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Light emitting diode (LED) is widely used due to its energy-saving,environment friendly and long life and so on.But the low electro-optic conversion efficiency and the pursuit of low cost always make the LED devices wording under a high junction temperature condition,this can significantly affect the reliability of LED device.As a result,it is necessary to carry out the heat dissipation performance improvements by the structure design and optimization of LED device.The thermal resistance characteristics of LED were studied by combining T3ster thermal resistance test and the theoretical calculation.Two groups of LED devices with different printed circuit board substrate parameters (with different copper layer thickness and different substrate area) are tested,the results show that the thermal resistance of LED with different substrate area showed a difference as high as 13.3%.For this group,the dimension difference is the two screw holes.The thermal resistance of LED with different copper layer thickness (35μm and 70 μm) shows a difference as high as 32%.The theoretical calculation analysis show that the area of substrate as cooling area can affect the thermal spreading resistance and convection thermal resistance,especially the thickness of copper layer has a great influence on the thermal spreading resistance,the difference of thermal resistance mainly be caused by the difference of thermal spreading resistance;Thermal spreading resistance decrease significantly with the increase of the copper layer thickness.The theoretical calculation analysis also shows that with the same substrate,the thermal spreading resistance can be obviously affected by the layout of the LED device on the substrate.So in order to improve the heat dissipation performances of LED device,not only the choice of material with high thermal conductivity,but the device structure and the layout optimizing of the LED device on the substrate is also important.