Via Tenting And Plugging Design Rules

A via is a hole through the board, and unless it is covered it is also an opening that solder, flux and cleaning fluid can enter. The decision about what to do with it is made in the fabrication notes, and it is often left to the fabricator by default. The default is not always the right answer, and the consequences appear in the assembly yield or in the field.

This article compares the options, explains what each one does to the assembly process, and sets out how to specify them per via type rather than with a single blanket instruction.

What The Options Are

Tenting is the application of the solder mask over the via so that it is covered on one or both sides. Plugging is the filling of the hole with a mask material or a resin before the mask is applied, so that the via is closed rather than covered. Filling is the complete filling of the barrel, usually with a conductive or a non conductive material, followed by plating over the top so that the surface is flat.

The three are often confused because they all result in a via that is not open, and they are not equivalent. Tenting leaves a void inside the barrel, plugging closes the hole with a material that may not be flush with the surface, and filling produces a surface that can be soldered over as if it were a solid pad.

Tenting Over A Via

A tented via has mask across its opening, which prevents solder and flux from entering during assembly. On a standard board this is the normal treatment for vias that are not in a pad, and it costs nothing extra because the mask is applied anyway. The mask over a small hole is supported by the rim, and it survives the assembly process if the hole is small relative to the mask thickness.

The difficulty appears with a large hole. A mask tent over a hole of a few millimetres cannot be supported and it tears during development or during assembly, leaving an opening that is worse than an untented via because the torn mask traps material. The via diameter at which tenting stops being reliable depends on the fabricator and on the mask, and the limit should be taken from the fabrication notes rather than assumed.

Plugging And Filling

Plugging is used where the via has to be closed but the surface does not have to be flat, for example a via that sits close to a pad where an open hole would wick solder away. The material is screened or stencilled into the hole from one side, and the fabricator has to be told which side, because the plug is not always visible afterwards.

Filling is used where the via is in a pad and has to be soldered over, which is the case for a thermal pad on a power device or a via in a ball grid array pad. The barrel is filled and the surface is plated and planarised, so that the paste and the component sit on a flat surface. The requirement should be stated as a flatness and a fill percentage rather than as a process, because the process is the fabricator choice. The design of those features is described for via in pad.

Vias In A Thermal Pad

A thermal pad under a power device carries a large number of vias, and the treatment of those vias determines how the paste behaves during printing. An open via wicks paste into the barrel, which starves the joint and can also carry paste through to the other side of the board. A tented via on the underside traps air, and the expanding air pushes the molten solder out of the joint and produces a void.

The usual compromise is to plug or fill the vias so that the barrel is closed while the pad remains solderable, and to keep the vias small and closely spaced so that the loss of paste is limited even if a small amount enters each one. The relationship between the via treatment and the void content of the joint is direct, and the assembly consequences are described under solder joint voiding.

Outgassing And Assembly Effects

Outgassing is the release of moisture and volatiles from the laminate during reflow. A via that is open gives those volatiles a path out of the board, while a via that is plugged on both sides traps them, and the pressure can lift the mask or blow a bubble in a molten joint above it. The effect is larger in a thick board and in a board that has absorbed moisture before assembly.

The practical consequence is that the vias near a thermal pad should not be sealed on both sides unless the board has been dried, and that a board that is baked before assembly outgasses less. The requirement should be written into the fabrication notes as a per via class instruction, because a single blanket rule that satisfies the fine pitch area may be wrong for the thermal pad.

Choosing For A Design

The specification should list each via class and the treatment it requires. Signal vias that are not in a pad are normally tented on both sides. Vias that sit near a pad and carry a risk of solder wicking are plugged. Vias that are in a pad and have to be soldered over are filled and planarised. Vias under a thermal pad are plugged or filled with the barrel closed and the surface flat.

Each of those instructions has a cost, and the cost should be weighed against the function. Tenting is free, plugging adds a process step, and filling adds another. A design that specifies filling everywhere pays for a capability it does not use, and a design that specifies nothing accepts whatever the fabricator does by default. The stack up context is described in via stack selection and in the fabrication process.

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.

Deliberate attention to via plugging 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 plugging explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, via plugging is the item that decides how the rest of the board is arranged. 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. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

FAQ

Is a tented via sealed against moisture? No. Tenting covers the opening with mask but leaves the barrel empty, and the mask is permeable to moisture over time. It prevents solder entry during assembly rather than sealing the board.

Why does an open via in a thermal pad cause problems? It wicks paste away from the joint during printing and can push material through to the other side, which starves the joint and contaminates the assembly.

Should every via be filled for a fine pitch design? Only the vias that are in a pad and have to be soldered over. Filling the others adds cost without improving the assembly.