Along with a reduction in size and an increase in functionality of electronic equipment such as mobile phones, circuit devices including multiple wiring layers have become dominant among those housed inside the electronic equipment. This technique is described in Japanese Patent Application Publication No. 2003-324263, for example. Referring to FIG. 8, a circuit device including a multi-layered substrate 107 will be described.
Here, the circuit device is configured by mounting a circuit element such as a package 105 on a first wiring layer 102A formed on an upper surface of the multi-layered substrate 107.
In the multi-layered substrate 107, wiring layers are formed on an upper surface and a back surface of a base 101 made of glass epoxy resin. Here, a first wiring layer 102A and a second wiring layer 102B are formed on the upper surface of the base 101. The first wiring layer 102A and the second wiring layer 102B are stacked with an insulating layer 103 interposed therebetween. On the lower surface of the base 101, a third wiring layer 102C and a fourth wiring layer 102D are stacked with another insulating layer 103 interposed therebetween. The wiring layers are connected to each other at predetermined positions by connection parts 104 formed by penetrating the insulating layer 103.
To the first wiring layer 102A, which is the uppermost layer, a package 105 is fixed. Here, the package 105 having a semiconductor element 105A therein and sealed with resin is surface mounted on the first wiring layer 102A through connection electrodes 106.
However, since the base 101 included in the multi-layered substrate 107 having the above-described structure is made of resin, heat generated from the package 105 is difficult to release to the outside.
Moreover, to improve the mechanical strength and also the heat dissipation of the multi-layered substrate 101, the base 101 is highly filled with filler such as alumina. However, resin mixed with a large amount of filler tends to be brittle, and thus cracks frequently occur in the base 101 in a transport process and the like.
Further, when a conductive foil made of a plated metal core material is used for the base 101, a problem consequently arises that the mechanical strength of the entire base 101 becomes poor because such a conductive foil has poor mechanical strength.