Annular Ring Tolerances a Volume Shop Actually Holds
Volume PCB shops hold a real-world minimum annular ring of 0.002 inches for Class 2 designs when artwork grants 0.005 inches of nominal land margin over tool size.
Manufacturing Allowance
Tolerance Allocation
Board design specifications incorporate dimensional offsets to compensate for material shrinkage and registration errors during fabrication. This adjustment, known as manufacturing allowance, ensures that the finished copper traces and holes match the nominal dimensions required for reliable electrical function. Designers increase the pad diameters and trace widths in the Gerber files to offset the loss of material during chemical etching.
This practice prevents the board from falling below the minimum copper thickness and width requirements.
Drill Registration
Misalignment between the drilling machine and the internal copper layers can result in drilled holes that cut through the edge of internal pads. Allocating a manufacturing allowance to the annular rings around vias compensates for this mechanical drift, ensuring that the drill hole remains fully enclosed by copper. This allowance prevents open circuits and ensures a secure electrical connection between different signal layers.
It is especially important for dense multilayer boards where conductor spacing is extremely tight.
Design Optimisation
Correct application of these dimensional compensations allows the fabricator to achieve high yields without sacrificing board density. Setting the manufacturing allowance too high increases the board area, whereas setting it too low leads to high defect rates and scrap. Balances must be struck between the capabilities of the fabricator and the performance needs of the circuit.