Pete, on your next lot of PWBs, perhaps you can ask the vendor to
provide you with a small quantity of boards with an alternate finish
such as IAg. See if you have the same issues with that lot. Use it as a
qualifying lot. If the fractures show up in that lot, you can be pretty
sure you are dealing with stress issues unrelated to the ENIG finish. If
they don't, you can assume the finish is causative. This is not a
black/white issue, because most of the time ENIG issues such as BN and
BP are also dependent on stress to bring them about. But it is often a
pretty good indicator.

-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of Pete Houwen
Sent: Wednesday, January 30, 2008 10:52 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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