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, 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.
1.1 Packaging process for material performance requirements
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.
1.2 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.
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 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.
1.4 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.
2 Research Status of Power LED Packaging Materials
2.1 Silicone/Epoxy Resin Packaging Materials
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.
Considering that the LED chip heating is the main cause of the aging of the packaging material, some packaging manufacturers use the inner layer of the sheet to brush the silicone material, and the outer lens material selects the epoxy resin, PC, PMMA, and the like. However, in actual application, it was found that epoxy resin, PC, and PMMA have insufficient aging resistance, and the inner layer silicone material also has poor interface compatibility.
When the silicone modified epoxy resin is used as a packaging material, the toughness and high and low temperature resistance of the material are significantly improved. It has been reported in the literature that an addition of silicone and epoxy resin is used as a packaging material, and a mixture of a polyorganosiloxane containing a vinyl group and a Si-OH group and a ring-vanadium resin having a specific structure is used as a base polymer, and a crosslinking agent is added. , catalyst, thinner, formulated into packaging materials for LED packaging, heat-resistant test does not change color, a 40-120 oC cold and thermal impact without peeling and cracking occurs, LED luminous efficiency is high. There are also reports of mixed encapsulants using epoxy-modified polyorganooxane ring vanadium compounds], which are mixed with an epoxy-modified polyorganosiloxane and an aliphatic or alicyclic epoxy compound. The packaged material has the characteristics of resistance to UVV aging, thermal shock resistance, high transparency, high hardness and good adhesion to the substrate, and is suitable for the packaging of blue and white LEDs with wavelengths below 500 nm. The specific structure of epoxy resin and polyorganosiloxane I, ED encapsulant can improve the heat resistance of epoxy resin, resistance to uV photoaging, and improve the adhesion of silicone materials, surface adhesion Attachment is a development path worthy of attention.
Japan Shin-Etsu Co., Ltd. uses a mixture of silicone resin, silicone oil and silicone rubber containing silicon hydroxy group to react with epoxy resin, and finally obtains 70 (Shore D), 1.51 (nd, 25 ° C) modified resin material by plastic molding. It can resist nearly a thousand thermal shock tests.
The US GE company patent reported that the silane will be co-hydrolyzed and polycondensed to obtain a hydroxyl-containing silicone resin, and the silicone resin will be compounded with the silicone-modified epoxy resin. The refractive index of the packaged material prepared after high-temperature vulcanization can be up to 1.60. The material (5mm thickness) is still higher than 80% after being irradiated by light wave with a wavelength of 380nm for 500 h or ultraviolet light at a wavelength of 400-450 nm for 500 h at 150 °C.
Quartz powder, single crystal silicon, aluminum powder, zinc powder, glass fiber, etc. are commonly used to improve the heat resistance and thermal conductivity of LED packaging materials. The US patent report adds nano-scale silica and nano-sized spherical glass powder to silicone-modified epoxy resin. The prepared packaging material has a refractive index of up to 1.56 and a light transmittance of over 95%. After 200 thermal shocks. The damage rate does not exceed 15%.
2.2 Silicone packaging materials
A large number of research reports have proved that silicone modification can improve the performance of epoxy resin encapsulants, but the molecular structure of silicone modified materials contains epoxy groups, which is unfavorable to the radiation resistance of materials, and there is a problem of yellowing. The optical clarity and thermal stability of silicone materials play an important role in high brightness and high reliability LED applications. Silicone materials are rapidly replacing epoxy and other organic materials, providing a wide range of potting materials, lens materials, adhesives, sealants and protective coatings for a variety of LED applications.
There are two types of silicone sealing materials currently on the market: high refractive index type and common refractive index type silicone materials, including gel, silicone rubber and silicone resin. The ordinary refractive index type silicone is mainly dimethyl siloxane, and the high refractive index type organic silicon is mainly phenyl methyl guanidin. The three major silicone companies in the world, Dow Corning, Momentive and Shin-Etsu products, are mainly high-refractive-index silicone packaging materials, and have an absolute advantage in the domestic high-end market.
At present, several mainstream suppliers of silicone LED packaging materials on the market are Japan's Shin-Etsu, Dow Corning, Momentive and Nusil Technologies, which have successively launched silica gel and silicone products with a refractive index of more than 1.50. Among them, Dow Corning has been researching polymer polymerization technology for 120 years. The research on LED packaging materials is at the leading level. The product range is complete, the market price is 5000-6000 kg. The main grades of high refractive index products are: SR7010, OE26336, OE6550 , JCR6175 and so on. Japan's Shin-Etsu company's products have superior aging resistance and higher positioning. The market price is 7,000 yuan / kg. The main brands are: SCR-1012, KER-2500, LPS-5547 and so on. Momentive's silicone has outstanding operability. The market price is 5,000 yuan/kg. It is mainly used in IVS series, with grades 5332, 5862, 4542, 4622 and so on.
