(I) 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, epoxy resin accounts for a large proportion in the domestic packaging materials market, because the resin itself has excellent electrical insulation, adhesion, dielectric properties, transparency, and good adhesion; at the same time, storage stability, flexible formulation, operation Simple, but the higher working temperature and ultraviolet radiation make the transparency of epoxy resin seriously degrade, it is difficult to meet the requirements of high-power LED packaging . Many experts even believe that the backwardness of packaging materials and processes has already bottlenecked the development of LED industry. effect. Packaging process requirements for material properties In order to meet the needs of the LED assembly process, the packaging material should have suitable viscosity, adhesion and temperature resistance, including: (1) physical properties before curing, general characteristics after curing. The physical properties before curing are related to operability, where viscosity and curing properties are particularly important. Due to the high expansion rate of the polymer material, after heat curing, the material cooling shrinks significantly, resulting in stress at the interface with the surrounding material, which in turn causes peeling and cracking of the material, so it is cured as low temperature as possible. (2) Surface adhesion. The exposed sealing material on the surface of the package is sticky, which causes the sealing materials to adhere to each other. This failure to peel off from the material selection machine results in a decrease in operability. In addition, during use, it can also cause dust to stick and reduce brightness. From the viewpoint of peeling resistance and crack resistance, a relatively soft encapsulating material is required, but in general, a softer material has a higher viscosity, and therefore, a material having a good balance between the two is required. (3) Lead-free countercurrent. In recent years, the requirements for the 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. Light transmittance The absorption of visible light by the LED packaging material leads to a decrease in the light extraction rate, and the packaging material has a low light absorption rate and high transparency. Silicone resins are more transparent than epoxy resins. The transmittance of the silicone resin-based encapsulating material in the ultraviolet region can be more than 95%, and the light transmittance and luminous intensity of the LED device are greatly improved. 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 to be as close as possible to the refractive index of the LED chip, which is favorable for light transmission. In general, the refractive index of the LED chip (/7, = 2.2-2.4) is much higher than the refractive index of the silicone packaging material (/7, = 1.41). When the chip emits light through the package material, it will occur at its interface. The total reflection effect causes most of the light to be reflected back inside the chip, which cannot be effectively exported, and the brightness performance is directly impaired. To solve this problem, the refractive index of the encapsulating material must be increased 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 importance of developing a high refractive index transparent material to reduce the refractive index difference between the chip and the packaging material is obvious. Heat aging and light aging resistance In high-power high-brightness LEDs, the packaging material is not only subject to intense illumination, but also affected by the heat dissipation of the chip. Therefore, the packaging 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. (II) Research status of power LED packaging materials Silicone/epoxy encapsulant As the power and brightness of LEDs become larger and larger, it is difficult to meet the packaging requirements in terms of light resistance and heat aging of epoxy resins. However, epoxy resins have excellent dielectric and bonding properties, especially at low cost and low cost. So in the past, researchers have not abandoned the use of epoxy resin, but instead used silicone to modify the epoxy resin to develop packaging materials that combine the advantages of both materials.
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