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October 2001

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Subject:
From:
Geoff Layhe <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 17 Jul 2001 14:15:10 +0100
Content-Type:
text/plain
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text/plain (79 lines)
Paul,
I don't think 5 plies of 7628 in 0.35" is the reason for the difference. 5 x
7628 calculates to a dielectric constant of 4.84. I just ran a quick manual
calculation using Er 4.84 and got the almost the same results as your Polar
(65.2 ohms.

Assuming track width and dielectric spacings are OK the only way to get an
impedance of 72 is by using an Er of 3.9. You aren't using BT Epoxy are you.

Is the 72 Ohms measured on a test coupon or on a track in the board. Could
there be an something in the circuit affecting the results?

Geoff Layhe
www.Lamar-uk.co.uk

-----Original Message-----
From: tech [mailto:[log in to unmask]]
Sent: 17 July 2001 13:33
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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