Microstrip

What Is a Microstrip?

A microstrip is a type of transmission line formed by a copper trace on an outer layer of a PCB with a reference (ground) plane on the adjacent inner layer. The trace, the dielectric material between the trace and the reference plane, and the reference plane itself form a complete transmission line structure with a characteristic impedance determined by the trace width, dielectric thickness (height above the reference plane), copper thickness, and the dielectric constant of the PCB material.

Microstrip traces are the most accessible transmission line configuration in PCB design because they are visible and easy to probe for debugging, and they do not require the trace to be sandwiched between two planes. However, microstrip has higher radiation loss than stripline because the electromagnetic field is partially exposed to air above the trace. This makes microstrip traces more susceptible to crosstalk from adjacent traces and more likely to contribute to electromagnetic emissions. For extremely sensitive signals or designs with strict EMC requirements, stripline routing on inner layers may be preferred.

Impedance-Accurate Microstrip Routing

Achieving accurate microstrip impedance requires precise control of trace width relative to the dielectric height and material properties — parameters that change with the stackup and can vary across different regions of the board if the dielectric thickness is not uniform. Physics-driven AI layout tools calculate the required trace width for target impedance on each layer in real time, adjusting routing geometry as traces move between different stackup regions or transition between layers. This dynamic impedance management ensures controlled-impedance performance without requiring the manual width calculations and cross-referencing that slow down traditional high-speed layout.

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