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Re: [BP] Dielectric Weave effects



Title:
Coming from the fabricator side, I agree with Joel's description below.  There are really two separate issues that we are folding into the dielectric effects trail.  One is true weave skew, such that a really long trace, when running parallel with surrounding weave.  However, this is typically minimal unless you're broadside coupled.  The other is inhomogeneous effects as described below.  This is the crushed bundles or resin voids etc that result from fabrication.  This is more prominent with a single ply construction.  Much of the telecom/datacom segment has regulatory restrictions on the use of single ply constructions to avoid such a void causing a catastrophic failure.  Other segments do not have this same level of regulatory restrictions, from my understanding. There are people reading this email that know a whole lot more about this than I do though.  Regardless of regulatory requirements, 2-ply constructions significantly limit the electrical/performance impacts of imperfections that result from the fab (crushed glass and voids).  
 
Again, we are back to a discussion that we had during the definition of "improved FR4".  Costs!!!  To move forward we have to make reasonable assumptions about designers' willingness to use improved implementations in their designs.  Nearly all backplanes we do use 2-ply construction.  So I think this probably a larger issue on the line card side where costs/volume really come into play.  But if the weave effects are more noticeable on longer tracks, I don't know that it really effects the line cards either?  
 
So I believe we can make much better progress on avoiding these performance degradations by having the improved constructions such as the resin rich 2-ply's as that is good design practice.
 
 
Just a couple of thoughts.
 
Mike
-----Original Message-----
From: owner-stds-802-3-blade@ieee.org [mailto:owner-stds-802-3-blade@ieee.org]On Behalf Of Joel Goergen
Sent: Sunday, June 12, 2005 8:55 PM
To: STDS-802-3-BLADE@listserv.ieee.org
Subject: Re: [BP] Dielectric Weave effects

What you describe is primarily the result of voids or dry glass in single ply construction.  That's one of the reasons that the carrier world uses two ply resin rich glass.  Also, broad side coupling makes the issue a real one.

What it comes down to is implementation.

And design practice.

Using the basis of the material definition adopted early on as an informative point to start from, you would not see the issue described.

Again, resin voids and broken or crushed bundles are a design issue and can easily be dealt with.  I have never seen the problem in all the traces I have examined (1000's) simply because of the glass construction and the resin I select.  I discussed this early on in the standards meetings and is one of the reasons I pushed so hard for the informative material definition we have for improved fr-4.

If you want to change this, then you are going to have to change a lot of things.

I, for one, am not up for a 50ps skew when there are known methods for avoiding this issue.
-joel

Mellitz, Richard wrote:

I somewhat disagree. Typically, you won’t find a diff pair that suffers from the weave effect. The probability is low. But once in a while, “in a galaxy far far away…” Oh no no no J . Anyhow, when you do, the effect is right at the top of the Pareto list. It can be hundreds of ps on a long route. I have personally seen 50 ps on 8 inch routes on computer -like boards.  For many years, I tossed this off as “anecdotal” but have come to understand the cause was inhomogeneous dielectric effects. Not withstanding the weave effect, I can compensate most conductor geometries with “good” design practice and yes, even twists, turns, and connectors. Most of the time these design practices serendipitously even be choke out the weave effect.

 

The good news is that back plane have wider trace widths which are less sensitive. The line cards are a different story.

 

… Rich

 


From: owner-stds-802-3-blade@ieee.org [mailto:owner-stds-802-3-blade@ieee.org] On Behalf Of Joel Goergen
Sent: Friday, June 10, 2005 6:37 PM
To: STDS-802-3-BLADE@listserv.ieee.org
Subject: Re: [BP] Dielectric Weave effects

 

All,
The differential pair skew per inch is from many factors, the least of which is glass construction.  The greatest, outside of the connector, is trace length mismatch from route direction changes (45s or rounds).  The second greatest is from length matching where a toroidal effect is not considered.

I would offer that there comes a time when we have to stop writing the standard for poor design practices.  A designer should always be targeting less then 5ps point to point.  My feeling is that 20ps is fine and does not violate any US patents in any system I have been involved with.  I wouldn't want to even try and debug a system with 50ps skew ... that would be horrid and unpredictable.

As I said ... sooner or later we have to consider reasonable engineering as a solution.

-joel

Scott McMorrow wrote:

I am not currently aware of any systematic study of differential pair skew on production backplanes, due to the problem of accurate instrumentation of hundreds of channels.  However, there are occasional anecdotal measurements that have been made by some connector vendors on customer backplanes which have shown 40 to 50 ps skew from time to time on high BER channels.  Teradyne and Teraspeed Consulting will begin conducting a systematic study of backplane length differential pair skew this summer, on a variety of standard backplane stackup and trace configurations.  That data should be available by fall, and allow us to set some fundamental skew limits on weave-parallel routed tracks and off-angle tracks.  I believe that Intel Labs is also involved in a similar study.   However, I believe thate Rich is correct. A theoretical upper skew bound of 6 ps/in does currently exist for many stackup, material and trace geometry combinations.  In addition, a lower skew asymptotic bound for optimal routing strategies w.r.t. the weave has been shown to exist, but has not been adequately quantified by anyone that I am aware of.  However, this may eventually prove to be elusive due to low level defects in the fiberglass structure and non-uniform epoxy Er.

It is my belief that when laminate weave skew is placed into the mix and common mode conversion is properly accounted for, non-NRZ encoding schemes may have enough UI headroom to ultimately outperform NRZ signaling, in the worst cases.  However, this is only a hunch and not yet substantiated.  However, I think it behooves the committee to consider laminate weave skew in the specification.  It is my considered position that a total differential skew specification of 20 ps in an end-to-end system is not achievable on epoxy/glass composites without extra ordinary efforts and license of (or violation of) at least one major patent in this area.

There is yet another possibility that may hold promise.  Electronic skew pre-compensation may very well allow channels to work in the presence of moderate amounts of differential skew.

Regards,

Scott

Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax

http://www.teraspeed.com

Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC



b_panos wrote:


Yea, S McMorrow et al.
The thing is, it really depends on things like material, bundle size, glass
dia. to trace width and separation, as well as the orientation and the
length of trace. To be quite honest, if skew is bad enough, it wouldn't meet
ISI levels and the eye wouldn't be recoverable. The RT of a 10G signal is
going to be on the order of 50ps. so any skew approaching this level, your
RL is going to look terrible, not to mention your S21 results. Just my
2cents

Regards
Bill


-----Original Message-----
From: owner-stds-802-3-blade@ieee.org
[mailto:owner-stds-802-3-blade@ieee.org]On Behalf Of Mellitz, Richard
Sent: Friday, June 10, 2005 1:43 PM
To: STDS-802-3-BLADE@listserv.ieee.org
Subject: [BP] Dielectric Weave effects


Hi All,
I started reviewing the draft doc and came across a differential skew
spec of 20 ps. Has any one actually measured skew on the published
backplanes?

It's possible to get up 6ps/inch skew in a pair due to bundle weave. Do
we need to address this issue?

These effects have been published before.

Rich Mellitz
Intel Corporation