[SI-LIST] Re: AW: Multi-Giga-Hertz Rigid-Flex Feasibility

Jeff

 

Maybe from a skew standpoint the flex could be better and yes there are skew 
issues with ribbonized coax's that have to be considered. 

 

From what I am hearing about at conferences there are miniature coax's, single 
and ribbonized out there now that run up to 10-12 gb/s with <10% eye closure. 

 

One way around the flex stiffness problem is simply to fabricate with 1/4 or 
1/8 oz copper. Disc drive mfg's have been doing this for years. 

 

According to my cable loss calculator one can be off +/- 5% with  impedance and 
still have a good >20db return loss.  

 

Richard

 

 

 


 
> From: jeff.loyer@xxxxxxxxx
> To: ihirshtal@xxxxxxxxxx; si-list@xxxxxxxxxxxxx
> Date: Wed, 2 Dec 2009 12:48:25 -0800
> Subject: [SI-LIST] Re: AW: Multi-Giga-Hertz Rigid-Flex Feasibility
> 
> In my experience, both flex and coax have their pros/cons
> Perhaps some flex can change its dimensions and hence impedance when bent 
> tightly, but I haven't experienced this (maybe because I keep the designs 
> very thin?). And most applications can (and should) limit this or form the 
> bend permanently.
> 
> RigidFlex (or flex) does take a lot of care to keep from introducing 
> inadvertent problems. Some that I've fought through include:
> * too rigid - couldn't bend the thing without a hammer and vise. Stripline 
> that isn't hatched will do this, as will "book binding", in my experience (it 
> was supposed to make it more flexible, but failed miserably in my experience 
> - simple physics, as it turned out. You'd have to have the 2 parts of the 
> book-binding VERY close to each other, otherwise you've introduced a very 
> thick beam)
> * be sure to not pass over severe bends at a right angle. Haven't experienced 
> failures when not doing this (always did it), but heard from experts that 
> it's something to avoid.
> * impedance control over a hatched ground plane. Good luck modeling that one. 
> It's tough, and the design will probably have to be tweaked based on 
> empirical data. And, you'll have to be conscious of increased cross-talk, if 
> routing density is high.
> * low loss dielectric covered with high-loss coverlay, ending up with more 
> loss than expected.
> * you have to adjust trace width between the flex and rigid portions to keep 
> Z0 constant. Check this carefully, beforehand and afterwards. I've had 
> vendors apply the same aperture to my entire trace, undoing my careful 
> modeling.
> 
> In general, I'd say expect a spin or two to get it right. Plan on building 
> some test boards in your schedule.
> 
> But coax isn't without challenges, either.
> * You must be sure your manufacturer really knows how to control skew between 
> signals, especially diff. pairs. You might want to plan on a verification 
> build to ensure this is done, or paying handsomely.
> * At some point, you've got to attach that beautiful clean coax to your 
> board, and that attachment point deserves careful design, both for Signal 
> Integrity and strain relief reasons.
> 
> Again, I'd plan on a spin to verify your design if this is your first foray.
> 
> Regarding the original question (if I read it correctly), I think most 
> vendors will give you +/-10% for your design w/o too much cost adder. But 
> you're right, you'll have to treat the transition from flex to rigid-flex the 
> same way as a layer transition in your simulations - assume no correlation 
> between impedance of the two. But, at least there isn't a via. So, if the 
> design guide allows 2 or more layer transitions, you're golden. Else, you'll 
> be in the gray area.
> 
> My 2 cents - good luck,
> Jeff Loyer
> 
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
> Behalf Of Richard Jungert
> Sent: Wednesday, December 02, 2009 12:01 PM
> To: tom dogastino; hirsh itzack; Scott McMorrow; lee ritchey; 
> gert.havermann@xxxxxxxxxxx; si list freelist
> Subject: [SI-LIST] Re: AW: Multi-Giga-Hertz Rigid-Flex Feasibility
> 
> Guys.
> 
> 
> 
