That depends on your board layer stackup. Most high speed boards these days are multilayer boards which means the signal traces are forced to be either on an interior layer as a stripline or an asymmetric stripline or an embedded microstrip line or as a microstrip on the top layer. Depending on the location of the trace you need to design use a simulation tool to best model its trace characteristics.
Hi @cblair ,
Depending on the layer thicknesses, you have either a symmetrical or asymmetrical stripline situation. Theoretically, it does not matter if the top layer is a power plane or a ground plane when designing the trace width for some impedance value.
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