Copper Paste Plugging for Vias

Copper paste plugging fills a via with a conductive material so that a subsequent plating step can bridge over it and form a flat, connected surface. It is used where a via has to carry current or heat, or where a via has to be stacked under another via in an HDI structure. The process is closely related to non conductive filling, and the difference is that the plug must be electrically continuous with the copper around it.

Why a Conductive Plug

A non conductive fill gives a flat surface but no electrical connection through the via. A conductive plug adds the connection, which allows the via to be used as a stacked path between layers rather than only as a flat surface for printing.

The plug also carries heat. A via filled with a copper loaded paste conducts far better than an empty barrel, and it removes the void that would otherwise form inside the joint when solder wicks into the hole.

The third reason is mechanical. A filled barrel is supported along its length, which reduces the strain under thermal cycling, and the support is more complete than a coating of plating on the barrel wall.

The Paste and Its Behaviour

The paste is a mixture of copper or silver particles, a resin and a solvent. When it cures the solvent leaves and the resin crosslinks, and the volume falls. That shrinkage is the property that makes the process difficult.

Shrinkage of several percent in a small via is enough to leave a depression at the surface. The depression has to be filled by the plating that follows, and if it is too deep the plating cannot bridge it and the surface remains dished.

A paste with a high metal loading shrinks less and conducts more, but it is also harder to push into a small hole. The choice of paste is therefore made from the via diameter as well as from the electrical requirement.

Filling Methods

Squeegee filling presses paste across the panel from one side, so that the holes are filled by the pressure. It is fast and it handles a large number of vias, and it is less effective where the holes are small or the aspect ratio is high.

Vacuum assisted filling draws the paste into the holes, which improves the fill on a high aspect ratio via and reduces the trapped air. It costs a process step and it needs a fixture that can apply the vacuum across the panel.

Dispensing fills one via at a time with a needle. It is used for a small number of vias, for a repair and for a via that is too large or too deep for the squeegee method, and it is slow by comparison.

Curing and Shrinkage Control

The cure profile has to remove the solvent without trapping it and without cracking the plug. A fast ramp produces a skin that seals the surface while the solvent beneath it is still there, and the trapped solvent becomes a void.

A two stage cure is common: a lower temperature stage to remove the solvent slowly and a higher stage to complete the crosslinking. The stages should be set from the paste data rather than from a general profile.

A second fill after the first cure is sometimes used to compensate for the shrinkage. The second application tops up the depression and is cured before the planarization, and it produces a flatter surface than a single fill.

Planarization and Plating

Planarization removes the cured paste and any plating that stand above the surface, and it is done by mechanical grinding or by chemical mechanical polishing. The target is a flat surface at the level of the copper pad.

Plating follows the planarization and covers the plug. The plating has to be thick enough to bridge any remaining depression and to carry the current of the via, and it has to bond to the copper pad as well as to the plug.

A plug that has pulled away from the barrel wall during cure leaves a gap that the plating solution enters. The trapped chemistry is a reliability risk, and the adhesion between the plug and the plating on the barrel is one of the parameters that should be verified on a sample.

Voids and Their Consequences

A void inside a conductive plug raises the resistance and the thermal resistance of the via, and it creates a point where the thermal expansion of the paste and the metal differ. The result is a crack that grows under cycling.

The void content is measured by a section or by X ray, and it is usually specified as a maximum percentage. The measurement is difficult because the void is small and the sample preparation can introduce artefacts.

The most common source of voids is the trapped air from the filling operation, followed by the solvent that remains after an incomplete cure. Improving the vacuum and lengthening the low temperature stage are the two responses that address the two causes.

Where It Is Used

Copper paste plugging is used in HDI structures that stack vias, in boards that carry a high current through a via array, and in assemblies where a via has to be both flat and connected.

It competes with a plated and filled via produced entirely by plating, which gives a pure copper plug with no shrinkage and a higher cost. The choice depends on the via size, the current and the number of connections that have to be stacked.

It also competes with a non conductive fill where the electrical connection is not needed. The decision should come from the function of the via rather than from a general preference, because the two processes have different costs and different failure modes.

Specification and Verification

The drawing should state which vias are plugged with a conductive material, the maximum void, the maximum depression and the plating thickness over the plug. Those four items describe the requirement in a way that can be measured.

Verification is by a section on a sample that contains the smallest via in the design, supported by a resistance measurement through a daisy chain of plugged and stacked vias. The resistance measurement is the one that detects a partial connection that a continuity test would pass.

The process should be qualified on the worst case combination of via diameter and stack depth, because the fill quality falls as the ratio of depth to diameter rises. A process that passes on a 0.2 mm via may not pass on a 0.1 mm via in the same panel.

Practical Rules

Choose the paste from the via size, cure in two stages to control the shrinkage and top up where the depression is too deep. Specify the void, the depression and the plating thickness.

Record the fill and section data with the build records and the via filling plating data, and review the blind via filling and the hole copper specification when a stacked structure is defined.

FAQ

Why is shrinkage a problem? The paste loses volume as the solvent leaves and the resin cures. The depression that remains has to be bridged by plating, and a deep one cannot be covered.

How are voids detected? By a section or by X ray on a sample. The void is small, so the sample preparation has to be careful to avoid introducing artefacts.

When is a conductive plug needed? When the via carries current or heat or when it has to be stacked, which requires an electrical connection through the plug rather than only a flat surface.