[SI-LIST] Re: AW: Glass-Weave Skew / Fiber-Weave Effect
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- Date: Fri, 5 Dec 2014 16:59:44 -0500
Jeff
on the shining the light thing, that's called an optical densitometer. you
can make one with a calibrated flatbed scanner. Then it's a matter of
creating a table of optical density vs Er. It would make for a nice
academic study.
As for the dozens of materials I've characterized recently, I can tell you
that I have a method that provides a very accurate relative figure of merit
between weave/resin systems in one trace measurement per laminate stackup,
not accounting for non-orthogonality of board and weave. A few more traces
and I can adjust for weave/board misalignment.
As for something that gives statistically accurate skew spread
measurements ... nope. Care to fund the study? I know how to do it. Have
known for 10 years I know the quickest methodology to get the statistical
results. It's just a matter of sampling quite a few boards.
Scott McMorrow
Teraspeed® Consulting - A Division of Samtec
16 Stormy Brook Rd
Falmouth, ME 04105
(401) 284-1827 Business
http://www.teraspeed.com
On Fri, Dec 5, 2014 at 4:39 PM, Loyer, Jeff <jeff.loyer@xxxxxxxxx> wrote:
> Hi Scott,
> Can you share any of the studies you've performed to characterize the
> dozens of materials? I'm especially interested in clever techniques you
> might have used to ensure you measured the maximum for each material, given
> the statistical nature of the problem (whenever I think of performing such
> a study, I am put off by "how do I ensure my data includes some
> representative 'worst case'?").
>
> To all,
> I believe there is extreme ambiguity in the terms used for spread glass, I
> would be wary of any particular terminology (is "ultra" better than
> "hyper"?). And, even if you do standardize the definition of "spread", the
> net effect is going to vary significantly for the various glass styles
> (1080 will probably always be more problematic than 2116, for instance). I
> don't know of any method of easily quantifying the electrical properties of
> any particular spread glass. There is work on-going for a visible light
> test, but as far as I know those results have not yet been correlated to Vp
> differences.
> BTW, this might be a nice opportunity for a clever person to come up with
> an easy (cheap, HVM-compatible) method of shining a light through glass
> weaves and quantifying the difference between the brightest and darkest
> areas - ideal "spread glass" would have very little difference,
> corresponding to very little difference in Vp across the weave.
> And finally, I don't know of any silicon solutions, ours or otherwise,
> used explicitly to solve the fiberweave issue. Adaptive equalization may
> make it less of an issue than it was previously, however.
>
> Jeff Loyer
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Scott McMorrow
> Sent: Friday, December 05, 2014 5:42 AM
> To: Havermann, Gert
> Cc: billh@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: AW: Glass-Weave Skew / Fiber-Weave Effect
>
> Gert,
> Even flat weaves have skew, as easily evidenced by a picture of the weave
> that shows gaps in the overlap of fiber bundles in one direction.
>
> I've found that the killer problem is skew on line cards causing diff to
> common mode conversion that bursts as crosstalk within connector PTH via
> fields and connectors themselves, which are not designed to control common
> modes. This was seen specifically in a significant loss of NEXT margin at
> receivers from Tx or Rx card skew causing excessive crosstalk in the
> connector pin fields for a well-known standard. This required two
> solutions, one was to guardband margin to allow for the additional skew
> caused NEXT. The other was to use some skew abatement methods.
>
> Doing 1000's of sensitivity runs I've found that this skew sensitivity is
> much worse on cards than it is on backplanes. That is, the card skew
> allowance has much less tolerance than does the backplane or total
> end-to-end skew. Where the skew is located is much more important than how
> much skew there is.
>
> To put some numbers to this phenomena, a 10G link had 12 ps of skew, which
> translates to around -0.7 dB of additional insertion loss. That is
> generally not a serious issue for 99.9% of all designs. However when the
> effective loss due to crosstalk was factored in, due to diff to common mode
> and common mode to diff conversion, total additional eye loss was -3.9 dB.
> which is a huge impairment.
>
> In the end, the only solution for skew, especially at 28G rates and above,
> is to utilize special techniques to mitigate skew on backplanes, and
> skew-free materials on cards. I've measured and characterized dozens of
> materials and can say that I can easily demonstrate the potential for any
> woven PCB laminate to have significant skew, even those with spread weaves,
> except for those laminates that are specifically engineered for zero-skew
> with matched glass and resin dielectric constants.
>
> Regards,
>
> Scott
>
> Scott McMorrow
> Teraspeed® Consulting - A Division of Samtec
> 16 Stormy Brook Rd
> Falmouth, ME 04105
> (401) 284-1827 Business
> http://www.teraspeed.com
>
> On Fri, Dec 5, 2014 at 3:29 AM, Havermann, Gert <
> Gert.Havermann@xxxxxxxxxxx>
> wrote:
>
> > Hi Bill,
> > I've seen it quite often, but I never seen it killing System
> > performance as we work with good margin, and the Silicon accepts even
> > more skew that expected.
> > One Design was not usable due to weave effect skew. It was a TRL
> > Cal.Stucture where the shortest line hat skew exceeding the Phase
> > difference. After that I redesigned with different Material and flat
> > weave Glass, and that worked great.
> >
> > BR
> > Gert
> >
> >
> > ----------------------------------------
> > Absender ist HARTING Electronics GmbH, MarienwerderstraÃe 3, D-32339
> > Espelkamp; Registergericht: Amtsgericht Bad Oeynhausen; Register-Nr.:
> > HRB 8808; Vertretungsberechtigte Geschäftsführer: Dipl.-Kfm.
> > Edgar-Peter Düning, Dipl.-Ing. Torsten Ratzmann, Dipl.-Wirtschaftsing.
> > Ralf Martin Klein
> >
> > -----Ursprüngliche Nachricht-----
> > Von: si-list-bounce@xxxxxxxxxxxxx
> > [mailto:si-list-bounce@xxxxxxxxxxxxx]
> > Im Auftrag von Bill Hargin (Nan Ya, USA)
> > Gesendet: Freitag, 5. Dezember 2014 06:04
> > An: si-list@xxxxxxxxxxxxx
> > Betreff: [SI-LIST] Glass-Weave Skew / Fiber-Weave Effect
> >
> > Hi Folks â¦
> >
> > Iâm doing a bit of research on glass-weave skew / the fiber-weave effect.
> > Iâve read the articles/presentations, and understand the nature of the
> > beast, but Iâm interested in getting data from the design trenches.
> >
> > Are you (or do you believe youâve) seen it in your designs? What
> > happened, and how did you resolve it? (E.g., angled routing, low-Dk
> > glass, homogeneous resin/glass, etc.) What were the characteristics
> > of the material and signals?
> >
> > I have no idea what Iâm going to hear in response, but if you respond
> > offline, Iâll hold the info you provide in confidence. If you reply
> > publicly â I promise not to share your secrets beyond the SI-List â¦
> >
> > Bill Hargin
> > Director of North American Sales / Marketing Nan Ya Copper-Clad Laminates
> > billh@xxxxxxxxxxxxx ⪠425-301-4425 ⪠Skype: bill.hargin
> >
> >
> >
> >
> >
> >
> >
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