Another problem is that FR4 is not uniform. Digging into how many Er's
exist in a stackup will make your head spin. C-stage has one resin
and therefore an Er. B-stage (partially cured material) has a different
resin content and therefore another Er. Both B-stage and C-stage are
comprised of glass bundles (another Er) that criss-cross through the
material with resin (another Er) in between.
Take all these lovely variables into play and you may conclude that
specifying a single value of Er is quite hopeless.
Further, the traces we are so careful about are not rectangles, they are
trapezoids with "teeth" on the c-stage side of the trace that can
for as much as 30% of the thickness of the line. The b-stage side of
trace is "roughed up" during the etching process to help adhere to the
Solution?: Let the vendor be responsible for Zo with a tolerance.
a line width and overall board thickness and let the vendor do the rest.
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