Via Tenting Versus Via Exposure on a PCB

The vias on a finished board are either covered by the solder mask or left open, and the choice is rarely made deliberately. It is inherited from a template, or copied from the previous product, or decided by whoever generated the fabrication data. The consequence is a board that works but has a mask pattern that is more expensive to produce and harder to assemble than it needed to be.

What Tenting Actually Means

Tenting means covering a via with the solder mask so that the annular ring and the barrel opening are sealed under a layer of ink. The term describes the intent, not the result: on a large via the mask tends to sag into the hole or crack over the edge, so a practical tent is often a plug plus a covering layer rather than a single printed film.

Understanding which of those two things is being asked for matters, because a fabricator who is asked for a tent over a large via will either refuse or deliver something that fails. The useful instruction is a diameter threshold above which vias are plugged and below which they are covered, and that threshold belongs in the fabrication notes.

The Case for Covering Vias

Covering a via keeps solder away from the barrel. Solder that wicks into a via can travel to the other side of the board, create a short to a neighbouring feature or leave a void in the joint, and the risk is highest where the via sits close to a pad on the same net. Covering also reduces the chance of a flux residue being trapped in the barrel, which matters for boards that must be cleaned.

For vias that are not inside a pad, covering also prevents accidental contact with a probe or a test fixture and reduces the exposure of the copper to the environment. On a high-density board where the mask layer is the only protection over a via, that is a meaningful reliability argument.

The Case for Leaving Vias Open

Open vias are easier to manufacture and easier to inspect. A mask opening over a via removes the risk of the mask cracking at the edge of a hole, removes the risk of trapped solvent in a plugged barrel, and makes the plating visible for inspection. Where a via is used as a test point, opening it is the only way to make contact reliably.

There is also a thermal argument. A via that is open to the air dissipates slightly better than one buried beneath ink, which is a minor effect on a single via and a measurable one on an array used to move heat out of a component.

Via in Pad and Paste Bleed

A via placed inside a component pad changes the problem. If it is left open, solder paste printed on the pad will flow into the barrel during reflow, leaving less metal to form the joint and creating the voiding pattern known as paste bleed. The accepted solutions are to plug and plate the via so that its surface is flat enough to print on, or to move it.

Plugging and plating is a process step rather than a mask decision, and it has to be specified in the fabrication notes rather than inferred from the mask layer. Our notes on PCB pad design standards describe how pad geometry and via placement interact when the two cannot be separated.

Assembly and Test Consequences

From the assembly point of view, the difference appears in the stencil design and in the reflow result. An open via near a pad gives paste somewhere to go, and the void that results is often attributed to the paste rather than to the via. A tented via is invisible, which is safer for assembly and less helpful for test.

Test access is the counter-argument. In-circuit test needs a defined contact area, and a covered via is not one. Boards that rely on vias for test access have to keep those vias open and, ideally, identify them in the documentation so that the mask pattern is not changed by a later revision.

Choosing Per Via, Not Per Board

The most practical approach is to decide per class of via rather than once for the whole board. Signal vias under a soldered area are usually covered, test vias are open, thermal arrays are often open, and vias in pads are plugged and plated. That produces a mask pattern that is purposeful and a set of instructions that a fabricator can follow.

The rule is easy to express as a note on the fabrication drawing, and it removes the ambiguity that otherwise leads to a uniform decision applied across a design that needs several different ones.

Documentation and Fabrication Notes

The fabrication note should state the diameter threshold, what happens above and below it, and whether vias in pads are plugged. It should also state whether the surface of a plugged via has to be plated flat, because that specification determines the process and the price, and it is not visible on the artwork.

Our notes on moisture sensitive devices on PCBs describe how the assembly process interacts with board features below the surface, which is the same reasoning applied to the barrel of a via.

Defects That Follow the Wrong Choice

Choosing to tent a via that is too large produces cracked mask and, in some cases, a lifted ring of ink around the hole. Choosing to open a via that sits under a fine pitch component produces solder bridges and voiding that appear as assembly defects. Both failures look like process problems and are actually design decisions.

A third case is a plugged via that was not specified as plated flat, which leaves a dimple that holds paste away from the joint. The defect appears as an intermittent open on one pad of one component, and it is one of the harder faults to trace back to its cause.

Process Control Around Mask Openings

Where vias are opened deliberately, the mask opening has to be controlled like any other feature. An opening that is too small leaves a ring of ink over the annulus; one that is too large exposes laminate between pads and can affect a controlled impedance line. Our notes on PCB manufacturing environmental controls describe how the printing process is held stable so that these openings are repeatable.

The simplest way to keep the pattern predictable is to make the rule explicit and to keep it the same across products. At gopcb, via treatment is agreed with the fabrication notes before the mask artwork is generated, so that assembly yield and test access are both considered rather than one being discovered at the expense of the other.

Additional Considerations for This Build

Practical attention to via tenting pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating via tenting explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

FAQ

Should all vias be tented? No. Tenting suits small vias away from soldered areas, but large vias tent poorly and test vias have to remain open. A threshold in the fabrication notes handles both cases without a per-via decision.

Can a via in a pad be left open? It can, but paste will flow into the barrel and the joint will be less reliable. Plugging and plating the via, or moving it out of the pad, is the more robust choice for a soldered pad.

Does tenting affect impedance? Only marginally, but it does change the environment of an outer layer trace slightly because the mask sits over the copper. On a controlled impedance line the effect is accounted for in the stackup rather than decided by the via rule.