Wetting in the Au–Sn System

Abstract The isothermal spreading of liquid Sn on Au substrates was studied over the temperature range of 250–430 °C using the sessile drop technique in a gaseous flux atmosphere. Digital video imaging was used to record the spreading kinetics, and rapid quenching and metallographic cross-sectioning was used to characterize dissolution and compound formation. Observations showed that three temperature/composition regimes gave rise to their own unique spreading behaviors. At low temperatures/Au concentrations, limited spreading was accompanied by nearly complete isothermal solidification. At intermediate temperatures/Au concentrations, rapid spreading and concurrent appearance of a transient intermetallic compound were observed. At higher temperatures/Au concentrations, extensive spreading was observed in the form of a thick film (∼20 μm) supplied by a central dissolution well whose L/V interface undergoes a global curvature reversal. We discuss the contact line mobility and other mechanisms that affect wetting and spreading behaviors: fluid flow, mass transport and metallurgical reaction.