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April 2005

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Fri, 15 Apr 2005 07:32:16 -0700
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Guenter,

I've never seen a fully satisfying explanation either, but let me give
it a try.  Black pad involves a P-rich layer of EN that can be ~ 28 - 30
wt% P versus the bulk EN composition of  7 - 9 wt% P.  If one looks at
the Ni-P phase diagram one sees a series of Ni-P compounds (from less P
to higher P ... Ni3P, Ni5P2, Ni12P5, Ni2P, Ni5P4, etc) at compositions
above 15 wt% P.  On an atomic basis or volume basis, the percentage of
compound in the Ni goes to 100% at 15 wt% P.  Note that phase diagrams
apply to chemical equilibrium so the actual conditions in the P-rich
layer will have to be left to someone with the right type of equipment
to study it (e.g. TEM/SAD, XRD, etc).  One noticeable feature of
black-pad-syndrome is a nodular and irregular Ni-Sn IMC layer just above
the P-rich layer, and why wouldn't it be?  When solder wets to Ni a
Ni3Sn4 IMC layer forms, but in this case solder isn't wetting Ni but
rather Ni-P compound.  Ni-P compounds are stable so this inhibits
formation of the Ni3Sn4 IMC layer.  There are many other factors that
may be in play such as imbalanced diffusion (Kirkendall voiding at the
Ni-P/Ni3Sn4 interface), crystallographic issues such as coherency of the
Ni-P/Ni3Sn4 interface, and concentrations of impurities such as O, C,
other at this interface).  The failures occur exactly at the Ni-P/Ni3Sn4
interface.

This is my somewhat fuzzy theoretical understanding of the black pad
phenomena as supported by ~ 8 years of microsection and surface SEM/EDS
observations.  I'm looking forward to the day when someone writes it all
down with detailed scientific data to support the final determination.

Best regards,
Ed Hare, Ph.D.
VP SEM Lab, Inc.

---------------------------------------------------

               SEM Lab, Inc.
Scanning Electron Microscopy and Failure Analysis
               Snohomish, WA
           Ph. (425)335-4400
           Fax (603) 658-4519
           http://www.semlab.com
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Guenter Grossmann wrote:

>Ed
>
>I would really like to know the mechanism and reason leading to Black
>Pad. I mean not just how often one has to change the bath or replenish
>it or the movement of the sauces in the plating bath etc. Does someone
>have a clue what happens in the metallurgy and why?
>
>Best regards
>Guenter
>
>
>
>EMPA
>Swiss Federal Laboratories  for Materials Testing and Research
>Centre for Reliability
>Guenter Grossmann,  Senior Engineer
>
>8600 Duebendorf
>Switzerland
>
>Phone: xx41 1 823 4279
>Fax :     xx41 1 823 4054
>mail:     [log in to unmask]
>
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