Microstrip vs. Stripline vs. Coplanar Waveguide: Choosing an RF Trace Structure

Microstrip runs on an outer layer with a single reference plane below it, open to air (or solder mask) above. It is the easiest structure to route, probe, and debug because the trace is physically accessible, and it is the default choice for short RF runs, connector launches, and any section where test-point access matters.

The tradeoff is shielding: with only one reference plane, microstrip radiates slightly and is more susceptible to coupling from nearby traces or components than a fully enclosed structure. For short, well-isolated runs this is rarely a problem; for dense boards with RF and digital sections close together, it is worth a second look.

Stripline runs on an internal layer, sandwiched between two reference planes. The field is fully enclosed, which reduces radiation and makes stripline traces less susceptible to external coupling and less likely to couple into adjacent circuitry—useful for sensitive RF sections routed near noisy digital layers.

The cost is accessibility: stripline cannot be probed without via transitions to an accessible layer, which themselves introduce small discontinuities that need to be accounted for in layout. Stripline traces are also generally narrower than microstrip for the same impedance target on the same dielectric, because the field is more tightly confined between two nearby planes.

Coplanar waveguide adds ground pour on either side of the signal trace, in the same layer, in addition to the plane below (grounded CPW) or between two planes (stripline CPW). This adds a third geometry variable—the gap between the trace and the adjacent ground pour—alongside trace width, which gives an extra degree of freedom to hit a target impedance in tight spaces, such as near a connector footprint where trace width alone cannot be widened enough.

The requirement that gets skipped: the adjacent ground pour needs via stitching back to the main reference plane at intervals short compared to the wavelength of interest. An unstitched coplanar ground pour can act as an unintended resonant structure, coupling energy instead of containing it—turning a shielding feature into a source of unexpected behavior. If your layout tool added coplanar ground pour automatically, verify stitching vias were added with it, not assumed.

Default to microstrip for short, well-isolated RF runs where probe access and simplicity matter, and the design is not dense enough for coupling to be a real risk. Move to stripline when a sensitive RF trace must cross a noisy area of the board, or when enclosure and isolation outweigh the cost of via transitions for test access. Use coplanar structures where routing density forces a narrower channel than microstrip alone allows, most commonly right at connector launches—and treat via stitching as a required part of that structure, not an optional extra.