Deat "Techies":

I have a "GOLDEN" opportunity for the experts to shine.  I have in my possession
some SMT assemblies built on gold flash fabs that exhibit gold residing on the
surface of the fillet, rather than being dissolved in the solder.  The fillet is
fully formed with no graininess or other visual anomilies.  The phenomenon
occurs on ONE end of a MELF diode, not on both ends.  To answer the obvious
question, it is NOT dependant on the diode polarity; it happens on either end
but on one end only.

I have digitally captured a couple of photos of the phenomenon.  If you need
pictorial representation, please e-mail me directly and I can attach it to a
return e-mail.

As you will see,  the gold remains on the surface of the fillet, rather than
being dissolved in the fillet.   A slight scraping of the fillet surface will
uncover the base solder with NO visual evidence of gold inside.  Other
terminations on other components do NOT exhibit this phenomenon.  The assembly
subcontractor is using standard NO-CLEAN solder paste (OMG Americas 500ADV
NO-CLEAN 63SN). Unsoldered parts show a tin/lead termination, indicating that
the gold has possibly migrated over the entire termination.

My question:  What mechanism is at work to keep the gold from totally dissolving
into the Sn/Pb paste after it leaves the fab during reflow?

Thanks for the help.

Bill Fabry, Sr. Manufacturing Engineer
Plantronics, Inc.
345 Encinal Street
Santa Cruz, Ca. 95060
(408) 458-7555

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