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.
Etching Undercut
Chemical Erosion
Lateral erosion occurs beneath the edges of the protective resist during the removal of unwanted metal. The resulting etching undercut describes the distance the chemical etchant penetrates horizontally into the copper trace, creating a trapezoidal cross section rather than a rectangular one. This degree of metal removal is measured as the difference between the width of the resist and the final width of the copper at the top of the feature.
It defines the practical limit for fine line width and spacing on a printed circuit board.
Etch Efficiency
Ratio of the downward etch depth to the lateral undercut distance determines the efficiency of the chemical process. Spray pressure and chemical concentration influence how aggressively the solution moves sideways relative to its vertical progress. High spray pressure can increase the rate of etching undercut if the nozzles are not calibrated for the specific copper thickness.
Additives in the etching solution are used to provide a protective film on the sidewalls of the copper to slow down this lateral movement.
Structural Integrity
Excessive removal of metal from the sides of a conductor reduces the bond area between the trace and the substrate. This loss of surface area increases the risk of the copper lifting during the high temperatures of the assembly process.