Design,fabrication And Performance Of Ultrathin Vapor Chamber With Gas-liquid Coplanar Structure

The emergence and rapid development of the fifth-generation mobile communication technology(5G)has triggered the development of electronic devices,especially smart phones and tablets PC,towards high performance,high integration and miniaturization,further leading to ultra high heat flux in extremely narrow space.Efficient thermal management in narrow space has become a major technical challenge for the development and application of modern advanced electronic devices.Ultrathin vapor chambers(UTVCs),with the advantages of outstanding thermal performance,large heat transfer area and good temperature uniformity,have become the current focus in the industry and academia.However,the conventional gasliquid hetero-planar ultrathin vapor chambers(HUTVCs)are difficult to achieve at such extreme small thickness owing to the extremely high vapor flow resistance.At present,the smallest thickness of UTVCs in mass production has always been at about 0.4mm.In 5G era,the demand for UTVCs with thickness of less than 0.4mm is increasingly prominent.In view of this,a novel gas-liquid coplanar ultrathin vapor chamber(CUTVC)with the thickness of0.27 mm is proposed in this thesis.The operation mechanism,structure design,manufacturing method and performance characterization of the CUTVCs were systematically studied.The main contents are as follows:(1)Design and fabrication of gas-liquid coplanar ultrathin vapor chambersTo satisfy the requirements of efficient heat dissipation in extreme narrow space for microelectronic devices under 5G penetration,a novel gas-liquid coplanar ultrathin vapor chamber is proposed.The stress-deformation and heat transfer characteristics of ultra-thin soaking plate are studied theoretically.It is found that the strip support posts exhibit outstanding anti-collapse deformation performance,and CUTVCs present a lower vapor flow resistance at extremely thin thickness.Based on the theoretical model,the optimal designs of the container,support posts,wick and the gas-liquid arrangement of CUTVCs are proposed.The structure,material and processing of CUTVCs are discussed comprehensively,and the manufacturing methods of container,support posts,wick and the gas-liquid arrangement of UTVCs are also developed.(2)Surface morphology control and performance characterization of wickTo enhance the wicking performance of the wicks,a micro-nano manufacture technology,oxidation treatment is proposed to control the surface morphology.The surface morphology and component of the wicks are characterized by scanning electron microscope(SEM)and Energy Dispersive x-ray Spectroscopy(EDS).The wettability of the wick is tested by contact angle measure devices.An accurate and reliable wicking performance evaluation method is proposed based capillary wicking dynamics.The wicking performance test setup is established,and the wicking performances of the wicks are also investigated.The experiment results indicate that oxidation treatment can significantly increase the surface roughness and the number of microchannels,further to improve the surface wettability,and hence enhancement of the wicking capability of the wick.In addition,the wicking performance of fibers is enhanced with the increase of oxidation time,but the additional wicking performance enhancement decrease as the oxidation time increase.Moreover,the long oxidation time will reduce the flexibility of the wicks,increase the risk of fiber fracture,further resulting in inconsecutive liquid flow channels.The wicking performance of fiber reduces with the decrease of its flattening thickness.When the thickness of fiber decreases to 0.15 mm or less,its wicking performance will significantly degrade.The optimal treatment parameters for fiber wicks are oxidation time of 40 min and thickness of 0.18 mm.Increasing the width of microgrooves will reduce the capillary force for liquid flowing and the resistance of vapor flowing.The optimal width of the microgroove is 0.2mm in the study.(3)Investigations on thermal performance and mechanism of CUTVCsThe maximum heat transfer capability theory and thermal resistance network are proposed for CUTVCs.The theoretical predicted maximum heat transfer capability and thermal resistance of the CUTVCs are 3.18 W and 0.93 ?/W,respectively.Thermal performance test apparatus is established.The thermal performances,including the temperature uniformity,maximum heat transfer capability,thermal resistance and effective thermal conductivity,of CUTVCs are systematically studied,and have been compared with vapor chambers in other literatures.The results show that CUTVCs with thickness of only 0.27 mm exhibit a maximum heat transfer capability of 3.5 W and thermal resistance of 1.2-1.5 ?/W.The placement has little effect on the thermal performance of CUTVCs.CUTVCs,with an effective thermal conductivity of more than 10000 W/(m·K),present a high competitiveness compared with the UTVCs in other literatures.Moreover,CUTVCs enable to effectively reduce the hot spot temperature of the cooling module,exhibit a wide prospect in heat dissipation for ultrathin smart mobile phone under the 5G era.(4)Optimization of process and structure parameters of CUTVCsThe effects of various process parameters(liquid injection and oxidation treatment)and structure parameters(distance between two support posts and thickness of vapor space)on thermal response performance and steady-state thermal performance are studied to optimize the thermal performance of CUTVCs.The results show that CUTVCs with optimal liquid injection of 30 mg can prevent occurs of too thick liquid film which will block the vapor flow and reduce the thermal response performance and temperature uniformity,and also delay the local dry-out and increase the maximum heat transfer capability.Oxidation treatment for fibers can improve the wicking capability,and hence enhancement of maximum heat transfer capability.Oxidation treatment for microgrooves enables the liquid to rapidly flow to fibers,preventing the vapor channels from blocking,and further degrading temperature uniformity.The increase of the distance between support posts can reduce the vapor flow resistance,but also will lead to the collapse deformation of containers and thereby increasing the vapor flow resistance.In addition,the effects of flexible bending deformation on thermal performance of CUTVCs are also investigated.The results indicate that CUTVCs with a high thermal conductivity of 7000W/(m·K)under high bending angles present high potential in the field of heat dissipation for flexible electronic devices.