Plated Hole Size Tolerance and Drill Selection

Drill Size Versus Finished Hole

The drill that makes a hole and the hole that a component sees are not the same dimension. Plating adds copper to the wall of every plated hole, so the finished diameter is smaller than the drilled diameter by twice the plating thickness, and the amount varies with the process, the aspect ratio and the position on the panel. A design that specifies the drill size is specifying a process input; a design that specifies the hole a pin must fit is specifying a result. The fabricator needs the second and derives the first, which is why the hole callout on a drawing should always be the finished size unless there is a specific reason to do otherwise.

Plating Thickness and Its Variation

Plating thickness is normally specified as a minimum on the barrel wall, often around 20 to 25 micrometres for a general-purpose board and more for a board that will see thermal cycling or a press-fit connector. The actual thickness is not uniform: it is thicker at the entry and exit of the hole and thinner in the middle, and the difference grows as the aspect ratio rises. That means the finished hole is not a cylinder but a slightly barrel-shaped bore, and the tightest point is usually near the middle. For a connector that has to fit through the whole board, the smallest diameter is the one that matters, so a specification written only on the surface dimension is incomplete.

Drill Wear and Hole Quality

A drill bit wears, and as it wears the hole it produces becomes slightly smaller, the wall becomes rougher and the entry burr becomes larger. Shops manage this by specifying a bit life and by resharpening or replacing the tool on a schedule, but the consequence for the design is that a hole has a natural spread even without any change to the artwork. A design that leaves no margin for that spread will produce a percentage of holes outside the tolerance, usually at the end of a drilling run. Where the hole is critical, the drill life should be part of the process agreement rather than left to the shop’s own standard.

Registration and the Annular Ring

Hole position varies because of the drilling accuracy and because the copper pattern has its own registration error. The annular ring that remains after both is what carries the connection, and it has to be positive at the worst corner of the stack, not at nominal. A hole that moves to the edge of its pad leaves a thin ring, which is a reliability risk even when it is not a reject, and a hole that moves further produces a breakout. Designing with a generous ring is cheaper than asking for tighter registration, because the second requires a different process rather than a different callout.

Aspect Ratio and What It Limits

The aspect ratio is the board thickness divided by the drill diameter, and it governs how uniformly the barrel can be plated. As the ratio rises, the plating in the middle of the barrel becomes thinner and the hole becomes more barrel-shaped, and beyond the process limit the plating may not be continuous. This is the reason a thick board cannot use arbitrarily small holes even when the layout would like to. Where the design needs a small hole in a thick board, the options are to use a build-up structure with a shallower via, to increase the drill diameter, or to accept a lower hole count on that layer.

Specifying for Connectors and Press-Fit

Where a hole accepts a component rather than being soldered, the specification has to follow the component datasheet, which usually gives a recommended finished hole size and a tolerance band. A press-fit pin, a self-clinching fastener, a rivet and a plastic barb all have their own requirements, and all of them are expressed on the finished hole. The tolerance in those bands is often tight, which is a signal that the board may need a plating specification that supports it: a hole tolerance of a few hundredths of a millimetre has to be built on a controlled plating process rather than on a general one. Where the design also needs a fine annular ring, the two requirements have to be reconciled before the data is released.

Verification

The finished hole is verified by measurement on the panel, usually with a pin gauge or an optical system on a sample, and by cross section where the barrel plating has to be confirmed as well. A pin gauge checks whether the hole is within its band but says nothing about the wall; a cross section shows the wall thickness and the shape at the cost of destroying the sample. For a critical hole, the useful combination is a gauge check on a sample of holes across the panel together with a section from the worst position, which is usually the centre of the panel or the corner with the highest aspect ratio.

Hole Tolerance in the Larger Assembly

The hole is one dimension in a chain that ends at the mating part, and it is rarely the only one that matters. A connector that has to align with a panel cut-out, a screw that has to pass through a board and a chassis, a pin that has to enter a socket: each of these is a stack of tolerances in which the hole is one element. Designing the hole to the tightest tolerance available and leaving the other elements loose is a common mistake, because the assembly is limited by the worst element rather than the best. The productive approach is to list the dimensions in the chain, assign a realistic tolerance to each, and check that the sum still allows the parts to fit; only then decide whether the hole needs a tighter band. Where one element has to be tight, it is usually cheaper to make it a mechanical feature such as a fixture or a locating pin than to tighten the plating process on every hole in the board.

FAQ

Should I specify the drill size or the finished hole size? The finished hole size, because that is what the component or the pin has to fit, and the fabricator selects the drill accordingly.

How much does plating reduce a hole? By twice the plating thickness, typically around 0.04 to 0.05 mm in diameter for a general-purpose barrel.

Is the hole the same diameter all the way through? No. Plating is thicker at the ends and thinner in the middle, so the smallest diameter is usually near the centre.

What limits how small a hole can be in a thick board? The aspect ratio, because the plating has to be continuous and uniform along the barrel.

How is a finished hole verified? By pin gauge or optical measurement on samples, with cross sections where the barrel plating also has to be confirmed.

Conclusion

A hole is a produced dimension, not a drawn one, so the specification should be written on the finished size with the tolerance that the component requires and the aspect ratio that the process can plate. Leave margin for plating thickness, drill wear and registration, and verify the result on the worst position of the panel. Drilling and plating capability is described under PCB capabilities, the process steps are covered in PCB manufacturing, and the pad and ring rules belong to PCB design and layout. Connector fits are normally confirmed during PCB assembly in 2026.