Bill,

I have recently seen a number of instances where gold-tin intermetallic compound (IMC) was on the
surface of solder joints but not in the bulk solder.  I believe the reason is that the density of
the solid IMC is less than the molten tin-lead and there the IMC floats to the surface prior to
solidification.

Ed Hare
--

               SEM Lab, Inc.
Scanning Electron Microscopy and Failure Analysis
               Snohomish, WA
               (425)335-4400
           http://www.sem-lab.com

Bill Fabry wrote:

> 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
>
> [log in to unmask]
>
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