I detected a shortcircuit between two vias in one of our PCBs. The shortcircuit was caused because an offset on the solder mask leaves part of the via's ring "half opened". When the PCB was assembled, a little amount of tin produced an electrical contact between the vias. The PCB has PTH and SMT components.
I think there should not be any offset on the solder mask at all. I will contact the PCB manufacturer. But I have some questions regarding this issue.
Is this offset error on the solder mask a remarkable manufacturing mistake?
In other PCBs without offset on the solder mask, it is still possible to have electrical contact on the vias, because the via's hole is open. But I have seen some PCBs with the vias completely hidden, filled with solder mask.
In case filling the vias were desired is there a special manufacturing process for it, or that depends only on the diameter of the via's hole?