Wetting of low melting point alloys on metal substrates

The spreading drop technique was used to experimentally evaluate the spreading performance of Sn(L) on Au, Cu, Bi and Au coated Cu substrates over a temperature range of 250 < T < 427°C. The experiments were performed on an isothermal hot stage in an environmental chamber containing a mixture of formic acid and nitrogen gases. The spreading events were recorded using two orthogonally mounted digital cameras, which simultaneously acquired top and side views of the spreading. The dynamic contact line radius and dynamic contact angle were then measured using digital image processing techniques. In addition, selected substrates were removed from the hot stage and quenched at prescribed times. The cross-sections of these samples were examined using scanning electron microscopy and optical metallurgical techniques to determine the extent of dissolution and intermetallic formation. Compositional information was obtained using wavelength dispersive spectrometry.; Spreading in the Bi-Sn system, which forms no intermetallics but has high solubility (Bi in Sn(L)) was compared with the Au-Sn and Cu-Sn systems, both of which can form intermetallics. Generally, the spreading was found to be dependent upon many factors, including temperature, substrate solubility and the formation of reaction products (intermetallic compounds). Rapid spreading, substantial intermetallic formation and re-liquefication of the intermetallic was observed in the Au-Sn system for temperatures where the δ phase (AuSn) was in equilibrium with Au rich liquid (289 $\lesssim$ T $\lesssim$ 412°C). At lower temperatures (250°C), Sn spreading on Bi was greater than on either Au or Cu. Spreading Sn(L) on Au coated Cu exhibited linear R4 (radius) versus τ (time) behavior for 1, 0.4 and 0.2 μm Au thicknesses over the 250 $\lesssim$ T $\lesssim$ 338°C temperature range. Fractal contact line morphology was observed for Sn spreading on Au coated Cu over the temperature range 368 $\lesssim$ T $\lesssim$ 407°C. The fractal spreading temperature regime overlays the rapid spreading regime temperature range observed in the Au-Sn system.