PURPOSE: To enable formation of a conductive pattern to a mold board without taking much time by forming a circuit pattern to the mold board with a trench whereto solder can be immersed through capillary phenomenon, by immersion the mold board into a solder bath and by making the solder immersed inside the groove of the circuit pattern.

CONSTITUTION: An electronic parts 14 and 16 are mounted on a mold board 10. Resist is attached to an area of the mold board 10 excepting a circuit pattern 12 to avoid solder thereon. Then, the mold board 10 is reversed, and if it is immersed into a solder bath, solder is immersed into a trench of the circuit pattern 12. Since resist is attached to an area of the mold board 10 excepting the circuit pattern 12, the solder does not mount on an area excepting the circuit pattern 12. The solder is thereby filled inside the trench of the circuit pattern 12 and a conductive pattern is formed to the mold board 10. In this way a conductive pattern can be formed readily without taking much time.