Key Technology Analysis of Organic Silicon Materials for LED Package
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LED device performance 50% depends on the chip, 50% depends on the package and its materials. Packaging materials mainly play a protective chip and the output of visible light, the luminous efficiency of LED devices, brightness, life and so plays a key role. As technology advances, LED power, brightness, luminous efficiency continues to rise, and then the packaging material also made new requirements - the packaging process requires its high bonding strength, heat resistance, viscosity before curing appropriate ; LED performance requirements in terms of its high refractive index, high transmittance, heat aging, UV aging resistance, low stress, low moisture absorption, LED packaging materials has become the current constraints on the development of power LED key issues.
Currently LED packaging materials are commonly used epoxy and silicone materials. Epoxy resin has become the mainstream material of low-power LED package because of its excellent adhesion, electrical insulation, adhesion and dielectric properties, low cost, flexible formula, easy molding, high production efficiency. For power LED, due to hygroscopicity of epoxy resin, easy to aging, poor heat resistance and other inherent defects directly affect LED life; and in high temperature and short-wave light color, and thus affect the luminous efficiency; and its curing a certain amount of Toxicity and other shortcomings, has been far unable to meet the packaging materials in the high refractive index, low stress, high thermal conductivity, high UV resistance and high temperature aging performance requirements, it is not suitable for power LED packaging materials. Silicone material is excellent in heat aging resistance and UV aging resistance, and has the advantages of high light transmittance and low internal stress. It is considered as the best matrix resin for high refractive index silicone material for LED package, Power LED packaging materials research hot spots.
Development of silicone materials for encapsulation
The main chain of the silicone material is Si-O-Si bond, the side chains are connected with different functional groups, and the whole molecular chain is in a spiral shape. The special hetero chain molecular structure gives many excellent properties: low temperature resistance, heat resistance Excellent stability and weatherability, wide operating temperature range (-50-250 ℃), good hydrophobicity and very low moisture absorption (<0.2%), can effectively prevent the solution and moisture intrusion inside, thereby enhancing the LED The service life. Silicone material in addition to the above characteristics, but also has high light transmittance, UV resistance and other advantages, and the transmittance and refractive index can be adjusted by the ratio of phenyl and organic groups, its performance is significantly better than the epoxy resin, Is the ideal LED packaging materials.
With the development of power LED, epoxy resin has been unable to meet the requirements, but as LED packaging materials with good adhesion properties, dielectric properties, and low prices, easy to operate, in view of the advantages and reduce the performance of silicone materials Cost considerations, through the physical blending and chemical copolymerization methods to make silicone modified epoxy resin has become a number of research directions. Toughening modified epoxy resin by silicone material can improve the flexibility of its molecular chain, reduce its internal stress, and thus improve the cracking problem; the use of good heat resistance and strong UV-resistant silicone modified to improve the epoxy Resin aging resistance, poor heat resistance, UV resistance and other issues.
However, the epoxy resin contains an aromatic ring that absorbs UV rays, absorbs UV rays, oxidizes to generate carbonyl groups and forms a luminous color group to discolor the resin, and discolors after preheating, which in turn causes the epoxy resin to emit light in the near UV wavelength range Transmittance decreased, the LED luminous intensity greater impact. LED outdoor use contains a lot of UV, indoor use, a small amount of UV will make it yellow, and yellowing of epoxy resin is the main reason for the decrease of LED output light intensity, epoxy resin curing crosslinked high density, Large internal stress, brittleness, poor impact resistance and other shortcomings, therefore, silicone-modified epoxy resin is not the best choice for power LED packaging materials.
In recent years, people's research focus gradually shifted to the high refractive index, high thermal conductivity, high transmittance silicone packaging materials. At present, the power LED chips are mostly gallium nitride (GaN), which has a high refractive index of about 2.2, while the silicone packaging material has a relatively low refractive index of about 1.4, the difference between the refractive index of them Light rate has a great impact. When the chip glows through the packaging material, it will have a total reflection effect on its interface, resulting in most of the light reflected back inside, can not be effectively derived, the brightness performance is directly impaired. In order to reduce the light loss caused by the interface refraction more effectively and improve the light extraction efficiency as much as possible, the refractive index of the silicone and the lens material should be as high as possible. If the refractive index is increased from 1.5 to 1.6, the light extraction efficiency can be increased by about 20% . The refractive index of the ideal packaging material should be as close as possible to the refractive index of GaN. Therefore, the high refractive index transparent LED package with silicone material to reduce the chip and packaging materials refractive index difference is crucial.
With the continuous improvement of LED power, the heat dissipation problem of LED becomes more and more prominent. The larger the input power is, the larger the heat effect is. The higher the temperature, the lower the LED device performance will be directly reduced or attenuated, which will seriously affect the LED optoelectronic performance, .
The key technology of encapsulating organic silicon material
2.1 high refractive index
LED packaging technology is the biggest challenge to improve the LED chip to the air light extraction rate, according to Snell's equation:
Where i is the optical critical angle at the interface of the chip and the encapsulation material, n1 is the refractive index of the encapsulating material, n2 is the refractive index of the LED chip, and η0 is the light extraction rate. From the formula (1), (2) can be seen, only when the smaller the difference between n1 and n2, i closer to 180? , The greater the light extraction rate. Therefore, power LED device packaging materials have high refractive index requirements, the need for> 1.5.
