EMC Question of the Week: March 4, 2019
A printed circuit board trace on an external layer with a width approximately equal to its height above a current-return plane has a characteristic impedance of approximately,
- 10 Ω
- 50 Ω
- 75 Ω
- 100 Ω
Answer
The correct answer is "c." The exact characteristic impedance is a function of the exact trace width and height, as well as the dielectric permittivity and solder mask. But generally, for boards using an FR-4 dielectric, the impedance is about 75 Ω.
50-Ω controlled impedance traces are wider than their height above the plane. Narrow traces can have impedances on the order of 100 Ω. Traces on inner layers have lower characteristic impedances, because they are surrounded by the dielectric.
Precise calculations of the characteristic impedance can be done using 2D numerical modeling codes. Approximate values can be obtained using calculators such as the one here.
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