Annular Ring Calculation and Breakout Limits in PCB Design

The annular ring is the ring of copper that surrounds a drilled hole on a printed circuit board. It is what connects the barrel of the hole to the trace and what holds the pad together, and its width is the result of a calculation that combines the pad size, the drill size and the tolerances of every process that touches the area. Annular ring calculation is therefore one of the first design rules a layout engineer has to apply, and one of the most common reasons a design is rejected at fabrication review.

What the Annular Ring Is

The ring is measured from the edge of the drilled hole to the edge of the copper pad, on the plane where the pad exists. The minimum ring is the smallest width around the hole, and that minimum is what the specification limits, not the average. A hole that is slightly off centre reduces the ring on one side without changing it on the other.

The ring carries the current between the barrel and the trace and it also holds the pad to the laminate during thermal cycling. If it is too narrow, the pad can lift, the barrel can separate, or the connection can open during assembly. The mechanical and electrical functions are served by the same copper, so the width has to satisfy both.

The Calculation and Its Inputs

The basic calculation subtracts the maximum drilled hole diameter from the minimum pad diameter and divides by two. Each of those values is a limit rather than a nominal figure, and using nominal values is the most common mistake in the calculation, because it ignores exactly the variation the limit is meant to cover.

The pad diameter comes from the design with its own tolerance, and the hole diameter has to allow for drill wear, drill runout and the plating that will be added afterwards. The finished hole is smaller than the drilled hole because the plating reduces it, but the annular ring is measured at the drilled diameter, which is why the calculation has to be done carefully.

Drill Tolerance and Registration Error

The drill does not follow a perfect path. The machine has positional accuracy limits, the panel can shift on the tooling pins and the drill itself wanders as it enters the material, particularly in a thick stack. Each of those effects moves the hole away from its nominal position and takes material off one side of the ring, and they apply in addition to the dimensional tolerance of the pad itself.

Registration error adds to the same problem on the inner layers. Each layer is imaged and etched with its own tolerance, and the layers are stacked with a further allowance, so the copper pad on an inner layer is not exactly where the drawing says it is. The calculation has to include all of these contributions, and the sum is often much larger than designers expect.

Breakout, Tangency and Internal Layers

A drill breakout occurs when the hole is so far off centre that the ring disappears on one side and the hole touches or crosses the pad edge. The cause is normally the sum of drill position error and layer to layer registration rather than a single gross mistake. A tangency is the borderline case where the hole just meets the edge of the pad, leaving no measurable ring at that point. Both are defects on an outer layer, and on certain structures they can be functional failures.

On internal layers the situation differs because there is no plating on the surface and the pad is defined by the etch. A breakout on an inner layer can disconnect the plane connection entirely, while the same geometric condition on an outer layer may still have a continuous barrel. This is why the rules for inner layers are usually tighter, and the structure of the finished hole is described in the guide to hole copper.

Pad Size and Design Rules

The practical way to guarantee the ring is to specify a pad diameter that leaves enough copper after all the tolerances have been applied. Shops publish a table that relates hole size to minimum pad size for their process, and the design should be built from that table rather than from a general rule.

Where space is tight, the alternative is to reduce the hole size or to move to a different via structure. Reducing the hole increases the aspect ratio, which affects plating, and the trade-off between hole size and reliability is discussed in the guide to aspect ratio. The design has to balance the space available against the process capability, and the balance should be made with data rather than optimism.

Teardrops and Their Effect

A teardrop is a fillet of copper added where a trace meets a pad, which widens the connection and increases the effective ring on that side. Teardrops are particularly useful on small pads and on pads that will experience thermal stress, because they reduce the concentration of stress at the junction and give the etch more room to work.

The cost is space. A teardrop needs area around the pad, so it cannot be used everywhere on a dense board, and it changes the impedance of the trace slightly at the junction. The decision should be made per feature rather than as a global setting, and the reason should be recorded so that a later revision does not remove them by accident.

Effect of the Finish and the Hole

The surface finish sits on top of the copper and does not add to the ring, but it does affect how the pad behaves during assembly. A finish that consumes copper, such as a thick hot air levelled coating, changes the effective thickness of the pad and can make a marginal ring worse if the copper beneath is already thin.

The hole itself also changes with plating thickness. A hole that is plated to the upper end of its tolerance becomes smaller, which is good for the ring but can make component insertion difficult. The dimensional chain that runs from the drill through plating to the finished hole should be understood by both the designer and the shop.

Inspecting the Ring

Inspection is normally done on a coupon or a sample panel by sectioning the hole and measuring the copper on each side. The measurement is made at the plane of the pad, and the smallest value around the circumference is what counts. On production panels, the same measurement can be made optically from the top for outer layers where the pad is visible.

A coupon that represents the worst case, including the smallest pad and the largest hole, is the most useful test vehicle. If the ring is adequate on the coupon, the production panels are likely to be acceptable, and the coupon also serves as a record that the process was capable at the time the order was built.

Design Rules and Fabrication Notes

The design should state the minimum annular ring that is required, the pad sizes used and the layer on which the tightest features appear. It should also state whether breakout is permitted on non functional pads, because the answer changes the cost of the panel significantly.

The notes should be written before the design is released, and they should be checked against the shop’s capability rather than assumed. A design that uses pads below the shop’s minimum will either be accepted with a deviation or returned for revision, and both outcomes cost time. The checks that prevent that situation are listed in the guide to the fabrication notes checklist.

FAQ

What is a typical minimum annular ring? It depends on the shop and on the layer, and values between 0.05 and 0.15 millimetres are common for standard production. The number that matters is the one the shop can hold after accounting for drill tolerance and registration, and it should be taken from their capability table.

Is breakout always a defect? On a functional pad it is, because the connection between the barrel and the trace may be reduced or lost. On a non functional or mechanical hole it may be acceptable, provided the drawing says so and the limits are defined.

Do teardrops remove the need for a larger ring? No. A teardrop strengthens the trace to pad junction and increases the effective copper on one side, but it does not fix a hole that is off centre. The ring calculation still has to be satisfied around the whole circumference.