At Hitachi, we are considering moving to BGA technology during our 
     next model design phase.  We are not very familiar with the 
     manufacturing process for this technology.  We have come up with a 
     question we have not yet been able to resolve (as follows).
     
     Solder joints on QFP's resist cracking due to thermal expansion 
     because the leads have some give due to a "spring" effect.  With BGA's 
     there is no give between the board and the part except for the solder 
     joints.  If there is a difference between the board's coefficient of 
     expansion and the part, the solder joint will crack.
     
     1. Has anyone experienced this problem?  How do you resolve it?
     
     2. We are considering some sort of sealant material to protect the     
        solder joints and provide some thermal relief.  Does anyone have    
        any good ideas for materials?
     
     Anyone who can do a data dump on this for me will be greatly 
     appreciated! Thanks in advance,
     
     Roger Held
     Hitachi Computer Products
     Norman, OK
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