Via Plugging With Ink Versus Conductive Paste

When a via has to be closed, two materials are used: a non conductive solder mask ink, and a conductive paste filled with metal or carbon. Both fill the barrel, and they behave very differently in the finished board. The choice is normally decided by whether the via has to conduct heat or current, and by whether the surface over it will be soldered.

This article compares the two, how each is applied, and how planarity and shrinkage are controlled.

What Via Plugging Has To Achieve

The first requirement is that the barrel is closed, so that flux, cleaning chemistry and moisture cannot enter it and so that no gas escapes through it during reflow. The second is that the surface is flat enough for whatever will be placed over it: a solder joint, a coating or a stencil seal. The third, where the via is a thermal or an electrical path, is that the fill does not add significant resistance to the path.

Those requirements interact. A material that fills completely and shrinks on cure may leave a dimple that ruins the planarity; a material that does not shrink may be difficult to apply without voids. The way a via is configured in the first place, and whether it is tented, plugged or plated over, is described under via in pad or plated through.

Non Conductive Ink

Mask ink is applied by screen printing over the whole panel and forced into the barrels with a squeegee, then cured. It is inexpensive, it uses the same material as the mask, and it is adequate for a via that carries a signal and is not soldered over. Because it is not conductive, it does not change the electrical behaviour of the barrel, which still carries the current through its plating.

Its weaknesses are shrinkage and trapping. Ink shrinks as it cures, so a large barrel can end up with a dimple in the middle even when the fill was complete. And because the ink is applied over the surface, air can be trapped in the barrel, producing a void under the cured surface that appears when the board is heated. Where the via will be soldered over, both effects matter, and a second print or a vacuum assist is often needed to achieve a filled and flat result.

Conductive Paste

Conductive paste is filled with silver, copper or carbon particles in a polymer binder. It is applied by stencil printing into the barrels and cured, and because the cured material conducts, it turns the via into a thermal or an electrical path that is not limited to the plating. A copper filled paste is used where thermal conduction matters; a silver filled one where the electrical resistance has to be low.

The paste brings its own problems. Its expansion is different from the copper and the laminate, so a large filled via develops stress as the board is heated, and the fill can pull away from the barrel wall. Its cure also shrinks, and a paste with a high solvent content shrinks more than a low one. The metallic filler settles if the paste is stored too long, which changes the conductivity of the fill in a way that is not visible from the surface.

Planarity And Cure Shrinkage

Planarity is measured over the filled via in the pad it sits in, and the requirement is set by the solder process. A dimple of a few tens of micrometres changes the volume of paste in the deposit and therefore the volume of the joint, and a bump causes the component to stand off. The acceptance limit is normally stated as a maximum deviation from the pad surface, measured on a sample.

Shrinkage is controlled by the formulation and by the cure. A filled material with a low solvent content and a controlled cure shrinks less, and a cure that is too fast or too hot produces a surface skin while the interior is still fluid, which leads to a crater after cooling. Where planarity is critical, the fill is applied in two passes or is planarised by a light grinding or plating step. The plating route is described under electroplating and via filling for HDI.

Thermal Vias And Heat

A thermal via is a barrel that carries heat from a pad to an internal or a far side plane, and its function depends on the cross section of copper in the path. A plated barrel contains a thin wall of copper, and its thermal conductance is set by that wall, not by the hole diameter. Filling the barrel with a conductive paste adds a parallel path through the fill, which raises the conductance, but the fill is less conductive than copper by an order of magnitude or more.

Filling with copper by plating is therefore the best thermal solution, because the barrel becomes solid copper. Where that is not available, a paste with a high metal content helps, and an unfilled or ink filled via helps least. The difference is measurable: a stack of thermal vias filled with copper transfers substantially more heat than the same stack with an air filled barrel, and the choice should be made from a calculation of the thermal resistance rather than assumed.

Specifying And Checking

The drawing should state, for each via or class of via, whether the barrel is filled, with which material family, whether the fill is conductive, and the planarity requirement. It should also state whether the surface over the via will be soldered, since that is the requirement that drives the planarity figure. A generic note about plugging leaves the supplier to choose, and the choice will be made on cost.

Checking is by section for the fill and the barrel wall, by an optical or a profilometer measurement for the planarity, and by thermal or electrical measurement where the via is functional. The section shows voids and separation from the barrel, which the surface inspection cannot see, and it should be taken on a sample from each lot rather than once at qualification. The rules that keep these requirements consistent with the rest of the design are gathered under design guidelines for manufacturability.

Where the requirement is not stated, the result is usually decided by whichever process the shop runs by default, which may be a simple tent or a partial fill. Both can look acceptable on inspection and fail later, either as a void under a soldered pad or as a path for moisture into the barrel. Stating the requirement costs a line on the drawing; discovering it after assembly costs a batch of boards.

FAQ

Can mask ink be used in a via that will be soldered over? Only if the fill is complete and flat after cure. Ink shrinks and can trap air, so a second print or a different material is usually needed for a via inside a component pad.

Does a conductive fill carry the signal instead of the plating? It carries part of it in parallel with the plating. The current divides according to the conductance of each, and the plating normally carries most of it unless the fill is copper plated over.

Which fill is best for a thermal via? Plated copper gives the best result because the barrel becomes solid metal. A metal filled paste is the next best option, and an unfilled or ink filled barrel gives the least benefit.