Reflow Soldering

Reflow soldering is a process in which solder paste, a sticky mixture of powdered solder and flux, is used to temporarily attach anywhere from one to thousands of tiny electrical components to their contact pads, after which the whole assembly is subjected to controlled heat that melts the paste into permanent solder joints; heating is typically done by passing the assembly through a reflow oven, though an infrared lamp or a hand-held hot air pencil can also be used, mainly for prototyping. It is the preferred method for soldering surface-mount components to a printed circuit board, using long industrial convection ovens whose segments are each regulated to the specific thermal needs of the assembly passing through, and a conventional reflow process runs through four distinct thermal zones: preheat, thermal soak, reflow and cooling. The goal is to bring the solder paste to its eutectic temperature, at which it becomes a molten, adhesive liquid capable of wetting the surfaces it joins, a necessary condition for forming an acceptable solder joint under industry criteria; the oven temperature profile must be tuned to each board size, layer count and component mix so the solder reflows properly without overheating and damaging nearby components.