Electronic – Why some vias have solder mask and others do not


Here's an area of a commercial PCB.

I'd like to know if the "missing" solder mask is a deliberate choice made by the PCB designer, and if so, why?

edit: changed graphic to highlight representative vias.
Blue circle – solder mask (gold-colored)
Red circle – no mask (silver colored)

enter image description here

Best Answer

It's an option in most PCB design software, both as a default and on a per-via basis.

The advantage of leaving the vias uncovered is you can probe them with test probes without having to scrape away the solder-mask first. The advantage of covering them is reduced risk of shorts or solder-stealing.

Personally I tend to start with them uncovered, but then cover the ones close to components.

I wonder if in your case the designer has made them covered by default on the power/ground nets, but uncovered by default on signal nets. That could make some sense as power/ground vias are less likely to need to be probed and are more likely to be in tight locations.

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