[SI-LIST] Re: AW: Fiber weave effect modeling

Gert,

Thanks for the feedback.

I would prefer to use the terms "high resin content" and "low resin content"
instead since neither trace sees pure epoxy or pure glass.

Yes, in my opinion the "high resin content"(minus tolerance)would be the
lowest Dk value to use.  It represents the bulk Dk of 106 style glass,
loosest weave and has the highest resin content of common laminates used.
Since 7628 has the lowest resin content and tightest weave, it would be the
highest Dk value(plus tolerance). 

The problem with only using 7628 style is it's thickness of 8 mils. Most
high density layouts use cores and prepregs of 4-5 mils for 50 ohm in the
stack-up which means they are made up of a combination of 106 (2 mils) and
1080 (3 mils). 

There has been some discussion lately about using 3313 style cloth which is
comparable to 1080 in pitch except that the threads have been flattened to
achieve a tighter weave much like 7628. I have not found a data sheet to
know the thickness but I presume it will also be comparable to 1080 in
thickness.

My purpose for the study was to develop a practical methodology to establish
a worst case delta Dk to model and simulate timing skew due to fiber weave
effect. Based on the statistical data from Intel, this methodology
correlated quite well.

Best regards,

Lambert (Bert) Simonovich
Consultant and Founder
LAMSIM Enterprises Inc.
Web Site: http://lamsimenterprises.com
Blog: http://blog.lamsimenterprises.com/      

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Havermann, Gert
Sent: January-12-11 9:32 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] AW: Fiber weave effect modeling

this is a great paper, many thanks for sharing it with the si-list.

please allow me a question. I understand that the 106 and 7628 Prepregs are
used to predict the dk values of "pure epoxy" and "close proximity to the
Glass bundle". 
Do you think that the "pure epoxy" value is always the worst case that I
have to expect for my diff pair? If I (for instance) would only use 7628
Style everywhere, will there even be areas "in pure epoxy", or is the weave
dense enough that the worst dk is somewhere between the calculated min. and
max. value?

BR
Gert


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Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] Im
Auftrag von Bert Simonovich
Gesendet: Freitag, 7. Januar 2011 21:55
An: si-list@xxxxxxxxxxxxx
Betreff: [SI-LIST] Fiber weave effect modeling

Hi all,

Recently there were discussions on PCB fiber weave effect. I recently did a
study and published a White Paper titled, "Practical Fiber Weave Effect
Modeling".

Abstract: 
Fiber weave effect is becoming more of an issue as bit rates continue to
sore upwards to 5GB/s and beyond. Due to the non-homogenous nature of
printed circuit board laminates, the fiberglass weave pattern causes signals
to propagate at different speeds within differential pair traces; causing
timing skew and mode conversion at the receiver; leading to reduced
bit-error-rate (BER) performance; and increased EMI radiation. The relative
dielectric constant (Dk) surrounding a trace ultimately determines its
propagation delay. This paper delves into the issue and presents a novel
approach to practically establish worst case min/max values for Dk and use
them to model this effect using ADS circuit modeling software. A PCIe CEM
Rev2 case study is used to practically demonstrate the model and to explore
the design space.

Here is the link: http://lamsimenterprises.com/White_Papers.html

Thanks to Jeff Loyer, Istvan Novak and Gustavo Blando for there help in
clarifying some results of their prior published work on the subject. 

I hope you find it useful.

-Bert

Lambert (Bert) Simonovich
Consultant and Founder
LAMSIM Enterprises Inc.
Web Site: http://lamsimenterprises.com
Blog: http://blog.lamsimenterprises.com/




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