Surface tension driven molten metal flow over flat and/or grooved reactive surfaces during brazing and soldering

State-of-the-art brazing and soldering processes depend on favorable spreading conditions of molten metal over a reactive surface. Depending on the surface state, molten filler under specified processing conditions flows over a flat interface controlled by capillary, viscous and gravity forces. This flow is characterized by a relatively slow front movement. In contrast, single or multiple surface grooves, ranging from micro to mezzo in size, can significantly enhance this capillary flow over macro domain distances prior to joint formation. Furthermore, transport of a filler metal through the grooves may facilitate a delivery of a molten metal to a remote location (that is, away from the location where it is melted). This experimental study is devoted to a phenomenological investigation of molten metal flow over both flat surfaces and through the micro grooves for: (1) Al alloy systems over an Al substrate, and (2) Sn - Pb systems over a Cu substrate. Associated aluminum brazing and cooper soldering were performed under tightly controlled process conditions. Real time monitoring of molten metal flow is performed using either the hot stage microscopy or a brazing furnace with a transparent hot zone. Empirical data were corroborated with modeling of spreading using: (1) a modified Tanner's law correlation devised from the lubrication theory based on Navier-Stokes equations, and (2) a theory based on a modified capillary flow model of Washburn, advanced by Yost et al.