Dear Paul,

I agree with some of the other respondents
that you may be seeing effects because of
high resin content, - in FR4 the resin is around Er
of 3 and the Glass is Er of 6, so glass resin ratio will
certainly alter the finished impedance.

This effect is often more apparent with differential traces
where the field often crosses an area which is almost filled
with pure resin.

Have a look at this application note to see what is happening:


www.polarinstruments.com/support/cits/AP139.html


Hope this sheds some more light on your question

Kind regards
Martyn Gaudion
Polar Instruments
[log in to unmask]
www.polarinstruments.com

At 13:33 17/07/2001, you wrote:
>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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