In a reflow oven, thermal energy is conveyed in a direct and well defined way onto printed circuit board assemblies populated with SMD components. This leads to heating up the assembly, whereby certain guide lines and limits have to be observed. The maximum temperature the assembly reaches has to be higher than the melting point of the applied paste, to assure that a complete melt of all the paste on the board is melted. The characteristic temperature / time progression measured on the assembly is either termed as a linear profile or a saddle profile. The transfer of the thermal energy in modern reflow systems is obtained through recirculated hot air or nitrogen, therefore the name convection reflow. In earlier times of the SMD technology, the thermal transfer was through IR radiation. IR had the substantial disadvantage that the absorption of radiation was subject to the component package surface. Absorption was uneven, so that not all components heated up to the same degree. These temperature difference need to be avoided or at least kept to a minimum. Thermal transfer through convection offers clear advantages, and that is why convection reflow has become the standard method to assemble SMT populated boards.