In order to increase the refractive index of the encapsulating material, it has been studied that a high refractive index phenyl silicone-containing hydrogen-containing silicone oil and a platinum-based catalyst can be used for the curing reaction of the base polymer, and the refractive index of the material can be remarkably improved, and The mechanical strength and thermal resistance of the material are also improved to varying degrees. It is also possible to reduce the shrinkage of such silicone materials by increasing the mass fraction of phenyl groups in the encapsulating material, and to improve the thermal and thermal cycle impact resistance, excellent mechanical properties and bonding properties. In addition, some people have added liquid silicone rubber by injection molding, and experiments have shown that the shrinkage of rubber materials can be reduced. For resin materials, injection molding is also suitable. It has been reported that when preparing materials of the same hardness, injection molding can increase the elastic modulus of the material by about 3% without affecting the light transmittance of the material, and the addition of inorganic filler can also Effectively improve the heat resistance and radiation resistance of silicone rubber. Recently, some people have studied the problems of long curing cycle and high curing temperature of packaging materials. Adding sensitizer to the formulation and curing with visible light or ultraviolet light can be cured completely in 15-20 minutes, while other properties are not affected. The platinum material for LED encapsulation generally uses a platinum catalyst, and the platinum catalyst has a short storage time and is prone to yellowing, which is unfavorable for the light transmission property of the encapsulating material. For this reason, a phenyl group-containing siloxane is used as a ligand. A platinum catalyst which is not easily discolored is studied. The silicone rubber encapsulant prepared by using the catalyst has a high refractive index and a light transmission performance exceeding 95%.
From the current market point of view, the amount of phenyl silicone resin encapsulant added to the silicone packaging material has a significant increasing trend. The silicone resin is easy to process, has a high refractive index, good light transmission performance and bonding performance, and is resistant to ultraviolet light. It has strong heat aging ability, and the addition of two-functional chain segment can also improve its impact resistance to a certain extent, and can be used for LED packaging or lens materials. In addition, the use of ultraviolet light to cure the silicone resin can also obtain a packaging material with excellent performance, especially the material has excellent anti-tarnishing ability, and can be used for packaging white power type LED.
In combination with the above-mentioned silicone materials for high refractive index LED packaging prepared abroad, it is not difficult to see that they select a phenyl group-containing polysiloxane when selecting a base polymer. At present, domestically produced phenyl-containing polysiloxanes can only be used to produce low-end products with low performance requirements, while high-performance phenyl-containing polysiloxanes still rely on imports. It is for this reason that there are not many reports on domestic high-refractive index silicone materials, but some progress has been made.
The Institute of Chemistry of the Chinese Academy of Sciences has prepared a silicone oil with a refractive index of 1.56 in the laboratory. Hangzhou Normal University uses a mixed ring siloxane for co-ring polymerization, and is prepared by cation exchange resin at 40-80 ° C in a toluene solvent. A colorless transparent silicone oil containing hydrogen at the terminal group having a refractive index of 1.39 to 1.51 (25 ° C) is obtained. The Institute of Chemistry of the Chinese Academy of Sciences has developed a silicone-modified epoxy resin LED lens material, but the material is not as resistant to ultraviolet light and heat aging as silicone. Recently, Shenzhen Fangda Guoke Optoelectronic Technology Co., Ltd. reported that the composite silicone resin and silicone oil are mixed, and the transmittance of the packaging material prepared by the hydrosilylation reaction can be as high as 98%, which has achieved good application effect on high-power white LED. .
3 Conclusion
With the in-depth study of the luminous efficiency of LED devices, new requirements have been put forward for the light aging, heat aging, refractive index and light transmittance of packaging materials. At present, LED packaging companies at home and abroad have begun to use transparent silicone materials with excellent UV aging resistance and heat aging resistance instead of epoxy resin as packaging materials. These materials have become the research and product development of several large foreign companies and research institutions. Hot spot.
Undoubtedly, silicone packaging materials are ideal for LED packaging requirements and are rapidly replacing epoxy and other organic materials, providing a wide range of potting materials, lens materials, adhesives, sealants and applications for a variety of LED applications. Protective coating products. However, with the rapid development of the LED industry, the need for diversified development of brightness, use, packaging process, design, etc. has created demand for different hardness and larger refractive index packaging materials, and at the same time, in order to ensure the reliability after packaging, choose the right one. Hardness and cohesive materials are also very important. At present, the main problems of silicone materials for LED packaging are as follows: (1) From the viewpoint of chemical structure and composition of organic silicon, high refractive index is difficult, and high-refractive-index ultra-fine nanoparticles are used for modification. Has received much attention. (2) The thermal conductivity of the silicone material is low. In order to improve the thermal conductivity, it is necessary to increase the filling rate of high conductivity auxiliary materials such as alumina and silver. However, transparency cannot be obtained by using only the prior art, and further technological breakthroughs are required. (3) Compared with other LED packaging materials, silicone has many advantages, but it does not have strong mechanical strength and high thermal expansion rate. To compensate for this defect, the material needs to be modified. The above points will be a very serious challenge for the silicone manufacturing industry.
Edit: Cedar
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