As far as I know, the theoretical calculations have never matched the actual board. Probably a combination of measurement techniques, raw material variation and manufacturing process. As I recall, 5% discrepancy between theoretical and actual was usual. I also think that the plane/trace relationship will dominate at this dielectric spacing. I would focus more on the L2/L3 and L4/L5 spacing. The other issue is: how long of an isolated reduction in trace width or height will give a spike in impedance? If I were looking to understand this in depth, I would start with the metrology. If your measurements are in question, forget the rest. Rigorous metrology is the key to impedance testing. The Polar software is a good start. Steve At 05:33 AM 7/17/2001, tech wrote: >I have a 6 layer design with layers 2 and 5 being planes and 3 and 4 >signals. In this particular case , due to overall thickness and outer >layer impedance requirements we have .010 from the planes to the signals >and .035 between the signals. 1 Oz. copper and finished lines at .006. >My impedance software ( Polar CITS25) is giving a number of 65.02 ohms >but the actual results we're getting are all at the 72+ ohm range.We do >many impedance designs and have not seen a discrepency like this. The >.035 dielectric between the signals is 5 pieces of 7628 which is an >unusually high amount of prepreg for us and the .010 from planes to >signals are cores. All 140Tg FR4. Can the high thickness (resin content) >of prepreg be creating the problem ? Sections on the coupons are very >close to design so we don't appear to have a process issue. > Any input appreciated. >Paul Greene > >--------------------------------------------------------------------------------- >Technet Mail List provided as a free service by IPC using LISTSERV 1.8d >To unsubscribe, send a message to [log in to unmask] with following text in >the BODY (NOT the subject field): SIGNOFF Technet >To temporarily halt delivery of Technet send the following message: SET >Technet NOMAIL >Search previous postings at: www.ipc.org > On-Line Resources & Databases > >E-mail Archives >Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional >information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 >ext.5315 >--------------------------------------------------------------------------------- --------------------------------------------------------------------------------- Technet Mail List provided as a free service by IPC using LISTSERV 1.8d To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt delivery of Technet send the following message: SET Technet NOMAIL Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ---------------------------------------------------------------------------------