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October 2001

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Subject:
From:
"McGlaughlin, Jeffrey A" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 17 Jul 2001 10:22:24 -0400
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The problem may be the assumed dielectric constant of the FR4 prepreg
material.  FR4 has a dielectric constant that changes with respect to
frequency, making it necessary to find the effective Er for impedance
calculations, not the base Er.  These calculations appear in several of
Richard Hartley's presentations on high speed design. These presentations
should be in the proceedings for either PCB Design conference from last year
if you have them (if not let me know and I will try to find my copy). I have
found that based on these calculations the Er of FR4 *can* vary from around
4.2-5.0 when using "garden variety" material.  When doing boards where the
controlled impedance is critical I have found it to be a good idea to
include not only a stack-up but a list of assumed values and the calculator
used. This allows fabricator to make the necessary adjustments to my data to
meet the critical values, based on their processes.

Good Luck,

Jeffrey A. McGlaughlin, C.I.D.
Sr. PCB Designer
Battelle Memorial Institute
Columbus Ohio
[log in to unmask]



-----Original Message-----
From: tech [mailto:[log in to unmask]]
Sent: Tuesday, July 17, 2001 8:33 AM
To: [log in to unmask]
Subject: [TN] Impedance Discrepency


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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