EMC Question of the Week: December 11, 2017
A printed circuit board trace on an outer layer whose width is approximately equal to its distance from a solid plane on the layer below has a characteristic impedance of about,
- 50 Ω
- 70 Ω
- 100 Ω
- 150 Ω
Answer
The correct answer is "b". A microstrip trace on an FR-4 dielectric (εr ≈ 4) whose width equals its height above the plane will have a characteristic impedance of about 65 to 70 ohms. 50-Ω traces will have a width that is about twice their height above the plane. Controlled-impedance traces with impedances higher than 100 Ω are not common, because they require thin traces and/or large distances from the ground plane.
Traces on inner layers will have lower characteristic impedances than traces of the same width and distance from the plane(s) on outer layers. It can be helpful to experiment with a Trace Impedance Calculator, to see how changes in various circuit board parameters affect the characteristic impedance and other properties of circuit board transmission lines.
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