Electronic component built-in substrate and method of manufacturing the same

An electronic component built-in substrate, comprising:

• a first wiring substrate having a first wiring substrate main body and a first wiring pattern provided on a first surface of the first wiring substrate main body;
• a first electronic component surface-mounted on the first wiring pattern via flip-chip connection with bumps;
• a second wiring substrate having a second wiring substrate main body and a second wiring pattern provided on a first surface of the second wiring substrate main body, the second wiring substrate being arranged under the first wiring substrate such that the first surface of the first wiring substrate main body opposes to the first surface of the second wiring substrate main body;
• a second electronic component surface-mounted on the second wiring pattern via flip-chip connection with bumps, and arranged to oppose to the first electronic component;
• an insulating material made of a glass fiber impregnated with resin, located between the first and the second wiring substrates where the first wiring substrate directly opposes the second wiring substrate;
• an underfill material made of a resin which does not include a glass fiber, located between the first and the second electronic components, between the first electronic component and the first wiring substrate, and between the second electronic component and the second wiring substrate;
• and
• an electrode that electrically connects the first wiring substrate with the second wiring substrate, the electrode provided in a portion of the insulating material.