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

Thanks.


Bob


Robert Barr
Manufacturing Engineering
Formation, Inc.
Voice: 856-234-5020 x3035
Fax: 856-234-6679
email: [log in to unmask]

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Douglas Pauls
Sent: Friday, February 23, 2001 2:19 PM
To: [log in to unmask]
Subject: Re: [TN] SMT adhesive


Good day TechNet!
I'm looking for recommendations today.  I need an adhesive that I can dot
dispense on circuit boards (Dispense head on a GSM-1) and cure in the same
oven profile as a solder paste reflow profile.  I've got a couple of parts
that need a bit of extra grip on the board (for vibration resistance).  The
parts are SMT paste attached, so the glue will have to cure at the same
time as the paste reflows.

*Graham, I recommend that you NOT take this approach.  At CSL, we saw a
number of cases where this practice caused metal migration and other
assorted failures under SMT components.  The problem lies in the fact that
at one point you have a semi-cured plastic material (the adhesive) and hot
flux residues from the paste.  They can combine to form a non-removable
residue or undesirable changes in topology of the adhesive structure.  In
addition, a solder reflow profile may be much more aggressive than those
recommended by the adhesive manufacturer for adhesive cure.  If you cure
the adhesive too fast, you can get a skinning effect on the adhesive,
entrapping volatiles that then blast their way out other places, leaving
voids to suck in flux residues.  For whichever adhesive you pick, look at
the recommended cure profile and compare to your reflow profile.

Doug Pauls
Rockwell Collins

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