Abstract: A flux consisting of: (i) 30 to 80 wt % of one or more different acidic oligoesters having an acid number in the range from 100 to 300 mg KOH/g and having a weight-average molecular weight Mw in the range from 300 to 600, (ii) 10 to 50 wt % of at least one organic solvent, (iii) 0 to 15 wt % of one or more amines, and (iv) 0 to 10 wt % of one or more components other than components (i) to (iii); and, solder paste consisting of 80 to 92 wt % of one or more different solders and 8 to 20 wt % of the flux.