In general, wetting is the ability of a liquid to flow across a surface as opposed to sticking to itself. Wetting occurs when the attraction between liquid and surface is greater than the surface energy of the liquid, drawing a molecularly thin layer across itself. Wetting in soldering applies to molten solder spreading along the base metal/metallisation surfaces to produce a relatively uniform, smooth, unbroken and adherent film of solder. A good intermetallic bond between surfaces is formed. One action of flux is to reduce the surface tension of the solder to enhance wetting.Good wetting is indicated by a low â€˜contact angleâ€™ (positive wetting angle) between the solder fillet and the base metal/metallisation.