Tented Via: 5 Rules for Mask Coverage
A tented via is a via whose opening is covered by solder mask, so the hole and its annular ring are sealed under the coating instead of being left open. The purpose is simple. An open via next to a pad can wick solder through the barrel during wave soldering or reflow, and the solder that travels through the hole can appear on the far side as a short or a stray ball. Via tenting closes that path.
The tent is only as good as its coverage. Mask that fails to bridge the hole sags into it, cures thin over the middle, and cracks when the air inside expands in the oven. Where the via is large, or where a test point or a solderable feature sits on it, tenting is the wrong choice and the via has to be plugged or filled instead.
What Tenting Does for a Via
Tenting does three jobs at once. It stops solder from entering the barrel, it keeps flux and cleaning fluid from being trapped inside the hole, and it prevents the bare copper of the annular ring from being exposed to the environment. On a dense board those three benefits are usually enough to justify the extra mask work.
The coverage also improves the appearance of the finished board and removes a place where surface contamination can collect. For a no clean process, keeping the hole closed means the residue that remains is only on the surfaces that were meant to be fluxed, which makes the ionic cleanliness reading easier to interpret.
Tented, Plugged and Filled Vias Compared
Tenting is the lightest of the three options. Plugging pushes mask or a dedicated plugging material into the barrel, and filling uses a resin or copper to occupy the hole completely before the surface is plated or capped. The IPC via protection types give each of these a name so that a drawing can specify one without ambiguity.
The choice follows the via diameter and the surface. Small vias, typically below about four tenths of a millimetre, tent reliably. Larger vias need plugging, because a mask dam cannot bridge a wide hole with enough thickness to survive assembly. Vias that sit inside a pad on a ball grid array need filling and capping, since the surface must stay flat enough for the solder ball to sit on it.
Tent Diameter and Mask Registration
The tent has to overlap the annular ring on every side, and the overlap is what absorbs registration error. If the mask image shifts by the same amount as the overlap, one side of the ring is exposed and the tent has failed at that edge. The usual rule is to add a margin of about a tenth of a millimetre on each side, then confirm it against the shop’s registration capability. Mask coverage that stops short on one side is worse than no tent at all, because the exposed ring still looks protected at a glance.
Registration is not the only variable. The mask image also changes size during development and cure, so a tent that is correct on the artwork can be smaller on the finished board. Where the vias are close together, the mask between them becomes a narrow dam that has its own limits, and the solder mask development step decides whether that dam survives.
Mask Thickness Over the Via
Liquid mask flows, and a small amount always sags into an open hole. The result is a film over the middle of the via that is thinner than the film on the surrounding laminate. Thickness is what gives the tent its mechanical strength, so a thin centre is a weak tent even when the coverage looks complete from above.
Two things help. The first is a solder mask viscosity that is high enough to resist running into the hole. The second is to reduce the diameter that has to be bridged, either by plugging the via first or by choosing a smaller finished hole. Where the design cannot change, a dry film mask applied as a sheet can give a more uniform tent than a screen printed liquid.
Why Tents Crack in Reflow
Every via contains air, and that air expands when the board is heated. Under a tent, the expanding air pushes against the film from below, and a film that is thin or poorly adhered lifts or splits. The failure often appears as a small blister beside the via or as a flake of mask sitting on a neighbouring pad.
The same mechanism explains why tented vias near a large thermal mass are the first to fail. Those areas stay hot longest, so the pressure has more time to build. Keeping moisture out of the laminate before assembly reduces the effect, and the solder mask peeling article covers the adhesion side of the same problem.
Tenting and Test Points
A test point has to stay exposed, so a via used as a test point must not be tented on the side that the probe touches. On a double sided test strategy, the same via may need to be open on both faces, which rules tenting out entirely and pushes the design toward a filled via with a plated cap.
Test point vias should also be kept away from the mask dam limits. A probe lands with some force, and mask beside the pad can be crushed if the opening is tight. The probe access requirements should be written into the fabrication notes at design release rather than added by the assembly house later.
Vias Under Components and BGA Areas
Vias placed inside a land pattern are the hardest case. Solder paste is printed over them, and any void or open hole lets paste drain away from the joint during reflow. Those vias must be filled and capped so that the surface is flat and solderable, and the cap must survive the same thermal cycle as the joint.
Outside the land, a tented via is usually acceptable under a component body, provided the mask is fully cured and the tent is not in contact with the part. Adhesive or underfill that flows over a tent can pull it away when the material cures, so the clearance between the via and the bond line should be checked during layout.
Documentation for the Assembly House
The fabrication drawing should state the required protection for each via class, not simply mark the board tented. A note that says all vias tented is misleading when some of them are test points or sit inside pads, and the shop has no way to know which is which without being told.
The clearest approach is to name the protection type, give the diameter range it applies to, and list the exceptions with their locations. Where the assembly process uses a pin in paste or a selective solder operation, the note should also say which vias must stay open, because those operations change the wicking risk that tenting was meant to remove.
Inspection and Acceptance Limits
Inspection of a tented via is mostly visual and is done with magnification or with automated optical inspection. What matters is that no hole is visible through the mask, that no tent is lifted at the edge, and that no mask has flowed onto a pad that has to be soldered.
Cross sections are used during qualification to confirm the mask thickness over the via and the profile of the film inside the hole. A tent that passes visual inspection but measures thin in section will fail in the field, and the section is the only check that sees it. Sample sections should be taken from a coupon that carries the same via sizes as the product.
Design Rules That Prevent Rework
Most tenting problems are created in layout. Keeping the smallest vias away from pads, leaving the mask dam wide enough, and reserving test point locations before the routing is finished removes the reasons for rework later. A short review of the via list against the fabrication notes at mask design stage costs very little.
Where a design must use tenting on a via that is marginal, the decision should be recorded with the reason, so that the next revision can change it deliberately. A rule that is applied everywhere without thought is the one that produces a field failure that nobody can explain.
FAQ
Does a tented via need to be plugged as well? No, but the two are often confused. Tenting covers the opening with mask and relies on the film to bridge the hole. Plugging fills the barrel with material, which gives a much stronger result on large vias. A via that is both plugged and covered is the most reliable option and the most expensive.
Can a tented via still be probed in circuit test? Not through the mask, because the probe needs metal to contact. Where a via has to act as a test point it must be left open on that side, and the tent should be applied only to the opposite face if the design allows it.
Why do tents fail after assembly even when they look perfect before? The usual cause is air or moisture trapped in the barrel. Heating the board expands it, and a thin film over the hole lifts. Larger vias, thinner mask and vias close to heavy copper are the conditions where this happens most, and the fix is to plug the via rather than to add more mask.
