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 > --------------------------------------------------------------------------------- Paul I have seen the material prepreg can vary your impedance a great deal as the glass content to resin ratio is increased. You need to ask the vendor if they used REG # FR-4 11 7628 material that has a 40 % resin which has a high glass content and a DK of 4.75. --------------------------------------------------------------------------------- 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 ---------------------------------------------------------------------------------