Resin Plugging Versus Copper Filled Vias

A via that must not wick solder away from a pad, or that must remain flat under a ball grid array, cannot simply be drilled and plated. The barrel has to be closed, and there are two established ways to close it: filling with a conductive paste that becomes solid copper, or plugging with a non-conductive resin and covering the result with a plated cap. The two are often treated as interchangeable, and they are not.

This article compares the processes, explains what each one is good for, and sets out the design and documentation details that decide whether the result is reliable.

Both processes were developed for the same reason, which is that an open via inside a solder pad acts as a capillary during reflow and draws paste away from the joint.

What Each Process Does

Copper filling deposits a conductive paste into the via from one side and then planarises the surface, so the finished via is solid metal flush with the pad. The result is a good electrical and thermal conductor, it can carry current, and it behaves thermally much like a solid via. The process requires tighter control, since the paste must be completely cured and any void left inside becomes a reliability risk.

Resin plugging fills the barrel with an insulating epoxy, cures it, and then plates a cap over the top so that the surface presented to the solder is metal. The fill is not conductive, so the via carries no current through the plug. It is more tolerant of imperfect filling and it is cheaper, which is why it dominates in consumer products.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/Prototype-PCB-Assembly-Manufacturers.jpg.webp" alt="Cross section of a copper filled via in a pad” />

Where The Choice Matters

The decision usually turns on three questions. Does the via have to conduct heat away from a hot component? Does the via sit inside a pad that will be printed with paste? Does the design need the surface to be perfectly planar across a large area, as under a fine pitch array?

Copper filling answers yes to all three and costs more. Resin plugging answers yes to the second and third, at a lower price, provided the via does not need to move heat or current. Where a thermal via is required under a power device, resin plugging removes the path that the via was placed to provide, and the thermal performance of the board changes accordingly.

Tenting And Mask Over The Via

Before either process is applied, a common alternative is tenting, where the solder mask is printed across the via opening so that the paste cannot enter. On small vias this works and costs nothing beyond the mask print. It fails when the via is large enough that the mask tents imperfectly, or when the mask sags into the hole during cure and later cracks.

Tenting also changes the electrical behaviour only slightly, but it changes the inspection story: the via is no longer visible, so any defect underneath is invisible until it appears as a failure. Where the via must be inspected, or where it later has to be probed, an open or filled via is better than a tented one.

Outgassing And Voiding

The reliability of a filled via depends on how completely it is filled and cured. Moisture trapped in the fill, or a solvent that was not fully driven off, will expand during reflow and push material out of the barrel, producing a void under the pad or a blowhole beside it. This is the failure mode that makes resin plugging look unreliable when the process is at fault rather than the concept.

The countermeasures are procedural: a controlled fill depth, a cure profile matched to the material, and a baking step before assembly when the boards have absorbed moisture in storage. The design contributes by keeping the via diameter within the range the process can fill reliably, and by not stacking a small via on top of a large one where the fill has to bridge a step.

Via In Pad And Assembly

Where the via is inside a solder pad, both processes must leave a surface that the paste can print onto without sinking. A copper filled and planarised via achieves this directly. A resin plugged via needs the cap plating to be flat and free of dimples, which is a process capability rather than a design choice, so it should be qualified before the layout assumes it.

The stencil also has to suit the surface. A pad with a via in the middle needs an aperture that places paste around the hole rather than over it, and the via in pad decision therefore reaches into the stencil design as well as the fabrication specification.

Documenting The Requirement

Whichever process is chosen, the drawing must say so explicitly, including which vias it applies to. A note that says all vias are plugged and capped is ambiguous about whether the plug is conductive, how deep the fill is, and whether the surface must be planar. Those three details determine both the cost and the reliability.

It is also worth stating what happens to the vias that are not in pads. Tenting them, leaving them open, or plugging them all are three different processes with three different costs, and leaving the choice to the shop produces an inconsistent result across a production run. Where the vias carry current, the fill process interacts with the plating and via filling sequence, and the via design rules that govern the rest of the board apply with more force once a fill is involved.

Cost And Where It Belongs

Resin plugging is the default for cost sensitive products because it is a mature process with wide capability. Copper filling is specified where the via must conduct, where the flatness requirement is severe, or where the assembly uses a package with a large number of vias inside the pad area and the process window for plugging is too narrow.

The decision should be made per via class rather than for the whole board. A design that uses copper filling only under the power device and resin plugging everywhere else gets the thermal performance it needs and pays for it in one place.

Process Control and Verification

Reviewing the design before the data is released is far cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

FAQ

Can a resin plugged via carry current? Not through the plug. The current path is the plated barrel around the plug, which is thinner than a solid filled via, so the current capacity and the thermal path are both reduced.

Is plugging necessary if the via is not in a pad? No, unless the via must be masked off for cleanliness or the board will be coated. An open via away from a pad is normally left open or tented.

How can the filling be verified? By cross sectioning a sample, and by X-ray inspection for voids in the filled barrel. A visual inspection of the surface proves nothing about the interior.