Solder Mask Plug: Design Rules and Process Limits

A solder mask plug is mask material that is pushed into a via rather than draped over it. The distinction matters because a tent can crack and pull away, while a plug survives the assembly process and gives a surface that can be printed or probed. The process is more demanding than tenting and less demanding than a full conductive fill, which is why it is a common middle ground on a dense board.

What a Mask Plug Does

The plug fills the barrel with the same material as the surface mask, so the via is closed from both sides and the surface over it is nearly flat.

The main purpose is to stop solder wicking down the barrel during reflow, which is the same purpose as a tent and a more robust way of achieving it.

The second purpose is to give a flat surface so that the next process can work. A paste deposit above a plugged via is uniform, while a deposit above an open via slumps into the hole.

The third purpose is protection, since the plug keeps flux and cleaning chemistry out of the barrel and prevents the copper inside from corroding.

Plugging Versus Tenting

Via plugging and tenting both aim to close the barrel, and they differ in how the material is placed. A tent is a film of mask over the pad and the hole, supported only at the edges. It bridges the opening and it sags or cracks when the hole is large or the mask is thin.

A plug fills the hole, so it has material behind it and it does not depend on the film spanning an unsupported span. It survives thermal shock and handling better than a tent of the same thickness.

The cost is higher, because the process needs a squeegee pass or a vacuum to push the material into the holes, and the surface has to be planarised afterwards.

The two techniques are often mixed on one board: tenting for the small signal vias and plugging for the larger ones or for the vias inside a pad.

Process Sequence

The mask is applied over the whole board, then a squeegee pushes material into the open vias. The excess is removed and the surface is left with the mask in the holes.

A vacuum assist is used on a thick board, where the material would otherwise not reach the bottom of the barrel. The vacuum draws the mask through the hole from the far side.

The plug is then cured, and the surface is planarised by a light sanding or a buffing step so that the mask over the via is level with the surrounding surface.

The sequence belongs to the fabrication process rather than to the artwork, but the artwork has to leave the vias open in the mask layer for the plug to be possible.

Planarisation and Fill

Planarisation is what makes a plug usable under a pad. Without it the mask forms a dome over the via and the paste deposit above it is uneven.

The planarising step removes material from the whole surface, so the mask thickness over the copper falls as well. The process has to be controlled so that the final thickness is still within the specification.

A plugged via that is not fully filled leaves a dimple, which is a reservoir for flux and a place where a void forms during reflow.

A plugged via also leaves a small mark on the surface, and on a product where the appearance matters the mark should be checked with the customer before the process is fixed.

The material used for the plug is usually the same as the surface mask, although a dedicated plugging paste exists. The two have different flow characteristics and the choice affects the fill quality.

Design Rules

The via diameter that can be plugged reliably has an upper limit, which is set by the process. A hole that is too large will not fill before the material cures.

The via and the pad should be designed with the hole types and the plug in mind, and the via construction choice should be made together with the plugging decision.

The mask opening over a via that is to be plugged has to be larger than the hole so that the material can enter. A mask opening that is smaller than the hole leaves a ring that cannot be filled.

A via in a pad needs the plug to be flush, so the planarisation rule is the critical one. A via in a ground pad that is not flush produces a solder void under the component.

The fabrication notes should state which vias are plugged, which are tented and which are left open, and the note should be organised by size or by net class.

Risks and Defects

The first defect is an incomplete fill, which leaves a dimple or a hole that traps material. It is found on a cross section rather than by looking at the surface.

The second is a plug that shrinks away from the barrel wall during the cure, leaving a gap that is a path for chemistry. The gap is invisible from above.

A plug that is slightly recessed is acceptable on a via that is only used for a signal, because the small depression has no effect on anything that touches it. On a via inside a pad the same depression changes the paste volume and becomes a defect.

The third is the mask that lifts over the via after thermal cycling, which is the same failure mode as a cracked tent but with a different appearance.

The last risk is the interaction with the surface finish, since a plug that is applied before the finish can be attacked by the plating chemistry and one that is applied after may not cure fully. The order is a question for the shop.

The fourth is a plug that is pulled out during a rework operation, which leaves an open barrel that must be cleaned before the new part is fitted.

Practical Rules

Decide per via class whether the via is plugged, tented or open, and put the decision in the fabrication notes rather than in a general statement.

Keep the mask opening over a plugged via larger than the hole, and specify planarisation where the via is inside a pad.

Check a cross section of the plugged area on the first article, since the fill quality cannot be judged from the surface.

Record the result with the quality record, and check the plugged vias again after any change of mask material.

FAQ

What is the difference between a plug and a tent? A tent is a film of mask over the hole, while a plug fills the barrel with material. A plug is more robust and more expensive.

Why does a via in a pad need planarisation? Because the paste deposit above the via has to be uniform. A dome or a dimple in the mask changes the volume of paste and the void content.

Can any via be plugged? There is an upper size limit set by the process, and a via that is too large will not fill completely. The limit should come from the fabricator.