A flux comprising: an organic acid; a nonionic surfactant; a weak cationic surfactant; and a solvent, wherein: the flux does not contain water, the organic acid includes 1,2,3-propanetricarboxylic acid, the nonionic surfactant is one or more selected from the group consisting of a polyethylene glycol, an ethylene oxide-resorcin copolymer, polyoxyalkylene acetylene glycols, polyoxyalkylene glyceryl ether, polyoxyalkylene alkyl ether, polyoxyalkylene ester, polyoxyalkylene alkyl amine, polyoxyalkylene alkylamide, an aliphatic alcohol polyoxyethylene adduct, an aromatic alcohol polyoxyethylene adduct, and a polyvalent alcohol polyoxyethylene adduct, the weak cationic surfactant is one or more selected from the group consisting of terminal diamine polyethylene glycol, a terminal diamine polyethylene glycol-polypropylene glycol copolymer, an aliphatic amine polyoxyethylene adduct, an aromatic amine polyoxyethylene adduct, and a polyvalent amine polyoxyethylene adduct, a content of the 1,2,3-propanetricarboxylic acid is 1% by mass or more and 15% by mass or less with respect to a total amount of the entire flux, a total content of the nonionic surfactant and the weak cationic surfactant is 30% by mass or more and 65% by mass or less with respect to the total amount of the entire flux, and a total content of the solvent is 30% by mass or more and 65% by mass or less with respect to the total amount of an entire flux. an organic acid; a nonionic surfactant; a weak cationic surfactant; and a solvent, wherein: the flux does not contain water, the organic acid includes 1,2,3-propanetricarboxylic acid, the nonionic surfactant is one or more selected from the group consisting of a polyethylene glycol, an ethylene oxide-resorcin copolymer, polyoxyalkylene acetylene glycols, polyoxyalkylene glyceryl ether, polyoxyalkylene alkyl ether, polyoxyalkylene ester, polyoxyalkylene alkyl amine, polyoxyalkylene alkylamide, an aliphatic alcohol polyoxyethylene adduct, an aromatic alcohol polyoxyethylene adduct, and a polyvalent alcohol polyoxyethylene adduct, the weak cationic surfactant is one or more selected from the group consisting of terminal diamine polyethylene glycol, a terminal diamine polyethylene glycol-polypropylene glycol copolymer, an aliphatic amine polyoxyethylene adduct, an aromatic amine polyoxyethylene adduct, and a polyvalent amine polyoxyethylene adduct, a content of the 1,2,3-propanetricarboxylic acid is 1% by mass or more and 15% by mass or less with respect to a total amount of the entire flux, a total content of the nonionic surfactant and the weak cationic surfactant is 30% by mass or more and 65% by mass or less with respect to the total amount of the entire flux, and a total content of the solvent is 30% by mass or more and 65% by mass or less with respect to the total amount of an entire flux. the flux does not contain water, the organic acid includes 1,2,3-propanetricarboxylic acid, the nonionic surfactant is one or more selected from the group consisting of a polyethylene glycol, an ethylene oxide-resorcin copolymer, polyoxyalkylene acetylene glycols, polyoxyalkylene glyceryl ether, polyoxyalkylene alkyl ether, polyoxyalkylene ester, polyoxyalkylene alkyl amine, polyoxyalkylene alkylamide, an aliphatic alcohol polyoxyethylene adduct, an aromatic alcohol polyoxyethylene adduct, and a polyvalent alcohol polyoxyethylene adduct, the weak cationic surfactant is one or more selected from the group consisting of terminal diamine polyethylene glycol, a terminal diamine polyethylene glycol-polypropylene glycol copolymer, an aliphatic amine polyoxyethylene adduct, an aromatic amine polyoxyethylene adduct, and a polyvalent amine polyoxyethylene adduct, a content of the 1,2,3-propanetricarboxylic acid is 1% by mass or more and 15% by mass or less with respect to a total amount of the entire flux, a total content of the nonionic surfactant and the weak cationic surfactant is 30% by mass or more and 65% by mass or less with respect to the total amount of the entire flux, and a total content of the solvent is 30% by mass or more and 65% by mass or less with respect to the total amount of an entire flux.