Circuit board and electronic device

A circuit board comprising:

• a metal circuit plate;
• a plurality of metallic heat diffusing plates disposed below the metal circuit plate and arranged in an up-down direction;
• a metallic heat dissipating plate below the plurality of heat diffusing plates;
• a first insulating substrate disposed between the metal circuit plate and an uppermost heat diffusing plate of the plurality of heat diffusing plates and comprising an upper surface bonded to a lower surface of the metal circuit plate and a lower surface bonded to an upper surface of the uppermost heat diffusing plate;
• a second insulating substrate disposed between a lowermost heat diffusing plate of the plurality of heat diffusing plates and the heat dissipating plate and comprising an upper surface bonded to a lower surface of the lowermost heat diffusing plate and a lower surface bonded to an upper surface of the heat dissipating plate;
• and
• a third insulating substrate disposed between a pair of heat diffusing plates adjacent to each other in the up-down direction and comprising an upper surface bonded to a lower surface of an upper heat diffusing plate of the pair of heat diffusing plates and a lower surface bonded to an upper surface of a lower heat diffusing plate of the pair of heat diffusing plates,
• wherein a grain diameter of metal grains contained in each of the heat diffusing plates decreases from each of the upper surface and the lower surface of the heat diffusing plate toward a center portion of the heat diffusing plate in a thickness direction.