Impedance Is a Stackup Decision
A width rule such as 0.15 mm has no universal impedance meaning. The dielectric spacing, material properties, copper geometry and reference planes define the transmission-line cross-section. Begin with a manufacturable stackup instead of a width copied from an unrelated board.
Move from estimate to controlled geometry
Use the single-ended tool to explore width and plane distance. Then inspect differential coupling with the edge-to-edge gap definition. These simple expressions help reveal sensitivity but omit mask, etch shape and frequency-dependent details. Send the target impedance and tolerance to the supplier and agree on the final dimensions and coupon report. On the layout, preserve the reference path through connectors and layer transitions. A nominally correct width cannot compensate for a broken return plane or a long via stub.
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