Solder Mask Plug and Via in Pad on Dense Boards
A via that is left open in the solder mask is a small hole with a large appetite. Paste, flux and wash liquid all find their way into it, and the consequences show up on the assembly line rather than at the fabricator. A solder mask plug closes that hole so the surface behaves as if the via were not there.
The decision sounds cosmetic and is not. Whether a via is tented, plugged or filled changes how much paste the stencil delivers, how flat the pad is, and whether any process gas escapes through the barrel during reflow. Getting the classification right on the fabrication drawing is the cheapest quality improvement available on a dense board.
What a Solder Mask Plug Achieves
The purpose of a solder mask plug is to prevent material from entering the barrel while leaving the pad area flat enough for assembly. Ink is pushed into the hole from one or both sides, cured, and then planarised so that the surface above the via does not sit proud of the surrounding mask. The result is a barrier rather than a structural fill.
That distinction matters when the via sits under a component pad. A plug that is simply screened over the hole can leave a dome or a dimple, and both of them disturb the stencil gasket. The stencil has to seal against the mask, and any step in that surface lets paste escape sideways, which shows up later as a solder bridge or as a starved joint on the adjacent pins. The same sealing behaviour is what the rules in pad design standards are written to protect.
Tented, Plugged and Filled Vias
A tented via is the simplest option. Mask is printed across the annular ring and the hole, and no attempt is made to fill the barrel. Tenting is adequate for a via in a field of copper that carries no paste and sees no rework, but the mask can crack over the hole during thermal cycling and it offers only a thin film of protection.
A plugged via goes further, with material driven into the barrel and cured before the outer layers are finished. The plug protects the barrel from flux and from cleaning chemistry, and it removes the open cavity that would otherwise trap liquid. Filling is the heaviest version and is reserved for vias that carry current or that must be planar for a pad above them, and it is usually paired with copper plating as described in copper plating and via filling.
Via in Pad and Paste Loss
When a via sits inside a component pad the open barrel acts as a drain. Paste is printed over the aperture, and during reflow the paste wicks into the hole instead of forming a fillet. The joint ends up thin, and the void that remains in the barrel can outgas and blow a bubble through the molten solder, which is the classic via in pad defect.
The remedy is a filled and capped via that presents solid copper at the surface. Paste then sits on a flat pad with nothing to drain into, and the stencil aperture can be sized on its own merits. This is the same logic that governs any fine pitch aperture, and it is why the area ratio numbers apply to capped vias without modification. Designers who ignore this step often blame the stencil for a defect that was created at the fabrication stage.
Planarity and the Assembly Window
Planarity is measured across the pad plus the surrounding mask, not across the via alone. A cap that is 20 micrometres low creates a pocket where paste collects, and a cap that is high lifts the stencil away from the neighbouring pad. Either way the paste volume on the adjacent apertures changes, and the change is not uniform across the panel.
The practical tolerance is tight enough that the fabricator should confirm the target with the assembler. Most shops work to a surface that is flat within about 25 micrometres over the pad area, and they will say so if the design asks for something the process cannot hold. That conversation is far cheaper than a reflow yield problem found during a build, and it belongs in the same review as the rest of the fabrication notes.
Process Control at the Fabricator
Plugging is done with a squeegee or a dedicated head that drives ink into the barrel under pressure, followed by a cure and a grinding or planarising step. The variables are the ink viscosity, the pressure, the number of passes and the cure profile, and each of them can leave a void that only appears after the surface is ground back.
Inspection is therefore destructive in the sample phase and visual afterwards. A cross section on a coupon proves that the barrel is filled to the required depth, and the production check confirms that the surface is at the right height. Buyers who want evidence should ask for the coupon result, since a visual check cannot see a void that sits in the middle of the barrel. The same sampling discipline applies to the parameters collected in the manufacturing tolerances list.
Design Rules and Documentation
The drawing has to say which vias are tented, which are plugged and which are filled and capped, because the three are priced differently and a default assumption will be applied if the note is missing. A table keyed to the via classes is clearer than a sentence on the drawing.
Vias under thermal pads should be capped whenever the pad must be soldered, and vias in a ground plane that carries no paste can usually stay tented. Where a designer wants to be certain, the safest route is to specify fill and cap for everything under a component and tenting elsewhere, then confirm with the fabricator that the mask dam around each via is wide enough to survive the process.
Thermal Behaviour of a Filled Via
A filled via carries heat differently from an open one. Copper in the barrel conducts from the inner layers to the surface, and a solid fill gives that heat a continuous path rather than a thin plated wall. On a board with a thermal pad this is often the reason the via exists at all, and the fill removes the air pocket that would otherwise sit under the pad and insulate it.
The same fill changes the way the assembly heats. A plugged via that is not fully cured can release solvent during reflow, and the gas escapes into the molten joint as a void. This is one of the mechanisms behind paste wicking failures that appear only on the first panels of a build, once the ink has absorbed moisture from the shop air.
Step Height and Measurement
Step height is the difference between the capped surface and the surrounding mask, and it is measured with a profilometer or an optical gauge on a coupon rather than on the production panel. The target is a surface that is level within a few micrometres, and a shop that cannot measure it cannot control it.
Where the equipment is not available, the alternative is a process that is known to be stable and a coupon that is sectioned for the record. A buyer who asks for the number should also ask how it is obtained, because an unmeasured specification is a claim rather than a control.
A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.
Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.
A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.
Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.
FAQ
Does a plugged via cost more than a tented one? Yes, because it adds a print, a cure and a planarising step, and the price difference is modest on a panel basis but noticeable on a design with thousands of vias.
Can a via in pad be left unfilled if the aperture is small? It can be attempted, but the risk of wicking and voiding remains, and the failure is intermittent enough that it usually appears after the design has been released.
How is the fill inspected in production? By cross section on a coupon built with the panel, supported by a height measurement on the surface, since an external view cannot detect an internal void.
