Throwing Power and Plating Uniformity in Deep PCB Holes

Plating a hole uniformly is harder than plating a flat surface, and the difficulty grows with the depth of the hole relative to its diameter. The reason is that the plating solution has to be exchanged inside the hole while the current is flowing, and the geometry works against both. Throwing power is the property that describes how well a process overcomes this, and it is the number that determines whether a stack-up can be plated reliably.

The Electrolyte Problem Inside a Hole

Plating consumes copper ions at the cathode surface. On a flat panel the solution is replenished by convection, and the ion concentration at the surface stays close to the bulk value. Inside a hole the exchange is much slower, so the concentration at the wall falls as plating proceeds and the deposition rate at the centre of the barrel drops below the rate at the surface of the board.

The result is a plating thickness distribution in which the copper on the board surface is thicker than the copper at the middle of the hole wall. The ratio between the two is what a fabricator measures when qualifying a process for a given aspect ratio, and it is the reason a deep, small hole may be specified as unplatable rather than merely difficult.

Throwing Power and Aspect Ratio

Aspect ratio is the depth of the hole divided by its diameter, and it is the single most useful predictor of plating difficulty. As the ratio rises, the barrel becomes a long thin tube, convection at the centre becomes weaker, and the difference between the surface plating and the barrel plating grows. Throwing power describes the ability of the chemistry and the current waveform to compensate for that, and a process with high throwing power can plate a deeper hole with an acceptable thickness ratio.

The practical consequence for design is that the smallest via diameter must be chosen against the finished board thickness, not in isolation. A via that is easy to plate on a thin board can be marginal on a thick one, and the fabricator’s capability chart should be consulted before a minimum via size is fixed.

Getting the Solution Into a Through Hole

In vertical plating, the board is immersed in tanks and the flow of solution through a through hole depends on the pressure difference between the two faces. That pressure difference is what drives the liquid into the hole and displaces the air that is trapped in it. For holes with a high aspect ratio, the pressure difference matters more, because the resistance to flow inside a narrow tube rises sharply.

If the solution does not enter the hole, the hole is plated only partially, and the defect appears later as a barrel crack during thermal cycling or as an open circuit after assembly. Wetting the hole thoroughly before the plating current is applied is therefore as important as the plating itself. Our hole wall quality notes cover the defects that follow poor wetting.

<img src="https://www.gopcba.com/wp-content/uploads/2026/05/IoT-Gateway-Controller-PCBA.png" alt="Blind via cross-section in a high density interconnect printed circuit board” />

Blind Vias Make Everything Harder

A blind via is open at one surface and closed at the bottom, so the trapped air has only one direction in which to escape and the solution has only one direction from which to enter. Exchange is therefore much weaker than in a through hole of the same dimensions, and the difficulty rises quickly as the depth increases. Where the depth is small or the aspect ratio is low, all the usual techniques work, including horizontal plating, and the result is a reliable connection.

Where the blind via is deep, horizontal processing struggles. The solution is delivered from above, the air has to be displaced downward against the flow, and in the worst case the electrolyte does not enter at all. Periodic inversion of the board helps, and vertical processing with a controlled pressure difference is generally superior to horizontal processing for blind vias with a high aspect ratio. As a working limit, blind vias beyond an aspect ratio of roughly 0.8 become difficult to process reliably.

The Techniques Available

There are four standard responses to a hard plating task. The first is to increase the exchange rate inside the hole, either by increasing the flow rate or by agitating the board. The second is to raise the current density inside the hole, which is generally impractical because raising the current density at the hole also raises it at the surface, making the thickness difference worse rather than better.

The third is to reduce the current density and the copper concentration in the electrolyte together while increasing the exchange rate, which narrows the concentration difference between the surface and the hole wall at the cost of productivity. The fourth is pulse plating, in which the current is applied in short pulses with a defined off time, chosen according to the aspect ratio. The off period allows the ion concentration at the wall to recover, and a well-tuned pulse regime can bring the surface and barrel thicknesses close to parity. Our hole copper notes describe what the finished barrel has to achieve.

Designing for a Reliable Barrel

The designer’s contribution is to keep the plating task inside the process window. Choose via diameters that match the finished thickness, avoid the smallest available drill on the thickest panels, specify blind vias with an aspect ratio the process can support, and place vias in areas where the board can be processed from both sides rather than in a region that will be inaccessible. The aspect ratio guidance explains how the limit is calculated, the blind and buried via material covers the HDI structures that depend on it, and the design release checklist includes the via sizing checks.

Measuring the Result

Plating uniformity is verified on a coupon rather than on the board, because the measurement requires a cross-section. The coupon is plated alongside the production panel, sectioned and measured, and the result is reported as the copper thickness at the surface, at the barrel mid-point and at the inner-layer connection. A process is considered capable when the thinnest measurement in the barrel exceeds the specified minimum across the range of aspect ratios in the panel.

FAQ

What plating thickness should a barrel have? The requirement is normally expressed as a minimum copper thickness on the barrel wall, commonly around 20 to 25 micrometres for a general-purpose board, with a higher figure for boards that will be thermally cycled or used in a high-reliability application. The critical number is the minimum, not the average, because the barrel fails where it is thinnest. A coupon measured after plating is the usual way to confirm the minimum has been met.

Why does a via crack after assembly rather than before? Because the crack is caused by thermal expansion. The laminate expands much more than the copper along the axis of the hole, so each heating cycle stretches the barrel. If the barrel is thin at any point, or if the plating is brittle, that point accumulates damage and eventually separates. The defect is present from the beginning and becomes visible only after enough cycles.

How does gopcb decide whether a via is platable? The decision is based on the finished thickness, the drilled diameter and the aspect ratio, compared against our process capability for the plating method being used. We review this during design release, because a via that is outside the window cannot be corrected by adjusting the process afterwards; the design has to be revised while it is still a drawing.