Hi, Gurus, I have couple of questions regarding the edge coupled coated microstrip differential impedance. Recently, I have designed a board with 100 Ohm differential impedance microstrip. The design track width is 7mil, 4mil space edge to edge, 8.9 mil distance to the reference plane. The track thickness is 0.5oz plus 1oz plating. one local board shop made the boards for us. The actual track width is 7mil, 4mil space and 8..4 distance to the reference plane. The track thickness is 0.5oz plus 1 oz plating. The measurement for these differential pairs on a coupon (6 inches) is 101 Ohm, which is good. We switched another board shop for the second spin. they told us that the spacing have to be adjust from 4mil to 7mil in order to make 100 ohm microstrip differential pair. Their calculations show the impedance with my original design would give around 80 ohm. the parameters they were using are, track width 7mil, spacing 4mil, 9.5mil distance to the reference plane, track thickness 0.5mi plus 1.5 plating, 0.8mil solder mask. The dielectric constant is 4.7 for their calculation. Here are my questions: 1) why is the calculation and result so different between these two board shop? 2) how much will solder mask effect the differential impedance on microstrips? I know solder mask has some impact to the microstrip impedance. Is that possible to be 20% difference? 3) If the solder mask or material or processing have such big impact to microstrips, why the first board shop doesn't need to adjust to get 100 ohm? My calculation is based on the edge coupled microstrip without counting the solder mask coating. I thought it would be close enough for board shop to adjust tracks according to their processing and make make boards. But this makes me to rethink about this idea. Anyone has such experience? thanks and regards. Jerry Hu ------------------------------------------------------------------ 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