PURPOSE: To suppress the damage in solder bumps by a method wherein a substrate loaded with an electronic component is heated at the temperature exceeding the setting point of a thermo-setting resin and then cooled down at the temperature not exceeding the melting point of solder for the reinforcement of solder bumps.

CONSTITUTION: The periphery of a circuit pattern 2 is coated with a mixture 10 of a thermosetting resin and a solder activator. Next, an electronic component 3 is shifted on a substrate 1 so as to position solder bumps 4 on the circuit pattern 2. Next, the substrate 1 is put in a reflowing device to be heated. When the solder reaches the melted down temperature, the solder bumps 4 are melted down in liquid state to be junctioned with the circuit pattern 2 leaving the mixture 10 in the pasty state intact. Next, when the temperature reaches the setting point of thermosetting resin, the mixture 10 is set leaving the solder bumps 4 in the liquid state intact to be firmly reinforced. Finally, after reaching the maximum temperature, the substrate 1 is cooled down leaving the solder bumps 4 in solid state to be cooled down at the ordinary temperature later.