You cannot overcure an epoxy board.
You can, however, oxidize the board by overbaking in air. You can also cause
decomposition of the epoxy at a high enough temperature. Too much baking too
hot can be a bad thing.
Usually such boards will be discolored. Tg will be lower after the damaging
bake.
Effects on drilling are hard to predict. 
If you were undercured before and your bake is now giving you full cure, you
will have a rise in Tg. This could lead to a more brittle product and lead to
cracking or crazing on drilling. Tg data would help.
J S Sallo
Sallo Consulting
Phone or Fax 760 346 6717

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