Calculation of the Differential Impedance of Tracks on FR4 substrates

There is a discrepancy between calculated and measured values of impedance for differential transmission lines
on FR4. This is especially noticeable in the case of surface microstrip configurations. The anomaly is shown to
be due to the nature of the substrate material. This needs to be considered as a layered structure of epoxy resin
and glass fibre. Calculations, using Boundary Element field methods, show that the distribution of the electric
field within this layered structure determines the apparent dielectric constant and therefore affects the
impedance. Thus FR4 cannot be considered to be uniform dielectric when calculating differential impedance.