Hi team - I agree with George/Richard - looks like a standard solder joint
fracture, more contribution from mechanical stress than thermal stress. The
solder joint microstructure exhibits "coarsening" where enough energy has
allowed the lead phases and tin phases of the solder microstructure to
reorganize into "bands or regions" of tin and lead. The solder joint crack
tends to run along the tin phase grain boundary's. If there was a large or
long duration thermal cycle input then the microstructure coarsening would
be much more evident. If the microstructure coarsening is more subdued then
the root cause tends to be more mechanical stress root caused (which is
what I see in the pictures). Note that sometimes both thermal and
mechanical stresses contribute making a detailed knowledge of the product
use environment critical for root cause resolution. The only thing I don't
quite agree with from the postings is the importance of the soldermask
interference point. Solder isn't a stress concentrator sensitive material
but the maximum shear zone on a BGA solder joint is typically at the
pad/solder interfaces where the soldermask and solder joint reside. Richard
and I are on the same page but with slightly different explanations. Bottom
line is that the failure is a fatigue type response and not a brittle shock
response. Great cross-sections and photos! So Chris, can you now fill in
the "rest of the story" for us to some extend?

Dave Hillman
Rockwell Collins
[log in to unmask]


On Wed, Aug 6, 2014 at 1:48 PM, Chris Mahanna <[log in to unmask]> 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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