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

A substrate for mounting electronic element, comprising:

• a first substrate comprising a first surface and a second surface opposite to the first surface;
• a second substrate comprising a third surface and a fourth surface opposite to the third surface;
• and
• a plurality of heat dissipation bodies each comprising a fifth surface and a sixth surface opposite to the fifth surface,
• the first substrate comprising at least one mounting portion for at least one electronic element at the first surface, and the at least one mounting portion being a rectangular shape,
• the second substrate being made of metal, having a quadrangular shape,
• the plurality of heat dissipation bodies being made of a carbon material, and the fifth surface being connected to at least the second surface at location overlapped with the at least one mounting portion in a transparent plan view,
• heat conduction of the plurality of heat dissipation bodies in a direction perpendicular to a longitudinal direction of the at least one mounting portion and perpendicular to a direction along opposite sides of the second substrate being greater than heat conduction of the plurality of heat dissipation bodies in the longitudinal direction of the at least one mounting portion and in the direction along opposite sides of the second substrate in a transparent plan view of the substrate for mounting electronic element.