Non-Plated Holes: Tolerance, Burrs and Breakout Control

Non-plated holes are drilled through the board after the plating steps, so nothing closes them back up afterwards. They exist for screws, connector clearance, isolation and tooling, and because they carry no electrical function they are often specified with less care than the plated ones.

That is a mistake. A non-plated hole that is undersized stops a screw from passing, one that is oversize removes material from a narrow web, and a burr left on the exit side stops a connector seating flat. The rules that matter are the finished dimension, the copper clearance, the burr limit and the distance from the panel edge, and all four are decided at design time.

What Makes a Non-Plated Hole Different

A plated hole is drilled undersize and then built up by copper, so its finished dimension is the drill diameter plus twice the wall. A non-plated hole has no such compensation, and its finished size is the drill diameter adjusted only by the spring-back of the laminate.

The wall is the second difference. There is no copper to cover a rough surface, so a non-plated hole shows the drilled surface including fibre pull-out, it cannot be used as a test point, and solder mask must be kept out of it.

Drill Oversize and the Finished Dimension

Drills cut slightly oversize in laminate, and the amount depends on the tool, the spindle speed and the feed rate. On a 3.2 mm hole an oversize of 0.05 to 0.08 mm is typical, which is wider than the tolerance band of many drawings.

The consequence is that the drawing should state the finished hole size rather than the drill size, and the shop should record what it actually drills. Where drill data arrives with a tool diameter quoted for a non-plated hole, the finished dimension has to be confirmed on the first article before the lot runs.

Tolerance Classes and What They Cost

Hole tolerance is quoted in classes because cost rises quickly as the band narrows. A clearance hole for a screw can be held at plus or minus 0.10 mm on a routed panel without special effort, while a hole that locates a press-fit bushing or a connector peg may need plus or minus 0.05 mm and a first article on every lot.

The tighter class usually demands a dedicated tool, a controlled stack height and an inspection step, and those costs belong in the quotation. Asking for a tight tolerance on a hole that only passes a screw pays for accuracy that carries no benefit. The band should also be quoted against the finished dimension, because that is the dimension the mating part sees.

Burrs on Entry and Exit

Drilling produces a burr on the exit side unless the backup material and the parameters are chosen to prevent it. The burr is a thin lip of copper or laminate standing proud of the surface, and on a non-plated hole nothing plates over it, as described in the notes on entry material and burrs.

A general limit is 0.05 mm or less, with 0.025 mm where a connector or a heatsink has to sit flat. Deburring uses a brush, a pumice scrub or a controlled chamfer, and the operation has to be verified because an aggressive brush rounds the hole edge and changes the finished dimension.

Copper Clearance Around the Hole

A non-plated hole passes through the copper as well as the laminate, so the copper has to stop short of the finished edge. A clearance of about 0.25 mm from copper to hole edge is a common minimum, and it must hold on every layer the hole crosses, which is a registration question as much as a design one.

The check belongs on the drill data rather than on the artwork alone, because the clearance is drawn from the finished hole while the drill follows the tool diameter. A hole drilled slightly oversize can turn a legal clearance into a short, and the registration budget decides how much margin is left.

Breakout at the Panel Edge

A hole placed close to the board edge can break out through the edge, leaving an open slot instead of a closed circle. The usual rule is that the hole edge stays at least one radius from the board edge, and many designs allow breakout over a limited angle where the hole is only used for clearance.

Breakout through copper is a different matter and is treated as a reject. The distinction should be written into the drawing with an angle and a direction, because the acceptance question comes up at inspection, and the routing behaviour behind it is covered in the notes on edge quality.

Countersinks, Slots and Mixed Features

A countersink or counterbore is machined after drilling and carries its own depth and diameter tolerances, and it must not cut into the surrounding circuitry. The depth is usually specified with a tolerance of plus or minus 0.05 mm, and the tool is dressed on the same schedule as the drill, as set out in the notes on countersink and counterbore.

Slots and oblong holes are routed rather than drilled, so their tolerance follows the router. Where a slot has to be plated on part of its wall and not on the rest, the sequence becomes a plated hole followed by a routing operation, which is a different and more expensive process.

Measurement and Incoming Checks

Non-plated holes are measured with pin gauges, with a video measuring system, or on a section for the burr and the wall condition. A pin gauge set gives a fast go and no-go answer, provided the set is calibrated and the sizes are recorded against the drawing.

The first article should cover the smallest and the largest non-plated hole on the panel and any hole with a tight tolerance, with the results filed alongside the tool numbers. Sampling afterwards follows from the tolerance class, with tight holes checked far more often than clearance holes. Where a hole is critical to assembly the check is made on every panel rather than on a sample, and the gauge used is recorded with the result.

Design Rules That Avoid Rework

Four rules remove most of the problems: specify the finished size rather than the drill, hold the copper clearance at 0.25 mm or more on every layer, keep the hole at least one radius from the board edge unless breakout is allowed, and reserve tight tolerances for the holes that need them.

Where a hole has to locate something precisely, a plated hole with a reamed bore or a separate machined feature is often a better answer than a non-plated hole held to an unrealistic tolerance. The acceptance limits for the finished board are the ones in the applicable workmanship specification, not the ones invented at the drill.

FAQ

Should a drawing give the drill size or the finished size? The finished size. A drill cuts oversize in laminate, so a tool diameter quoted as the finished dimension usually produces a hole that is out of tolerance on the large side.

What burr height is acceptable? Around 0.05 mm for a general product and 0.025 mm where a connector or heatsink must sit flat. On a non-plated hole nothing plates over the burr, so burr removal is a step of its own that has to be verified on the finished board.

How much copper clearance is needed around a non-plated hole? About 0.25 mm from the copper to the finished hole edge is a common minimum, checked on every layer and against the drill data rather than the artwork.