Conventionally, in a multilayer printed circuit board in which an electronic component is mounted, various structures for releasing heat generated from the electronic component has been suggested. For example, heat generated from an electronic component is released outside from an electrode (land) that is directly coupled to the electronic component. In another example, an electrode is coupled with a case that houses a multilayer circuit board therein to release heat to outside. In other words, heat generated from the electronic component is released to air from a surface of the case through the electrode. The case is generally set at a ground potential. However, depending on a type of the electronic component, an electrode may have a potential (for example, a positive potential) different from the ground potential. In this case, the electrode and the case at the different potentials cannot be coupled.
In order to solve the above-described issue, JP-A-2008-130684 discloses that an internal layer heat-transfer conductor is disposed in a multilayer printed circuit board so as to overlap a surface heat-transfer conductor (first electrode). In this method, heat generated from an electronic component is transferred from the surface heat-transfer conductor to the internal layer heat-transfer conductor through an insulating substrate. Then, the heat is transferred to a case or the insulating substrate that forms the multilayer printed circuit board and is released to outside.
In the above-described method, heat transfer from the surface heat-transfer conductor to the internal layer heat-transfer is performed between planes opposite to each other through the insulating substrate. In general, a thermal conductivity of an insulating substrate is smaller than a thermal conductivity of metal. For example, a thermal conductivity of prepreg, which is often used as an insulating substrate, is 4.40×10−4 W/(mm·K), which is about 1/1000 of thermal conductivity 4.01×10−1 W/(mm·K) of copper. In other words, a heat transfer performance from the surface heat-transfer conductor to the internal layer heat-transfer conductor is low. As a method of improving the heat transfer performance from the surface heat-transfer conductor to the internal layer heat-transfer conductor, opposing areas of the surface heat-transfer conductor and the internal layer heat-transfer conductor through the insulating substrate may be increased. However, when the area of the surface heat-transfer conductor increases, the number of components mounted on the multilayer printed circuit board may decrease, a dimension of the multilayer printed circuit board may increase, and a cost may increase.