Typically, when ordering an impedance controlled board, you don't specify the Er.
You specify the desired impedance, the thickness of copper and dielectric, and the trace width.

If you need to know the Er for a specific material for some reason, you can look up the ratio of glass to resin from the material spec sheet. Glass has a certain Er and resin has a certain Er, and it is easy to calculate the "result" based on the proportion.

BUT... BE WARNED!
The Er you come up with may not be the same Er as they are using in their calculations! Sometimes fab vendors use a "fudge factor" Er to make the calculated result match the real historical data they are getting from their process. For example, if they are using a certain software that always gives a predicted result of 50 ohms, but every time the make a board using those numbers the result is 47 ohms, they may "adjust" the Er in the calculator to match the real world. (And if you are not using the same software, you may get a different result)

Anyway, don't specify the Er, specify the impedance you want.

Good Luck,
Jack

(p.s. If you need a lower Er than FR4, you can specify a different material. For example I have used Rogers 4350 quite often, and the answer to your question is YES, the core and prepreg will both be the same material, they wont use 4350 cores and FR4 prepreg, if that is what you were worried about!)


                -----Original Message-----
                From:   Harris, George [mailto:[log in to unmask]]
                Sent:   Thursday, May 11, 2000 10:50 AM
                Subject:        Er of prepreg vs FR4

                Can somebody tell me how Er of the FR4 layer correlates with Er of the
                prepreg layer?

                If we specify Er=3.8, does the board shop have special low-Er prepreg to go
                with it?
                Or is there just one standard prepreg that everybody uses?

                thanks,

                -George