In response to Dave Hoover's request concerning pcb warpage during BGA 
     repair:
     
     Our analysis of BGA rework processes involved use of a 10"x12", 10 
     layer, .062" thick FR4 test board containing (12) PBGA 225 OMPAC 
     devices.  Several manufacturers' rework systems were evaluated, 
     allowing comparison of rework processes utilizing both highly focused 
     and wide area bottom side heating methods.  
     
     We also experienced the problem of severe local area warpage during 
     the reflow process, particularly where highly focused bottom heat was 
     used. In some instances, the board surface at the center of the BGA 
     dropped more than .015". 
     
     Our best success dealing with the local warpage problem was achieved 
     with the use of controlled wide area convective bottom side heating 
     combined with adequate board support.  The entire pc assembly was 
     bottom heated prior to and during reflow with a fairly gentle thermal 
     profile not exceeding 1.5 degrees C/sec. ramp rate. The profile's 
     thermal events were precisely controlled with thermocouple feedback 
     from the pcb, BGA, and top & bottom heat sources. The entire 
     preheat/reflow cycle averaged 6 minutes, followed by cool down at room 
     temperature.  This process eliminated any significant local warping, 
     but keep in mind it was a controlled test using a single side test pcb 
     with only BGA's attached.  
     
     Dave, I hope this info helps.  Sounds like your customer is headed in 
     the right direction.
     
     
     Dennis Measor
     Senior Manufacturing Engineer
     Allen-Bradley Co, Cleveland, Ohio
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     Ph: 216.487.6170