Via Plugging and Solder Mask Process Control

Two process steps are often considered together because they both concern the way a via is treated after it has been plated. Via plugging fills the barrel with a material so that the surface above it can be used, and solder mask defines what remains exposed after the coating is applied. Together they determine whether a board can host a component directly over a via and whether the vias are protected from the environment.

Both steps are process sensitive in ways that are not always visible in the finished board. A plugged via that is not completely filled traps chemistry that later causes a blister, and a solder mask process that is not under control produces bridges that fail in reflow or dams that lift. Control therefore depends on the process window rather than on the drawing.

Why Vias Are Plugged

The original reason was to prevent solder from wicking down a through hole during wave soldering, which could produce voids and unreliable joints on the component side. A plugged and tented via also prevents flux and cleaning chemistry from being trapped in the barrel, which matters for assemblies that must be cleaned to a specified ionic contamination level.

A second reason has become more important: via in pad. Placing a component directly over a via requires the via to be filled and the surface to be planar, because a void below the pad produces a joint with a void, and an unfilled barrel allows paste to drain away from the joint. This is now the dominant driver for plugging on high density boards, particularly under ball grid arrays and thermal pads, where the space between pads leaves nowhere else to put the via.

Plugging Materials and Methods

The traditional approach is a resin plug, applied by screen printing or by vacuum assistance, cured and then planarised so that the copper above it can be plated. The requirement is that the material fills the barrel without voids, adheres to the plated wall, and has an expansion coefficient close enough to the surrounding material that thermal cycling does not open a gap at the interface.

Conductive pastes are used where the via has to carry current through the filled volume, and copper paste or silver filled epoxy are the common choices. Their resistance is higher than plated copper, so the design has to account for the additional resistance and for the heat generated in a high current via. A third method is plating the barrel closed, which produces a fully metallic fill but requires very thick plating and is limited in aspect ratio.

Process Control for Plugging

The main defects are incomplete fill, voids, and resin that bleeds onto the surface and interferes with the subsequent plating or mask step. Incomplete fill is caused by insufficient material, by a barrel that has not been dried, or by a vacuum step that is too short. Voids form when the material cures from the outside in, trapping solvent in the centre, which is controlled by the cure profile rather than by the fill step alone.

Verification is by cross section on a coupon that is processed with the production panel. The section shows the fill, the interface with the barrel wall and the planarity of the surface above it. Where the via will carry a component, the planarity matters as much as the fill, and it is measured as the difference in height between the filled via and the surrounding pad. The same reasoning as any other via filling process applies, with the difference that the material here is a polymer rather than plated copper.

Solder Mask and Plugged Vias Together

The mask and the plug interact at the via. A via that is plugged and then tented with mask has two layers of protection, which is the most robust arrangement for a via that must not be exposed. A via that is plugged and left open so that it can be used as a test point or a thermal path has only the plug, and the mask opening around it has to be large enough that the mask does not sit over the joint area but small enough to keep the dam between adjacent vias.

Where a via is plugged for via in pad, the mask must not encroach on the pad, so the via is defined as an open feature and the mask dam is placed between the pads rather than over the via. That places the mask dam requirement and the plugging tolerance in conflict on a fine pitch device, and both have to be satisfied with the process window that the fabricator actually has. Reviewing the two together, rather than as separate rules, is what avoids a design that is manufacturable in principle and not in practice.

Inspection and Yield

Inspection of a plugged via is difficult because the feature is internal. Cross sections on coupons give the definitive answer, and X-ray can show a void in a metallic fill but not reliably in a resin fill, since the resin and the surrounding material have similar absorption. Where the via is used for a component joint, the joint itself can be inspected after assembly, which is the most relevant check for the electrical and thermal performance.

Yield losses from the two processes are usually attributed separately but often interact. A resin bleed that interferes with the mask adhesion shows up as a mask defect, while the cause is in the plugging step. Recording the two processes together, with the cross section results and the mask inspection results from the same panel, is what allows the cause to be identified rather than the symptom. Doing so is part of managing the overall yield and quality control of a high density line.

Solder Mask Types and Their Effect on Vias

The mask itself is applied as a liquid, imaged and developed, or screen printed. Liquid photo imageable mask gives the best resolution and the best control of the dam width, which matters when the opening around a via has to be small and accurately placed. Screen printed mask is cheaper but its thickness and its resolution are poorer, and it is used where the features are coarse.

The mask thickness determines how well a tent over a via survives. A tent is a mask bridge across the via opening, and it has to withstand the pressure of the assembly process and the thermal expansion of the air trapped inside the barrel. Where the via is small and the mask is thick, the tent holds; where the via is large, the tent sags and may rupture. This is why a large via that must be sealed is plugged first and tented afterwards rather than relying on the mask alone.

Tenting, Opening and Their Selection

A via that is fully covered by mask is tented, one that is covered on both sides is double tented, and one that is left exposed is open. Tenting is the cheapest protection and is adequate for a signal via that carries no current and needs no probing. Opening the via is needed when it has to be used as a test point, when it carries high current and needs the additional copper area, or when it must dissipate heat into the surface.

The choice is usually made from the function of the via rather than from a general rule, and it should be recorded in the fabrication drawing because it changes the mask artwork. A via that is intended to be tented and is accidentally left open exposes the barrel to flux and to the environment, while one that is intended to be open and is accidentally tented cannot be probed. Marking the intent explicitly, rather than leaving it to be inferred from the pad shapes, removes a recurring source of ambiguity.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Does every via need to be plugged? No. Plugging is only needed where solder wicking, contamination or via in pad is a concern. Many vias are simply tented with mask, which is cheaper and adequate for a via that carries only a signal.

Can a plugged via be reworked? Not usually. Once the resin is cured and the surface is plated or masked, there is no practical way to reopen the via without damaging the surrounding layers.

Why does a plugged via blister after reflow? Because chemistry or moisture was trapped in the barrel during filling. The cure profile and the drying step before filling are the controls that prevent it.