Substrate for mounting electronic element, electronic device, and electronic module

A substrate comprising:

• a first member;
• and
• a second member covered by the first member, wherein
• the second member includes a first plate material and a second plate material each of which is a plate-like member comprising a carbon material,
• the first plate material and the second plate material are bonded to each other in a state of being laminated in a first direction which is a thickness direction of the second member,
• the second member has a first bonding portion between the first plate material and the second plate material on a side surface extending along the first direction,
• the second member further includes a third plate material which is bonded to the second plate material in a state of being laminated from a side opposite to the first plate material, and a second bonding portion between the second plate material and the third plate material,
• the first member covers the first bonding portion not to be exposed to an outside and covers the second bonding portion not to be exposed to the outside,
• one direction intersecting the first direction is a second direction, and another direction intersecting the first direction and the second direction is a third direction,
• in the first direction,
• thermal conductivities of both the first plate material and the second plate material are greater than a thermal conductivity of the first member, and
• a thermal conductivity of the third plate material is greater than the thermal conductivity of the first member,
• the thermal conductivity of the first plate material in the first direction is greater than a thermal conductivity of the first plate material in the second direction,
• the thermal conductivity of the second plate material in the first direction is greater than a thermal conductivity of the second plate material in the third direction,
• a thermal conductivity of the third plate material in the first direction is greater than a thermal conductivity of the third plate material in the second direction,
• in the second direction,
• the thermal conductivities of the first plate material and the third plate material are smaller than the thermal conductivity of the first member, and
• the thermal conductivity of the second plate material is greater than the thermal conductivity of the first member, and
• in the third direction:
• thermal conductivities of the first plate material and the third plate material are greater than a thermal conductivity of the first member, and
• the thermal conductivity of the second plate material is smaller than the thermal conductivity of the first member.