Reflow soldering uses a solder paste to temporarily attach electrical components to their contact pads. Subsequently, the entire assembly is subjected to controlled heat. As a result, the solder melts, permanently connecting the joint. Heating can be accomplished by passing the assembly through a reflow oven, under an infrared lamp, or by soldering individual joints with a hot air pencil.
Reflow soldering is the most common method of attaching surface mount components to a circuit board. It can also be used for through-hole components by filling the holes with solder paste and inserting the component leads through the paste. Reflow is not generally used on pure through-hole boards because wave soldering can be simpler and cheaper. When used on boards containing a mix of SMT and THT components, through-hole reflow allows the wave soldering step to be eliminated from the assembly process, potentially reducing assembly costs.
The goal of the reflow process is to melt the solder and heat the adjoining surfaces, without overheating and damaging the electrical components. In the conventional reflow soldering process, there are usually four stages, called “zones,” each having a distinct thermal profile. These are preheat, thermal soak (often shortened to just soak), reflow, and cooling.