Consult your PWB fabricators and ask them what is most commonly used. > -----Original Message----- > From: Hurst, Joe [SMTP:[log in to unmask]] > Sent: Tuesday, March 31, 1998 7:56 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 ################################################################