This relationship is supported by extensive data acquired for FR4 laminates and has been
shown to be accurate within a few percent of experimental measurement over the range 1
kHz to 1 GHz. Since this equation was derived empirically, caution should be taken when
extrapolating to frequencies above 1 GHz [
]. It has been determined through
experimental measurement that the glass cloth reinforcement experiences no dielectric
constant variation in this frequency range; subsequently, the equation is only a function of
the frequency and the resin content.
The PCB manufacturer should be able to provide the volume content of the resin. However,
if this information is not attainable, it can be estimated as
is the total thickness of the glass cloth and H is the total thickness of the dielectric
layer. When PCBs are manufactured, the dielectric layers are built up to the desired
thickness by stacking several layers of glass cloth and gluing everything together with the
epoxy resin. The manufacturer can provide the type and thickness of glass cloth used and
the number of layers used, which will yield H
. The total layer thickness can either be
measured via cross-sectioning techniques or obtained from the manufacturer.
However, if this information is not attainable, it can be estimated as (4.20) where H gls is the total thickness of the glass cloth and H is the total thickness of the dielectric layer.