Assemblies to be soldered are preheated prior to making contact with the liquid solder, firstly to initiate the chemical action of the activator found in the flux, and secondly to evaporate the solvent or the water. In order to avoid introducing mechanical stresses into the printed circuit board on account of a difference in the temperature distribution, this preheating should be gentle and homogeneous. The maximum preheat temperature is determined by the thermal energy required by the board, the thermal stability of the flux and the thermal rating of the components.