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 PCB 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.
It is important to distinguish between covering and filling. Covering simply covers the opening; the cavity remains. Filling involves completely filling the via with paste. For via-in-pad, filling is absolutely essential, as otherwise the solder will flow into the hole and the solder joint will remain incomplete.
The paste is usually applied by screen printing or using a dispenser. This is followed by a curing step. The material must be compatible with the laminate and the surface; otherwise, cracks or adhesion problems may occur later.
When filling, flatness is of paramount importance. If the paste protrudes or lies too low, the flatness is compromised. With fine structures and dense pad grids, this can interfere with component placement.
Another consideration is degassing. Air trapped within the via can outgas during soldering, causing blisters or voids. Working cleanly and ensuring the correct paste viscosity prevent this.
Not every printed circuit board requires this step. It is particularly worthwhile in cases of high drilling density, vacuum mounting or via-in-pad designs. In simple cases, covering the area with solder resist is often sufficient.