I have a customer who is experiencing excessive warpage
          after reworking BGAs. The PCB is a .062" thk - 10 lyr made
          from 140'C Tg FR-4. He uses forced hot air under the BGA
          device that heats only the region/area where the device is
          located. I discussed with him the issues
          (stress/TCE/expansion/contraction) that he may be
          encountering by heating only specific areas. I also stressed
          issues associated with controlled heat rise and cooldown of
          PWAs. His next tests will be to heat the entire board
          slightly below component degradation temperatures then
          perform his BGA rework procedure. Once rendered he was going
          to allow the entire PWA to cooldown at a slow rate to
          minimize any potential stresses that could happen during
          rapid cooling.
               Since I'm a fabricator and don't completely understand
          the assembly/rework issues, I thought I would throw this up
          for suggestions.


          Any responses will be greatly appreciated.

          Thanks in advance.

          Dave Hoover
          Senior Process Engineer
          Hadco Corp/Tech Center Two-Watsonville, CA
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