Yes, 
Two different companies called me in the past few weeks where they were
seeing an increase in MLCCC chip caps cracking. The first one had the
problem with a new design, but they did not associate it with the fact
that the process had changed simultaneously to a Pb-free process. The
second company, they simply changed an existing power supply design over
to a Pb-free process.
They thought it was a fault of their board layout, but I explained to
them that with the hotter reflow temperatures there is a longer cooldown
from solidus to room temperature. Once at solidus the caps are locked
in. But as the board continues to cool it imparts a major stress. Being
that ceramic caps often have a much different CTE than the new pwb
materials, they crack. The cracks are sometimes visible under 100X
magnification, but are present immediately after reflow. They may fail
electrical test immediately, or they will fail later.
In both cases the same exact lots of capacitors did not exhibit this
problem with standard 63/37 reflow profiles on other pwbs that (I
suspect) are more forgiving. I had not thought of the lesser elasticity
in the SAC alloy until you mentioned it, but all of these things work
together.
 
The more I see of the reliability issues related to Pb-free, the more it
is impressed upon me that all of these failures are seldom the result of
a single root cause, but rather a combination of different factors.
 
So, another fallout from RoHS.

________________________________

From: [log in to unmask] [mailto:[log in to unmask]] 
Sent: Thursday, October 19, 2006 8:58 AM
To: Stadem, Richard D.; [log in to unmask]
Subject: Re: [TN] Tombstoning and nitrogen


Hi Richard,
You are absolutely correct. Large fillets also cause chip cracking,
particularly with the stiffer LF-solders.

Werner


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