In order to prevent various soldering defects as well as to provide a more solid and conductive joint, PCBs are preheated in most soldering applications today. For vapor phase, this is usually done by hovering the boards directly above the vapor layer in the phase chamber for a predetermined amount of time in order to sufficiently heat the solder paste and the components. The methods for doing this can vary from machine to machine. In many machines, there is a secondary vapor phase which is produced at a cooler temperature than the main vapor layer. No soldering takes place here, only a temperature raise. Other machines rely on IR preheating or air temps above the vapor to heat the element and prepare for soldering. In any case, there must be some board temperature conditioning before entering a temperature at which the solder liquefies.
The actual soldering process on the board as it enters the vapor level begins with a condensing of the vapor onto the board, which induces a heat transfer between the vapor and the entire board structure, including the solder. The heat transfer is very uniform and the risk of overheating is virtually nil. The heat transfer rate can be controlled by the heating elements under the liquid base, which can increase the vapor generation rate but will not increase the actually temperature. As the board is completely immersed in the vapor it continues to condense until the PCB is at the same temperature as the vapor. This happens much more quickly than IR or air heated boards due to the excellent heat transfer capabilities of the vapors in use today. After a time, the board is removed from the vapor level, and the condensed chemicals evaporate off the still-heated components, leaving a residue-free and fully soldered unit.