Hi Ted,
Let get things straight.
The laminate layers in PWBs are fully cured during the initial lamination 
process at the laminator.
The prepreg layers are B-stage cured [that means on heating they will 
liquefy] and on further heat exposure during the PWB lamination process will further 
cure towards complete molecular cross-linking, but will in most cases be not 
fully cured; additional processing does not substantially contribute to 
completing curing; thus, the resin also does not shrink [if you look at drilled hole 
walls after soldering, you will see that the hole walls of laminate and 
prepreg layers align [if what you said were true, you would have a visible effect 
looking like a larger positive etchback].
By the way, there is no such thing as overcuring, once complete cross-linking 
is achieved the process stops; also, otherwise all laminate layers would be 
'overcured.'
You say that "As a board reaches its Tg it expands in the Z axis not the x or 
y axis"—that is true away from any holes, but not at the holes, where the 
resin puts the Cu barrel wall under compression [picture a submerged submarine 
under hydro-static (resin-static in this case) pressure] because it tries to 
collapse the hole walls [that is why you see bulged-in Cu barrels particularly 
where non-functional lands have been removed].
It is clear that prepreg layers have a higher resin content than laminate 
layers, and result together with the somewhat incomplete degree of cure in a 
larger z-CTE [that is why barrel cracks predominately occur where prepreg layers 
are and not laminate layers].

Werner



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