The choice between covering a via with solder mask and plugging it is not arbitrary.
 
- Solder Mask Over Via (SMO) : Solder mask is applied to the surface of the via to cover it. This process is simple and low-cost, offering protection against oxidation and preventing solder overflow. However, the inside of the via remains hollow.
- Solder Mask Plugged Via (SMP) : The via is completely filled with solder mask first, followed by a solder mask coating on its surface. This method thoroughly blocks solder from flowing into the via, preventing short circuits and enhancing reliability.
 
Therefore, solder mask over is sufficient for ordinary PCBs. However, solder mask plugged is mandatory for BGA (Ball Grid Array) dense areas; otherwise, the rework rate will be extremely high.