Hole Wall Voids: Causes and Plating Coverage Control

A hole wall void is a break in the plated copper that covers the barrel of a drilled hole. The break may expose the laminate underneath or it may be a thin spot where the plating is discontinuous. Either way the barrel no longer has a continuous conductor connecting the outer layers to the inner ones, and the joint relies on whatever copper remains. Voids are found by sectioning, because nothing on the surface of the board indicates that they exist.

What a Hole Wall Void Is

Voids are described by their position, their size and their shape. A void at the interface between the base copper and the plated layer is a microvoid; one that runs around the barrel is a ring void; one that exposes the laminate is an open void. Each points to a different part of the process.

The distinction between a barrel void and a thin spot matters for acceptance. A void interrupts the conductor, while a thin area merely reduces the cross-section. Specifications treat the first as a defect and the second as a thickness measurement.

Where Voids Originate

Four process steps can produce them. The drilling step leaves a surface that has to be prepared properly. The desmear and activation steps have to cover that surface completely. The plating bath has to deposit uniformly into the hole. And the laminate itself has to be free of trapped gas that can be released during plating.

Because the causes are distributed across the process, a void found in a section is a symptom rather than a diagnosis. The position of the void in the barrel and its relation to the interface narrow the field, and the section should be examined at a magnification high enough to see the interface clearly.

Drilling Contribution

The surface left by drilling is what the plating has to cover. A hole with resin smear on the wall, with torn glass fibres or with a rough wall offers a poor surface for the subsequent steps, and the smear in particular can remain in place behind the plated copper if it is not fully removed.

Drill parameters influence the wall quality directly. A high hit count, an excessive spindle speed or an inadequate feed all raise the temperature at the cutting edge and increase the amount of resin that is smeared. Changing the drill supplier without re-qualifying the parameters is a common cause of a sudden change in wall quality.

Desmear and Activation

Desmear removes the resin that drilling left behind, and it also roughens the wall so that the subsequent chemistry can bond to it. An incomplete desmear leaves smear, which becomes a void once the copper covers it. An excessive desmear attacks the glass bundles and leaves a wall that the activation step cannot cover evenly.

Activation is the step that deposits the catalyst the electroless copper grows from. A catalyst that does not reach part of the hole leaves that part unplated, and the resulting void is a patch rather than a ring. Air bubbles trapped in a deep hole during activation produce the same result at the same place on every board.

Plating Chemistry and Coverage

Electroless plating coverage is the foundation; the electrolytic plating that follows can only build on what is there. Where electroless coverage is incomplete, the electrolytic copper bridges the gap from both ends and leaves a void in the middle of the barrel.

The electrolytic bath then determines the thickness and the uniformity. Low copper microvoid levels depend on the bath chemistry and the additive balance, and a bath that is out of balance produces a deposit with small voids at the interface even when the coverage is complete. The plating thickness has to be measured in the hole and not only on the surface for any of this to be visible.

Void Shape and What It Reveals

A ring void that follows the circumference of the barrel and sits at a consistent depth usually points to a bubble trapped at that position, which is a process or a hole geometry issue. A void at the base copper interface, running along the barrel, points to the activation and electroless step.

A large void that occupies a section of the barrel opposite the entry side points to a supply problem: the plating solution is not exchanging inside the hole. Agitation, hole diameter and aspect ratio all contribute, and the same board plated with better flow usually shows the void reduced rather than displaced.

Measurement and Acceptance

Voids are measured by sectioning holes and examining them under a microscope, with a magnification that resolves the interface. The number of holes examined and their distribution across the panel have to be stated, because a void rate quoted without a sample size is not comparable.

Acceptance criteria usually state the maximum permitted void area and whether a void may expose the laminate. Where the requirement is a minimum plating thickness in the barrel, the measurement and the void inspection are done on the same section, so the microsection preparation has to be good enough for both.

Effect on Reliability

A void interrupts the conductor, so the effect on electrical performance depends on how much copper remains around it and on how the current redistributes into that remaining section. A barrel with a small void may still pass electrical test and fail later, because the remaining copper is thinner and the current density locally is higher, which accelerates electromigration and thermal fatigue.

Mechanically, a void acts as a stress riser. The barrel cracks at the void edge during thermal cycling, and the crack propagates around the circumference until the connection is lost. This is the mechanism that makes a small void a reliability problem rather than a cosmetic one, and it is why hole wall quality is judged on a section rather than by electrical test.

Records and Corrective Action

The record should carry the drilling parameters and hit count, the desmear condition, the electroless and electrolytic bath analyses, the copper thickness in the hole and the void observation from the section. Those fields cover the four steps that can produce a void, and they make it possible to act without a trial.

Corrective action should follow the shape of the void rather than its size. A ring void at a consistent depth is attacked through agitation and hole geometry; an interface void through activation and electroless control; a scattered void through drilling and desmear. Treating every void as a plating problem is the most common reason a void rate stays where it is.

FAQ

What causes a void in a plated hole wall? Incomplete desmear, incomplete activation, poor electroless coverage, trapped bubbles during plating or a rough drilled wall. The shape and position of the void indicate which step is responsible.

Can a void be seen without sectioning? Generally not. A void is internal to the plating, so a section or a scanning method is required. Electrical test finds only the voids that interrupt the conductor completely.

Is a small void acceptable? It depends on the specification and on the area permitted. A void acts as a stress riser and increases the local current density, so it is treated as a reliability concern rather than as a cosmetic one.