(1) 45 degree inclined crack with grain boundary sliding - some  
kindly of cyclic stress due to possibly thermal expansion difference  
between the module and board.  (either a large chip, or multi-chip  
stack on module side... compare to the board of cte of 17ppm or so  
for example).  The module must be very rigid or very flexable... -  
look like rigid would be apply... just a guess.
(2) significant ppt of high melting phase prior to eutectic  
formation, the solder ball material look  like is off eutectic Pb/Sn  
with reballing cooling at low rate. - weak structually speaking.
(3) the operating temperature and environment may got something to do  
with it.  - no info provided based on the email below.
just a guess.
        jk
On Aug 6, 2014, at 2:48 PM, Chris Mahanna wrote:

> 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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