The dielectric constant of FR-4 epoxy material is derived by the ratio of glass to resin. Dielectric Constant of E-Glass: 6.2 Dielectric Constant of Resin: 3.0 Total 9.2 Assume standard FR-4 to be 50% glass and 50% resin, then divide by two. The result is a dielectric constant of 4.6. Though I have always used 4.5, for microstrip and embedded microstrip calculations, you can use 4.6 if you wish. However, when calculating stripline, the traces are resin rich on the edges and sandwiched between two planes. This changes the dielectric constant for stripline to 3.5. These numbers have been verified by thousands of actual designs requiring impedance control. The thinner glass styles contain a higher percentage of resins and the thicker glass styles contain less resins. Therefore, the thinner glass styles have a lower dielectric constant and the thicker glass styles have a higher dielectric constant. To manufacture a controlled impedance board, your vendor must average the various dielectric constants of the various glass styles to obtain the correct dielectric constant to use. This is why you may find the dielectric thickness actually used to construct the board, a little different than your stack-up used to obtain your original calculations. You will find that the final calculations come very close to the TDR readings on the actual manufactured board. Hope this helps. Norm Einarson