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June 1998

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From:
David D Hillman <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 23 Jun 1998 14:39:44 -0500
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Hi Bill - Please email me the photos but I have a couple of questions:

1) If gold is truely on the solder joint surface then it had to come from
somewhere - what surfaces are in the vicinity of the diode during soldering
that are gold coated?

2) When does this "gold" surface coating appear? After solder processes
(reflow) or after ???????

Sorry for the obvious questions but having gold on the surface of a solder
joint as the result of a reflow process is pretty tough to accomplish. Gold
has a very fast diffusion rate in solder so if the solder joint had a gold
coating to start with or was in contact with a gold finish then the gold
would go directly into the solder joint itself rather than "deposit" on the
solder joint surface. Getting gold (or other metals) to migrate onto a
solder joint surface would take an electrochemical reaction - some
reasonable temperature(s) (but not molten!)  coupled with moisture (e.g.
humidity). You gave one clue to what you might have - the use of a low
residue material in a no-clean process mode. It has been documented  that
flux residues can give a solder joint a "golden" appearance (many of the
old RMA fluxes if not removed properly could give this appearance). I
recommend getting a scanning electron microscopy xray analysis of the
solder joint - it will tell you definitavely if you truely have a "gold"
coating or if you have an flux residue coating. Good Luck.

Dave Hillman
Rockwell Collins
[log in to unmask]





Bill Fabry <[log in to unmask]> on 06/23/98 02:00:16 PM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
      to [log in to unmask]

To:   [log in to unmask]
cc:    (bcc: David D Hillman/CedarRapids/Collins/Rockwell)
Subject:  [TN] Gold Coated Solder




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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