Hi All, 
A) Does anyone use Sunstar reworkable underfill 991U (reworkable)
before?
http://www.eleshow.com/contents/sunster/rewunderfill/enindex.html

Recently, I used this material to underfill a dummy Pb-free BGA (1156
balls on ENIG) which showed a number of "open circuit" during ESS
testing. Post measurement showed that the "open circuit" solder joints
closed back, hence was not able to determine the cause of the problem
(Pending cross-section). 

Intend to use thermal shock (-45 to 85 oC) to "break" the joint
permanently, is it the right way?

Any suggestion what might be the cause?

Thank in advance...

Regards
GA Tan 

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