Ed - The kinetics of the oxide reaction on fine lines can be compromised by a
galvanic cell set up between the copper and underlying foil treatment.  This
treatment, usually zinc, is exposed after etching.  It's only a fine line at
the copper epoxy interface but the galvanic cell is strong enough and results
in little or no oxide formation.  Ground planes and/or larger copper areas
have enough copper surface that the oxide reaction overcomes the galvanic
cell and the oxide forms properly.

There are several ways to overcome this "cell" problem.  Mostly, improved
surface preparation is the answer.

Roger Mouton
INSULECTRO

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