Hi, hope all is well.

Here is a direct quote from the article I referenced in an earlier post.

"The immersion gold (IG) process provides a direct replacement of nickel
surface atoms with gold atoms. The process is self limiting (diffusion is
limited by nickel to the liquid interface) and provides a highly porous free
protective barrier to passivation. Thus the IG process can be considered
(loosely) as a controlled corrosion process.

"There have been many investigations in recent years into the wettability of
EN/IG surface finishes, also into the reliability, and failure modes of the
EN/IG solder joints. The conclusions of these reports have, in general, had
confounding results and no root cause for predominate failure mode(s) could
be defined. The investigations did have one point in common in that they all
reported a condition called "black pad". This condition was first reported
by Puttlitz as defective joints that exhibit a dark gray-to-black appearance
that would only partially wet.

"This paper describes, in detail, the microstructure of the black pad defect
and identifies as its root cause failure mode. The black pad defect was
found to be due to a hyperactive "corrosive" IG process that changes the
near surface microstructure of P-Ni into one with a marginal to a
non-wetting state. The black pad defect shall be classified in terms of
hyperactive corrosive activity. Furthermore, evidence shall be presented
which shows that the black pad defect does not occur in a random of sporadic
manner. Scanning electron microscopy/energy dispersive spectroscopy
(SEM/EDS), Auger spectroscopy and focus ion beam microbe (FIBM) are used to
gain an understanding of this root cause effect."

:Later in the article

"It has been shown that the black pad defect is due to a hyper corrosive
activity of the immersion gold process near the nickel surface. Eight levels
of the defect activity have been identified. The lower activity levels do
not present solder joint reliability issues. The highest activity level,
black pad, creates an enriched phosphorous layer called black band. It has
been shown that black band phosphorous enrichment is distinguishable from a
thin layer of natural phosphorous enrichment created during Ni/Sn
intermettalic formation."

In the article there are many pictures of their results. They also managed
to induce the phenomenon. Though they don't offer any solutions to prevent
this from happening, I recommend anyone having this problem to get a copy of
the article. It is on page 133 of "Future Circuits International", issue 5.

Robert Peterson
mailto:[log in to unmask]

Alternate Final Finishes, Inc.
166 Middlesex St.
N. Chelmsford MA 01863
Phone (978) 251-7429     Fax (978) 251-7219
http://www.AlternateFinalFinishes.com


-----Original Message-----
From: Richard Tilbrook [mailto:[log in to unmask]]
Sent: Wednesday, October 13, 1999 3:57 AM
To: [log in to unmask]
Subject: [TN] Black Plague


Forgive my ignorance, please define BLACK PLAGUE.

Is it something I should be looking out for?

Is it something we've already got and are suffering from???!!!

Richard Tilbrook
British Aerospace

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