Fabrication Process And Thermal Performance Analysis Of Ultra-thin Flattened Heat Pipes

With the development of microelectronic devices such as smart phones and tablet PCs heading towards the direction of ultra-thin and high-performance, high heat flux problem due to enhanced performance of electronic chips has brought a serious impact on the performance and stability of the electronic devices. Using efficient heat transfer component to reduce heat flux has become the key to the development and application of high-performance electronic devcies.The manufacture process of ultra-thin flattened heat pipes with 0.6mm thickess was proposed and process parameters were determined. Three kinds of composite wick strutures suitable for ultra-thin flattened heat pipes, including copper foam-mesh, two-layers mesh and three-layers mesh composite wicks, were investigated and the corresponding manufacture proess was also proposed.Two key manufacturing processes of ultra-thin flattened heat pipes, namely, die extrusion tube shrinking process and phase change flattening process were mainly analyzed. Mathematical models of deformation and stress analysis for the two processes were carried out. Finite element models of the two processes were also established. The surface quality, stress and strain of ultra-thin heat pipes and the force of the die were studied.Based on the basic theory of heat pipe, the characteristics of fluid and steam flow and main heat transfer limits were analyzed. It was pointed out the main heat transfer limits of the ultra-thin heat pipes were capillary limit and boiling limit. The heat transfer limits of ultra-thin heat pipes were calculated based on the structure parameters of the composite wick structures and the physical properties of the working fluid. Result shows that capillary limit is the main limitation to the ultra-thin heat pipes.An experimental testing platform was set up to test the start-up and steady thermal performances of ultra-thin heat pipes with different composite wick structures under different filling ratios. The heat transfer characteristics of the ultra-thin flattened heat pipes with different composite wick structures were compared. The results of the start-up test indicate that the response time of composite ultra-thin heat pipes is about 10 s and composite ultra-thin heat pipes can start up within 30 s. The results of the steady test indicate that the optimum filling ratios were 110% for copper foam-mesh composite wick structure, 90% for the two-layers mesh composite wick structure and 100% for the three-layers mesh composite wick structure. The maximum heat transport capacities were 9W,8W and 8.5W, respectively.