Soldering flux and solder paste

When the amount of thixotropy required to obtain the effect of suppressing sagging of heating is included in the flux， the amount of flux residue increases， and when applied to applications used without cleaning， a large amount of flux residue derived from thixotropy around the soldering area Remains， affecting chemical and electrical reliability. In addition， in applications where the flux residue is washed and used， the washability is poor. Therefore， the flux for soldering contains nanofibers composed of at least one of polysaccharides， modified polysaccharides， or incompletely modified polysaccharides that are in the process of being modified from polysaccharides to modified polysaccharides. The present invention addresses the problem that: if a flux contains an amount of thixotropic agent necessary for obtaining the effect of suppressing a flow due to heating， the amount of flux residue increases， and， if applied for uses that do not involve washing， a large amount of flux residue derived from the thixotropic agent remains around the soldered portions， thereby affecting chemical and electrical reliability; and that washing performance is poor in uses involving washing of the flux residue. Accordingly， this soldering flux contains nanofibers of one or more kinds from among polysaccharides， modified polysaccharides， and incompletely modified polysaccharides being modified from polysaccharides into modified polysaccharides， by an amount of not less than 50 wt ppm and not more than 3000 wt ppm with respect to a total amount of flux. The present invention addresses the problem that: if a flux contains an amount of thixotropic agent necessary for obtaining the effect of suppressing a flow due to heating， the amount of flux residue increases， and， if applied for uses that do not involve washing， a large amount of flux residue derived from the thixotropic agent remains around the soldered portions， thereby affecting chemical and electrical reliability; and that washing performance is poor in uses involving washing of the flux residue. Accordingly， this soldering flux contains nanofibers of one or more kinds from among polysaccharides， modified polysaccharides， and incompletely modified polysaccharides being modified from polysaccharides into modified polysaccharides， by an amount of not less than 50 wt ppm and not more than 3000 wt ppm with respect to a total amount of flux.