Inner layer separation can occur between the electroless copper and the inner layer foil, between the electroless copper and the electrolytic copper and totally within the electroless deposit - that is if the boards were built with electroless copper.  Each of these has their own set of causes, which is a very long list. 

Assembly operations can cause inner layer separation on good boards, but this is very rare.  Excessive temperatures can do this expecially on a pin that has multiple ground and power plane connections and is difficult to remove and replace.  However, this is rare.  If separation shows up in assemblies there was most likely a weakness there to begin with in the bare board.

Perhaps if you tell us where the separation occurs or send a picture of a micrograph, we can give you all of the possible process steps that might have an effect.

Susan Mansilla
Robisan Lab ---------------------------------------------------
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