Challenges and Advances of Heat Pipes in Cooling Notebook Systems

Notebooks represent an increasing percentage of PC client market with growth surpassing that of desktop computers. Heat pipe has been an integral part of notebook computer system cooling and will remain so for the foreseeable future. Heat pipe allows for efficient transport of heat from the CPU and other high power components to a location where there is more room for accommodating motherboard cutout for a fan and a heat exchanger. The thermal resistance along this path must be minimized to enable maximum cooling. This paper first briefly describes the contributing resistance in a heat pipe and ways to measure them for a notebook thermal solution. Since there are several parameters that can affect the performance of the heat pipes, we use an experimental procedure utilizing DOE (Design of Experiments) to first understand the sensitivities of these design, manufacturing and usage parameters on performance and then to arrive at an optimum level of these parameters to minimize various resistances in a heat pipe. We show that for various different wick technologies, it is possible to optimize the heat pipes to achieve an evaporator performance of the level of 0.1 C-cm2/W. Furthermore, we show some simple design rules to minimize the condenser resistance and also results of a design study to optimize the design of heat pipe block at the CPU end to minimize the evaporator resistance. We want to encourage the heat pipe vendor community to use these methods to optimize their products for performance as well as process enhancements to produce higher performing parts, at lower cost.