> If you bend the flex cable the Zo changes too. BTW. Bend it some more while 
> observing the eye pattern on the scope and watch the eye pattern open and 
> close at higher speeds.
> 
> 
> 
> One can fabricate a flex with 1/4 oz copper. We did a 100 ohm differential, 
> 12 inches long a couple of years ago with 1oz. copper and we could not bend 
> it without great force. Not good and the performance was not acceptable 
> either. Flex's are generally made for low speed apps like signals running 
> inside disc drives between motors.
> 
> 
> 
> I would recommend to anyone not to do a flex but to use minature coax's or 
> ribbonized coax's for high speed apps and signaling between rigid boards.
> 
> 
> 
> Richard Jungert
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> > From: tom@xxxxxxxxxxxxx
> > To: ihirshtal@xxxxxxxxxx; scott@xxxxxxxxxxxxx; leeritchey@xxxxxxxxxxxxx
> > CC: Gert.Havermann@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] Re: AW: Multi-Giga-Hertz Rigid-Flex Feasibility
> > Date: Wed, 2 Dec 2009 10:23:29 -0800
> >
> > Itzhak
> >
> > Think about what determines Zo. It is the relationship between the signal
> > path and its return path(s). If the signal path and return path do not
> > change from rigid to flex to rigid then Zo does no change. If you add extra
> > metal or insulation at the boundaries that effect the fields then you will
> > get changes in Zo. It all depends on how you design your system.
> >
> > Tom Dagostino
> > Teraspeed(R) Labs
> > 13610 SW Harness Lane
> > Beaverton, OR 97008
> > 503-430-1065
> > 503-430-1285 FAX
> > tom@xxxxxxxxxxxxx
> > www.teraspeed.com
> >
> > Teraspeed Consulting Group LLC
> > 121 North River Drive
> > Narragansett, RI 02882
> > 401-284-1827
> >
> >
> > -----Original Message-----
> > From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> > Behalf Of Hirshtal Itzhak
> > Sent: Wednesday, December 02, 2009 1:37 AM
> > To: Scott McMorrow; Lee Ritchey
> > Cc: Gert Havermann; si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] Re: AW: Multi-Giga-Hertz Rigid-Flex Feasibility
> >
> > Hi Scott
> >
> >
> > As I've already told a few times in my emails, my main concern is impedance
> > continuity across the rigid-flex-rigid combination.
> >
> >
> >
> > If the tolerance of the 3 sections is independent of each other, and if the
> > flex tolerance is +/-20%, assuming a standard +/-10% tolerance on the rigid
> > boards, this could create a substantial impedance discontinuity, e.g:
> > 120-Ohm on the Flex and 90-Ohm on the Rigids.
> >
> >
> >
> > I just wanted to know if and how it's practical, and over what flex-length,
> > to transfer a few GHz (up to 3.125GHz) signals from one rigid board to the
> > other
> >
> >
> >
> > Thanks
> >
> >
> >
> > Itzhak
> >
> >
> >
> > ________________________________
> >
> > From: Scott McMorrow [mailto:scott@xxxxxxxxxxxxx]
> > Sent: Tuesday, December 01, 2009 4:05 PM
> > To: Lee Ritchey
> > Cc: Hirshtal Itzhak; Gert Havermann; si-list@xxxxxxxxxxxxx
> > Subject: Re: [SI-LIST] Re: AW: Multi-Giga-Hertz Rigid-Flex Feasibility
> >
> >
> >
> > I agree with Lee, microstrip works just fine for flex circuits with a cover
> > layer. High performance stripline is also achievable in a flexible assembly
> > if you crosshatch the planes. To do this, you need to do a bit of
> > engineering homework in order to achieve controlled impedance and low loss.
> >
> > scott
> >
> >
> > Lee Ritchey wrote:
> >
> > Itzhtal,
> >
> > I use both polyamide based flex material and Liquid Crystal Polymer from
> > Rogers. Both work well. LCP has lower loss.
> >
> > The reason that stripline is usually not used in flex circuits is that
> > makes a 3 layer assembly which is not very flexible. There is nothing
> > wrong with microstrip. Works fine.
> >
> > Lee Ritchey
> >
> >
> >
> >
> >
> >
> > [Original Message]
> > From: Hirshtal Itzhak <ihirshtal@xxxxxxxxxx>
> > <mailto:ihirshtal@xxxxxxxxxx>
> > To: Havermann, Gert <Gert.Havermann@xxxxxxxxxxx>
> > <mailto:Gert.Havermann@xxxxxxxxxxx> ; <si-list@xxxxxxxxxxxxx>
