Mark, Dr. Eric Bogatin had covered this subject in his monthly column "No Myths Allowed" in the July 2006 issue of Printed Circuit Design & Manufacture. The trace width selection is a compromise between higher dielectric losses for wide traces and higher conductor loss for narrow traces. His conclusion: 7 mils is the optimum trace width for GHz interconnects on FR4. Regards, Ravinder Server PCB Development Hitachi Global Storage Technologies Email: Ravinder.Ajmani@xxxxxxxxxxxxxx "Mark Burford" <Mark_Burford@xxxxxxxxxxx> Sent by: si-list-bounce@xxxxxxxxxxxxx 11/20/2006 08:15 AM Please respond to Mark_Burford@xxxxxxxxxxx To <si-list@xxxxxxxxxxxxx> cc Subject [SI-LIST] Thick vs. thin diff. pairs Dear all, I would like to ask opinions, on the merits and disadvantages of using wide or narrow microstrip lines. From where I am sitting it looks like narrow microstrip diff. pairs have it won hands down. Thick lines: More area...which gives more dielectric loss as dielectric loss over takes skin effect losses at higher frequencies. More S11 because any corner on the line will give a greater area and more capacitance. Thicker substrates to keep the impedance at the right value. As thicker microstrip lines go round a bend, the inter-pair skew will be more than for two narrow lines. There is also the possibility of increased EMI from thicker substrates (been reading antenna design books). So please someone tell me why should we use thicker lines? And also please could someone tell me the trip ups of HDI (high density interconnect) such as how small can one realistically go with back plane and inter-chip routing? Thanks Mark ------------------------------------------------------------------ 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ 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