I have seen this.  I published two short columns on the phenomena in
Circuits Assembly Magazine a few years back.  In short, the problem had
occurred when the assembler decided to save time and use the reflow profile
to cure the adhesive.  Most adhesives desire a 2-4C/min maximum ramp rate.
Reflow profiles are more on the order of 10C/min.  The adhesive skinned
over, the entrapped volatiles inside the adhesive had to come out somewhere
and they blew channels in the adhesive.  The water soluble flux from the
solder paste wicked into these "microcanyons" and could not be removed.
Electrical leakage, corrosion, and metal migration occurred.  When the
assembler cured the glue first at the proper ramp rate, the problem
disappeared.  You can always make something faster and cheaper if quality
is no big concern.

Doug Pauls




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             10/20/2004 09:26                                      Subject
             PM                        Re: [TN] Solderball Trapped in Flux
                                       Residues

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                Afri Singh
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Hi all
I was wonder if anyone had come across a situation where the adhesive
under the surface mount components had flux residue trapped which was
than attacking the soldermask and copper underneath.
Also saw some very small solderballs.

Any idea what would be the cause and how to avoid this ?
Thanking you

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