Provided are a flux that does not contain a reducing agent and an activator and can make a solder joint portion residue-free, and a solder paste using the flux. The flux of the present invention is a first solvent having a temperature at which the mass becomes zero at a temperature of 180 ° C. or higher and lower than 260 ° C. in a thermal weight measurement at a heating rate of 10 ° C. A second solvent in which the temperature at which the mass becomes zero in a thermal weight measurement at a temperature rise rate of 10 ° C. at a nitrogen flow rate of 0.2 to 0.3 L / min is 100 ° C. or higher and lower than 220 ° C.; Including. The content of the first solvent is less than the content of the second solvent. It does not contain a reducing agent for reducing and removing the surface oxide film of the solder and an activator for improving the reducing property, and the mixture with the solder powder can be a solder paste.
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