Mail*Link¨ SMTP               [TECHNET] BGA DFM and thermal management

     A PCB design engineer asked me: What potential difficulties will he
     see by placing numerous components under a BGA? My answers are:

     - Affect BGA solder joint reliability due to less compliance. - Can
     cause insufficient support at repair if too dense.
     - The bottom component can affect x-ray image quality (especially
     Metal
     Cap.)
     - In case that no solder mask is required to plug the vias in design,
     then difficult to apply masking tape before wave soldering if needed

     However, due to density restrictions, he has to do it. I think the
     above are desired but not mandatory, is it right?

     He also asked: does having four power planes affect the thermal
     transfer at all? Any answers?

     Thanks,

     Yuan