I few years ago I had good results with reheating/cooling assembled 
warped boards.  When the board was held (fixtured) flat and heated just long 
enough so that the core temp is just short of the Tg of the laminate, it 
worked well.  It was equally important to let it cool very sloowwly so that 
new stresses were not created.
    You mentioned using weight with poor results.  It would seem difficult 
to evenly distribute weight on an assembled board.  Also, assuming the 
weight only contacted parts of the board, the cooling rate would not be 
equal becuase of the differences in thermal mass.
    The fixture I used was made from 1/2 bar stock.  The board was clamped 
onto the frame and extra short pieces were added as needed to pull the 
center area flat.   The bar stock kept the board flat and allowed air 
circulation around both sides.
    Being assembled, check the high temp rating on the components.  Either 
remove the ones that will melt or run at a lower temp.  If your temp. is too 
low, the stresses within the laminate will not be fully relieved.

Norm Dill
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 ----------
From: TechNet-request
To: TechNet Forum
Subject: Don't want to scrap it.
Date: Wednesday, March 27, 1996 12:06PM


Greetings,

Does anyone have a good procedure for removing a warp/twist from a pcb?  I
am dealing with a .093 thick assembly that is 7 inches by 20 inches.  The
board currenly has smt components placed and soldered (topside only) but,
the through hole components have not yet been mounted/soldered.  I have
tried heating the board and letting it cool while applying weight, that did
not do the job.  Any suggestions ? please respond.

Thanks,

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