Hi Mike -

Here's my two cents worth, from an assemblers viewpoint:

I've tried to spec the same thing in the years gone by, only to learn that the
HASL process doesn't have the degree of control that many folks would like.
After a number of attempts, we finally learned that the paramount
charactaristics are SOLDERABILITY and UNIFORMITY.   Specifying minimum
thickness led our suppliers to be absolutely sure they had enough thickness to
meet our minimums.  This often gave us boards with tall solder crowns, which
affected both past application and component placement.  We also found
substantial variations: on individual boards, from side to side; on the same
side; from board to board; and from one production lot to the next.

To make a long story short, we now spec solderability requirements and tell
the supplier our preference on thickness, without it being mandatory.

Regards - Kelly

PS - we're still looking at the white tins by Omikron (Florida CirTech) and
Dexter in hopes of getting that ultimate solderable board with flat pads!!!!