How do decide the size of your annular ring?

Design and Layout Design for Manufacturing
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We often this issue when running DFM checks so I thought of starting a conversation here on the topic of annular ring size.

Quick recap on annular ring requirements: the minimum annular ring width should always meet or exceed what’s defined in the PCB design, since anything smaller can compromise component attachment and lead to pad breakout (where the annular ring is effectively zero).

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When that happens, the trace enters directly into the pad, reducing current-carrying capacity and reliability. In practice, laser-drilled vias can go as low as a 2-mil annular ring, while mechanically drilled vias require at least 3 mils.

Talk to your account manager but recommended drill sizes are usually 5 mil for laser-drilled holes and 5.9 mil for through-hole vias. For rigid PCBs, standard fabrication allows a process pad diameter of D + 0.012″ (about a 1-mil annular ring), while advanced and micro fab push closer to tangency at D + 0.008″. For flex and rigid-flex designs, both flex and rigid-flex processes typically require D + 0.014″ to maintain a 1-mil annular ring.

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For tolerances, I want to add that when drilling vias, a drill wander of about 2 mils can be expected, so the annular ring should be at least 2 mils for laser-drilled microvias and 4 mils for mechanically drilled vias, keeping in mind that annular ring values shown in CAM are based on the drilled (not finished) hole size since holes are intentionally drilled larger to allow for plating and surface finish.

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Finally, teardropping reinforces pad-to-trace connections and can repair via breakout by adding copper to reestablish continuity, which is especially critical for traces under 20 mils and for flex boards where added structural integrity helps resist shear forces and vibration.

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Obviously… It depends!

Discuss with your fab house.

I’d recommend you read the annular ring section of Sierra’s Knowledge Base: https://www.protoexpress.com/kb/annular-ring/

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