Qian Keyuan, Liu Hongtao, Semiconductor Lighting Laboratory, Shenzhen Graduate School, Tsinghua University, Shenzhen 5180558,
Abstract: LED road lighting is one of the most demanding products in LED lighting products. The reliability of the system is an important aspect that affects the application of LED road lighting. The reliability of LED light source is the key factor that limits the life of the lighting. Through the analysis of the thermal resistance of the LED light source and the failure mechanism analysis, the method of judging the reliability of the LED light source device is explored, and the key points that limit the life of the actual LED product are found to provide reference for designing the high-efficiency and high-reliability LED road lighting system.
Keywords: power LED, reliability, performance degradation
1. Introduction <br> In recent years, in the field of outdoor lighting, new LED street lights are attracting the attention of many industries and investors. Hundreds of companies across the country have launched their own LED street lighting products, almost all cities have more or more Less LED street lights have been installed on the main roads of the city. However, in many product sequences, the products are still not perfect, and some of the products are still not perfect. The biggest problem is the overall reliability of the LED street lamps. LED road lighting systems differ from traditional lighting fixtures in that they are much more complex and have many factors that affect overall reliability. See Figure (1).
The inherent characteristics must be considered when studying the reliability of their systems.
1. The technical specifications of the light source module itself are subject to various factors.
2. The parameters of the light source module are very discrete
3. Device reliability is highly dependent on heat dissipation
4. LED drive power design life is far behind LED light source
Among the many factors, the performance index of the LED light-emitting device itself is undoubtedly the most important component to determine the performance of the lamp, such as light efficiency and life, and is also the most concerned research aspect.
In order to improve performance and improve reliability, we analyze and study the main factors affecting the reliability of LED light-emitting devices, and initially discuss the failure mechanism of LED light sources.
2. Solid-state thermal resistance analysis of LED chip <br> The solid-state thermal resistance is the thermal resistance introduced by the solid crystal layer between the chip and the pedestal, which has a great influence on the heat dissipation effect of the chip, and is the main thermal resistance of the package. As a component, the analysis of the solid-state thermal resistance of different materials has been stuck in the numerical simulation stage [11] [12]. The existing thermal resistance test equipment can not distinguish the thermal resistance introduced by each part, and the actual process often needs to evaluate the solid crystal thermal resistance. For this reason, the dynamic junction temperature test method developed from the forward voltage method can be effective. A new method for analyzing the thermal resistance of the solid crystal layer.
Many types of solid crystal materials are currently encountered. The thermal conductivity of these materials ranges from 0.2 to 60 W/K·m. The thermal resistance of the solid crystal is different when using different materials. In order to study the thermal resistance of the solid crystal with thermal conductivity. The relationship between the rate changes, we did a numerical simulation analysis of the solid crystal thermal resistance in a simple model, ignoring the temperature gradient of the chip itself, the 1x1x0.1mm 1W LED chip is solidified on a 30x30x5mm aluminum substrate, and the ambient temperature is At 20 ° C, the solid crystal layer was 25 μm thick. When the thermal conductivity of the solid crystal material is 18W/Km, the local temperature distribution of the chip and the substrate is simulated by professional thermal simulation software. As shown in Fig. 2, the chip temperature is 54.06 ° C, and the substrate temperature is 52.77 ° C. At this time, the solid crystal thermal resistance is 1.29 K/W. The thermal conductivity of the solid crystal layer material is changed from 2 to 60 W/Km, and the solid crystal thermal resistance is simulated as shown in Fig. 3.
Figure 2 Thermal simulation temperature profile
Figure (3) Solid-state thermal resistance changes with the thermal conductivity of the die-bonding material (solid crystal layer thickness 25 μm)
It can be seen that the thermal resistance of the solid crystal is related to the thermal conductivity of the solid crystal material. When the thickness of the solid crystal layer is 25 μm, the thermal conductivity of the solid crystal material increases as the thermal conductivity of the solid crystal material decreases, and the thermal conductivity of the material reaches 22 Above, the thermal resistance is less than 1, and then the thermal resistance changes with the increase of the thermal conductivity of the material is not as obvious as the previous period.
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