Re: [SI-LIST] : Different board vendors different impedances

Chris Heard ([email protected])
Mon, 29 Mar 1999 10:06:56 -0800

Specifying Er of FR4 is not an easy task. It depends on resin content,
which changes with dielectric thickness and it also changes with
Should we specify the fundamental bandwidth of the TDR risetime that is
used to measure Zo? (Example, Tr of TDR = 35ps, which slows to around
140ps the the test trace, so 0.35/140ps = 2.5GHz). I think not.

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.

Chris Heard
3Com Corp
Marlboro Ma

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