Wes
In the "olden days" there was a problem with Z axis shorting, or "punch
through" between layers in a multilayer board
In order to prevent this, the mil spec required 2 layers of prepreg. .
This became a standard call out in the U S because of the mil spec
Agruable whether it was ever really needed, but that is the history.
Do you need it now? Depends on your board, but you should be open to dropping
the requirement if you don't need it, or the added cost
Good Luck
Jerry Sallo
Sallo Consulting Services

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