Ed,
When you say, "surface of the solder joint," do you mean a surface adjacent
to the original gold/tin location or had the IMC moved away to another
location?
Syed.
-----Original Message-----
From: SEM Lab, Inc. [SMTP:[log in to unmask]]
Sent: Thursday, June 25, 1998 12:27 PM
To: [log in to unmask]
Subject: Re: [TN] Gold Coated Solder
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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