The process in a reflow oven involves heating the assembly gradually, first to remove flux volatiles and start flux activation, then to bring the materials to be joined to a temperature sufficiently high that a final burst of energy can perform the soldering operation. Then, when the solder paste has formed a single liquid solder volume for each joint, the assembly is cooled to solidify the solder joints.
This process occurs on a continuous basis, with a belt or other conveyor carrying the assembly through the series of heated and cooled zones into which the oven is split. Most ovens blow jets of air (or nitrogen) onto the assembly, either to heat or cool the joints, although some of the heating comes through infrared radiation from the tunnel walls. The air velocity within a convection furnace is quite high, but typically the gas is blown vertically down on the board, so that components remain in position.
Within each of the control zones in the oven, the air is recirculated, with a small percentage of fresh air (or nitrogen) being added to replace the spent gas, contaminated by resin by-products, that is extracted generally from the hottest zone of the furnace. This recirculation improves the thermal efficiency of the system.
The rate of heating is determined by the flow rate of the gas and by its temperature. Typically in a convection furnace the difference in temperature between gas and assembly is kept to a minimum, in order to avoid overheating small components.
The bulk of the heat transfer takes place at the beginning of the heated zone, when the assembly is introduced into the oven, and at the reflow stage, where a fast ramp-up allows soldering to take place in a short time. However, the central section between these two points is often quite long. This is because effective soldering needs all components to reach almost soldering temperature before the final burst of heat is applied.