If you only considered one of several factors, contributing to impedance
control, as dielectric thickness tolerance, you would understand the
variation. We all know it is typical for dielectric thicknesses of core
and laminate material to be in the range of + or -10% tolerance. If you
add to that etched trace width tolerances as (whatever the supplier'
process manages), you can see impedance changes on the order you
describe. Using TDR and X-Sectional analysis together, it is possible to
build better statistical models for, at least, some materials and
suppliers to get closer to your requirements.

Earl Moon ([log in to unmask])

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