Solder Flux and Solder Paste

If the flux contains the amount of thixotropic agent necessary to obtain the effect of suppressing the heating deflection， the amount of the flux residue increases， and when applied to the use without washing， a large amount of the flux residue derived from the thixotropy agent around the soldering site Will remain， affecting chemical and electrical reliability. Moreover， in the use which wash | cleans and uses a flux residue， washability is bad. Therefore， the soldering flux contains 50 wt ppm or more and 3000 wt ppm or less of nanofibers composed of one or more of incompletely modified polysaccharides which are in the process of being modified from polysaccharides， modified polysaccharides or modified 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.