Via Tenting: Preparation, Placement and Process Control

Via tenting is one of the cheapest decisions on a printed circuit board, and it is also one of the most misunderstood. A tented via is simply a via whose pad and hole are both covered by solder mask, so the mask bridges the barrel like a small roof. Whether that roof survives assembly depends on the hole size and on the mask.

What Via Tenting Is

Via tenting is the practice of covering a via with solder mask so that the mask spans the pad and the hole. The mask forms a tent over the barrel, and the via is closed to the outside world. The purpose is to stop solder from running down the barrel during reflow, and to keep flux and contamination out of the hole.

A tented via is not the same thing as a plugged or a filled via. Tenting is a mask operation, so it adds no material inside the hole and no extra process step beyond the mask itself. That is why it is popular with cost sensitive designs: the feature is almost free.

The mask has to bridge the hole without sagging or cracking. On a small via the film can span the opening cleanly, but on a large hole the mask slumps into the barrel and leaves a thin spot. That thin spot is where a crack appears after the thermal shock of soldering.

Tenting, Plugging and Filling Compared

Via plugging pushes a paste or a resin into the barrel, and via filling goes further by filling the hole flush with the pad so a component can be placed over it. Both processes add material, and both add a process step with its own cure or planarisation.

Tenting is the cheapest of the three, and it is enough for a via that carries a signal and has no component above it. Where a via sits inside a thermal pad or under a BGA, plugging or filling is usually required, because a tent over a large hole will not survive the assembly.

The three treatments are often mixed on the same board, and the fabrication drawing has to say which via gets which. A note that says tent all vias will be applied literally, including the ones that need to stay open. The relevant via types should be listed by size or by net class.

If the distinction is left to the shop, the result is a board that tests well and fails in the field, because the via that had to be plugged was tented instead. The cost difference is small, but the reliability difference is not.

Solder Wicking and the Barrel

Solder wicking is the movement of molten solder down a via barrel under capillary action. On a via that is not covered, the solder can travel from the pad on one side to the other and form a blob that shorts a neighbouring net or lifts a component.

A tented via blocks the capillary path at the surface, which is where the solder would enter. The mask does not have to be perfect to be effective: even a partial tent reduces the volume of solder that can travel, because the opening is much smaller than the pad.

Wicking matters most on a wave soldered or a selectively soldered assembly, where a large volume of solder is available. In a reflow process the effect is smaller, but a via in a thermal pad can still drink enough solder to leave a void under the component.

Where a via must stay open for a test probe, the wicking risk is handled by keeping the via away from the solder source and by using a small hole. A via with a small diameter wicks far less than a large one, and the difference can be an order of magnitude.

Process Limits and Registration

Mask registration is the limit that catches most designs. The mask has to cover the whole pad, and the mask image is aligned to the copper within the registration tolerance of the shop. A tent on a pad that is barely larger than the hole has no margin for that shift.

The usual rule is to tent vias of about 0.3 mm or less, and to keep the pad at least 0.1 mm larger than the hole on each side. Below that the mask is unsupported and the tent is a film of a few tens of microns over a hole.

Thin mask films crack at the edge of the tent during thermal cycling, and the crack lets flux and moisture into the barrel. A mask that is applied as a liquid photoimageable film behaves differently from a dry film, and the shop should be asked which one is used.

The fabrication notes should state the tenting rule, the maximum via size that may be tented, and the vias that must remain open. A rule without a size limit is not a rule, because the shop has no way to apply it.

When Not to Tent

A via that is used as a test point must stay open. A probe needs to reach the copper, and a tent over the pad will either be punctured by the probe or will insulate it. Test vias should be excluded from the tenting rule by net class.

Thermal vias under a power device should not be tented on the side that faces the component, because the mask adds a thermal resistance and can trap flux. They are normally left open on one side and tented or plugged on the other.

A via that carries a high current should not be tented either, for the same reason as a thermal via. The mask is a polymer with a low thermal conductivity, and a tent over a current carrying via is a hot spot in an otherwise well designed plane.

Vias inside a fine pitch BGA field need special care, because the tent has to survive an assembly that is already tight. In that region a plugged via is the safer choice, even though the process costs more.

Inspection and Reliability

Tented vias are inspected visually and by the mask thickness over the hole. A tent that has collapsed shows as a dimple in the mask, and a tent that has cracked shows as a ring around the hole. Both are visible under a low power microscope.

The joint between the mask and the copper is the weak point, so adhesion matters more than thickness. A surface that is contaminated before the mask is applied will delaminate at the tent, and the delamination spreads along the barrel.

Where a via must survive a solderability test, the test should be run on a coupon that carries the same via size and the same mask. A coupon is the only way to prove the tent before the production panel is produced.

The mask finish also interacts with the surface finish on the exposed pads, so the surface finish and the mask should be chosen together rather than one at a time.

FAQ

Does tenting a via cost anything? It is essentially free, because the mask artwork defines it. There is a small yield cost when a shop has to hold a tight registration to keep the tent on the pad.

Can a tented via be probed in circuit test? Only if the tent is opened on the test side. The usual practice is to leave the test vias open and to tent the rest of the board.

What via size can be tented reliably? Around 0.3 mm or smaller is the common limit, with the pad large enough to give the mask support. Larger vias should be plugged instead.