Doug,

Your response to Graham's question has got me thinking about some boards we
will be building soon. These boards have surface mount actives on both
sides. On a sample board I have seen, a few of the components on the bottom
side have glue dots since apparently solder surface tension alone will not
keep these particular parts on during reflow of the top side. The glue
looks
like regular SMT adhesive and I am sure it was cured when the board went
into the oven for bottom side reflow. I think this is fairly common
practice
when glue is required.

*I don't know that I would say it is a common practice.  I'm sure it is
done, but most places that I interacted with had a separate glue cure
profile prior to any reflow operations.

How serious (or common) is the problem you describe
and should I be thinking of a different way of processing these boards?

*How serious?  It was catastrophic in the cases I examined.  If you go to
the Circuits Assembly archives, you can look for the two articles I wrote
in the Process Rx column:  "Microcanyons" and "Become One With The Glue".
They give more details on the problem.  And yes, there was a fair amount of
late night liquid engineering in the selection of those column titles.  In
short, the manufacturer tried to cure the glue at twice the manufacturers
recommended rate, leading to the skinning effect and the entrapment of
volatiles.  When the glue was cured at the recommended rate, surprise, the
problem went away.  This is why I don't recommend doing glue cure and paste
reflow  (or wave solder) at the same time.  Bad karma.

Do you have a specific glue recommendation?

*I have always been a fan of the Loctite 3609, but that is more from a
cleanliness perspective (very low chloride content), than any other reason.

Doug Pauls

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