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.
Tool Diameter
Instrument Width
Cutting instrument dimension determines the minimum size of a drilled hole or a routed slot in the substrate. A tool diameter is a critical factor in the setup of automated drilling and routing machines used in board fabrication. This dimension is usually slightly larger than the target finished hole size to account for the thickness of the plating that will be added later.
Selection of the correct bit size ensures that the mechanical and electrical requirements of the design are met.
Material Removal
Speed and feed rate of the machining process are adjusted based on the specific tool diameter being used. Smaller tools are more fragile and require higher rotational speeds but lower feed rates to prevent breakage. The kerf, or the width of the material removed by a router bit, is directly controlled by this diameter.
In dense designs, the smallest available tool diameter limits the minimum size of the vias and the spacing between copper features.
Wear Compensation
Continuous use of a cutting tool causes the edges to dull and the diameter to decrease slightly over time. Manufacturers must monitor this reduction to ensure that the holes do not fall below the minimum tolerance.