PURPOSE: To allow the application of a reflow method to circuit boards by incorporating the thermally reversible reaction product of any of carboxylic acid, triazole, thiazole, thiadiazole or imidazoles and ethanol amine into the flux.

CONSTITUTION: The thermally reversible reaction product of any of the carboxylic acid, triazole, thiazole, thiadiazole or imidazoles and the ethanol amine is incorporated into the flux for cream soldering. Any of β-methoxyethyl amine, methoxypropy propyl amine, ethoxypropyl amine, and promethoxypropylamine is used in place of ethanol amine. The cream solder is constituted by kneading 92 to 80 pts. wt. powder solder and 8 to 20 pts. wt. fluxes mentioned above. The corrosion of soldered parts is prevented in this way even if the flux remains at these parts.