I wanted to close off this topic and to thank all those who provided some
very insightful input into our problem.  We chose to tackle this problem
from two angles.  Assemblies were sent to Sentec in Toronto for microsection
and SEM analysis and we also contacted the subcontract fabricator of the
PCB's to see if there were any processing variances that would be of
concern.  Both avenues of inquiry rapidly arrived at the same conclusion - a
problem with the nickel layer.  The initial microsections showed cracks in
the joints on assemblies that were not shock tested.

Thanks again,

John Parsons

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Parsons
Sent: Friday, November 19, 2010 3:32 PM
To: [log in to unmask]
Subject: [TN] BGA Falling off PCA

I am not seeing a lot of the usual Friday quiz traffic so I know you
guys-n-gals are just itching for some real work :o).

We are a PCB fabricator and one of our customers has returned some
assemblies to us for feedback.  What they noticed when someone accidentally
dropped an assembly is that the BGA (352 pin Cirrus Logic EP9312-CBZ) popped
off the board.  They then proceeded to drop a few more from a height of
about a foot with the same results.  All the other components on this
Pb-free assembly stay fast.  

We have received three PCA's and 5 BGA's.  On one of the BGA's a single ball
has remained on the PCB.  The rest of them all the balls have broken cleanly
from the PCB leaving a very flat (under x10 magnification) dull gray
surface.  I sure wish that I could take photo's but lighting is critical and
I don't think that I have the resources to get photo's that would do justice
to what the naked eye is seeing.

Also of note is that the BGA lands are mask defined with a 31mil circuit pad
and 26mil solder mask relief.

Does anyone have any ideas as to how we can determine the root cause of this
failure.  This part has been built for some time but previously the surface
finish on the PCB was a solder finish.  This is the first run of the boards
that has an ENIG finish.  This doesn't look to me like a raw board issue but
that is where everyone looks first isn't it ;o).

Regards

John Parsons


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