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Mounting holes - how to set copper pour clearance.

In DesignSpark PCB a mounting hole is simply a through hole pad with the required hole size and copper pad area.

But what is a “correct” mounting hole implementation?

The choice is yours as the designer...

  1. Many designers recommend for a professional design to use a through plated pad with the copper pad diameter just greater than the mounting bolt head diameter. This can be further ruggidised by vias within the pad area to strengthen the structure and reduce any crushing effect.
    The copper pad provides a 'biting' surface for anti-shake washers, so this is the most robust solution for high vibration environments.
    Electrically the pad can also provide a grounding route to a metal assembly.
  2. A straight forward through hole pad as above but without vias is also frequently used particularly for light weight PCB assemblies.
  3. The third option is a simple hole. This is often used where the PCB is mounted in a plastic enclosure when electrical grounding cannot be applied. Self-tapping screws are often used biting into the palstic case mounting bush and is ideal for light weight PCB assemblies.

 

As described in the title this article discusses mounting holes in relation to a copper pour.
Mounting hole options 1 and 2 behave exactly as any pad so the copper pour will be isolated from the pads.

Using a mounting hole without a pad (option 3) however results in the copper pour covering the area up to the hole with a gap as set in the Design Technology spacing for "Shape to Pad".

Mounting hole as viewed in DesignSpark PCB

Obviously you do not want to change this as it will change all the copper to pad gaps.

The recommended solution is to add a "Keepout" for the copper pour. There are additional pour shapes available from the menu bar and the circle is the ideal.

Keep out menu as viewed in DesignSpark PCB

 

The circle diameter is set to provide clearance to the bolt head or washer to keep the copper out of this area. Place the Keepout shape and right click to view the properties, ensure it is set as a keepout.

Keep out property as viewed in DesignSpark PCB

Now perform the copper pour as normal and you will have the required clearance around the mounting hole.

Keep out clearance setting as viewed in DesignSpark PCB

 

The Pour Keepout approach can be applied in any situation where you require to customise the copper pour.

See also:

How do I create a custom PCB - "mounting holes"

 

 

RS Components / Allied Electronics Support for DesignSpark PCB

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