Reflow soldering is the making of solder joints by (re) melting previously applied solder, usually in the form of a paste. A re-flow machine is a conveyorised oven that transports the pasted and component-mounted PCB through a well-controlled heating system. This heats the solder paste, activates the flux suspended in the paste, and raises the temperature of the solder for a short duration (reflow zone) above the melting point of the metal powder. The liquid solder forms a set of soldered joints between the component connections and the PCB lands. After reflow, the PCB continues through a final cooling zone. The challenge in the reflow soldering process is ensuring that all components, terminations and PCB pads are brought to a re-flow temperature simultaneously, as each has a different heat absorbing capacity, or thermal mass.
Smaller, darker coloured materials have a lower thermal mass than larger lighter ones and therefore, heat up more quickly. If the board is not brought up to the re-flow temperature in a gradual and controlled temperature gradient, components with a smaller thermal mass rapidly overheat and suffer permanent damage. The cool down process after the peak re-flow temperature is controlled, as too rapid cooling can also introduce component failures. A compromise exists here, as generally the faster the cooling; the stronger is the solder joint although the ductility may be impaired. A recommended cooling rate is 2 - 4C per second.
Reflow ovens utilise conduction, radiation and convection to transfer heat onto the components, printed circuit board and solder paste. All have their advantages and disadvantages, but whatever the process used, the key parameters are the control of maximum component temperature and temperature gradients when heating and cooling.
All re-flow ovens take the boards through a series of heating compartments or zones; preheat, soak (activation), re-flow and cool down. The circulated heat is directed onto the topside of the PCB during its travel through the oven.
