Via filling is the process of filling the vias with a non-conductive paste in order to close them. Usually this is required for PCBs with a large amount of drills when the fixation in assembly is done by vacuum lifter. By closing the via the runoff of solder is also prevented. Furthermore, via filling is used on inner layers when buried vias are present.
Depending on the application, there are various methods of covering and filling vias: they can be tented, filled or plugged from one or both sides, or for via-in-pad-locked solutions sealed with a metalized cover (capped). The different versions of via covering and filling are specified in international standard IPC 4761