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When looking at PCBs in consumer products or PCBs I have ordered from manufacturers myself, I notice that, while many of the vias are covered in soldermask, it seems like a few random vias are missing it for no apparent reason. This is true even if I specify soldermask to cover all of the vias.

some vias seem to be missing soldermask

Why is this? Does the soldermask get wicked down inside the vias during manufacturing? Is there a way to prevent this?

All of the vias on my projects are a minimum of 15 mils. If I used smaller vias would this not happen?

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    \$\begingroup\$ That PCB was designed with no relief for any of the vias. Its just that on some of them, the surface tension 'bubble' popped before the solder mask dried. \$\endgroup\$
    – brhans
    Jul 14, 2017 at 22:16

2 Answers 2

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Could be sloppy design, could be the solder mask just didn't stick.

However, it is generally better practice to relieve the solder mask around vias especially if the board is to be flow soldered. Solder filling the via improves the quality of the connection through the board.

If you WANT the mask to cover the holes, you are asking a bit much of a fluid to consistently bridge over air. Add in heating processes during soldering and the chances of a few popping is pretty high.

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  • \$\begingroup\$ Thanks for the perspective. I've heard about pros and cons to covering vias and not covering them. Are there some books that go into more depth on this topic? \$\endgroup\$
    – John M
    Jul 14, 2017 at 19:34
  • \$\begingroup\$ @johnny_boy more experience than anything else. But unless you have a specific reason to block the holes, maybe as part of a hermetic sealing process or something, they are better left open and clear or soldered through. A bad through hole plating is really hard to fix if its plugged with solder mask, and rework becomes harder too. \$\endgroup\$
    – Trevor_G
    Jul 14, 2017 at 19:37
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Judging by thus picture there are plugged vias and just normal ones. In normal via usually solder mask leaves a small uncovered margin to ensure solder mask doesn't get inside the via.

Edit: on this picture its even more like all vias are of same stackup (one side plugged), but some are "on top", others "on bottom".

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  • \$\begingroup\$ And what's wrong with solder mask getting inside the vias? \$\endgroup\$
    – John M
    Jul 14, 2017 at 19:34
  • \$\begingroup\$ Good question. First of all, the guy on assembly line may get mad on me, so have to cleanup all vias... Ok, if solder mask covers the via from both sides, or from one side it is the mask and a component from another side, yhere is an air bubble which will expand in the oven and cause some damage, reducing yield. Besides, some people claim air flow through via serves for cooling. I am a little sceptic about this. \$\endgroup\$
    – user76844
    Jul 14, 2017 at 19:38

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