Hi Ingemar,
The corner epoxying serves not so much a mechanical fixation in the 
x/y-directions, but a way of preventing the solder balls from collapsing, and thus 
improving thermal cycling creep-fatigue reliability by (1) increasing h [with N  
h**2.2], and (2) improving the solder joint shape from squashed collapsed 
spherical to near-hour-glass. I also rather doubt that the 4 epoxy corners would 
be anywhere near strong enough to prevent the warping of superBGAs that 
thermally asymmetrically designed. 


Regards,
Werner Engelmaier
可靠性先生
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 386-437-8747, Cell: 386-316-5904
E-mail: [log in to unmask], Website: www.engelmaier.com



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