I have to respectfully disagree with this method, a bit. In some instances it is a very good thing to get definitive fab geometries, dielectric constants, and all the sometimes obscure data you must take into account to build an accurate model to determine the properties of your stackup. But, in my personal experience, that's not a task to be taken lightly. Unless you have a specific need to delve into the nitty-gritty, I would opt for relying on the fab vendor's empirical data to determine stackup to achieve the right impedance, for instance. Of course, this depends on the vendor's data collection method and their analysis, but they should have the data and experience. If they don't, and/or you opt to go through the exercise of precisely determining all the relevant factors yourself, be prepared for a lengthy study. Jeff Loyer -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of David Hoover Sent: Friday, February 20, 2004 6:28 AM To: cadpro2k@xxxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Layout Cross Section The best thing to do is make sure you understand your fabricators true finished product parameters. The trapezoidal shape of the etched copper conductor makes a huge difference in the calculated impedance results. (Hopefully your calculator has that as an entry) Next, you need to use the effective relative permittivity (Er') of the material at the frequency that the TDR sees (based on the risetime of the TDR. Usually around 1 to 1.75 GHz nominally for FR4) You can't just use the bulk material values (around 4.5 at 1 MHz). Those are fine for 2 sided because there is so many sheets of material stacked up. But multilayers have higher resin to glass ratios and end up with a much lower Er' value. Each type of material is different, so ask your fabricator for a list of Dk values at different frequencies to help you better understand the variables. Removing parameters and adjusting the variables to get the calculated result to match the measured is not the best thing to do. Trust me I KNOW! The fabricators have done it for years and some may still be doing this. Ever hear the words "Empirical Data Adjustment"? (It's like a mental slide rule) The 2D and 3D Field Solvers today have the ability to accurately calculate and solve the mystery. That coupled with entering the real values will produce results so close that there is no need to adjust parameters to "tweak" the results. Of course if you only have a static equation calculator (like the MECL stuff) then I guess your stuck fidgeting with the numbers. : / David Hoover Sr. Field Application Engineer Multilayer Technology, Inc. ----- Original Message -----=20 From: <Aubrey_Sparkman@xxxxxxxx> To: <cadpro2k@xxxxxxxxxx>; <si-list@xxxxxxxxxxxxx> Sent: Thursday, February 19, 2004 10:36 AM Subject: [SI-LIST] Re: Layout Cross Section > Uri, Mitch, > You don't need to mess with your material.dat file; adding a layer of FR4 > works. Before you do your testing, delete your interconn.iml file; you want > to make sure Allegro re-computes the parasitics. I opened a test board I > have, selected "DISPLAY/PARASITICS", and clicked on a trace on the top layer > and got: > 57.113 ohms > 36.207 nH > 11.1 pF > 0.633954 nS > > I then removed the FR4 layer and did the "DISPLAY/PARASITICS" thing again: > 64.334 ohms > 36.2065 nH > 8.74793 pF > 0.562789 nS > > To me this says everything changed about the right amount in the right > direction. > > Aubrey Sparkman > Enterprise Engineering Signal Integrity Team > Dell, Inc. > Aubrey_Sparkman@xxxxxxxx > (512) 723-3592 > > > > -----Original Message----- > > From: si-list-bounce@xxxxxxxxxxxxx > > [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Mitch S. Morey > > Sent: Thursday, February 19, 2004 10:48 AM > > To: si-list@xxxxxxxxxxxxx > > Subject: [SI-LIST] Re: Layout Cross Section > > > > > > Hi Uri, > > > > Before you get into material.dat file, and the parameters and > > what-not, I > > experimented with changing some variables in my stackup and > > the impedance > > DIDN'T change. I tried FR4 over top; conformal coat; etc; changed > > thicknesses; Dk and when I do DISPLAY/PARASITICS on my Clines nothing > > changes. Might be worth your time to experiment first. :) > > > > Mitch > > > > > > Hi ! > > > Thanks for all answer me Kim, Mitch ( nice joke :) and Jeff. > > > Do you know what parameters of soldermask ? > > > Now I know that possible edit material.dat and now I need > > to do this. > > > Uri > > > > > > -----Original Message----- > > > From: Jeff Seeger [mailto:jseeger@xxxxxxxxxxxxxx] > > > Sent: Wednesday, February 18, 2004 8:38 PM > > > To: si-list@xxxxxxxxxxxxx > > > Subject: [SI-LIST] Re: Layout Cross Section > > > > > > > > > Uri Chaplianka wrote: > > >> > > >> snip < > > > > > > > > ----------------------------------------- > > This email was sent using DACafeMail. > > Get Your FREE 10 MB eMail Account Now. > > http://cafemail.pcbcafe.com > > ------------------------------------------------------------------ > > 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.org > > > > 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.org > > 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.org 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.org 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