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.