I agree with Gert's comments about the glass weave misalignment wrt edge of
board.
My paper Gert referred to can be found here:
http://lamsimenterprises.com/Practical_Fiber_Weave_Modeling_Iss3_Mar2_12.pdf
At the time of writing, it was meant to corroborate the work Jeff Loyer did
while he was at Intel and try and find a way to practically model the
effect. Since then there has been other work done including the recent
DesignCon paper cited below.
Although this method of modeling would give worst case it always bothered me
why we didn't see more FWE issues in real life. My theory, at least for
stripline, was that because there are mostly 2 or more dielectric sheets
between the signal layers and each reference plane, it would be a very rare
occasion that you would find one trace coincident with fiberglass bundles of
each dielectric layer for the entire length while the other trace saw mostly
resin. This is because the bundles for each dielectric sheet will line up
randomly between traces and reference planes. Of course there would be some
skew, just not worst case. There is some evidence to support this in the
Dcon paper Table 2. For single sheet of 1067 non-spread glass, for example,
had 7.6ps/in, but 2 sheets of the same dielectric showed only 1.2ps/in.
On microstrip, with a single sheet of dielectric, you do have the
possibility of exact alignment. The later work done in Eric et al's DCon
papers is quantifying how much skew you can expect with a controlled
experiment. In fact there are some examples that agree with Intel's original
work that showed worst case on the order of +/-8ps/in from nominal
propagation delay.
Bert Simonovich
Signal/Power Integrity Practitioner | Backplane Specialist | 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: 17-Mar-17 9:07 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Glass Weave Skew
Hi Boris,
Most Laminate Vendors are aware oft he skew problem and try to solve it in
different ways. Some solutions were just found "by accident" (spread
Glass/flat weave was initially designed to improve Laser drilling!), some
were done on purpose (matching the dk of Glass and Epoxy).
Routing at an angle to eliminate the risk of skew has been around since many
years. I recall a Paper from Siemens more than 10 years ago, and they found
15° angle to be perfect, but this comes at a cost because you loose Panel
space, and at some angles you create periodic loading.
Erics paper was about how to quantify the amount of skew a certain trace
geometry will see in the worst case (some years ago Bert Simonowich had a
nice paper about another attempt to quantify that, it must be still on his
webpage). Once you know the worst case skew, you can calculate the maximum
length of a straight routed diff-pair and you can check your routing for
potential issues.
The "artefact" that Eric talks about in the summary is a well known effect
in textile industry and should be known by every person who ever owned a
cheap T-Shirt: the fabric isn't perfect, and isn't equal in warp and fill
direction. And based on the process and common Loom machines that are out
there, I doubt that there is any way to make the fabric inhomogeneous in
both directions. I believe that one could invent a loom that does that, but
given the low quantity needed, I doubt that anyone ever spend some Millions
on this new technology.
Just my two cents.
BR
Gert
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-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Boris Bakshan
Sent: Friday, March 17, 2017 12:23 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Glass Weave Skew
Hi experts,
Following Eric's paper on designcon and the conclusions regarding the
inherent misalignment of the glass bundles with respect to the edge of the
board, could it be that some of the laminate manufacturers took initiative
to help mitigating the skew problem caused by the way fabrics are built for
so many years?
(
https://www.signalintegrityjournal.com/ext/resources/article-images-2017/410
3/4103_SIJ_GlassWeave.pdf
)
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