Via Filling or Tenting: Choosing a Via Treatment

A via is a hole connecting two layers, and once plating is complete the question becomes what to do with the opening. Via filling puts material into the barrel, while via tenting covers the opening with solder mask, and the two treatments solve different problems at different costs. The decision also changes the assembly process, because a filled via and a tented via present different surfaces to the stencil.

The choice is driven by what sits above the via: a pad for a component, a test point, a thermal path or nothing at all. Each case leads to a different answer, and the answer belongs at the design stage because it changes the fabrication process.

What a Via Has to Do After Drilling

Mechanically a via only has to carry a connection between layers, and a covered opening satisfies that requirement. Electrically a via is a small stub that adds inductance and capacitance to the net it belongs to. The stub also carries current, and its resistance is part of the net.

Thermally a via can be a heat path, and a filled via conducts heat far better than an empty one. The treatment therefore follows the function, and a board that asks the same via to serve several functions is where the compromises appear. Splitting the functions between two vias is usually cheaper than asking one via to do both.

Via Tenting with Solder Mask

Tenting covers the opening with the solder mask that is already being applied. The mask bridges the hole, and the barrel is left empty and sealed at both ends on a board processed on both sides. The tent is a film over a hole, and its strength comes from the mask rather than from the copper.

The quality of the tent depends on the mask, on the hole diameter and on the development step. A large hole with a thin mask produces a tent that sags or tears, while a small hole with a thick mask produces a reliable one. The imaging side is described in the notes on solder mask development. The limit is best expressed as a maximum hole diameter for a given mask thickness.

Filling and Plugging

Via filling places material into the barrel, and via plugging places it only in the opening. The distinction matters because a filled via is flush or nearly flush with the surface, while a plugged one is a shallow cap over an empty barrel. The two treatments are sometimes combined, with a plug over a partial fill.

Filling requires the material to reach the bottom of the hole without voids, which is done by vacuum assisted dispensing or by screen printing with a squeegee. Plugging is faster and is used where the requirement is only to stop solder wicking through the hole. Printing scales to a whole panel, while dispensing suits a small number of large vias.

Conductive Versus Non-Conductive Fill

Conductive fill is used where the via is part of a thermal or an electrical path and the resistance of the barrel alone is not enough. Non-conductive fill is used where the requirement is mechanical: a flat surface, a sealed barrel and a protected via wall. The choice also affects rework, because a conductive fill can be soldered and a non-conductive one cannot.

The two materials behave differently through the rest of the process. A conductive fill expands and contracts with the board, while a non-conductive one usually has a coefficient closer to that of the laminate, which changes the stress at the interface.

Planarity and Assembly

Assembly is where planarity becomes visible. A via that is not filled leaves a dimple in the pad above it, and paste printed over a dimple produces a smaller volume or, at fine pitch, an incomplete deposit. The dimple is small, and it is the reason a filled via is specified in the first place.

For a via in a pad, filling is the usual answer, and the fill has to be flush enough for the stencil to sit on a plane. The mask thickness over the same area is discussed in the notes on solder mask thickness.

Thermal Effects and Outgassing

An empty barrel traps air and process chemistry, and both are released when the board is heated. The release appears as a blowhole in a soldered joint or as a blister in the mask above the via. The trapped volume is the barrel volume, which grows with the square of the hole diameter.

A filled via removes most of that volume and the problem with it. A tented via leaves the volume in place and relies on the mask to contain the pressure, which is why a tent above a large via is the combination most often associated with blisters. Cure completeness affects the same property, and it is described in the notes on solder mask curing.

Choosing by Via Type

The type of via drives the choice. A small via away from any pad is normally tented, a via in a pad is filled, and a via used as a thermal path is filled with a conductive material and often plated over. The decision is recorded on the fabrication drawing rather than left to the fabricator.

A via on a fine pitch land pattern is the difficult case, because the fill has to be flush without leaving a bump that lifts the stencil. That requirement is a process capability rather than a preference, and it is confirmed on a test panel before the design is released.

Inspection and Acceptance

Inspection differs by treatment. A tent is checked for coverage and for tears, a fill is checked for voids and for recession below the surface, and a plugged via is checked for the depth of the plug and for the absence of a dimple.

A cross section on a sample shows the fill and the voids together, and it is the only method that shows what happened inside the barrel. Surface inspection shows the part of the result that the assembly process will see. The two methods answer different questions, and both are needed for a complete picture.

Cost and Process Control

Cost rises with the treatment. Tenting is nearly free because it uses the existing mask step, plugging adds a printing operation, and filling adds a material, a dispense or print step and often a planarisation step.

The cost is justified by the function rather than by appearance. A board that fills every via gains nothing on the vias that carry no thermal load, and a board that tents a via in a pad pays for the decision at assembly. The fill material itself is controlled like any other process material, with its lot and its cure recorded. A cheaper treatment that fails at assembly is not cheaper.

FAQ

Can a tented via be used under a pad? It can be used where the pad is large and the paste volume is generous. At fine pitch the dimple reduces the deposit and the risk rises quickly.

Do filled vias need to be plated over? Not always. Plating over the fill is used where the via is also a pad and where the surface has to be flat for assembly.

Which treatment is best for thermal vias? Filling with a conductive material, because the fill carries heat that an empty barrel does not.