RE>FEB: ceramic BGA cracking after wave...     6/13/97

Sheila,
In a past life, I worked on a problem where we had ceramic PGAs that cracked
after going through a wave soldering operation.   (I understand that your
problem is with BGAs, but I thought that I'd share this anyway!)  The
problems were traced to 3 main contributing factors:
1) Excessive cooling rate;
2) Design problem:  the ceramic that was beneath the die was only about 15
mils thick - cracks occurred in these areas readily;
3) Material incompatability:  an aluminum heat sink was attached to the
alumina ceramic package body.  There is a huge CTE mismatch between the two
materials, which result in large stresses being applied to the ceramic during
thermal excursions.  These stresses were easily transmitted by a very stiff
adhesive that held the heat sink in place.  

The third factor was found to be the easiest to address for our application. 
A more compliant adhesive was found to mitigate some of the CTE
mismatch-generated stresses, but thermal analysis found that the heat sinks
weren't actually needed.  They were removed and the problem went away.

Best regards,
Greg Bartlett
Mercury Computer Systems
Chelmsford, MA
[log in to unmask]
--------------------------------------
Date: 6/12/97 5:47 PM
To: Greg Bartlett
From: Jim Herard
I recommend you thermocouple the ceramic, and also have it checked for
moisture
content.  Something seems wrong.

I'd expect the solder joints to fail before the ceramic if its a thermal
expansion stress problem.

You could have a bad batch of ceramics.  Also, you may want to look at
columns
instead of balls.

Jim Herard
IBM Microelectronics
Quality Engineering

---------------------- Forwarded by Jim Herard/Endicott/IBM on 06-12-97 05:38
PM
 ---------------------------

        ssmith @ ael.com
        06-11-97 05:22 PM
Please respond to [log in to unmask] @ internet


To: TechNet @ ipc.org @ internet
cc:
Subject: FEB:  ceramic BGA cracking after wave solder

We are experiencing cracking of large (1.8" on a side) ceramic BGA cracking
(like in half) following wave solder.  The devices are on polyimide boards.
This is a new program -- any advice on things we might do better or
different ... bake the boards before we stuff them?  fixture boards during
wave solder? go back to our board supplier for resolution?  Thanks for any
help...

Sheila Smith, Tracor AES

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