Peter, 

I do not think corrosion is as big an issue as crack propagation. You
say that you see <5% failure due to fully fractured joints but due you
know what percentage of joints have cracks and how severe the cracks
are? 

There is no doubt that the cracks will propagate over time but the
acceleration of the crack propagation needs to be determined based on
your low stress environment. I would imagine that the cracks are related
to your high strain component selection. Can you share any information
on the component and PCB material? 

Also, you mentioned the Hi-P ENIG but I am pretty sure the standard ENIG
chemistry is considered Medium P such as the Atotech's Aurotech ENIG
chemistry used on PCB's for a final finish. You also mentioned that the
P is <10% so that is in line with the medium phosphorus system. 

You mentioned the cross-section SEM and EDX testing showed very good
intermetallic formation. How thick was the average intermetallic of the
crack joints and were the cracks actually in the intermetallic or in the
bulk solder? Can you share the results of the intermetallic EDX?

What was your finish specification - specified thickness of electroless
nickel and specified thickness of immersion gold? Were these thicknesses
verified on an unsoldered pad in your analysis or by the manufacture? 

You mentioned SAC alloy - what type of SAC was used? Was the assembly
profiled properly using thermal couples and not just on a solder sample
but on a loaded board? If so, what was the thermal profile at the site
where solder joint defects where found?

As for a reliability projection, that's a tough one. You would need to
perform a detail analysis to build a good acceleration model to predict
long term reliability for your specific environment.

Thanks
Tony Cosentino
Tekelec Hardware Engineering


-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of Pete Houwen
Sent: Wednesday, January 30, 2008 11:52 AM
To: [log in to unmask]
Subject: [LF] Reliability projection for BGA interfacial cracking?

We are experiencing the common mechanical stress induced BGA corner
cracking in our assembly plant.  I have identified a number of potential
corroctive actions, but I can't realistically expect the problem to be
eliminated.  What I am being asked to provide in order to allow the
product to be released is reliability projections for the BGAs.

We have experienced <5% failure rate due to fully fractured joints.
What we can't determine is how many partially fractured joints are being
created, and what may progress in these joints in use.

The product will not see severe environmental conditions, as they are
used in indoor retail locations.  Do I need to be worried about
potential corrosion in partially fractured joints?  Should I be
concerned about partial fractures continuing to grow until they create
an open joint?

Cross section SEM and EDX testing shows that the joints (Hi-P ENiG and
SAC) are very good, IMCs are formed, <10% P.

I'm not finding too many studies on long term reliabliity assessment,
and I don't know that I will be able to convince anyone to do regular
cross sectional analysis.  But they're going to want my yea/nay on
continuing production.

Thanks,

Pete

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