Wetting and spreading of liquid metals: the rôle of surface composition

Quantitative measurements of the surface compositional changes which occur during the wetting of copper substrates in vacuum by liquid SnPb solder alloys have been made in situ, by Auger electron spectroscopy. Five different spatial regions associated with the wetting front have been identified. Away from the spreading front, the liquid solder shows a surface enhancement of lead. Approaching the front, adjacent bands of Cu3Sn intermetallic compound, and tin‐depleted liquid occur, both of width ∼ 20 μm. The lead‐enriched surface of the solder extends over these two regions, and out over the copper to form approximately 50% monolayer ‘halo’ of ∼ 80 μm width. Beyond the halo, in the fifth region, very small amounts of tin and lead were detected. The lead halo forms rapidly by surface diffusion, and precedes wetting. In the absence of other limitations, it is the reduction of the copper substrate surface energy by the lead halo that restricts the spread of the molten alloy. Intermetallic compound formation is relatively slow and only takes place once spreading has ceased and a quasiequilibrium reached.