Paul, I don't think 5 plies of 7628 in 0.35" is the reason for the difference. 5 x 7628 calculates to a dielectric constant of 4.84. I just ran a quick manual calculation using Er 4.84 and got the almost the same results as your Polar (65.2 ohms. Assuming track width and dielectric spacings are OK the only way to get an impedance of 72 is by using an Er of 3.9. You aren't using BT Epoxy are you. Is the 72 Ohms measured on a test coupon or on a track in the board. Could there be an something in the circuit affecting the results? Geoff Layhe www.Lamar-uk.co.uk -----Original Message----- From: tech [mailto:[log in to unmask]] Sent: 17 July 2001 13:33 To: [log in to unmask] Subject: [TN] Impedance Discrepency 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 ---------------------------------------------------------------------------- ----- Private & Confidential: This e-mail message is confidential and is intended solely for the person or organisation to whom it is addressed. If the message is received by anyone other than the addressee please return the message to the sender by replying to it and then delete the message from your computer. --------------------------------------------------------------------------------- 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 ---------------------------------------------------------------------------------