Doug,

Thank you for your input and the pictures. Just to clarify, conformal
coating was never intended to be applied AS underfill. The NASA paper
suggested EITHER underfill or conformal coating.

In your experiment did you apply conformal coating before reflow? Or was
the coating applied after reflow and injected  under BGAs. Are you
saying if you need to conformal coat a board, you first need underfill
material to prevent the conformal coating to damage the BGA?

Leif Erik Laerum
Quality Assurance Manager
Texas Memory Systems
[log in to unmask]
Tel: (713) 266-3200 x468
www.texmemsys.com


On 7/8/2010 7:01 AM, [log in to unmask] wrote:
> 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]>
> Sent by: TechNet<[log in to unmask]>
> 07/07/2010 05:11 PM
> Please respond to
> TechNet E-Mail Forum<[log in to unmask]>; Please respond to
> Leif Laerum<[log in to unmask]>
>
>
> To
> [log in to unmask]
> cc
>
> Subject
> [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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