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The annular ring on a printed circuit board (PCB) is the area of copper pad that extends around the circumference of a drilled and finished (usually plated) hole. This ring is vital for providing a reliable connection between the hole (via or through-hole) and the conducting layers of the PCB.

Here are some key considerations and rules regarding annular ring design:

Size

The size of the annular ring is critical. If it's too small, there might be a poor connection or no connection at all. If it's too large, it may unnecessarily consume PCB real estate. The minimum annular ring size is generally defined by the manufacturer's capabilities and is often in the range of 1-6 mils (0.001-0.006 inches) for outer layers and 1-1.5 mils for inner layers.

Tolerance

Fabrication tolerances, including drilling accuracy and alignment, also play a role in defining the annular ring size. There can be a positional variance during drilling, so a larger annular ring allows for these potential misalignments.

Plating

Most vias are plated with copper to electrically connect different layers of the PCB. The plating process can reduce the size of the annular ring, so this must be taken into consideration when defining the initial size.

Mechanical Stress

Annular rings are more vulnerable to mechanical stress than other parts of the PCB. High levels of mechanical stress can cause the annular ring to fracture or separate from the PCB substrate, leading to circuit failure. Therefore, if your board is expected to experience high levels of mechanical stress (like in a high vibration environment), you might want to use larger annular rings.

Thermal Stress

Thermal cycles can cause expansion and contraction of the PCB material and the copper annular ring. This mismatch can cause cracks in the annular ring or separation from the substrate, leading to circuit failure. If your design is intended for a high-temperature environment or will experience thermal cycles, you may need to use larger annular rings.

Cost and Design Complexity

As with other aspects of PCB design, larger annular rings increase reliability but can also increase cost and decrease the density of the design. So, a balance must be struck between reliability, cost, and design complexity.

In conclusion, the size of the annular ring in your PCB design will depend on a number of factors, including manufacturing capabilities and tolerances, the expected mechanical and thermal environment, and cost and design considerations. Always consult with your manufacturer to understand their specific requirements and capabilities.