It is to plug the via holes by solder mask or by resin performed in a clean room.(Or fill the via holes with copper,which is another process).
![](https://pcbweller.com/app/uploads/2020/11/PCB-Via-hole-plugging-process.png)
How to do this? By screen printing to push soldermask into holes – a separate operation using an aluminium stencil.
![](https://pcbweller.com/app/uploads/2020/11/PCB-Via-hole-plugging.png)
WELLER perform the plugging criteria with 70% filling per hole. This is to eliminate the risk of “broken” circuit path after high temperature in assembly process.
![](https://pcbweller.com/app/uploads/2020/11/PCB-Via-hole-plugging-quality.png)