Joe - The question that you need to answer first is how accurate does the impedance of the trace need to be. If +/- 20% is accurate enough for your application, you can use the numbers derived manufacturer's frequency/relative permittivity graph and be done with it. This should give you a value of between 4.4 at 1MHz and 4.0 at 1000MHz. If you want to be more accurate than that allows contact your fabricator and have them supply you with TDR derived numbers based on processed coupons. This may better than half the tolerance required on your calculations. Beyond that may not be possible, most of the Fabricators I have worked with will only guarantee the accuracy of the boards to +/- 10% . Jeffrey A. McGlaughlin PWB Designer Battelle Memorial Institute Columbus Ohio -----Original Message----- From: Hurst, Joe [SMTP:[log in to unmask]] Sent: Tuesday, March 31, 1998 8:56 AM To: [log in to unmask] Subject: [DC] FR4 Relative Permittivity Hello Folks: Since I often design PWBs having controlled impedances, the question often arises as to what value to use for the relative permittivity (Er) of FR4 material. 1. Some consultants have said to use 4.8 for outside layers and 3.5 for inside layers. Impedance calculations based on these numbers give results closely matching actual measurements. 2. Another source has said that due to the pressing process the Er around the traces on inside layers actually is 0.5 less than the Er of the FR4 material being used. 3. Others insist that an Er of 4.5 be used for all calculations. Considering that these materials and processes have been around so long the entire issue must have been settled by someone. What are other people using in these calculations? Thank You Joe Hurst ################################################################ DesignerCouncil E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c ################################################################ To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE DesignerCouncil <your full name> To unsubscribe: SIGNOFF DesignerCouncil ################################################################ Please visit IPC web site (http://jefry.ipc.org/forum.htm) for additional information. For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311 ################################################################ ################################################################ DesignerCouncil E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c ################################################################ To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE DesignerCouncil <your full name> To unsubscribe: SIGNOFF DesignerCouncil ################################################################ Please visit IPC web site (http://jefry.ipc.org/forum.htm) for additional information. For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311 ################################################################