Hi Wee Mei,
You sure do not want the package almost flat to the board after reflow. while
there is nothing in the assembly acceptable criteria, with the exception of a
possible minimum clearance for cleaning purposes, smaller solder joint stand
-off heights result in reduced reliability, everything else being equal. That
does not mean that it results necessarily in inadquate reliability--thtat will
depend on the detail, just reduced reliability. The details can be plugged
into the Design-for Reliability procedure in IPC-D-270 to determine whether or
not you have adequate reliability. In any case, the solder joint stand -off
height is one of the critical parameters for solder joint reliability. That is
why some people use high-melt solder balls on BGA corners to prevent collapse.

Regards,
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 386-437-8747, Fax: 386-437-8737, Cell: 386-316-5904
E-mail: [log in to unmask], Website: www.engelmaier.com

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