While we are on the subject of heat sinks, I have a question for TechNet
regarding the effect of thermal interface materials on solder joint
reliability:

When you hold a heat sink down on top of a BGA with a compliant thermal
interface pad between them, you need to maintain a certain amount of
pressure (say, 10-20 psi?) on the interface pad to maintain good thermal
contact.  If the heat sink is attached to the circuit board, the force from
the interface pad is transferred through the solder balls, and since the
cross sectional area of the solder is maybe 1/10th that of the top of the
part, the pressure in the solder is multiplied by some factor of ~10. This
would lead to a static compressive loading in the solder of 100-200 psi.
My limited knowledge of solder creep mechanisms has led me to believe that
this level of static loading would lead to creep in the solder and
presumably impact the solder joint reliability.

Some preliminary testing that we have done shows that the interface
materials do creep and the contact pressure goes down over time, so maybe
the interface pressure eventually gets small enough that it doesn't  have a
big effect on reliability (but that would probably mean that the thermal
peformance would also go down, wouldn' t it??).  So far I haven't been able
to get a clear answer from any of the interface material suppliers that I
have asked about this on how their materials could affect component
reliability.   In a fairly quick literature review I was also unable to
come up with any papers that directly addressed this issue.

I did run a limited study to try to see if a heat sink with thermal
interface material could affect the solder joint reliability and the
preliminary raw data seem to indicate that it did.  We need to finish
de-bug and cross sectioning to figure out what was really happening before
I would want to say too much more about that testing though.

Does anyone have a recommendation on how much static loading you can place
on a eutectic solder joint before you start to significantly affect the
thermal cycle reliability?  Is anyone familiar with any testing on the
effect of static loading due to thermal interface materials on BGA solder
joint reliability?

thanks,

ross

===================================
Ross Wilcoxon
Senior Mechanical Engineer
Advanced Technology Center
Rockwell Collins, Inc.
400 Collins Road NE, M/S 108-101
Cedar Rapids, IA 52498
319-295-7139, fax: 319-295-3751

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