Relative Permittivity

UltraScale Architecture PCB Design User Guide (UG583)

Document ID
UG583
Release Date
2023-11-14
Revision
1.27 English

Relative permittivity is a measure of the effect of the dielectric on the capacitance of a conductor. The higher the relative permittivity, the slower a signal travels on a trace and the lower the impedance of a given trace geometry. A lower er is almost always preferred.

Although the relative permittivity varies with frequency in all materials, FR4 exhibits wide variations in er with frequency. Because er affects impedance directly, FR4 traces can have a spread of impedance values with increasing frequency. While this spread can be insignificant at 1.125 Gb/s, it can be a concern at 10 Gb/s operation.