Microvia Filling and Copper Capping Process Control for HDI

A microvia is a small hole, usually formed by laser, that connects one layer to the next without passing through the whole board. As the diameter falls and the depth stays the same, plating the barrel becomes the easy part and filling the hole becomes the hard part. Microvia filling is what allows the next layer to be built directly on top, and it is the step that decides whether a high density design can be stacked at all.

What Microvias Are and Why They Need Filling

In a sequential build, each layer pair is drilled, plated and then laminated into the next. If the microvia is not filled, the resin that flows during lamination is pushed into the open hole, and the surface above it cannot be planar. Some of that resin may also be expelled during reflow, creating a void under the pad above.

A filled and capped microvia gives the next layer a flat copper surface to build on. That flatness is what makes stacked vias and via in pad possible, and it is why filling is specified for designs that would otherwise be perfectly manufacturable with a conformal plated hole.

Conformal Plating versus Full Fill

Conformal plating follows the shape of the hole, depositing an even layer on the wall and leaving a hollow centre. It is simpler, cheaper and perfectly adequate for a via that is not stacked or used as a thermal path, and it remains the standard choice for many designs.

Full fill closes the hole completely from the bottom up. It requires a chemistry and a current programme that deposit faster inside the hole than on the surface, which is the opposite of what a conventional plating bath naturally does. The process is slower and more sensitive, and it is chosen when the design demands a planar surface.

<img src="https://www.gopcba.com/wp-content/uploads/2026/08/pcb.webp" alt="Microvia cross section showing filled copper and dimple depth” />

Plating Chemistry for Void-Free Fill

Fill plating relies on additives that suppress deposition on the surface and accelerate it inside the hole. The balance between suppressor, accelerator and leveller determines whether the deposit closes cleanly from the bottom or pinches shut at the top and traps a void in the middle of the via.

Chemistry control is continuous rather than periodic. Additives are consumed and their by-products accumulate, so the bath has to be analysed and dosed on a schedule derived from the charge passed through it. A bath that is inside its chemical specification can still fill badly if the organic balance has drifted.

Dimple Depth and Its Limits

Even a good fill leaves a shallow depression at the top of the via, known as the dimple. The depth of that dimple is measured and specified, because a dimple that is too deep leaves a cavity under the pad and a dimple that is absent usually means the copper has overflowed onto the surface.

The acceptable dimple depth depends on the layer above and on the laminate. A via that will be capped and used as a pad has a tighter requirement than one buried inside the stack, and the requirement should be stated on the fabrication drawing rather than assumed from a general specification.

Copper Capping and Planarisation

Copper capping is the step that closes the top of the filled via and provides a continuous copper surface for the next process. Capping can be done by plating after the fill, by laminating copper foil over the filled via, or by a combination of both, and the choice affects how flat the finished surface is.

The cap has to be thick enough to survive the subsequent etching and bonding steps and to carry the current of the net. Where the cap is too thin, the via can be exposed during etch or can delaminate during thermal stress even though the fill itself was sound.

Blind Via Reliability and Thermal Cycling

A filled microvia carries current and heat through a very small cross section, and it is the place where the expansion difference between the copper and the resin is concentrated. Thermal cycling is the test that exposes a poor fill, because the void and any thin cap are the points where a crack starts and propagates.

Reliability is therefore a function of the fill quality rather than of the via geometry alone. Two boards with identical stack-ups and identical via sizes can behave very differently if one was filled with a sound process and the other was filled with a bath that had drifted out of balance.

Inspection of Filled Microvias

Inspection combines cross sectioning with non-destructive checks. A microsection shows the fill profile, the dimple depth, the cap thickness and any void, and it remains the reference method for qualifying a process. It is destructive, so it is applied to coupons and to samples rather than to every panel.

Non-destructive methods look for the same features without cutting the board. The structure of these small features and the way they are formed is described in the guide to HDI blind and buried via processing, and the measurement of the copper itself is covered in the guide to plating thickness.

Common Defects and Their Causes

The typical defects are a central void, a partially filled via, an overfilled via with copper on the surface, a cracked cap after thermal stress and a delaminated interface between the fill and the barrel. Each points to a different part of the process, from the chemistry through the current programme to the lamination cycle.

Diagnosis starts with the section. The position of the void within the fill is the clue: a void at the bottom indicates a wetting or activation problem, a void in the middle indicates that the deposit closed at the top too early, and a void at the top usually indicates an incomplete fill that was capped over.

Process Windows and Control Charts

Because the process is sensitive, the parameters that matter should be charted: additive concentration, current programme, dimple depth from the coupon and cap thickness. Charts show the drift before a defect appears, which is the difference between controlling the process and reacting to it.

Control limits should come from the process capability rather than from the drawing tolerance, because a process that is only just inside the specification has no room to absorb normal variation. Where the capability is inadequate, the window has to be widened by process improvement, not by relaxing the acceptance criteria.

Additional Considerations for This Build

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

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Does every microvia have to be filled? No. Conformal plating is sufficient for vias that are not stacked on and are not used as a thermal path. Filling is specified when the layer above must be built on a flat surface, when the via is used as a pad, or when the design stacks vias from layer to layer.

How deep can a dimple be? The limit comes from the layer above and from the laminate, and it is normally stated as a fraction of the via depth or as an absolute value on the drawing. A dimple that is too deep leaves a cavity under the pad and becomes a starting point for a crack.

Why does a filled via still fail in thermal cycling? Usually because of a void inside the fill, a cap that is too thin or an interface that did not bond during lamination. The fill looks complete in a plan view, so the failure is only visible in a section or after the stress has opened a crack.