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 ################################################################ TechNet 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 TechNet <your full name> To unsubscribe: SIGNOFF TechNet ################################################################ 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 ################################################################