Hi Peter and TechNet Land! I can add a bit of science to Doug's reply which
may help clear up the picture a bit. One of the lessons we have learned the
hard way (e.g. Doug's reference to the Great PostCoat Massacre) is that
under standard/normal/typical conformal coating conditions/processing,
conformal coating neither adds to nor statistically improves the solder
joint thermal cycle fatigue life. However, the Z dimension CTE
characteristics of a conformal coating can be significantly different than
the X and Y dimension CTE characteristics. If you take an acrylic conformal
coating and intentionally force the material to underfill the BGA you will
have the conformal coating literally "push" the BGA away from the pwa. In
addition to this mechanical issue, you also have Doug's issue of trapped
solvent under the BGA leading to corrosion problems. Hope this adds to the
discussion.

Dave Hillman (is it CTE or TCE?)
Rockwell Collins
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[log in to unmask]@ipc.org> on 05/21/2002 08:05:03 AM

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Subject:    Re: [TN] Conformal coating BGAs


Peter,
We have research papers here at Rockwell from 1990, stemming from an
incident we lovingly call "The Great Post Coat Massacre of 1990", which
show conclusively that conformal coating mascarading as an underfill is a
REALLY bad idea.  Unfortunately, I cannot release that paper to the public.
Since you cure from the outside in, underfilling a component with conformal
coating means you will have entrapped solvent in the centermost portion of
the component.  Bad karma there.  You do not get a uniform material that is
uniformly cured throughout the bulk, as you do with an underfill.

Doug Pauls
Rockwell Collins

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