>How thin is too thin? As assemblers, when (at what thickness or process
>conditions) would you return boards to the fabricator?

I beleive for most people the solder on the board is purely for
solderability preservation, so if boards are assembled soon after
you make them you probably won't get many complaints no matter
how thin you go. However, boards often sit around for a significant
period before use, during which solderability will decrease due to
oxidation and intermetallic growth, both of which will alter the
composition of the solder and therefore its melting point. If the
solder dosn't melt then dispersion of the oxide layer will be
more difficult & therfore soldering will be more difficult, The
point at which this becomes a problem will depend on things such
as process temperature and flux activity, i.e. there is no single
answer to the question, but I am sure the practical experiences
of others will set some limits?

David Whalley