Resin Plugging vs Via Tenting: Choosing the Right Via Treatment

A via can be left open, covered with solder mask, filled with mask ink or filled with resin and capped. The choice looks like a detail on the fabrication drawing, but it decides whether solder balls form during assembly, whether a ball grid array pads can be printed, and how much the board costs. This article compares the four treatments and explains which one each situation needs.

What Each Treatment Involves

An open via has a solder mask clearance around it, so the copper annular ring and sometimes the hole itself are exposed. It is the cheapest option and it is fine on a slow net, but it can wick solder during wave or reflow processes.

Via tenting covers the barrel with solder mask on both faces, and the via is then described as tented. A tented via is covered by solder mask on both faces, with no opening in the mask artwork. The mask bridges over the hole, sealing it from the atmosphere. Between the two extremes sit mask plugged vias, where ink is pushed into the barrel, and resin plugging, where an epoxy is forced into the hole, cured and then ground flat or capped with copper.

When Tenting Works

Tenting suits vias that carry signals or planes and never need to be probed. Sealing the hole keeps contamination and flux out of the barrel, which is an advantage on fine pitch assemblies where a stray solder ball can bridge two pads. It also removes the annular ring from the solderable area, which reduces the chance of shorts.

The limits are practical. A tented via cannot be used as a test point, and a large hole under a thin mask bridge can burst during reflow as air and moisture expand underneath, leaving a hole in the mask. Fabricators therefore set a maximum diameter, often around 0.5 mm, above which tenting is not offered without plugging underneath.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/1.webp" alt="PCB cross section comparing a resin plugged via with a tented via” />

Mask Plugging and Its Interaction With Surface Finish

Mask plugged vias fill the barrel with the same ink used for the solder mask. It is inexpensive and works well with finishes such as immersion gold, immersion silver and organic coatings. The ink shrinks during cure, so it rarely fills the barrel completely, and a second pass is sometimes applied to close the remaining gap.

The finish that causes trouble is hot air solder levelling. If only one side of the via is plugged, molten solder can be trapped and expelled as a ball during the levelling step, which is why single sided plugging is avoided on HASL boards. Either both sides are plugged or the via is left open. The behaviour of the mask ink itself, including the thixotropy of solder mask ink, determines how well the barrel fills and how the surface levels afterwards.

Resin Plugging for Demanding Builds

Resin plugging uses an epoxy formulated for the purpose. It fills the barrel completely, cures to a hard material that does not outgas during reflow, and can be ground flush with the copper surface. That flatness is what makes it the standard treatment for via in pad construction, where the via sits inside a component pad and paste must be printed over it.

The process adds steps: filling, curing, grinding and, in the best case, plating a copper cap over the filled via. Each step costs money and adds a chance of a defect such as an incompletely filled barrel or a dimple that disturbs paste release. Where a ball grid array is involved, the added cost is usually unavoidable.

Choosing Between Them

Work from the requirement. If the via must be solderable or probed, leave it open with a mask clearance. If it only carries a net and will never be touched, tent it. If it sits in a pad, fill it with resin and cap it. If the board uses a finish that tolerates single sided plugging and the appearance of the via matters, mask plugging may be enough.

State the treatment on the fabrication drawing, per via class or per layer, rather than in a general note. A drawing that says “vias tented” while the layout shows a via inside a pad invites either a non-manufacturable board or a change made without the designer noticing.

Cost and Schedule Impact

Open or tented vias cost nothing extra. Mask plugging is a small adder and is usually done in the same pass as the solder mask. Resin plugging is a separate process with its own curing and planarising steps, and the price difference is significant on a dense board with thousands of vias.

The schedule is affected too, because resin filled panels cannot be processed as quickly as standard ones. Where a design has a small number of critical vias, it is common to fill only those and leave the rest tented, and that selective approach keeps most of the cost out of the board. Documentation for a mixed treatment follows the usual manufacturable design guidelines, with the via classes marked on the drill drawing.

Verification and Common Defects

Inspection should match the treatment. For a filled via, a cross section confirms that the barrel is fully filled and that no void reaches the surface. A dimple deeper than about 25 µm is usually rejected for via in pad because it disturbs paste deposition. For a tented via, the check is visual: a broken mask bridge is a defect and a trapped air bubble is a warning sign.

The most common field problem is a partially filled via that passes electrical test and later opens under thermal cycling. That is why the fill process is qualified by cross section at the start of a build rather than by electrical test alone, and why a change of resin supplier is treated as a process change.

Process Control on a Plugged Panel

Via tenting and plugging both depend on how the mask artwork is generated, so the first control is data preparation. The solder mask layer must be checked against the via list: every hole that should be covered needs a filled aperture, and every hole that should remain open needs a clearance ring. Automated tools do this reliably only when the via classes are named in the design.

Fabrication adds its own controls. Mask plugging is inspected by looking for a visible depression at the surface of the barrel, and a second print is applied when the first pass does not close the hole. Resin plugging is controlled by the fill pressure and the cure schedule, then by grinding to a flat surface and, where required, plating a cap over the top.

Assemblies should be validated with a thermal stress test rather than a simple continuity check. A partially closed via that measures fine at room temperature can open after three reflows, so the qualification should include the full thermal history the product will see. Where the via sits in a ball grid array pad, the paste release test on the first article is the most sensitive indicator of a filling problem.

FAQ

Can a tented via be used as a test point? No. The mask covers the copper, so a probe cannot make reliable contact. If a via must be probed, leave it open and make sure the probe does not damage the barrel.

Is resin plugging the same as mask plugging? No. Mask plugging uses solder mask ink, which shrinks and can outgas. Resin plugging uses a dedicated epoxy that fills completely and can be planarised for via in pad.

Why is single sided plugging a problem with hot air levelling? Because trapped solder can be blown out of the partially filled barrel during the levelling step, leaving balls on the surface. Plug both sides or leave the via open.