Hello George,
    If you are talking about the difference between the Er of a core versus
that of the "fill" between cores the answer depends.
There are several different glass styles and most laminators will use
different resin ratios for each style in order to get a certain  thickness.
For instance, they may use 2 sheets of 1080 glass for the core and 2 sheets
of 1678 glass for the fill.
The Ers will probably be different because the resin content is different.
The next shop may want to use a different glass or even the same glass but
from a different laminator which may have different resin to glass ratios.
What I suggest is that you
run your similator using an Er of 4.1 and then identify only the most
critical parameters that must be met (ie. overall board thickness, minimum
line width, or dielectric thickness). Please keep in mind that your life
will be much easier if you allow the board fabricator as much flexibility
as possible in dielectric thickness between layers and allow them to adjust
the line widths to some extent.

Jeff





"Harris, George" <[log in to unmask]>@IPC.ORG> on 05/11/2000 12:49:58
PM

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Subject:  [DC] 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