I have a 6 layer design with layers 2 and 5 being planes and 3 and 4
signals. In this particular case , due to overall thickness and outer
layer impedance requirements we have .010 from the planes to the signals
and .035 between the signals. 1 Oz. copper and finished lines at .006.
My impedance software ( Polar CITS25) is giving a number of 65.02 ohms
but the actual results we're getting are all at the 72+ ohm range.We do
many impedance designs and have not seen a discrepency like this. The
.035 dielectric between the signals is 5 pieces of 7628 which is an
unusually high amount of prepreg for us and the .010 from planes to
signals are cores. All 140Tg FR4. Can the high thickness (resin content)
of prepreg be creating the problem ? Sections on the coupons are very
close to design so we don't appear to have a process issue.
 Any input appreciated.
Paul Greene

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