Hi Bijesh, one important point not yet mentioned, is that tracks on external layers (which are then plated up) vary in thickness, depending upon the routing density in the local area (less tracks around the diff. pairs means more copper). This makes the impedance harder to control precisely, due to varying cross-section of tracks across the board. 'Thieving' pads can be applied to fill open areas (the PCB manufacturer will often ask if he can do this anyway, as it also means less copper to be removed in the etching process), but the problem exists nevertheless, even if you are ok with allowing this. To be on the safe side, you should confirm with your manufacturer what kind of influence on the tolerance range of controlled impedances it would have - for example, +/- 5% might be possible, but perhaps only on inner layers. For this reason, it is PREFERABLE to route critical sigs on inner = layers. Incidentally, on boards with multiple microvia layers (2-x-2 and above), tracks on some of the INNER layers may also be affected by the plating process - laser microvias have to be plated, of course - although with some processes, in some cases, such as the Neo-Manhatten (Bump) process, even the outer layers are not effected (with a pure SMT board). Mit freundlichen Gr=FC=DFen ____________________________________ Sol Tatlow, M.Eng. (Oxon) ProDesign Electronic & CAD Layout GmbH Product Developer Albert-Mayer-Str. 16 D-83052 Bruckmuehl Phone: +49 (0) 8062-808-302 Fax: +49 (0) 8062-808-333 Mailto:sol.tatlow@xxxxxxxxxxxxxxxxxxxx www.prodesign-europe.com ____________________________________=20 -----Urspr=FCngliche Nachricht----- Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] = Im Auftrag von bijesh.t.b Gesendet: Dienstag, 19. Oktober 2004 12:30 An: si-list@xxxxxxxxxxxxx Betreff: [SI-LIST] Signal Routing in outer layer Hi all, I would like to get some clarification reg. the high speed differential = signal routing in multilayer board. 1).Can we route high speed differential signal in outer layer?. 2).If we can route the differential signal in outer layer then, how do = we calculate the impedence. 3).What is the minimum tolereance which we can give for the outer layer impedece. 4).If the high speed differential signal can be routed in the outer = layer then,how do we control the EMI issues?. If anyone of you can help me on this,Please.. Thank you Bijesh ------------------------------------------------------------------ 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