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Randy's Comments:
IPC specs do not dictate the pad size. It's all about the finished minimum annular ring.
The IPC criteria is 2 mil finished annular ring on external layers and 1 mil minimum on the internal layers. Class 3 pad sizes > drill + 12mil are imposed by fabricators who have > 4 mil layer to layer tolerances.
It comes down to fabricators capability and process tolerances. Smaller pads can be used if the fabricator has tight layer to layer tolerances.
Just be aware that tighter (smaller) tolerances will impact yields and the costs will be passed to the customer.
The other noteworthy Class 3 requirement for vias is the via barrel wall plating must have an average minimum thickness of 1 mil. Stringent class 3 fab notes will specify an absolute minimum thickness for the barrel plating.
That's it !
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