Electronic element mounting substrate, electronic device, and electronic module

An electronic device, comprising:

• an electronic element mounting substrate including
• a first substrate comprising a first main surface and a second main surface opposite to the first main surface;
• and
• a plurality of second substrates each comprising a third main surface and a fourth main surface opposite to the third main surface,
• the first substrate comprising a mounting portion for mounting an electronic element, positioned on the first main surface,
• the mounting portion having a rectangular shape,
• one end portion of the mounting portion in a longitudinal direction being positioned at an outer edge portion of the first main surface,
• the plurality of second substrates being formed of a carbon material,
• the third main face of the second substrate which is positioned closest to the first substrate facing the second main surface,
• the plurality of second substrates are stacked in a thickness direction of the first substrate, wherein
• a thermal conduction of the mounting portion in a direction perpendicular to the longitudinal direction is greater than a thermal conduction of the mounting portion in the longitudinal direction, in the third main surface of the second substrate which is positioned closest to the first substrate, in plan view,
• two adjacent second substrates of the plurality of second substrates are so configured that a direction in which a thermal conductivity of one of the two adjacent second substrates increases and a direction in which a thermal conductivity of the other of the two adjacent second substrates increases are perpendicular to each other in plan view, and
• each of the plurality of second substrates comprises a first side surface on a side where the one end portion is located in plan view, and is so configured that a thermal conduction in the first side surface in a thickness direction is greater than a thermal conduction in a direction perpendicular to the thickness direction;
• an electronic element mounted on the mounting portion of the electronic element mounting substrate;
• and
• a wiring board or an electronic element housing package on which the electronic element mounting substrate is mounted.