3

I've noticed this more and more now that I'm designing PCBs often. Below is one example, from a motherboard. Note the eight vias around the through-hole used for screw mounting.

What is the purpose of these vias?

photo of printed circuit board with eight vias around a mechanical screw hole

I'm sure the vias are connected to the PCB's ground plane, but if their only purpose is to make a ground contact to the chassis (through the screw in the hole), why not just pull back the soldermask? That would provide even more contact with the screw. I've also seen this configuration in situations where the mechanical hole was plated-through. So I can't see grounding being the entire reason. Do the vias have some beneficial mechanical property?

user4718
  • 390
  • 3
  • 11
  • 4
    *Do the vias have some beneficial mechanical property?* So you do know ! They are there to keep the copper in place. Mechanical stress can easily dislodge the copper from the PCB material. With the vias in place the copper is sort of "bolted" to the PCB. Also the vias will get some solder on top making sure the electrical contact with the screw is in place. – Bimpelrekkie Jun 09 '16 at 13:25
  • Ok, yeah, it is a duplicate of the second half of that guy's question. But I wish he had posted a zoomed-in picture like I did :) I will not be pissed off if you close this as a duplicate. – user4718 Jun 09 '16 at 13:35
  • 1
    They've always appeared to me as if they would have a (minor) function in vibration-resistance as well, like shake-proof crenellated washers. I've seen similar where the vias as pictured are blobs of solder, which I assumed to serve a similar purpose. – John U Jun 09 '16 at 13:43
  • To tie them into the ground plane. – Alex Jun 09 '16 at 15:32
  • Aggressive acts on the hole with an oversized screw or a drill bit to assist with same will strip the through plating from the hole. The vias will make sure the top surface will still be connected to all the relevant ground planes/layers/tracks. The solder bumps are also there to make positive contact under the screw head if there is no handy hole outline with a large solder mask opening in the designers parts library. – KalleMP Aug 30 '16 at 05:34

1 Answers1

3

If the screw (which should be smaller than the hole) is exactly in the middle of the hole, it won't touch the plating. So you can't rely on this.

Instead, they put a few openings in the solder mask (green paint) around the hole, which must be covered by solder paste so the thickness increases and it raises above the mask. It is not necessary to make it a via, but it make a shorter path between the bottom/internal ground layers and the screw.

dim
  • 15,845
  • 3
  • 39
  • 84
  • Fair enough. But the boards I've seen where the hole was plated-through also had the soldermask pulled back as well (I've made a slight edit to the question to indicate that). Regarding covering with solder paste, that wasn't done on the board above... so isn't that a problem? – user4718 Jun 09 '16 at 13:31
  • I'd say it could be a problem, yes. If the screw is not tightened enough to squeeze the mask, it won't make contact, then. As to why they don't remove the mask underneath the whole screw footprint, if they use paste, it is just not necessary, and may simply be a matter of preference (and paste volume, eventually). But since they don't use paste... I'd be almost tempted to say someone forgot to do his job correctly. – dim Jun 09 '16 at 13:38