Janice,
I had occasion to investigate failures of BGA joints recently and found that
they failed at the ball/package interface.
I saw no surface markings like 'stars' on the original BGA.s although the
surfaces exhibited the usual dendritic/grain effects.
What I did find was that subsequent wave soldering of the underside of the
board raised the temperatures under the BGA.s to the liquidus temperature.
At this temperature the solder is either just melted or very weak. Any
flexing of the board would then cause cracking which would not self heal.
I cured it by supporting the board on all four sides and applying peelable
solder mask to the underside of the BGA vias. A bit OTT but it worked.
Hope this helps
Regards
Eric Dawson

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