1. Field of the Invention
The present invention relates to a substrate with built-in electronic component in which an electronic component is embedded in a substrate, and more particularly, to a substrate with built-in electronic component in which an electronic component is stored in a storage portion formed in a core layer.
2. Description of Related Art
Known examples of substrates with built-in electronic component include a “substrate not provided with a core layer” disclosed in Patent Document 1 below, for example, and a “substrate provided with a core layer” disclosed in Patent Document 2 below, for example. The “substrate not provided with a core layer” has a structure in which an electronic component is embedded in an insulating layer, and the electronic component is held in a proper place by being covered with a synthetic resin constituting the insulating layer. The “substrate provided with a core layer” has a structure in which an electronic component is stored in a storage portion that is formed so as to penetrate the core layer. When there is space between the electronic component and inner walls of the storage portion, the space is filled with a synthetic region, and the electronic component is held in a proper position by being covered by the synthetic resin filler.
In the “substrate provided with a core layer,” if the synthetic resin filler has higher moisture absorbency than moisture absorbency of the core layer, the following problems occur. That is, because various electronic components are generally mounted on a substrate with built-in electronic component by the reflow soldering method, due to the heat generated in the reflow soldering, moisture included in the synthetic resin filler is evaporated, and tries to escape to the outside. When insulating layers made of a synthetic resin and conductive layers made of a metal are alternately formed on one surface and the other surface of the core layer in the thickness direction, the vapor tries to escape to the outside from the end of each insulating layer closest to the core layer, travelling through the insulating layer. However, because the path through which the vapor travels to be released to the outside is long, possible problems can be caused by the vapor, such as a crack in the respective insulating layers closest to the core layer, layer separation between the insulating layer and the core layer, and layer separation between the insulating layer and a conductive layer closest to the core layer.