to connect the PCB to a chassis ground or provide EMI shielding. In contrast to PTH mounting holes, NPTH mounting holes are cheaper to produce. Overall, they are better for just mechanical fastening and offer electrical isolation.

Should Mounting Holes Be Connected to Ground?

If your design needs EMI filtering or electrostatic discharge (ESD) protection, create a ground connection for your mounting holes. If the mounting hole on the printed circuit board is connected to ground, the metal enclosure of the device gets grounded through the hole, and then makes the enclosure act as a shield. In some analog circuits, you may keep them isolated to avoid ground loops.

Do Mounting Holes Need Vias Around Them?

To strengthen the mechanical properties of the copper pad and ensure electrical continuity, vias should be added around the PCB mounting hole structures. By preventing copper peeling when a screw is tightened, the vias provide additional paths for current and heat to transfer between the board layers and the mounting hardware.

Final Thoughts

As a professional PCB manufacturing factory, PCBMay provides free DFM checks to optimize your mounting hole layout. We are equipped with precision CNC and laser drilling for high-accuracy hole processing. All our production processes comply with IPC Class 2/3 standards. Just send us your Gerber file today to discuss for your project.