Hi Joseph - take a look at the following paper:

"Intermetallic Compound Growth on Ni, Au/Ni, and Pd/Ni Substrates with
Sn/Pb, Sn/Ag,, and  Sn Solders", Blair/Pan/Nicolson, 1998 Electronic
Components and Technology Conference Proceedings, paper session 7, paper
#2.

It contains quite a bit of information that may be helpful to you. Good
Luck.

Dave Hillman
Rockwell Collins
[log in to unmask]




Joseph Furrer <[log in to unmask]> on 07/31/98 02:08:45 AM

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

To:   [log in to unmask]
cc:
Subject:  [TN] Intermetallic Phase: BGA soldering on Immersion Nickel Gold
      PCB




Dear TechNet forum member
Does any one has experience with the Intermetallic layer build between
Immersion Gold/Nickel and Tin Lead eutectic ? The Nickel (4-6 microns)
applied on the PCB surface contains 6-8% Phosphorus. The Goldflash is
0.08 microns thick.
We observed a 0.5 to 2 micron thick intermetallic layer between Nickel
and the Tin Lead Ball after Reflow soldering . It seems not to be the
well known Ni3Sn4 phase, because it is to thick and irregulary. Gold
could not be detected as well (SEM-Analysis). The reliability of the BGA
joints seems to be in a straight correlation to this brittle
intermetallic layer.


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