When designing PCBs with mounting holes for screws, there’s often debate about whether the holes should be plated or unplated. Plated holes are generally believed to offer better mechanical support for screws, reducing the risk of PCB damage during assembly or prolonged use.
However, it’s worth noting that many commonly used PCBs, such as the Arduino UNO, feature unplated mounting holes. This raises the question of whether plating is always necessary or if it depends on specific use cases or cost considerations.
Plated mounting holes don’t necessarily provide better mechanical support or protection. Since mounting holes are typically larger than the screws they accommodate, the material coating the interior of the hole doesn’t significantly impact performance. However, the decision to use non-plated holes can introduce additional manufacturing steps. Typically, PCBs are drilled, plated, and finished in a single streamlined process. If non-plated holes are required, an extra drilling run after plating becomes necessary, which can increase both time and cost.
Some manufacturers may handle this differently based on equipment availability. For instance, smaller holes for through-hole components might be drilled using one machine, while larger holes for mounting might use a separate machine. If the larger holes are drilled after plating, it doesn’t add a new step and saves on plating material, which could explain why PCBs like the Arduino UNO often feature non-plated mounting holes. It’s also important to distinguish between plating the interior of the hole and the presence of pads around the mounting hole. Pads around mounting holes serve a different purpose and could be a separate consideration entirely.
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Here are a few important considerations regarding plated mounting holes on PCBs:
Plating thickness is not controlled to extremely tight tolerances by board manufacturers. A general rule of thumb is around 0.001". This thickness is minimal and doesn’t provide substantial mechanical support. Always specify the finished hole size in your design. Since plating thickness can vary, it might lead to mounting holes being tighter than intended. Planning for this variability ensures proper fit.
If the plating connects to an internal ground plane and the screw is over-torqued during assembly, there’s a risk of damaging the plating, potentially breaking the connection. This highlights the importance of carefully managing tolerances and torque specifications. Some board houses might avoid plating holes that don’t have electrical connections. Ensure your design clearly specifies which holes need plating to avoid confusion or delays.
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Plating all mounting holes is generally a good practice to avoid potential manufacturing errors where holes might unintentionally remain unplated. This oversight could lead to major issues, so it’s better to err on the side of caution and ensure plating.
As for connecting mounting holes to the local PCB ground (0V), this depends on the application. While some designs may not benefit from this, many—especially those housed in metal enclosures—can gain from reduced noise and improved grounding. To keep flexibility, you can connect mounting holes to the ground plane through small jumper pads or similar provisions, allowing the connection to be bridged or left unconnected as needed.
This approach is particularly helpful during bench testing, as it offers a convenient grounding point for external connections like input 0V wires. It’s a small addition that can add significant utility without complicating the design.
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I do not see much issue with plated versus non-plated mounting holes. It all depends on what the design requires. I tend to lean towards plated for most situations.
One item that has not yet been discussed is if your assembly is wave soldered. Plated mounting holes generally need to be be shielded from the wave so the holes do not fill with solder. Generally if the holes need to remain open but the holes are plated, then a wave pallet or othe type of covering is required to keep holes clear. This is an added process step or tooling which in turn adds some cost at the assembly level. I do not worry much about this item, but just another thing to keep in mind if the design has no other compelling reason to choose plated versus non-plated.
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One consideration that hasn’t been mentioned often is the impact of plated mounting holes during wave soldering. If the board goes through a wave soldering process, plated mounting holes need to be masked with Kapton tape to prevent solder from filling the holes. This masking and subsequent removal add extra steps to assembly.
Using non-plated mounting holes with solder mask covering their bottom pads eliminates the need for masking. This simplifies the process and reduces assembly time and cost. A board assembler contractor recommended this approach to me years ago, and I’ve adopted it for all my designs since. It’s a practical solution worth considering for designs that involve wave soldering.
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When deciding between plated and unplated mounting holes, it’s essential to consider the intended use of the PCB:
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If the PCB is designed to be manipulated by end-users, such as a PC motherboard or a reusable module, plating damage caused by screw threads can be a concern. Scraps from damaged plating might contaminate the board, potentially leading to failures. In such cases, unplated mounting holes could be safer. For instance, the Arduino UNO features unplated mounting holes. Whether this was a deliberate decision or a legacy from its predecessor’s single-sided, DIY-friendly design, it aligns well with its use case as a reusable and frequently handled module.
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For boards that will be enclosed and remain untouched after assembly, plated mounting holes are often preferable. They provide added protection against mechanical stress during initial assembly.
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Non-plated through holes can add extra steps to the PCB fabrication process. While this might increase costs for high-volume production runs, it’s less significant for smaller batches, especially with online board houses that offer standardized processes.
Ultimately, the choice depends on the specific application and usage scenarios. Unplated holes are practical for boards expected to endure frequent handling, while plated holes are better for static or enclosed applications where durability during assembly is more critical.
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