Michael,
Exposed Copper is a major concern associated with OSP.  Those who
used it and love it have convinced them selves that the exposed
copper is not a defect.  They do have a point.

Before OSP, all the metal was solder coated, or some other
metalic coating (ni/au).  Any exposed copper was the result of
the copper surface rejecting the coating that was being applied
to it.  This is a bad situation.  Since copper will always allow
solder to wet it, exposed copper is evidence that there is
contamination on the board.  The boards are rejected for
contamination, AS EVIDENCED BY exposed copper.  They are not
rejected for exposed copper, per se.  Many have forgotten why we
reject boards for exposed copper.  "It's just the way we have
always done it."

Further evidence of this distinction is found in the old
Mil-P-55110 which allowed the vertical edges to have exposed
copper after tin-lead reflow.  Why?  Since they were never coated
with solder, visible copper is not evidence of contamination.

Now we have OSP.  (I think you can see where this is going.) 
Boards are shipped without solder coatings.  If the solder paste
does not cover the entire pad, it is likely there will be exposed
copper after fusing.  Is this evidence of contamination?  No. 
Therefore, the argument goes, no contamination means no defect.

On the other side, One asks, If there were contamination on the
board, how would I know?  The exposed copper is evidence of one
of two things: 1) the copper was never coated; or 2) the copper
rejected the solder because of contamination.  Since I can't tell
the difference, I will assume the worse case, and reject the
board.

[This argument goes back and forth a few more times, ie there are
ways to tell the difference]

There has been evidence that CLEAN exposed copper is just as good
as solder coated copper from a long term reliability point of
view.  Some indications are that exposed copper is better that
solder.

So the choice is yours, or your companys.  How will you define
"exposed copper", as an indicator of contamination, or a
rejectable cosmetic defect.  Keep in mind the impact this choice
can have on your companies future competitiveness.

Personally, I think it is unfair to apply the process indicators
(exposed copper) of one process (solder coating) to a
fundamentally different process (OSP).

-- 
George Franck
Principal Product Engineer
Raytheon E-Systems
Falls Church Va
"These views are my own. They may or may not be shared by my
company"

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