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PCB Minimum Annular Ring: Formula and Guidelines

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • The PCB minimum annular ring is a function of the assembly density.

  • The formula to determine the minimum land size depends on the minimum hole size, the minimum annular ring, and a fabrication allowance.

  • Increasing the annular ring can offer mechanical advantages for assembly.

View of a bare board with many traces, vias, and plated through-holes.

The PCB minimum annular ring size will vary depending on assembly density, and additional mechanical considerations.

PCB design would restrict boards to single-side assemblies without through-holes and adjacent technology. While through-hole packaging has fallen out of favor compared to surface mount devices with continual miniaturization efforts, these components still provide the necessary functionality for many designs. 

Beyond assembly, vias help compact designs and promote signal integrity with the most direct routing path possible. An annular ring creates the solderable area for soldering for all these through-holes and ensures a good connection between the trace and the plated barrel. PCB minimum annular ring rules detail the sizing guidelines for this feature, as dictated by the producibility level of the design.

IPC Guidelines for the PCB Minimum Annular Ring

The annular ring is the ring of via pad area left after drilling; finding the minimum annular ring size that does not impede general reliability leaves designers with more room for routing. Like most design features, the scope and capabilities extend from the IPC Performance Classes and Producibility Levels:

Performance Class

Producibility Level

Class 1 (general) - Consumer devices where manufacturers deem function over performance as successful.

Level A (general) - The most robust/tolerant producibility level, preferred by manufacturers due to a wide range of acceptable fabrication tools and materials.

Class 2 (dedicated) - Sophisticated telecommunications or industrial equipment where uninterruptable performance is preferred but not required.

Level B (moderate) - The standard producibility level; an increase in tooling leads to a proportional cost escalation.

Class 3 (high reliability) - Critical systems (think life support or aerospace) or devices in demanding environments where continuous, uninterrupted performance is a design requisite.

Level C (high) - The most stringent manufacturing demands for a design; the sophisticated fabrication further drives up costs.

There are some essential distinctions between the two: the producibility level is not a design requirement, and the level applies on a per-feature basis. Furthermore, while class and level are often correlated (a high-reliability design will usually have more demanding feature demands that depress yield, and vice versa), they are independent manufacturing aspects. The producibility level governs land pattern determination for all component package types, including those containing through-holes:

  • Level A (maximum land/lead-hole relationship) - This level supports the most producible solder application method with a wide window of acceptable process controls. Excellent for boards with low component density.

  • Level B (nominal land/lead-hole relationship) - The nominal condition provides good producibility for most solder application methods and is suitable for PCBAs with moderate component density.

  • Level C (least land/lead-hole relationship) - The most restrictive producibility level, geared toward high component density designs like portable devices and other small form features.

Determining the appropriate production level balances the minimum need of a design, which is highly variable depending on the device application and end-use conditions. While the producibility level is necessary for the minimum annular ring, the minimum PCB annular ring rule is a fixed 0.05 mm/2 mils regardless of manufacturing sophistication. The effective annular ring size will change depending on the minimum annular ring and two additional rules:

  • Minimum hole size - Through-holes must be larger than component leads as plating will shrink the hole diameter. Higher producibility levels allow for a tighter tolerance. Designers should indicate the final (post-plating) hole size on appropriate documents; the fabricator then will have leeway to drill to whichever pre-plated diameter best suits their tooling capabilities.

  • Minimum fabrication allowance - This general fabrication tolerance decreases with producibility.

These three factors combine in the equation

L  =  a  +  2b  +  c

where L, a, b, and c are the minimum land size, maximum finished hole diameter, the minimum annular ring, and standard fabrication allowance, respectively.

Necessary Values to Calculate a PCB Annular Ring

Level A

Level B

Level C

Minimum Hole Size

.25 mm/10 mils

.2 mm/8 mils

.15 mm/6 mils

Minimum Annular Ring

.05 mm/2 mils

.05 mm/2 mils

.05 mm/2 mils

Minimum Fabrication Allowance*

.4 mm/16 mils

.25 mm/10 mils

.2 mm/8 mils

*: For copper weights above 1 oz/sq. ft., add .05 mm/2 mils for each additional oz/sq. ft.; for boards with more than eight layers, add .05mm/2 mils.

When the Minimum Annular Ring May Not Be Enough

Annular ring rules apply to vias used solely for connectivity and plated through-holes that integrate specific components to the board during assembly. The minimum annular ring guidelines skew more general, allowing manufacturers and designers to modify these requirements upward if necessary.

  • Mechanical support - Larger components like connectors can benefit from larger annular rings to bolster structural integrity and minimize harmful effects like shock or vibration.

  • Assembly method - Manual assembly outcomes necessitated by larger or sensitive components may improve with annular ring sizes above the IPC suggested minimums.

  • Non-plated through-holes (NPTHs) - While NPTHs are now commonly used for mounting or tooling,  some designs may opt for a single pad on the side opposite mounting for solder joint formation (this is primarily reflective of single-sided PCB technology). A larger pad can better dissipate heat, as these pads are less mechanically resilient than their plated counterparts and can separate from the board during initial assembly or rework. On the other hand, an excessive pad size may interfere with solderability; speak with a fabricator to better deduce the ideal annular ring.

  • Teardrops - Designers can add filleting at the junction between a trace and annular ring to lessen the chance of breakout (the deviation of the drilled hole from the center of the pad that causes the minimum annular ring to shrink). There are additional mechanical benefits, too, as the extra pad area can protect against cracking at the copper-substrate interface due to flexure/thermal cycling and facilitate greater solder attachment.

Cadence Has ECAD Solutions for all DFM Features

While the PCB minimum annular ring rule will be constant across designs, there is a high level of variability in the resultant annular ring size according to the feature producibility. As designs become more intricate and form factors shrink, board layout will want to free up as much routing space as possible. At the same time, designers will want to assist their manufacturers and project budgets by selecting annular ring sizing that does not unnecessarily bloat costs. 

Cadence’s  PCB Design and Analysis Software suite provides engineers and design teams with a proficient DFM toolset to rapidly simulate design for prototyping and production. When it comes time for layout, OrCAD PCB Designer possesses a comprehensive DFM Constraint Manager to ensure compliance and manufacturing compatibility.

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