1. Field of the Invention
The present invention relates to a printed circuit board, and more particularly, it relates to a printed circuit board having a plurality of via-holes which are provided for effectively radiating the heat generated from circuit elements which are mounted on the printed circuit board.
2. Description of the Related Art
Recent technology for printed circuit boards more and more tends to prepare semiconductor elements with high integration and to mount them with high density on printed circuit boards. In accordance with this tendency, it is strongly required to cool elements or to radiate heat generated from elements mounted on the printed circuit boards.
In actuality, a cooling apparatus is provided for a large scale electronics apparatus (for example, a large scale computer) in order to cool circuit elements. However, in the case of electronic apparatuses which are required to be small in size, it is difficult to use such a cooling apparatus because of the high cost and large size thereof. Accordingly, conventionally, in such a small scale electronics apparatus, a heat sink, for example, a metal plate, has been used for radiating the heat generated from circuit elements (i.e., heater elements) in stead of the cooling apparatus.
In general, the printed circuit board is made by a resin, for example, an epoxide, and this material has low thermal conductivity so that the radiation of the heat becomes worse. Accordingly, it is necessary to mount a heat sink which has a good characteristic for radiating the heat because of the high thermal conductivity thereof.
On the other hand, the Japanese Unexamined Patent Publication No. 62-257786 and the Japanese U.M. Publication No. 62-42273 disclose another cooling method for a purpose of low cost and small size. According to this method, the heat sink is not provided, and a plurality of through-holes (which are called "thermally conductive through hole" or simply "thermal-via") each containing a thermally conductive material, for example, a metal, are provided on the printed circuit board in order to realize good thermal conductivity in the whole printed circuit board.
Further, in the former document (JPP-62-257786), the through-holes are provided so as to penetrate the printed circuit board by using a drill, and copper (Cu)-plating is provided on a side surface of each through-hole. According to this structure, since the air is filled within a space provided by the through-hole, the thermal conductivity becomes extremely worse. However, since the Cu-plating is provided on the side surface of each through-hole, it is possible to improve the thermal conductivity of the printed circuit board as the whole.
However, although the above method is preferable for the purpose of low cost and small size, there are some limitations, for example, the density of the through-hole and the restriction of a size of hole (i.e., diameter thereof) so that it is very difficult to obtain sufficient radiation of the heat. Accordingly, there is a problem which it is very difficult to utilize this method to elements which radiate large amount of heat.
Still further, in the latter document (JUMP-62-42273), although this discloses the thermally conductive through-holes, it does not disclose a concrete method for improving the thermal conductivity.