Definition

Coated microstrip is surface microstrip with solder mask covering the trace - the realistic condition for most production PCBs. The coating typically lowers impedance by 2-3 ohms depending on mask type, thickness, and coverage pattern. For differential pairs, solder mask can flood into the gap between traces, significantly affecting coupling and impedance. Field solvers like Polar Si8000m model coating thickness both above and beside traces. Always specify coated microstrip (not bare surface microstrip) when calculating impedance for production boards with solder mask.