Heat Pipe Heat Sinks With Matched Coefficient of Thermal Expansion Surfaces for Direct Silicon Attach

Die level power densities are hitting unprecedented fluxes of over 200W/cm², in turn driving the junction-to-case and case-to-sink resistances higher than the sink to ambient resistance. The biggest potential for thermal improvement now lies within the heat sink to device interface. Reduction in this critical interface resistance can be realized through direct attach of the heat sink base to the silicon. Challenges to this approach include the near matching of the coefficient of thermal expansion (CTE) of these two surfaces. Design concepts for a heat pipe heat sink with a CTE similar to that of silicon are investigated. Design concepts take into account materials, heat sink design configurations, and material compatibility with heat pipe working fluids. Prototype test articles were designed and fabricated using a copper/molybdenum/copper laminate as the heat input surface. Test data indicating an evaporator thermal resistance of 0.080 cm²·°C/W at 290W/cm² are presented.