Annular Ring: Key Checks Before Release

The annular ring is the band of copper that surrounds a drilled hole on a pad. It has to be wide enough that the hole can be drilled off centre by the full tolerance of the process and still leave copper all the way around, and it has to be small enough that the pad does not intrude on the routing space next to it.

This article explains how the ring is sized, which tolerances set the minimum, what a breakout means, and how the requirement differs between the inner and outer layers.

What The Ring Has To Provide

The ring exists to guarantee an electrical connection between the barrel and the pad or the inner layer copper. If the hole breaks through the edge of the pad, the barrel is still plated but the connection to that layer is reduced to a narrow neck or is lost entirely. The ring also carries the mechanical load of the joint, because the solder fillet pushes against the pad as it forms, and a pad with an inadequate ring is more easily lifted by thermal cycling.

The minimum ring is therefore stated as a requirement rather than as a preference, and it appears in the fabrication drawing as a minimum value and not as a nominal one. The nominal pad size is derived from the maximum hole size plus twice the minimum ring, and the pad is then drawn at that size with its own tolerance. A design that states only a pad diameter and leaves the ring to be inferred is a design that will eventually be built with an inadequate ring. The conventions for pad and hole sizing in general are set out under PCB pad design standards.

Tolerances That Reduce The Ring

Four tolerances act together. The drill tolerance is positional, and it depends on the machine and on the number of hits on the bit. The pad or inner layer pattern has its own registration tolerance relative to the layer it is on. The layers have a tolerance between them, which accumulates through lamination. And the hole itself has a size tolerance, so the hole can be larger than nominal.

The ring on the worst case hole is the nominal ring minus half the hole size tolerance minus the drill position error minus the layer to layer registration error. On a fine pitch inner layer the registration error is usually dominant, which is why the ring rules for inner layers are looser than for outer layers in many shops: a drilled hole that breaks out on an inner layer that is misaligned is a process reality, and the design has to allow for it. The barrel quality that results from the drilling is covered under copper plating defects prevention.

Breakout And Its Consequences

A breakout is the condition in which the hole is tangent to or outside the pad edge, so the barrel is exposed on one side. On an outer layer it is usually visible as a crescent of laminate beside the pad, and it is treated as a defect in most workmanship standards, although a small breakout on a non connected pad may be accepted by some specifications for a given class.

The electrical consequence depends on the layer. On an outer layer with a solder joint, a partial breakout reduces the contact area between the barrel and the pad and weakens the joint, but the joint may still be reliable if the remaining ring is adequate. On an inner layer, a breakout removes part of the connection between the barrel and the inner layer copper, and the current has to pass through a narrower path. Where a breakout is found on a power or ground plane connection, the effect on current capacity is the number that matters, and it is estimated from the remaining copper rather than from the ring dimension.

Design Rules And Process Capability

Design rules for the ring are written against the capability of the fabricator, and the capability differs between shops and between processes. A shop with a laser drilling line and good layer to layer registration can hold a smaller ring than one using mechanical drilling on a stacked panel. Quoting a rule without knowing the shop is the reason that a board can be designed inside a textbook rule and still fail at the manufacturer.

The practical approach is to agree the ring rule with the fabricator at the start, to state it as a minimum on the drawing, and to let the fabricator adjust the pad sizes within a stated limit if the rules cannot be met. Where a pad cannot be enlarged because of routing, the alternative is to reduce the hole, which may then require a smaller component lead or a change of via type. Seeing that trade before the artwork is released is much cheaper than finding it during fabrication, which is the discipline described under the PCBA development process.

Via Ring And Through Hole Ring

A via and a component hole do not have the same requirement. A component hole carries a solder joint and has to support the mechanical load of the lead, so its ring is sized for both. A via carries only current and is usually covered by mask, so its ring can be smaller. Where a via is placed in a pad and covered by a component, the via ring and the pad are the same feature and the requirement reverts to the component hole rule.

A via that is tented, meaning covered by mask on both sides, still needs a ring at the mask opening, because the mask has to seal against the pad and not against the hole. A via that is not tented but is plugged with ink has its own requirement, since the plug has to bond to the copper around the hole. Each of these variants has a minimum ring, and they should be tabulated in the design rules rather than inferred, because the difference between a tented and a plugged via is a single note on the drawing and a large difference on the shop floor.

Checking The Design Before Release

The check is a comparison of the drawn pad against the maximum hole size plus twice the minimum ring, performed on every layer, including the ones that carry only planes. A design rule check can be set to flag any pad that violates the rule, but the rule itself has to be entered correctly, with the tolerance contribution included rather than hidden in a margin.

The second check is visual and applies to fine pitch areas, where a pad that meets the ring rule may still leave inadequate clearance to the neighbouring pad. The two requirements pull in opposite directions: a larger pad gives a better ring and a smaller gap, and a smaller pad gives the opposite. The design is correct when both limits are satisfied at the same time, and where they cannot be, the pitch or the via type has to change. The complete set of fabrication constraints is gathered under design guidelines for manufacturability.

Additional Considerations for This Build

Practical attention to drill tolerance pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating drill tolerance explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

FAQ

Is a breakout always a reject? It depends on the layer, the class and the specification. A small breakout on an outer layer with a sound joint may be accepted under some workmanship classes, while a breakout on an inner layer connection is normally a defect because it reduces the connection area.

Why is the inner layer ring rule different from the outer? Because the registration between layers adds to the error on an inner layer, while the outer layer is registered to the drill with the pattern that is on the same surface. The inner layer therefore needs a larger allowance for the same process.

Can the ring be increased without moving the pad? Only by reducing the hole, which changes the component or the via. Where neither is possible, the pad has to grow into the routing space, and that trade is best made with the fabricator’s capability in hand.