Good morning Leif,

Conformal coating as an underfill material is a really, Really, REALLY bad
idea.  The primary problem is the coefficient of thermal expansion.  In
thermal cycling, there is enough strength to pop BGAs right off a board.
Dave Hillman and I did a study a few years ago where we intentionally did
what you suggest.  I will post some pictures on the newfangled Technet
picture site of what happens.  The one part solvent based acrylic we used
on one set had a 73% failure rate.  The water-based acrylic we used did
fine, but it had a lower CTE value.  I would expect silicones to be much
worse, due to their CTE.

Second, you have the problem of solvent entrapment under parts.  That
solvent has to get out somehow.  If you wait to let it diffuse out slowly,
you have horrendously long process times. If you try to force it out, you
get bubbles and voids.  May be a little different if you are using a
2-part coating or a 100% solids coating for this second aspect.

Loctite has a pretty good line of underfill material and we use and have
evaluated several.  Which one you pick depends on whether you need your
assemblies reworkable or not.

Doug Pauls



Leif Laerum <[log in to unmask]>
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[TN] Underfill as a Remedy for Electro Migration?






Technetters,

We have identified electromigration as the root cause of some field
failures we have been seeing. During the troubleshooting we came across
the following NASA paper. All of the details of this paper fits our
situation perfectly. We use the same package as described.

http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/40854/1/08-23.pdf

The executive summary is that the silver in the epoxy used to attach as
die attach migrated to create a short. The migration was triggered by
humidity.

The paper suggests using underfill or conformal coating to remedy the
problem. Currently we do not use either in our process. I have some
questions regarding these:

- What are the pros and cons using either of these for this particular
purpose? IMO underfill seems the best solution for us.
- Does anyone have any experience as to whether this is a good solution? I
am not quite understanding the "chemistry" here.
- Does anyone have a recommendation as to what type of underfill would
work well for this application?

Thank you for your input.



--
Leif Erik Laerum



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