Don:
               The thickness of solder from a solder level operation
          is dependent upon pad geometry, amongst other things.  Board
          fabbers walk the tightrope between blowing enough solder out
          of the holes yet leaving enough remaining on the surface
          mount pads.
               I would strongly urge you to specify a horizontal HASL
          process from your board vendors.  Boards should also be
          cocked at a 45 degree angle when going into the solder to
          offset a "north-south, east-west" bias on QFPS.
               Getting back to the pad geometry, the larger the pad,
          the thinner the solder coat is apt to be, because the solder
          has a larger surface over which to retreat.  Worst case
          would probably be discrete pads, where we have a stated
          minimum of 50 microinches at the pad center.  Maximum solder
          thickness can be held to .0015".
               Where the solder is measured is another issue.  We
          always ask our customers to measure from the pad centroid so
          that outgoing and incoming measurements are apples and
          apples.  The pad center may or may not be the crest, and it
          usually isn't.  When I see thicknesses called out in specs
          that say "as measured at the crest", I chuckle because it
          would take too long just to find the crest of a given pad.
          In all cases, your board fabricator should guarantee
          solderability.
               In summary, the most workable callout would be a solder
          thickness of .0001" nominal, .00005" minimum, .0015" maximum
          (all measurements taken at pad center).  Again,
          solderability is to be guaranteed.
          Regards,
          Tom Coyle
          Field Services Engineer
          HADCO Corporation

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