Flux for soldering and solder composition

A soldering flux containing an epoxy resin and an organic carboxylic acid, wherein said epoxy resin and said organic carboxylic acid are compounded at a ratio of 1.0 equivalent of epoxy group in said epoxy resin to 0.8-1.1 equivalent of carboxyl group in said organic carboxylic acid, and total amount of said epoxy resin and said organic carboxylic acid is 70% or more by weight based on said total weight of said soldering flux,

• wherein said carboxylic acid is selected from the group consisting of succinic acid, glutaric acid, adipic acid, azelaic acid, dodecanoic diacid, itaconic acid, mesaconic acid, cyclobutanedicarboxylic acid, cyclohexanedicarboxylic acid, cyclohexenedicarboxylic acid, cyclopentanetetracarboxylic acid, dimethylglutaric acid, methyladipic acid, glutamic acid, ethylene-diaminetetraacetic acid, citric acid, malic acid, tartaric acid, pyrazindicarboxylic acid, phenylenediacetic acid, catecholdiacetic acid, hydroquinonediacetic acid, thiopropionic acid, thiodibutyl acid, dithioglycolic acid and mixtures thereof,
• wherein a temperature at the top of an exothermic peak in flux-curing reaction in which said epoxy resin and said organic carboxylic acid are polymerized is 180-250° C. or an initiation temperature of flux curing reaction in which said epoxy resin and said organic carboxylic acid are polymerized is 180-230° C.