Study On Light-Emitting Diode (LED) Silicon Substrate Integrated With Micro Heat Pipe

Considering that the shortage of energy is becoming more serious around the world, conserving energy and reducing emissions have become important issues. As a new type of semiconductor cold light illuminator, the light-emitting diode (LED) is widely used in the lighting area, and is popular for its energy conservation and environmental protection. However, the current research showed about 15-30% of the input electricity is transformed into light, while the rest into heat. As the junction temperature becomes higher, the luminous decay happens earlier and the LED’s life is shorter. Thus how to improve the efficiency of the heat dissipation and keep the junction temperature stable are the key issues for the manufacturing of high power LED devices.Silicon with a high thermal conductivity and a mature manufacturing process is trending to be the substrate material of the LED module nowadays. Micro heat pipes (MHP) conduct heat by phase change and have a high thermal conductivity. Thus the subject used the LED silicon substrate integrated with the MHP to explore a new method of high efficiency heat dissipation and to decrease the thermal resistance between the chip and the atmosphere. The micro flat heat pipe (MFHP) and the vapor chamber heat pipe (VCHP) were integrated together on the silicon substrate to reduce the thermal interfaces. To dissipate the heat of LEDs with the power of 3-10 W, the microgrooves and LEDs’electrodes on the two sides of the silicon substrate were fabricated by MEMS machining, respectively. Then the substrate was bonded with a Pyrex 7740 glass or an aluminum fin to form the body construction of the LED silicon substrate integrated with the MHP. The MHP was vacuumed and then charged with a certain amount of working fluid (degassed deionized water). After that,1 W LED chips were bonded on the LED bases of the LED silicon substrate by conductive silver and connected with electrodes by gold thread using ultrasonic wire bonding method. The thermal test experiments of the fabricated silicon substrate in the vacuum adiabatic environment and atmosphere environment were carried out, respectively. The results showed the LED silicon substrate integrated with the MHP could dissipate the heat effectively. And the equivalent thermal conductivity of the non-parallel micro flat heat pipe was 5.65 times of that of the silicon.In order to investigate the influence of the structure, size and surface energy of the microgrooves on the performance of the MFHP, the trapezoid-shaped and side wall sub-shaped microgrooves were designed. The water contact measurements showed the microgrooves with those two kinds of structure were more hydrophilic than the parallel microgrooves with the same size. And thermal test results showed the equivalent thermal conductivities of the trapezoid-shaped MFHP and the side wall sub-shaped MFHP were enhanced by 18.52-151.83% and 7.71-9.62%, respectively, compared with the parallel-shaped MFHP. Electroforming copper pillars and depositing graphene on the silicon microgrooves could change the hydrophobicity of the microgrooves. The microgrooves modified with copper pillars were more hydrophilic, while the microgrooves modified with the electrospray depositing graphene sheet were more hydrophobic than the normal ones. Thermal test results showed that compared with the unmodified one, the MFHP modified with copper pillars worked as a vapor chamber and the stable temperature of the MFHP modified with graphene was decreased by 4.87%,. The copper microgrooves electroformed on a silicon wafer showed better hydrophilicity than the silicon microgrooves. Thermal test results showed the equivalent thermal conductivity of the MFHP with copper microgrooves was enhanced by about 17%, compared with the MFHP with silicon microgrooves. These methods improved the hydrophilicity of the microgrooves, and changed the capillary traction, which made the working fluid cycle effectively and improved the performance of the MHP. This work was significant to the research of the LED silicon substrate integrated with micro radiators.Above all, the LED silicon substrate integrated with the micro heat pipe was designed and fabricated for high power LED devices, and the structure and surface energy of the microgrooves were optimized in this paper. Results showed the fabricated substrate improved the heat transfer capability effectively, which were a guide and experience for the efficient heat transport and the junction temperature’s stability control of high power LEDs.