DC calculations are made for core material as such. This means the DC does
not change for it in whatever MLB structure.

For prepregs, DC is calculated based on expected effects after cure in the
press - in the book - in the MLB. Variations are minor, within but a percent
or two, for the same glass styles and resin contents used in the structures.

"Good" board shops will provide pre-calculated DC numbers both for core and
prepregs used. Still, the numbers vary minimally and your overall impedance
will be little affected. However, for very high speed stuff, this can be
critical though little FR4 is used in these designs. You can then calculate
the requirements based on DC, thickness, trace widths (for differential
pairs as stripline [boradside and edge rate requirements] and microstrip
requirements).

MoonMan

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