[SI-LIST] Re: Are flex pcbs now competing with ribbon cables in terms of signal integrity?
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: <james.f.peterson@xxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Sat, 17 Nov 2007 14:33:38 -0800
Hi Jim,
One of the biggest challenges I've found in modeling flex is to figure
out what material it really is. You'll have to get your complete
stackup (unique to each particular design) from the vendor before
building a t-line model, if loss is a concern.
2-layer (1 GND, 1 signal) polyimide has a low loss tangent (Df ~0.01)
compared to FR4 (0.02). But, if you add coverlay (flex equivalent of
soldermask), with its Df of 0.02, what's the new equivalent loss? The
problem gets even stickier (pardon the pun) when you go to more layers,
where you'll have adhesive between your core dielectrics. Those
adhesives may not have much information on them, making it especially
tough to figure out what the final net loss tangent is like.
A related topic under investigation is how much moisture will be
absorbed by the polyimide - it's more hygroscopic than FR4. That will
increase its loss tangent.
I would plan on building a simple test board (or adding coupons to the
existing design) with your actual construction to derive t-line models
from (or, at least verify your existing ones).
Figuring out the impedance (Er) hasn't been nearly as challenging -
vendors seem to be able to give good numbers to get this correct; they
haven't as much experience with Df.
Other factors that have come in to play:
* Flex lets me use fine trace widths (thinner dielectrics, finer etching
ability). High density is nice, but these fine traces are also more
lossy.
* If I'm using a multi-layer design, I may want to perforate my ground
planes to make the flex more flexible. Building a true model of this
which comprehends the correct impedance and crosstalk can be very
challenging - plan on a test board to verify your model.
* When designing with flex, you should become intimately familiar with
the mechanical aspects, since they'll play such a large role in your
design. Some have already been mentioned, but one other that I'd
mention is the fact that, given a choice, you won't want to "stretch"
(apply tension to) your traces - better to have them under compression
(inside of a curve) or on the neutral axis of your flex "beam" (in the
center of a 3-layer symmetric stripline). For thin designs, this isn't
such a problem.
Disclaimer:
The content of this message is my personal opinion only and although I
am an employee of Intel, the statements I make here in no way represent
Intel's position on the issue, nor am I authorized to speak on behalf of
Intel on this matter.=20
Jeff Loyer
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Peterson, James F (EHCOE)
Sent: Friday, November 09, 2007 5:02 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Are flex pcbs now competing with ribbon cables in
terms of signal integrity?
Bill,
I read the article. It had good fab info, but lean on the SI
perspective. I've been assuming that if I get good Er data, and control
the dimensions, that my approach to rigid flex SI analysis is similar to
a regular PCB. In fact, I've been just modeling these flex boards as
basic PCBs....
Anybody see a weakness in this approach?
Thanks,
Jim Peterson
Honeywell
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Hargin, Bill
Sent: Thursday, November 08, 2007 8:16 PM
To: tom_cip_11551@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Are flex pcbs now competing with ribbon calbes in
terms of signal integrity?
Hi Tom:
You may find this article on rigid-flex design fairly interesting:
http://pcdandf.com/cms/cms/content/view/2948/95/
Bill Hargin
High-Speed Marketing Manager
Mentor Graphics Corp.
Systems Design Division
425-301-4425 - Mobile=3D20
=3D20
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx=3D20
> [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of tom_cip_11551
> Sent: Thursday, November 08, 2007 10:16 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Are flex pcbs now competing with ribbon=3D20
> calbes in terms of signal integrity?
>=3D20
> Hi Si-List experts,
>=3D20
> I am working on an application that requres ribbon cable runs=3D20
> of about a meter, for differential pairs at 100 Ohms (50 Ohms=3D20
> odd mode) impedance. My data rate is about 1.5 Gb/s . I have=3D20
> had lots of experience specifying printed circuit boards over=3D20
> the years and I though that "flex" pcb circuits were limited=3D20
> to the panel sizes of a particular fab house. Recently, I=3D20
> have seen some data to the effect that "ribbons",=3D20
> manufactured like pcbs, are available now that can be run=3D20
> for many feet, because of "step and repeat" fabrication =
processes.=3D20
>=3D20
> Yes, I am aware of the limiting factors of long cable runs on=3D20
> high speed signals (dispersion, crosstalk, emi, etc) but I am=3D20
> wondering if FPC (flexible printed circuit) technology has=3D20
> advanced to the point where it can compete with ribbon cable=3D20
> technologies. I know that in terms of signal integrity, a pcb=3D20
> beats a cable.
>=3D20
> Thank You
> Tom
> tom_cip_11551@xxxxxxxxxxx
>=3D20
>=3D20
>=3D20
> ------------------------------------------------------------------
> To unsubscribe from si-list:
> si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>=3D20
> or to administer your membership from a web page, go to:
> //www.freelists.org/webpage/si-list
>=3D20
> For help:
> si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>=3D20
>=3D20
> List technical documents are available at:
> http://www.si-list.net
>=3D20
> List archives are viewable at: =3D20
> //www.freelists.org/archives/si-list
> or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
> Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
> =3D20
>=3D20
>=3D20
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at: =20
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
=20
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List technical documents are available at:
http://www.si-list.net
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
