1. Field of the Invention
The present invention relates generally to a heat radiating mechanism, and more particularly to a heat radiating mechanism that transmits the heat generated from an exoergic circuit element (referred to as “exoergic element” hereinafter) mounted on an electronic apparatus. The present invention is suitable, for example, for a heat spreader that radiates heat from a CPU mounted on a printed board (also referred to as a “system board” or “motherboard”) in a personal computer (referred to as “PC” hereinafter). Here, the “heat spreader” is a metal plate that efficiently transmits the heat from the exoergic element to the outside.
2. Description of the Related Art
A Ball Grid Array (“BGA”) package, one type of package board soldered to the printed board, has conventionally been proposed in order to meet recently increasing demands for supplies of high-performance electronic apparatuses. The BGA package realizes a narrower pitch and more pins (i.e., high-density leads) without enlarging the package than a Quad Flat Package (“QFP”) that has the Gullwing type leads at four sides. Thus, the BGA package provides the high-performance electronic apparatus through the high density of the package.
The BGA package is mounted with an IC and an LSI that generally serve as a CPU, and the improved performance and larger size of the LSI swells the calorific value, for example, up to 100 W to 150 W. Accordingly, in order to thermally protect the electronic circuit in the LSI, a radiator called a heat sink is thermally connected to the LSI via a heat spreader. The heat sink has cooling fins, is located near the CPU, and radiates the LSI through natural cooling. Use of a material having a high coefficient of thermal conductivity (e.g., about 16.5×10−6/° C.), such as copper, for the heat spreader would result in a significant difference in the coefficient of thermal conductivity between the heat spreader and the LSI made of silicon (which has a coefficient of thermal conductivity of about 4.2×10−6/° C.). In addition, there is also a significant difference in thickness between them because the LSI has a thickness of several hundred microns whereas the heat spreader has a thickness of several millimeters.
Prior art includes, for example, Japanese Patent Applications, Publication Nos. 6-169037, 2-47895, 62-183150 and 2002-184914, and Japanese Utility-Model Application, Publication No. 2-101535.
A jointing layer joints the LSI to the heat spreader, and is typically made of a material having no elasticity, such as epoxy resin and solder. For example, a Cu heat spreader is arranged on an Si chip via a heat-hardening adhesive as a jointing layer, and the jointing layer is hardened, for example, at 150° C. and then returned to room temperature. Then, the heat spreader and the LSI generate thermal strains, but the heat spreader is thicker than the LSI and difficult to deform in the thickness direction. Therefore, a large thermal stress is applied to the LSI. which can break the LSI and the jointing layer. In particular, the influence of the thermal stress applied to the LSI becomes conspicuous for the increased chip size.
One conceivable solution for this problem is to use for the jointing layer an elastic adhesive, such as a silicon adhesive, or a sheet or paste jointing material. However, the metallic jointing material, such as solder, has a coefficient of thermal conductivity of about 40 W/° C.·m, whereas these materials have such a small coefficient of thermal conductivity as 1 to 2 W/° C.·m, lowering the radiation efficiency of the CPU. The non-operating LSI can endure the temperature up to about 200° C., but the operating LSI's electronic circuit should be maintained at about 100° C. and protected thermally. The low radiation efficiency would cause the operating LSI's temperature to exceed 100° C. and possibly result in the thermal breakdown of the electronic circuit. In particular, as the CPU has recently increased the calorific value, it is undesirable to use the jointing material having a low coefficient of thermal conductivity for the CPU and the heat spreader.