Following from the recent discussion regarding the intermetallic growth of
Ni/Au and consequent soldering, the main intermetallic formation is that of Ni3
Sn4. The Ni layer acts as a diffusion barrier between the Cu and Sn. Sn will
diffuse relatively easily into Cu but the Ni barrier greatly inhibits the
formation of the intermetallics Cu3 Sn and Cu6 Sn5. The presence of a Ni layer
will therefore extend the 'lifetime' of the solder joint.

Martin Farrell (Chemist)
GEC-Marconi Avionics,
Edinburgh.