Completely agree with Dave. The combination of thermal stress and mechanical stress both from CTE mismatch between the BGA body and the PWB is a given.
But there are also many examples of additional stresses imparted from the soldermask.
They all work together to cause the failure, sometimes more so one way than the other. It depends on the design, the part, the board, the solder alloy, the.............

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of David Hillman
Sent: Wednesday, August 06, 2014 4:27 PM
To: [log in to unmask]
Subject: Re: [TN] SJ fracture signature

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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