Light-emitting diode LED appeared in the 1960s. In the early 1990s, due to its breakthrough in epitaxy and chip technology, full-colorization appeared. The input power and brightness of the device were greatly improved. At present, the LED industry has entered high power. The period of rapid development of brightness. According to reports, China's power-type and high-power LEDs have reached the advanced level of international industrialization. The packaging of downstream devices has achieved mass production and has become an important LED packaging base in the world. In the LED industry, the research and production of epitaxial wafers and chips are progressing rapidly, but the research on packaging materials is relatively neglected. For a long time, the process of LED packaging has not changed much, and there has been no revolutionary breakthrough in packaging materials.
China's research and production of LED packaging materials and processes started late, with few varieties, and the technical level and production scale are far from the international level. Now there are only epoxy resin packaging materials for low-power LEDs. At present, high-end LED devices and high-power LED packaged silicone materials are required to be imported, which is expensive and greatly restricts the development of China's LED industry. At present, domestic and organic silicon related research units and production companies lack understanding of the LED packaging industry, and there are few research and development work on LED packaging related materials for silicone materials. Existing domestic silicone packaging materials have some defects: refraction The defects such as low rate, poor heat resistance, weak ultraviolet radiation resistance, insufficient product adhesion, and low light transmittance directly affect the luminous efficiency and life of the LED device. In this paper, the performance requirements of LED devices for packaging materials are reviewed. The research status of high-power LED packaging materials at home and abroad in recent years is reviewed. The problems of materials in the current packaging of high-power LEDs and the next research directions are discussed.
1. Performance requirements for LED packaging materials
On the one hand, the packaging materials for LEDs must meet the requirements of the packaging process, and on the other hand, they must meet the working requirements of LEDs. At present, the traditional epoxy resin packaging materials can not meet the requirements of high-power LED packaging in terms of ultraviolet light resistance and heat aging resistance. Many experts even believe that the backwardness of packaging materials and processes has played a bottleneck role in the development of the LED industry. Therefore, it is necessary to understand the performance requirements of LED packaging materials.
1.1 Packaging process for material performance requirements
In order to meet the needs of the process of the actual assembly operation of the LED, the encapsulating material should have suitable viscosity, adhesion and heat resistance, including: a, physical properties before curing, and general characteristics after curing. The physical properties before curing are related to handling properties, where viscosity and curing properties are particularly important. Due to the high expansion rate of the polymer material, the material shrinks after heat curing, resulting in significant shrinkage at the interface with the surrounding material, which causes peeling and cracking of the material, so it is cured as low temperature as possible. b, surface adhesion; the sealing material exposed on the surface of the package is sticky, which causes the sealing materials to adhere to each other. This condition that cannot be peeled off from the material selection machine leads to a decrease in operability. In addition, during use, it can also cause dust to stick and reduce brightness. From the aspect of peeling resistance and crack resistance, we need a softer packaging material, but in general, the soft material has higher viscosity, so we need a material with good balance between the two. , c, lead-free countercurrent. In recent years, the requirements for surface treatment of lead-free solders have become higher and higher, which also indicates that the heat resistance requirements of packaging materials are getting higher and higher. In the case of high-temperature reverse flow, peeling due to coloring, severe thermal changes, cracks, wire breakage, and the like occur.
1.2 Light transmittance
The absorption of visible light by the LED packaging material leads to a low light extraction rate, and the packaging material has low absorbance and high transparency. Silicone resins have higher transparency than epoxy resins. At present, the encapsulating material prepared by using silicone resin has a transmittance of more than 95% in the ultraviolet region, which increases the light transmittance and luminous intensity of the high-power LED device.
1.3 refractive index
The difference in refractive index between the LED chip and the encapsulating material has a great influence on the light extraction rate, so the refractive index of the material is increased, so that it is as close as possible to the refractive index of the LED chip, which is favorable for light transmission, generally In other words, the refractive index of the LED chip (n=2.2-2.4) is much higher than the refractive index of the silicone packaging material (n=1.41). When the chip emits light through the package material, a total reflection effect occurs at the interface, resulting in Most of the light is reflected back inside the chip and cannot be effectively exported. The brightness performance is directly impaired. To solve this problem, it is necessary to increase the refractive index of the encapsulating material to reduce the total reflection loss. Studies have pointed out that as the refractive index of the packaging material increases, the brightness of the LED will be increased. In the case of a red LED device, when the refractive index of the package material is 1.7, the external light extraction efficiency can be increased by 44%. Therefore, the development of high refractive index transparent materials to reduce the refractive index difference between the chip and the packaging material is important.
1.4 Heat aging and light resistance
In high-power high-brightness LEDs, the packaging material is not only strongly exposed to light, but also affected by heat dissipation. Therefore, the package material needs to have both light resistance and heat resistance. Even if exposed to high temperature for a long time, the sealing material is required to ensure no discoloration and stable physical properties.
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