Preheating allows for the use of significantly lower and safer temperatures when conductive or convective tools are used for component installation or removals. Preheating is also required when installing area array components and large leaded devices. The application of heat from the bottom side of the PCB serves several functions: 1) it keeps the PCB from twisting or warping, 2) it maintains the planarity of the rework site, 3) it warms the PCB so heat applied by the top heater is not drawn away from the rework site, and 4) it ensures that homogenous temperatures across the package and PCB are maintained, allowing the use of safe, low temperatures for the top heater.
