All -

I have a situation where we are experiencing micro-voids in solder joints after SMT reflow.  Problem manifest itself primarily in BGA to solder ball interface ( at the device junction interface and not the PWB to solder ball interface) confirmed with cross sectioning.  BGA solder joints are weak resulting in electrical failures and removal with minimal mechanical stress.

We are using a Kester 256 HA (62-36-2) solder paste on a electrolytic PWB finish.  The same board with the same process using ENiG does not exhibit this problem.

Ionic, XRF and SIMS analysis does not reveal any surface or interlayer organic contamination.  I have seen old publications indicating this phenomena has been observed before with hard gold chemistries.

DOEs conducted by varying reflow dwell times / surface cleaning / pre-bake does not remedy the situation

Any thoughts or experience with salvaging built CCAs or reclaiming bare boards to eliminate this issue?



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