> > <mailto:si-list@xxxxxxxxxxxxx>
> > Date: 11/29/2009 4:00:51 AM
> > Subject: [SI-LIST] Re: AW: Multi-Giga-Hertz Rigid-Flex Feasibility
> >
> > Hello Gert
> >
> > First - thank you for your contribution.
> >
> > I have a few questions regarding your email:
> >
> > 1) You refer to a trace width of 0.8mm as "small". This is a 31.5mil
> >
> >
> > width, which I hardly call "small"! I regularly use 3-4 mil trace widths in
> > my designs. Is there something different in Flex circuits, which enforces
> > me to use such LARGE traces (compared to my regular traces)?
> >
> >
> > 2) I see the problem you mention for the Microstrip configuration.
> > Is it
> >
> >
> > common to use Stripline on a flex circuit? I was told this is impractical,
> > although I'm not sure what the reason is.
> >
> >
> > Thanks
> >
> > Itzhak Hirshtal
> > Elta
> >
> > -----Original Message-----
> > From: si-list-bounce@xxxxxxxxxxxxx
> > [mailto:si-list-bounce@xxxxxxxxxxxxx]
> >
> >
> > On Behalf Of Havermann, Gert
> >
> >
> > Sent: Tuesday, November 24, 2009 4:19 PM
> > To: si-list@xxxxxxxxxxxxx
> > Subject: [SI-LIST] AW: Multi-Giga-Hertz Rigid-Flex Feasibility
> >
> > Hello Itzhak ,
> >
> > All Polimide (Kapton) materials will do the job from a loss and
> >
> >
> > dispersion prospective. The tolerance you mention seems to be unreasonable
> > high for an etching tolerance. I did Flex designs with impedance control,
> > 100Ohm +/-10Ohm, 0.8mm tracewidth without problems (of corse I had to pay
> > extra for the small trace width).
> >
> >
> > If you refer to an impedance tolerance due to bending and
> > surrounding
> >
> >
> > material, thats another story. If you design it in microstrip (as you are
> > planning it), then everithing touching the flex might change your
> > impedance. Designing it in stripline decreases the flexibility of the flex
> > and increases cost.
> >
> >
> > Look for Dupont, Grace Electron or Shengyi Flex materials for more
> > info.
> >
> > BR
> > Gert
> >
> >
> >
> > --------------------------------------------------------------------------
> > Absender ist HARTING Electronics GmbH & Co. KG; Sitz der
> > Gesellschaft:
> >
> >
> > Espelkamp; Registergericht: Bad Oeynhausen; Register-Nr.: HRA 5596;
> > persönlich haftende Gesellschafterin: HARTING Electronics Management GmbH;
> > Sitz der Komplementär-GmbH: Espelkamp; Registergericht der
> > Komplementär-GmbH: Bad Oeynhausen; Register-Nr. der Komplementär-GmbH: HRB
> > 8808; Geschäftsführer: Torsten Ratzmann
> >
> >
> > -----Ursprüngliche Nachricht-----
> >
> > Von: si-list-bounce@xxxxxxxxxxxxx
> > [mailto:si-list-bounce@xxxxxxxxxxxxx]
> >
> >
> > Im Auftrag von Hirshtal Itzhak
> >
> >
> > Gesendet: Dienstag, 24. November 2009 14:21
> > An: si-list@xxxxxxxxxxxxx
> > Betreff: [SI-LIST] Multi-Giga-Hertz Rigid-Flex Feasibility
> >
> > Hello All
> >
> >
> > My project manager intends to use a rigid-flex design in our next
> >
> >
> > high-speed board.
> >
> >
> >
> >
> > He wants to route a few pairs of 3.125GHz from a rigid section to
> > another
> >
> >
> > one through a flex section.
> >
> >
> >
> >
> > Does anyone know if this is feasible? I found out that the 2
> > sections,
> >
> >
> > although spec'd to be 100-Ohm diff impedance, can still differ
> > substantially - as much as 30%, because the tolerance of the flex section
> > is +/-20%. Can such a design work for a substantial trace length? Should I
> > restrict the trace length on the flex section to be no more than an inch or
> > so, in order to reduce the impact of the supposed non-uniform impedance?
> >
> >
> >
> >
> > I intend to use a 2-layer flexible laminate with the pairs on one
> > layer
> >
> >
> > and a Ground reference on the other one.
> >
> >
> >
> >
> > Any advice on this matter would be helpful
> >
> >
> >
> > Thanks
> >
> >
> >
> > Itzhak hirshtal
> >
> >
> >
> >
> >
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> > --
> > Scott McMorrow
> > Teraspeed Consulting Group LLC
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