We do it exactly the same way. Never a problem. Your "excess of burnt flux" could indicate that the temperature is still too high. Perhaps a better desoldering braid would help ?  You can also try to work with a desoldering iron (with vacuumsucker) instead of a desoldering braid.

Daan 
  ----- Original Message ----- 
  From: Mario Dion 
  To: [log in to unmask] 
  Sent: Wednesday, September 01, 1999 10:18 PM
  Subject: [TN] BGA site preparation after parts removal.


  Bonjour Everybody,
   
  I am actually facing a problem when I am performing BGA repairs on boards. The problem is the following:
   
    a.. The solder mask surounding the BGA pads is removed while we are removing the solder residues after parts was taken off it's location. The way we are preparing the site is the following: We are depositing paste soldering flux over the area to rework and then with the use of a soldering iron and a desoldering braid we remove the solder. We are trying not to scratch the board with the desoldering braid and keep the iron tip temperature at the minimum workable temperature. After the completion of that cycle we are cleaning gently the board to remove excess of burnt flux. Is there any other known good method about how to prepare the BGA site before installing a BGA at the same site again ? (Note: the one using Bismuth alloy is not aceptable for us...)
  I am waiting anxiously for an answer so please contact me about this topic. Thanks !
   
  Best Regards
   
  Mario Dion