Annular Ring Requirements For Plated Holes

The annular ring is the width of copper that surrounds a drilled hole on the pad. It is the material that connects the plated barrel to the conductor on that layer, and it is the part of the pad that has to survive drilling, plating and thermal cycling.

The size of the ring follows from a simple subtraction: the pad diameter minus the finished hole diameter, divided by two. Everything that can move the hole or the pad reduces the ring, which is why the requirement is written as a minimum after all the tolerances have been applied.

Why The Ring Matters

The ring carries current between the barrel and the track, and it also anchors the barrel mechanically. A ring that is too narrow has a higher resistance, generates more heat under load, and is the first part of the joint to crack during thermal cycling.

It also protects the laminate. If the ring is lost, the drilling operation can expose the resin around the hole, and that exposed resin absorbs plating chemistry and later causes a failure between the barrel and the pad.

Minimum Ring By Class

Common requirements are 0.05 mm minimum external ring for a Class 2 product and 0.025 mm for a Class 3 product, measured after plating and after the worst case registration error. Some customers require a larger figure for high reliability work, and some specify no breakout at all.

The figure applies to the finished board rather than to the design. A design that gives 0.1 mm nominally may produce 0.04 mm on a bad panel, which is why the tolerance stack has to be evaluated rather than assumed.

Registration And Drill Tolerance

Registration is the alignment between the drilled hole and the pad on each layer. It is affected by the drill accuracy, by material movement during lamination, and by the accuracy of the artwork on the inner layers.

Drill tolerance adds to the problem, because the finished hole has a diameter range as well as a position range. A hole drilled at the top of its tolerance and shifted to one side removes more copper than either effect would on its own, and the two contributions must be added together. The via structures involved are described under via to trace clearance in multilayer boards.

Breakout And Its Consequences

Breakout is the condition where the hole extends beyond the pad edge and the ring is interrupted. A small breakout may still pass inspection, but it removes the anchor on one side and concentrates the stress on the remaining copper.

The consequence appears later. A breakout reduces the fatigue life of the joint, and the failure often occurs during thermal cycling rather than at assembly, which makes it expensive to detect and even more expensive to correct.

Calculating The Pad Size

The pad diameter is calculated from the finished hole size plus twice the required ring, plus an allowance for registration and drill tolerances. A typical allowance is 0.15 to 0.25 mm on the diameter for a standard process.

The calculation should use the maximum finished hole size and the worst case positional error rather than the nominal values. Using nominal figures is the most common reason a design that looks compliant produces boards that are not. The way pad sizes are set is described under PCB pad design standards.

Reliability And Thermal Cycling

The ring is one of the features that determines how many thermal cycles a plated hole survives. A wider ring distributes the stress over more copper, and the fatigue crack takes longer to propagate from the barrel to the edge of the pad.

The reliability benefit is most pronounced on thick boards and on holes that carry high current, where the temperature excursion is largest. How the requirement is written into the fabrication drawing is described under PCB design and fabrication.

Inner Layer Rings And Their Own Rules

Inner layer pads are not subject to the same drilling damage as the outer layers, but they have their own minimum ring requirement. Because the inner layers are patterned before lamination, the registration error accumulates from the artwork, from the press and from the drill, so the allowance is usually larger.

A common figure for an inner layer pad is 0.1 mm of ring after tolerances are applied, with a larger value on a thick board or on a layer that carries high current. The inner layer ring is also the feature a cross section shows most clearly, which makes it a convenient check on process control.

Non-Circular Pads And Thermal Relief

A rectangular or oval pad on a through hole component provides more copper in one direction than another, so the ring is not uniform. The measurement must be taken at the narrowest point, and the pad shape should be chosen so that the narrow direction still meets the requirement.

Thermal relief spokes complicate the picture further, since the spoke width and length affect how heat flows from the pad into the plane. The spokes should be wide enough to plate reliably and narrow enough to provide the thermal isolation that the soldering process needs.

Process Capability And Inspection

The achievable ring depends on the drill capability of the fabricator, on the lamination press and on the registration of the artwork. A supplier who holds 0.05 mm routinely will still quote a larger allowance for a board with many layers, because the accumulated error grows with the stack.

Inspection is by cross section on a sample, measured at the narrowest point on each layer, and the result is compared with the drawing. The measurement should be taken after plating and, where the board will be reflowed, after the first thermal excursion as well. How the requirement is called out is described under PCB design and fabrication.

Ring Requirements For Press Fit And High Current Holes

A press fit connector loads the plated hole far more than a soldered pin, because the pin is inserted with an interference fit and stays under stress for the life of the product. The pad and the ring have to be sized for that load, and a thick board with a high aspect ratio needs a proportionally larger allowance.

A high current hole carries heat into the pad as well as mechanical stress. The ring should be generous enough to spread that heat and keep the local temperature rise low, and the copper on the connecting layers should be sized to match the barrel rather than the other way round.

The same reasoning applies to a hole that will be reworked during assembly. A pad with a small ring is easily lifted when a joint is desoldered and replaced, and a lifted pad is usually a scrap item because it cannot be repaired reliably in production.

FAQ

What is the minimum acceptable annular ring? Many specifications use 0.05 mm for a Class 2 product and 0.025 mm for a Class 3 product, measured on the finished board at the worst case registration.

Is a small breakout always a reject? Not always. It depends on the standard and on how much of the ring has been lost. Specifying no breakout at all is the safest approach for a high reliability product.

How is the ring measured? It is measured on a cross section of a plated hole, at the narrowest point, after plating and after any reflow. The measurement should be recorded with the lot.