Some thoughts, in no particular order (and targeted for High Volume Manufacturing products). * A paper you might find pertinent is "System Level Impact of Stitching Vias and Capacitors for High-Speed Differential Links", available as: Electronic Components and Technology Conference, 2007. ECTC '07. Proceedings. 57th Publication Date: May 29 2007-June 1 2007 On page(s): 357-36 ISSN: 0569-5503 ISBN: 1-4244-0985-3 * Keep in mind how stackup-specific any controlled impedance via design will be. A via that appears >50 ohms in a thick, lower-layer count board (say 100 mils, 6 layers), might well appear capacitive (< 50 ohms) when the number of layers is increased. Plug-in cards (or desktop boards) are often 60 mils thick, and only have 4 layers. For a server design, you'd need different via designs for your baseboard versus risers and/or plug-in cards. * Any realistic, precise via design would have to account for manufacturing variation, giving a "family" of possible impedances, rather than a single one. The impedance variations might be too small to matter, but should be understood. For differential vias, for instance, drill accuracy might play a significant role in the impedance. * Of course, each controlled-impedance via is only valid for a particular layer entry/exit scheme. * These make controlled-impedance vias very challenging for actual products. * Add to these points the real-estate a controlled-impedance via might require (especially if you start talking about surrounding each signal via with multiple ground vias), and I end up being very skeptical that they are suitable for anything other than research or low-volume/high-performance/long-leadtime/high-price products. For designs where cost and TTM (Time-To-Market) are primary drivers, ugly vias will continue to be necessary evils in our design. Some things that can be done to minimize their impact (and I invite others to add to the list): o Floorplan your high speed busses first, to optimize their topology for layer transitions: * Minimize the number of transitions (vias) from driver to receiver, including connectors and risers. Let your kHz or low MHz signals jump around from layer to layer, while your GHz signals continue on their dedicated layers. * When transitioning, go all the way through the board, minimizing the stub. For multi-board designs, this can be very challenging, but those are probably where this will be the greatest issue, also. o Provide adequate ground stitching vias near transitions. I think applying guidelines like these, and absorbing the "hit" from the vias that are necessary, will be more realistic than complex, 3-D via design for most products. I'd also add my 2 cents about loosely versus tightly coupled... As you point out, neither is without shortcomings. I do believe, however, you'll want the two halves of a differential pair to be in close proximity at any transitions - they'll be more "tolerant" of the impedance discontinuity (and any other impedance discontinuities). I would agree with your comment regarding wider trace widths being an advantage to looser coupling, but am not aware of any degradation of risetime from tight coupling, except perhaps if the traces are narrower. 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. Jeff Loyer -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Joel Brown Sent: Wednesday, January 09, 2008 8:30 PM To: wolfgang.maichen@xxxxxxxxxxxx; luant@xxxxxxxxxxx Cc: si-list@xxxxxxxxxxxxx; si-list-bounce@xxxxxxxxxxxxx Subject: [SI-LIST] Re: 50 Ohm Via? Wolfgang, Your point about how much simulation is worthwhile is well taken. I work for a small company and wear a lot of hats, I am not a full time SI engineer. We do have some tools such as Hyperlynx and Hspice which in my opinion have been under utilized. I know Hyperlynx claims to have some GHz via modeling capability but I am not sure how accurate it is and I don't think it takes the return path such as stitching vias into account. I have been trying to do more simulation as time allows and learning along the way. It's certainly not easy to learn multiple simulation environments and all the pitfalls. I have yet to get to the point to where I can correlate measurements against simulations. How would I know what the prop delay through a via will be? To Chris: I have been reading several places that recommend using loosely coupled differential pairs, that is why I mentioned 50 ohms. I know there are religious beliefs about tightly coupled vs loosely coupled pairs. The material I read regarding loosely coupled pairs mentioned advantages such as wider trace widths for a given impedance and avoiding degradation of rise time caused by coupling between signals within a pair. Thanks - Joel -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of wolfgang.maichen@xxxxxxxxxxxx Sent: Wednesday, January 09, 2008 7:19 PM To: luant@xxxxxxxxxxx Cc: si-list@xxxxxxxxxxxxx; si-list-bounce@xxxxxxxxxxxxx Subject: [SI-LIST] Re: 50 Ohm Via? As a simple rule of thumb: Usually not very important if the prop delay through the via is less than about 1/6th of your signal rise time (you may be able to get away with 1/4th). Rise time is much more important than bit rate or clock frequency. As to the number of vias - this can of course aggravate the problem; but on the other hand, I wouldn't attempt to design a 10 Gb/s channel and put in more than maybe two vias... just my 2 cents Wolfgang "Tony Luan" <luant@xxxxxxxxxxx> Sent by: si-list-bounce@xxxxxxxxxxxxx 01/09/2008 07:06 PM Please respond to luant@xxxxxxxxxxx To <si-list@xxxxxxxxxxxxx> cc Subject [SI-LIST] Re: 50 Ohm Via? How critical the characteristic impedance of via transition is? It depends on the bit rate, channel insertion loss and the number of vias on each channel.=20 BR Tony -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Harry Selfridge Sent: Wednesday, January 09, 2008 6:50 PM To: 'SI LIST' Subject: [SI-LIST] Re: 50 Ohm Via? There was an article written about controlled impedance vias several=20 years ago by Thomas Neu of Texas Instruments. I haven't seen any=20 followup articles by anyone on the subject since. You can read Neu's=20 article online at: http://www.edn.com/index.asp?layout=3Darticle&articleid=3DCA324403 . Others may have experienced different results, but I've never found=20 controlled impedance vias to be necessary or useful. The distances=20 involved in a via are so short that any pretense of matching=20 impedance is negligible compared with other variations that you might=20 encounter over the full length of a signal path. One board we built=20 for a customer provided two signal paths, one with Neu's controlled=20 impedance vias, and duplicates without. Testing of the loaded board=20 showed no appreciable difference in performance, and the loss of=20 board space to the structure necessary to achieve the controlled=20 impedance vias was considerable. Regards - Harry At 05:51 PM 1/9/2008, you wrote: >Is there such a thing as a design methodology for designing a PCB via with >50 ohm impedance, or does it have to be done iteratively using a 3D field >solver? >Are controlled impedance vias necessary, worthwhile or helpful for >multi-gigabit serial links running at 1 to 5 Gbps? > > > >Thanks - Joel ------------------------------------------------------------------ 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 ------------------------------------------------------------------ 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