Nalin Jain asked whether it is necessary to bake an assembled board 
     before PBGA rework, and if yes what the recommended temperature & 
     humidity condition would be.
     
     The risk of heating a board assembly enough to melt solder is that 
     water dissolved in the resin will be evolved so rapidly as a vapor 
     that it creates voids, a condition which is referred to as measling.
     
     Measling can be thought of as a process indicator.  It has not been 
     shown to reduce reliability.  The amount of baking required to prevent 
     measling depends on how much water is dissolved in the resin, and on 
     how resistant the fiber-reinforced resin is to forming the measles.  
     For a particular board, neither of these can be known with much 
     precision, beyond the obvious: 
     
     1. The higher the relative humidity and the longer the assembly has 
     been exposed to it, the more likely will be measling when it is heated 
     enough to melt solder.
     
     2. The higher the bake temperature that the assembly can tolerate, the 
     less time is required to get enough water molecules to come out of the 
     resin peacefully (i.e., diffuse to a surface) so that soldering won't 
     cause the remaining molecules to create measles.
     
     One can set a limit on the baking conditions needed: Saturate board 
     test specimens with water (say by letting them sit in boiling water 
     for a day) and then experiment to find out how much baking is required 
     to prevent their measling under your soldering conditions.
     
     Gordon Davy

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