Wetting is a property of the alloy when within its specific eutectic temperature range. With sufficient flux, in the state of liquidus, molten solder alloys will exhibit a characteristic called 'wetting.' Molten solder alloy behaves much as water, with properties of cohesion and adhesion. At this specific temperature range, the molten alloy demonstrates properties of adhesion.

The goal of the reflow process is for the solder paste to reach the eutectic temperature at which the particular solder alloy undergoes a phase change to a liquid or molten state. The reflow soldering of through-hole components using solder paste in a convection oven process is called intrusive soldering. While this may be said of lead-tin solder pastes used previously, lead-free solder alloys such as SAC present a challenge in terms of the limits of oven temperature profile adjustment and requirements of specialized through-hole components that must be hand soldered with solder wire or cannot reasonably withstand the high temperatures directed at circuit boards as they travel on the conveyor of the reflow oven. When used on boards containing a mix of SMT and plated through-hole (PTH) components, through-hole reflow, when achievable by specifically modified paste stencils, may allow for the wave soldering step to be eliminated from the assembly process, potentially reducing assembly costs. Wave soldering however, has been the common method of soldering multi-leaded through-hole components onto a circuit board designed for surface-mount components. Reflow ovens meant specifically for the soldering of surface mount components may also be used for through-hole components by filling the holes with solder paste and inserting the component leads through the paste. Each segment of the oven has a regulated temperature, according to the specific thermal requirements of each assembly.

Reflow soldering with long industrial convection ovens is the preferred method of soldering surface mount technology components or SMT to a printed circuit board or PCB. Heating may be accomplished by passing the assembly through a reflow oven, under an infrared lamp, or (unconventionally) by soldering individual joints with a desoldering hot air pencil.

The solder paste reflows in a molten state, creating permanent solder joints. Reflow soldering is a process in which a solder paste (a sticky mixture of powdered solder and flux) is used to temporarily attach one or thousands of tiny electrical components to their contact pads, after which the entire assembly is subjected to controlled heat. Example of reflow soldering thermal profile.

Vapor phase reflow