Mounting holes are non-plated or plated through-holes in a printed circuit board (PCB) whose primary function is mechanical fixation rather than electrical connection. They serve as the critical interface points where the PCB is securely attached to an enclosure, chassis, heatsink, or another PCB within the final product. A well-designed mounting hole system is fundamental to the structural integrity, reliability, and manufacturability of any electronic assembly.

1. Core Functions and Importance

Mounting holes fulfill several vital roles in electronic product design:

• Physical Securement: The primary role is to mechanically fasten the PCB using screws, standoffs, or other hardware, preventing movement or vibration that could damage solder joints or components.

• Grounding and Shielding (when plated): Plated mounting holes connected to the PCB’s ground plane can serve as low-impedance grounding points to the metal chassis, enhancing electromagnetic compatibility (EMC) by reducing noise and providing a shield.

• Alignment and Registration: Holes can be used with dowel pins or specific hardware to ensure precise alignment of the PCB during assembly, which is critical for connectors or interfaces that mate with the enclosure.

• Heat Transfer: When a PCB is mounted directly to a metal chassis or heatsink, mounting holes in thermal pads can improve heat dissipation from high-power components.

• Stress Relief: Properly designed mounting schemes distribute mechanical stress (from screws, vibration, shock) across the board, preventing warping or cracking.

2. Types of Mounting Holes

Mounting holes are categorized by their plating and connection to the PCB’s internal layers:

3. Key Design Specifications and Standards

Accurate specification in PCB design files is crucial for fabrication and assembly. Key parameters include:

• Hole Diameter (Finished Hole Size): The final diameter of the hole after plating (for PTH) or drilling (for NPTH). It must be larger than the fastener’s outer diameter to allow for tolerance. A common rule is: Hole Diameter = Screw Diameter + 0.15mm to +0.3mm.

• Annular Ring: The width of the copper pad surrounding the hole. A sufficient annular ring (typically >0.15mm) is critical for PTH holes to ensure a reliable plating connection and prevent breakout during drilling. NPTH holes should have a solder mask dam between the hole and any copper to prevent short circuits.

• Clearance (Keep-Out Area): A designated zone around the mounting hole where no components, traces, or copper pours are allowed. This prevents electrical shorts from the fastener and provides space for tooling (screwdriver, wrench). A minimum clearance of 1.5 to 2 times the screw head diameter is a good practice.

• Standards: The IPC-2221 generic standard on PCB design and IPC-7351 for land pattern standards provide guidelines for mounting hole geometries, clearances, and reliability considerations.
  

4. Best Practices for Mounting Hole Design and Layout

• Quantity and Placement: Use at least three mounting holes to prevent the PCB from rocking (a principle of three-point support). Distribute them symmetrically or near corners/heavy components to balance stress. Avoid placing holes too close to the board edge to prevent cracking.

• Material and Hardware Selection:

  • Choose screw and standoff materials compatible with the operating environment (e.g., stainless steel for corrosion resistance).

  • Use shoulder washers or insulating bushings in NPTH to prevent the metal screw from contacting and potentially shorting copper layers on the inner board edge.

  • Specify proper torque for screws to avoid over-tightening, which can warp the PCB or strip the threads in standoffs.

• Thermal and Grounding Considerations:

  • For thermal mounting, ensure the hole pattern matches the heatsink and use thermal interface material.

  • For grounding, connect multiple plated mounting holes to a robust ground plane with several thermal relief spokes or solid connections, depending on current and thermal needs.

5. Common Pitfalls and How to Avoid Them

6. Frequently Asked Questions (FAQ)

Q1: Should mounting holes be plated or non-plated?
A: Choose NPTH for simple mechanical mounting where electrical isolation is paramount. Choose PTH when you need a stronger hole, a grounding connection to the chassis, or when the hole will be used with a threaded metal standoff that requires plating for durability.

Q2: What is the standard size for an M3 screw mounting hole?
A: For an M3 screw (3mm nominal diameter), a typical finished hole diameter is 3.2mm to 3.4mm. Always confirm with your fastener and standoff specifications, and consider your PCB fabricator’s drilling tolerances.

Q3: How do you define mounting holes in PCB design software (like Altium or KiCad)?
A: Mounting holes are placed as drilled holes (pad or via objects). Set the correct hole size and pad size (for annular ring). For NPTH, set the pad’s plating property to “non-plated” in the tool’s properties. Assign them to a specific “MECHANICAL” or “MOUNTING” component class for easy BOM management.

Q4: Can a via be used as a small mounting hole?
A: It is not recommended. Vias are designed for electrical connections, are typically small (<1mm), and have thin plating not meant to withstand mechanical stress from screws. Using a via as a mounting point will almost certainly lead to cracked plating and board failure.