Soldering esp printed circuits - using flux contg vehicle and reducing agent released as gas after evaporating vehicle

At least one of reducing agents is released as gas after evaporation of the vehicle in soldering process, esp. for printed circuits, in which a flux, consisting of a vehicle, pref. EtOH, and a reducing agemt, pref. formalin, dissolved in vehicle, is applied, before soldering, to positions to be soldered, and where vehicle is evaporated by heat on soldering and the metal oxides and hydroxides are partly reduced and rinsed off. Surface tension of solder may be controlled by providing a higher mol wt. organic halogen-free cpd., esp. succinic anhydride. Pref. flux contains 100 cc 96% EtOH, 0.2 cc formalin (35% HCHO) and 1.5 g succinic acid. No residues are left and bubble inclusions are not formed.