Via Tenting: Preparation, Placement and Process Control

A via that passes through a solderable area has to be dealt with. The two standard answers are tenting, where solder mask is printed over the hole, and plugging, where the hole is filled with a material before the mask is applied. The names are used loosely in practice, and that looseness causes a surprising number of arguments at the shops.

What Tenting Means

Tenting is a mask operation. The mask is printed over the via pad and the hole, and the film that spans the hole is cured. No material is added inside the barrel, and the via remains open in an electrical sense but covered in a mechanical one.

Because the mask is a thin film, the amount of protection depends on the hole diameter. A small via tents reliably, a large one leaves a sagging film that can crack during thermal cycling or be blown open during cleaning.

What Plugging Means

Plugging fills the via with a material, usually a solder mask based paste or a dedicated via fill compound, applied by stencil and cured before the outer mask. The filled via is then over plated or over coated, depending on the class of the board.

The result is a flat, closed surface that can be printed over and placed on. The process is more expensive because it adds a print, a cure and often a planarisation step, and the cost is justified by what is built on top of it.

Via in Pad and Its Requirements

A via inside a thermal or a fine pitch pad has to be filled and plated over, otherwise the paste printed on the pad drains into the hole during reflow and leaves a void or a starved joint.

This is the case that drives most of the specification work. Our article on via filling and plugging sets out how the fill is specified and inspected, and our article on thermal management design explains when the via array is needed in the first place.

Thermal Cycling and Reliability

A filled via behaves differently from a tented one. The fill material has its own expansion, its own cure shrinkage and its own adhesion to the barrel, and a poorly matched fill can pull the pad or crack the barrel over thermal cycles.

Tenting avoids the fill material but leaves the barrel exposed to flux, cleaning chemistry and any coating that is applied later. Where a coating is used for reliability, a tented via may still allow the coating to wick into the barrel.

Cleaning and Contamination

An open via traps flux and process chemistry. Water soluble processes are the worst case, because residue that stays in the barrel also stays on the board, and it is the residue that causes leakage and corrosion later.

Plugging closes that path, which is one reason the process is specified for high reliability assemblies. Tenting reduces the exposure but does not remove it, so the choice should follow from the cleaning chemistry rather than from habit.

Specifying on the Drawing

The drawing should say which vias are tented, which are plugged and what the plugged ones are filled with. A blanket note that covers the whole board usually produces one result on one side and a different one on the other.

Where a via sits in a pad, the note should also state whether the surface has to be planar and whether the fill has to be conductive. Our article on fabrication notes checklist lists the other notes that are commonly omitted.

Cost and Practicality

Tenting is essentially free, because it happens with the mask print. Plugging adds steps, and the cost scales with the number of vias and with the planarity requirement.

Where the product does not need conductive filled vias, a non conductive plug that is over plated only where a pad is required is the usual compromise. The decision should be recorded so that it is not revisited on every order.

Additional Considerations for This Build

Practical attention to via tenting pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating via tenting explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Checks Before Release

On a design of this kind, via in pad is the item that decides how the rest of the board is arranged. The sequence of operations is part of the specification, because a different order produces a different result from the same steps. Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one.

Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first. The narrowest feature on the board usually sets the process window for the whole product, so it deserves the closest attention at review.

Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to. Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

Where a decision is made by judgement, a boundary sample makes the judgement repeatable between operators and between shifts. Where a process is at the edge of its capability, the margin should be bought deliberately rather than discovered during production.

A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

Points to Confirm at First Article

On a design of this kind, via in pad is the item that decides how the rest of the board is arranged. Where the supplier and the user both measure the same property, they should agree on the method before the first delivery. Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record.

A change that is not recorded is a change that cannot be explained when the result moves, which is why the record is part of the process. The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.

FAQ

Can a tented via be used under a BGA? A small tented via that is not in a pad can be used, but a via inside the pad itself has to be filled and plated over, or the paste will drain during reflow.

Is plugging the same as filling? The terms overlap. Plugging usually means closing the hole, while filling implies filling the barrel and often plating it flush, which is what a via in pad requires.

Does a tented via need to be inspected? The film should be verified as intact, since a cracked or blown tent admits flux and cleaning chemistry into the barrel later in the process.