Nickel Tungsten Compound in CCD die attachment

[tinchi]

I am doing an analysis on mobile phone camera. The camera structure is that a CCD is attached on a silicon substrate while the Si substrate is attached to the Aluminum Oxide cage with the layers of Epoxy-silicon resin, Gold, Nickel, Nickel 4 Tungsten compound and Tungsten as shown in the attachment.

I want to know why Ni4W compound are used here and I am looking for some references on these attachment technology.