What's the reason for the following layout rule? Gentlemen: We have a question = about a strange layout rule as below. Background: PCB = Stack----- Signal/L1 = = 0.5oz = prepreg 4.1 = mil (Er=3D4) GND = = = 1oz = core = 4.1 mil = (Er=3D4) Signal/L2 = = 1oz = Prepreg 35 mil = (Er=3D4) POWER = = = 1oz = core = 4.1 mil = (Er=3D4) GND = = = 1 oz = Prepreg 4.1 = mil (Er=3D4) Signal/L3 = = 0.5oz DDR400 x 2 on one = memory bus (impedance 48~50). Part of DQ/DQS pair layout = rule A) On L1/L3 = only B) DQ: trace = width =3D 7mil, trace spacing =3D 12mil (if trace length from chipset to = last DIMM <=3D 5.7") &nbs= p; trace width =3D 11mil, trace spacing =3D 15 mil (if trace= length >5.7") C) DQS: trace = width =3D 7mil, trace spacing =3D 12mil (if trace length fromchipset to = last DIMM <=3D 5") &nbs= p; trace width =3D 7mil, trace spacing =3D = 15 mil (if 5"<trace length <5.7") &nbs= p; trace width =3D 11mil, trace spacing = =3D 15 mil (if trace length >5.7") DIMM to DIMM: = 0.2~0.6" Last DIMM to = termination: 1" (max) Termination = resistor: 56 Ohm. Question1 : What's = the reason for "longer trace then wider"? We knew wider trace = can have lower inductance but higher capacitance. Question2 : If we = route some DQ/DQS group on L2 but wanna have similar performance as on = L1/L3, what's your suggestion? &nbs= p; We = think a) 5 mils to match impedance but inductance will be higher. b) = trace length shorter than trace on L1/L3 for 1~2 inches since = signal &nbs= p; = speed is slower in inner layer. c) To increasethe signal spacing over = 20/25 mils to further reduce the cross-talk. Best = regards! Allen = Wang ASUSTeK Computer = Inc. RD8- server = centric division Manager Mobile: = 886-935481831 TEL: = 886-2-28943447 ext:3102 FAX: = 886-2-28907689 ------------------------------------------------------------------ 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 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