I have taken the picture, blown it up, modified it to show the contrast a little better, and now it is easier to see where the soldermask makes contact almost at the beginning of the point where the crack starts on the left side. I am hoping Steve can post it. The rest of you will not see the attachment because Technet strips all attachments.
During thermal cycling the masking is continually pushing against the solderball joint at the base near the edge of the pad. The pressure is both inward and upward. That is what causes the stress that leads to formation of the crack in the coarsened crystalline structure of the solder joint, and also what causes it to continue (the z-axis prying force upward). It is actually an extrusion process.
At least that is what I think, based only on what I can see in the picture.
dean

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Chris Mahanna
Sent: Wednesday, August 06, 2014 1:49 PM
To: [log in to unmask]
Subject: [TN] SJ fracture signature

We see hundreds of cracked solder joints every year, but I can't say that I've ever seen one quite like this.  I have some suspicions, but I'll keep them to myself, as to not prejudice the crowd.
EDS maps are forthcoming.

This is a caught in-the-wild field failure of a 48-pin BGA Reballed to SnPb ENIG on board side Ni barrier on component side

http://stevezeva.homestead.com/45268-1.JPG

http://stevezeva.homestead.com/45268-2.JPG

http://stevezeva.homestead.com/45268-8.JPG

Thoughts?  And Thanks to Steve!


Chris Mahanna
President, Technical Manager
Robisan Laboratory Inc.


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