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Here is a picture of 3 mounting holes on three different PCBs.

enter image description here

The red one is what I prepared on Eagle CAD and used "hole" button on the left side menu , something like this:

enter image description here

  • Do I need to use "via" in order to have the same style mounting hole on the other PCBs?
  • As far as I can see, they are vias on the blue PCBs. Isn't it better to isolate the mounting holes on a PCB? What I am thinking to have a risk to get an unwanted signal or voltage to your ground plane as it is unisolated? or Does it improve grounding or tying grounds of unconnected PCBs?
  • Is it a gold finish on the mounting holes? Why would I want to have a gold finish on a screw hole that would increase the cost?
  • The mounting holes on blue PCBs have several holes on it, what is the purpose of having holes on the screw holes? Fabrication-wise, it seems it would slow down the production a little bit as each individual mounting(or screw) hole requires drill holes on it.

The upper blue PCB is from a projector, the bottom one is from a harddisk of a PC.

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The mounting holes that have the large copper pads around the hole are fabricated that way because it is desired to have that connected circuit be conductive to the mounting screw. In most cases it is the GND of the circuit that connects to the mounting hole pad and there is a desire to have such GND connect into the metal chassis to which the board mounts.

The small via holes in the mounting hole pads are designed to electrically connect the mounting hole pad to the pad on the opposite side of the board. In the case of a multi layer board with an inner plane layer the vias may also be used to connect the mounting hole pad to that inner layer.

In the past it was much less common to see mounting hole pads with these via holes. Instead the mounting hole was built as as a large plated through hole to connect to the opposite side and/or inner layers of the board. However, as in the examples you show, note that the more modern type mounting holes do not have plating in the large diameter screw hole and thus the need for the vias to provide the electrical connection.

Plating removal in screw mounting holes is primarily done for one reason. The sharp threads of a screw in the hole can cause small particles of metal to come off the plated hole. This is particularly true when boards might be removed and re-installed during testing and/or repair. These small particles can come out and appear on the circuit board or float around in the electronics enclosure. In the advent of surface mount components with vary narrow lead spacings these small metal particles from the screw holes can lead to shorts on the circuit leading to either intermittent circuit operation or outright failure.

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    \$\begingroup\$ Vias around the screw hole are alsonuaed for mechanical stiffness. If there are no vias and the screw is overthightened it can start to lift the copper pad around the hole. As vias are through hole plated they give better mechanical stiffness reducing the possibility of damaging the pad around the screw hole if the screw is accidentally overthightened. \$\endgroup\$ – IgorEE Nov 6 '14 at 20:23
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To get the same holes in your eagle project go to your schematic and click on add (or type add in the text box and hit enter). Now to go the holes library, then you see there is a MOUNT-PAD-ROUND and a MOUNT-PAD-SQUARE library. These are the library's you are looking for. In the schematic you can connect the pad to a signal, which is mostly used to connect to ground (not sure about this). And yes on the blue PCB's they are also functioning as via's probably because the screws are connected to ground.

The holes in your mounting hole is probably there so more current can flow through. To get these holes also in your eagle project you probably need to make your own library for that. The golden finish on the mounting hole is there because it is considered as a via or regular hole.

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