I’ve noticed that various manufacturers recommend specific annular ring requirements for laser-drilled vias, mechanically-drilled vias, and component holes.
My question is: When these minimum annular ring dimensions are specified, are they calculated based on the finished hole size (after plating) or the initial drill size (before plating)?
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Minimum annular ring values shown in CAM are based on the hole size as drilled (before plating), not the finished plated hole size. At Sierra the holes are intentionally drilled larger to allow for plating and surface finish, and the CAM system uses the drilled hole size in its annular ring measurement.
In other words the drill size (pre-plating) is what your CAD/CAM and the manufacturer will use when checking annular ring minimums. The finished hole (after plating) ends up slightly smaller because the copper plating adds thickness inside the hole.
I say you should size pads based on the drilled hole plus desired annular ring clearance to ensure that, after plating and tolerances, there’s still enough copper around the via.
This is why you often see recommendations like “2 mil annular ring for microvias / laser drills” or larger values for mechanical drills. They’re accounting for drill wander and plating tolerances around the pre-plated hole size.
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I thought is was finished hole size…but I prefer to design towards class 3 annular ring and processing tolerances.
Mechanically drilled holes are oversized but the varies depending on a variety of variables.
Laser drilled starts out smaller and increases as the process goes on.
I recommend using more than minimum annular ring except when there is no other choice. Results are more robust when alowing the fabrication more flexability to do their job.
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Annular ring checks are performed against the pre-plated drill size, since that is the reference used during CAM and DFM validation.
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Treat annular ring requirements as a manufacturing control, not just a CAD geometry rule. If you design exactly to minimum values, any drill wander, registration error or plating variation can push you out of spec. Since CAM checks are based on the pre-plated drill size, designers should explicitly specify both the finished hole size (for electrical/assembly needs) and allow the fabricator to choose the appropriate drill oversize and plating process. Add margin to your annular rings, and confirm the fab’s process assumptions (drill oversize, plating thickness) in your stackup notes.
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Inner layer annular rings are typically held to tighter tolerances than outer layers, since you can’t visually inspect or correct them after lamination. This makes the pre-plated drill size reference even more critical for buried and blind vias, where registration errors compound across layers. If your design includes any, confirm your fab’s inner layer registration tolerance separately from their standard annular ring spec.
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Do PCB fabs apply different annular ring dimensions for inner layers and outer layers?
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Yes, many fabs specify different minimum annular ring values for inner and outer layers due to tighter registration constraints on inner layers. It’s best to confirm the layer-specific requirements with your fabricator rather than assuming a single value applies to all layers.
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