Via Plugging Versus Tenting: Which To Use

Three Treatments

Vias are covered in one of three ways, and the three are often confused. Tenting covers the via with solder mask on one or both sides without filling the barrel. Plugging pushes mask or a dedicated material into the barrel, sealing it. Filling, which is a separate process again, fills the barrel with a conductive or non-conductive material and is usually followed by a cap of plated copper. The distinction matters because the three have different costs, different effects on the surface and different abilities to stop solder.

Tenting

A tent is a mask film stretched over the via opening. It is the cheapest treatment, it needs no extra process step beyond the normal mask application, and it keeps solder away from the via during wave or selective soldering, which is its main purpose. Its limitations are that the mask over a hole is thin and can crack or lift, particularly if the via is large or if the board is thermally cycled, and that it leaves the barrel open beneath the film. A tented via is therefore a reasonable choice on a via that is not under a pad and not required to be probeable, and it should not be used where the via is large relative to the mask thickness.

Plugging

Plugging pushes mask into the barrel, which seals the hole rather than covering it. It is done as a separate operation, usually by screen printing, and the material is cured with the mask. The result is more robust than a tent, because the barrel is filled rather than bridged, and it prevents solder from entering during wave soldering and prevents flux and cleaning fluids from being trapped. Where a via must not be probeable and must not be solderable, plugging is the appropriate answer. The plug does not necessarily give a flat surface, so it is not a substitute for filling where a pad must be printed with paste.

Filling and Capping

Filling goes further and puts a specific material into the barrel, often with a plated cap over the top so that the pad is a continuous metal surface. This is what a via in pad requires, and it is the treatment that gives a flat, solderable surface. The materials and the planarity requirements are described separately, but the distinction from plugging is the presence of the cap and the flatness requirement. A filled and capped via costs more than a plugged one because it adds plating and planarisation steps.

Which Via Needs Which

The choice follows from what the via has to do. A via under a component pad that must take paste has to be filled and capped, because an open barrel wicks solder away from the joint. A via in a wave-soldered area that must not take solder can be tented or plugged. A via that must be probed needs to be left open or given a test pad. A via that carries heat needs a conductive fill. A via that is simply a routing connection and is not exposed to solder can be tented, which is the cheapest option. Applying one treatment to every via on a board, which is what happens when the rule is set globally, always costs more than necessary or leaves some vias with the wrong treatment.

Effect on Solder and Reflow

The reason vias matter at assembly is solder wicking. An open barrel under or beside a pad acts as a reservoir, and molten alloy flows into it under the pressure of the reflow and the capillary action of the barrel. The result is a starved joint, a joint that is open, or solder that appears on the opposite side of the board where it does not belong. Tenting stops the solder at the surface but can be defeated by a hole that is too large or by a mask that lifts; plugging and filling stop it inside the barrel. Where a via is close to a pad but not under it, the same mechanism applies at a smaller scale, and tenting is usually sufficient.

Specifying on the Drawing

Because global rules are wrong somewhere, the drawing should state which vias receive which treatment. The practical way is to mark the filled and capped vias, since they are the fewest and the most expensive, and to state a default for the rest. Vias that must remain open for probing should be marked as well. Where the shop is asked to choose, they will choose the cheapest option that satisfies the note, which is correct behaviour but may not be what the design assumed.

Via Treatment and Inspection

The treatment also decides what an inspector sees. A tented via is invisible under the mask, a plugged via is a slightly different colour in the plug area, and a filled and capped via appears as a shadow in an X-ray image of the joint above it. An inspector who does not know which vias have been filled will record the shadows as voids, and a report full of false voids is worse than no report because it hides the real ones. Marking the treated vias on the inspection documentation, and stating the default treatment for the rest, turns the inspection into a check against a known expectation rather than a judgement. The same information is useful in failure analysis months later, when the question is whether a joint failed because of a via treatment that was not applied or because of a process that changed.

FAQ

What is the difference between tenting and plugging? Tenting covers the via with mask without filling the barrel; plugging pushes material into the barrel and seals it.

When do I need filling? When the via is under a pad that must receive solder paste, where the barrel would otherwise wick the solder away.

Is a tented via reliable? For a small via that is not thermally cycled hard, yes. For a large via or a demanding environment, plugging or filling is more robust.

Does plugging make the surface flat? Not necessarily. Only a filled and capped via gives a flat, solderable surface.

Can I apply one treatment to all vias? You can, but it will either cost more than necessary or leave some vias unsuited to their function.

Conclusion

Tenting, plugging and filling are three different processes with three different purposes, and the correct choice depends on whether the via must take solder, must not take solder, or must be flat. Set a sensible default, mark the filled and probeable vias explicitly, and check the vias under component pads before the data is released. Via treatment capability is described under PCB capabilities, the plugging and filling steps belong to PCB manufacturing, and the placement of vias relative to pads is part of PCB design and layout. Via treatment is usually confirmed during SMT PCB assembly in 2026.