Au is so thin with ENIG, so if it did not dissolve into the solder
(incredibly rapid dissolution) it suggests an organic film on top of the Au.
This is not uncommon depending upon the storage environment and materials in
contact with the Au surface or off-gassing near the surface.  Solderability
is worse with the very high P content e'less Ni.  EDX spectra from the top
surface provides significant signal from a micron or two below the surface
(function of SEM e-beam accelerating voltage).  The P content can change
during deposition if the bath gets out of control.  So it may be possible
that the P content on the surface immediately under the Au is higher in P
than in the Ni through the entire sampling depth generating x-rays in the
SEM.  But even in this case one would expect the Au to dissolve and not be
present after dewetting.  Perhaps good wetting never occurred due to a
solder problem or reflow profile problem.


Best regards,
Leo

Leo M. Higgins III, Ph.D.
Director of Applications Engineering
ASAT, Inc.
3755 Capital of Texas Highway, Suite 100
Austin, Texas     78704

ph     512-383-4593
fx      512-383-1590
[log in to unmask]
www.asat.com


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-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Eddie Rocha
Sent: Thursday, June 09, 2005 10:45 AM
To: [log in to unmask]
Subject: [TN] ENIG solderability


Does anybody know what can be done to a properly plated ENIG
board to degrade the solderability.

i.e. Storage conditions? can I affect solderability by storing the
boards in a particular environment? If so, what are those conditions?

Improper cleaning before assembly? Can this degrade solderability?
what conditions?

Improper baking?

Improper handling?

I had some solderability issues with some ENIG boards and then
had them analyzed outside (EDX and SEM). The lab found nothing
wrong with the plating. Good phosphorous levels and no nickel
corrosion. Because there was still gold present on some pads after
reflow, that led them to believe that some barrier must be present on
the gold surface, preventing the solder from contacting the gold
surface.

Does anybody have any ideas on what could be wrong?

-- Thanks,
Eddie Rocha
South Bay Circuits
(480) 940-3125 x122

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