The difference between vias solder mask opening and covered in the PCB design

About the difference between vias solder mask opening and vias solder mask covered, in fact, it is about whether or not the vias are insulation treated.

Vias solder mask opening：In PCB design, vias are exposed without any treatment.

 (Vias solder mask opening)      

Vias solder mask covered：It means cover the vias with solder mask ink. For example, if your board solder mask color is blue, then cover the vias with the blue solder mask ink.       

 (Vias solder mask covered)

As seen in the pictures above, the up vias (on green board) have solder mask openings, the down vias (on blue board) covered with solder mask.

If there is no special requirement, vias solder mask opening and vias solder mask covered are both fine. If you need to test the vias with the multimeter, then you should choose the vias solder mask opening. Personally, we prefer vias solder mask covered, so that the overall appearance of the pcb board looks beautiful.