I believe IPC-6012 Classes 1 through 3 do not spec a HASL thickness at 
     all but only require that full coverage is made and that the board 
     meets solderability requirements.
     
     Frank Hinojos
     Watkins-Johnson Co.


______________________________ Reply Separator _________________________________
Subject: FAB: RE: Question on Pad Leveling
Author:  [log in to unmask] at INTERNET
Date:    6/12/97 10:47 AM


          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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