PURPOSE: To lessen a time required for bonding a heat dissipating slug and to enhance a semiconductor mounting board provided with the slug in insulation properties between an inner board and the slug by a method wherein a cavity of an upper board is smaller than that of an inner board, the cavities arc set concentric, and the heat dissipating slug is bonded separate from the cavity of the inner board by a certain space.

CONSTITUTION: Square hole-shaped cavities 11a, 12a, and 13a different in size are bored in an upper board 11, an intermediate board 12, and a lower board 13 respectively. The cavity 13a of the lower board 13 is set larger than the cavity 12a of the intermediate board 12, and the cavities 13a and 12a are stacked up to form a stepped structure. Furthermore, the cavity 11a of the upper board 11 is set smaller than the cavity 12a of the intermediate board 12, and the cavities 11a and 12a are arranged in concentricity. A guide tapered part 21 is provided to the root of the projection of a heat dissipating slug 20 inserted in and bonded to the cavities 11a, 12a, and 13a, so that the slug 20 is properly inserted into and bonded in position without coming into contact with a pattern 14 formed on the side wall of the cavity 12a of the intermediate board 12.