Douglas Mckean wrote: > > Just a suggestion, but would it be too much to ask people > who did some of the tremendous studies in simulations > and analysis scenerios with printed circuit boards > to do best case and worst case scenerios with a RANGE > of FR-4 values and then to have the Dk of the board > for the frequencies of interest actually MEASURED? > > I found that ALL simulations presented used the typical > Dk = 4.7 value that can vary with great latitude from > mfr to mfr (dependent upon mfr-ing techniques) and > can vary with frequency. > > I even took one person aside after their presentation, > (a very good one I might add) and discussed this point > with them. They hadn't thought of this. We're talking > at the Ph.D. level. Now maybe that isn't such a big > concern for them, but it is for me. The 4.7 value is > at best a fantasy. > > Also, my understanding is that "Dielectric Constant" is > done with DC voltage while the more general "Permittivity" > is done with frequency. > > Am I being to overly something-or-other? > > Discussion? Ideas? Flames? > > Regards, Doug >
Hello Doug, for my opinion there is no TYPICAL epsr value for FR4! In the past we have done many investigations to realized impedance values on PCB. We have measured the impedance (with TDR) and we have measured the geometry of the traces (copper width, copper thickness and thickness of dielectric) for microstrip and stripline structures. Than we have computed the impedance with a field sover and compare with the measurement. In this way we have found the epsr-values for our dielectrics. These computations agree very well with the data provided from our PCB manufacturer. The actual epsr value is a function of the thickness of the materials (that means of the ratio of glass and epoxy). Thinner materials have a lower epsr value (for example for a 130 my core about 3.85-4.0) than thicker materials (for a 310 my core or prepreg about 4.2). We used these values in our Signal Integrity simulations and have not found any problems or differences between simulation results and lab measurements. The above values are primarily valid for frequencies above 300 - 500 MHz (Please think about the timing relations of the TDR measurement) I hope this help a little bit. Best regards Hans-Joerg -- Dr.-Ing. Hans-Joerg John email: [email protected] Siemens Nixdorf Informationsystems phone: +49 5251 820 347 OEC ES DB3 fax: +49 5251 820 349 EMC and Signal Integrity Support Heinz-Nixdorf-Ring 1 D-33106 Paderborn Germany --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], or [email protected] (the list administrators).
