Relative permittivity of FR-4

FR-4 is woven glass cloth in epoxy resin, the substrate of almost every printed circuit board made since the 1970s, and "4.4" is the number designers plug into microstrip and stripline impedance formulas to hit 50 Ω. It deserves a wide error bar. The value depends on the glass-to-resin ratio, on which direction the trace runs relative to the weave, on temperature, and on frequency — it falls from roughly 4.7 at 1 MHz toward 4.2 at 1 GHz, and different suppliers' "FR-4" can differ by 10%.

The consequence for signal integrity is a propagation delay of about 6 ns per metre in stripline, and a loss tangent near 0.02 that makes FR-4 marginal above a few gigahertz — which is why fast serial links move to lower-loss laminates like Rogers or Megtron. Fibre-weave effect, where one differential trace sits over glass bundles and its partner over resin, causes skew that has broken more than one 10 Gb/s design.