The Development of a Bonded Fin Graphite/Epoxy Heat Sink for High Performance Servers

IBM’s has recently introduced a high performance server that utilizes multichip modules that dissipate very high heat loads. Each multichip module consists of four microprocessor chips encased by a copper cap that serves to spread the heat load over an area of roughly 113 mm × 113 mm. The module is air cooled by a single aluminum alloy bonded-fin fan sink. For applications requiring the microprocessors to operate at higher frequencies, the aluminum heat sink, with its lower thermal conductivity, cannot provide sufficient cooling; therefore, a copper heat sink must be employed. However, copper alloys have the disadvantage of a significant weight penalty (density ∼ 8.9 g/cm³), being 3.3 times heavier than aluminum (density ∼ 2.7 g/cm³), and is significantly more costly to manufacture. A novel approach for an improved heat sink has been developed using a new natural graphite-based/epoxy composite material. This material has low density (∼1.9 g/cm³) and anisotropic thermal conductivity (∼370 W/m-K in two directions, ∼ 7 W/m-K in the third direction). Bonded fin manufacturing methods have been developed to produce a heat sink that exploits the material’s high thermal conductivity when used in combination with a copper spreader module, such as used in the IBM server. Convective heat sink thermal performance approaching that of copper (e.g. 0.030 °C/W) has been achieved at a fraction of copper’s weight. Therefore, additional hardware required to allow the copper heat sinks to withstand shock and vibration standards, was not necessary with the lightweight graphite solution. Mechanical issues involved with using the lower strength graphite materials in a metal retrofit situation had to be resolved. Solutions included the use of aluminum end plates to provide edge protection to the heat sink with metal stiffeners inserted into the base for extra structural integrity. A variety of mechanical attachment methods was evaluated to join the graphite to the copper heat spreader. Lapping procedures were developed for the graphite heat sink to provide the required flatness necessary to minimize the temperature drop across the interface.