The question is one of what the shop would like to see, as all solder mask
requires some clearance, the PCB shop will have to determen the actuall pull
back based on there method of appling solder mask, for shops that use slik
screening they need at least .008 clearance both sides. for shops using
photo imagable they can get closer at .002-.003 each side. But it all
depends on the type of board, if its flex thaen we need more because it
moves more in processing and is hard to align. for longer boards the growth
is enough to give aligmnet problems at even .008 spacing.  If a board shop
gets a file with no clerance it has to use its software and change the size
of the solder mask openings ( pads reversed)to get the desired spacing, if
you supply a over size pad opening with out consulting the shop then they
may have to change it to make the board.
if there is no tolerance then you will get solder mask on the pads which is
a major problem in assembily.
 Remenber when we make a printed circuit board everything is moving around,
the best we can work to is about +-.003 to +-.004 tolerance any where to any
thing.
hope that helps   robert Tarzwell  Megadawn.com ,pcb books

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