The refractive index nd can be represented by the Lorentz-Lorentz equation:
Where, nd refractive index, RLL molar refractive index, V is the molar volume. From (3) can be seen, the refractive index and molar refractive index is proportional to the molecular molar volume is inversely proportional. Moore's refractive index is additive, therefore, the introduction of molecules in the molecular chain of molar refractive index and molecular volume ratio larger atoms or groups can increase the refractive index of the polymer, the refractive index of common atoms and the formation of chemical bonds when the refractive index increase See Table 1 for quantity.
As can be seen from Table 1, the increase of refractive index of halogen is larger, but the introduction of halogen will increase the density of organic silicon material, poor weatherability and yellowing easily, so the introduction of benzene, sulfur, nitrogen and other groups to improve the silicone However, Liu Jingang et al. Point out that the introduction of aromatic groups, sulfur atoms, halogen atoms other than fluorine, and organometallics will hardly exceed the refractive index of 1.8. As the benzene ring has a higher molar refractive index and a relatively small molecular volume, the high refractive index of the packaging material to phenyl-based silicone material, the refractive index of 1.40 to 1.7 within the change, is currently the most mature One of the ways. Studies have shown that: the larger the phenyl content, the higher the refractive index of the silicone encapsulation material, while reducing the shrinkage of the material, resistance to thermal cycling impact performance, the refractive index of the silicon material at a phenyl content of 40% 1.51, a refractive index of 1.54 at a phenyl content of 50%, and a refractive index of 1.57 at a total phenyl group; however, when the phenyl content is too high (over 50%), the light transmittance of the encapsulant decreases, Too large and useless product value, when W phenyl = 20% -40%, the overall performance of the product is relatively best.
Dow Corning Corporation OE-645O series are high refractive index two-component addition type silicone packaging materials, refractive index of 1.54; 0E-6630 series is also a high refractive index additive material, after curing resin, refractive index of 1.54, Shore D hardness of 33-52 degrees, elongation at break of 75% -100%. Miyoshi K and so on through the hydrolysis polycondensation synthesis of vinyl phenyl silicone resin, under the action of a platinum catalyst with a phenyl silicone oil cross-linking reaction, vulcanized to obtain a refractive index of 1.51 packaging material, the Shore D hardness of 75- 85 degrees, flexural strength of 95 ~ 135 MPa, tensile strength of 5.4 MPa, after 500 h ultraviolet light transmittance decreased from 95% to 92%. Joon-Soo Kim and others use a sol-gel method to synthesize a phenylvinylpolysiloxane through vinyltrimethoxysilane and diphenyldihydroxysilane and cross-react with a hydrosilylation catalyst under a platinum catalyst. The resulting resin The refractive index of 1.56, at 440 ℃ to maintain good thermal stability.
Yang Xiongfa et al. Methylphenyl dichlorosilane co-hydrolyzed with dimethyldichlorosilane, methylvinyl dichlorosilane and phenyltrichlorosilane and then copolymerized under KOH catalysis, sealed with trimethylchlorosilane Preparation of terminal methyl methyl vinyl silane containing methylphenyl vinyl resin and methyl phenyl hydrogen silicone oil by a certain percentage of platinum catalyst vulcanization molding LED encapsulated silicone resin products at 400 The transmittance at nm is> 90% and the refractive index is 1.52. Chen Zhidong and other methyl, vinyl, phenyl chlorosilane as raw materials, by hydrolysis of a polycondensation method of high refractive index silicone resin, a refractive index of 1.542 1, the light transmittance> 99%, and discussed the different processes on Effect of silicone resin properties. Komatsu will vinyl silicone polymer (composed of vinyl silicone resin, vinyl terminated polysiloxane), solid catalyst, hydrogen-containing polymer (composed of a polyhydridosiloxane, vinyl silicone resin Or a vinylhydrogen silicone), an inhibitor to synthesize a high refractive index silicone resin having a refractive index of 1.53, a light transmittance of 99% and a cure shrinkage of 2%, good UV resistance and moisture resistance.
The above studies are generally used to II platinum catalyst, studies have shown that the refractive index difference between any two components in the packaging material exceeds 0.06, it will affect the light transmittance and yellowing resistance of the packaging material, the refractive index of the platinum catalyst The rate will also have an impact on the system. Kato et al. Synthesized a 1,3-dimethyl-1,3-diphenyl-1,3-divinylsiloxane platinum complex by introducing a phenyl group-containing ligand so that the ratio of the catalyst to the The difference of refractive index is narrowed, the refractive index of the encapsulating material synthesized by using the catalyst is higher than 1.50, and the light transmittance is higher than 92%.
In recent years, many scholars have begun to pay attention to have a high refractive index, UV radiation resistance, high transmittance, good overall performance of nanocomposite silicone packaging materials. Such as: TiO2 and ZrO2 refractive index of 2.0 to 2.4, and the refractive index of LED chips close to its refractive index range beyond the phenyl-modified silicone material, is the ideal material for modified silicone materials. Wen-Chang Chen, etc. using hydrolysis condensation method, using phenyltrimethoxysilane phenyl silsesquioxane obtained, adding it to the n-butyl titanate condensation reaction, the final optical film, with the The Ti content varies from 0 to 54.8%, and the refractive index increases from 1.527 to 1.759 (corresponding to a wavelength of 277-322 nm). Taskar Nikhil R and so on using titanium butyl titanate to prepare nano-TiO2 particles, the outer coating of magnesium compounds, made of aluminum oxide or titanium oxide coated with a core-shell structure, the surface is modified and added to the silicone package Materials, get refractive index of about 1.7 nm modified LED packaging materials, its optical absorption is less, can slow down the LED light attenuation, increase the LED light extraction efficiency and extend the service life, but the preparation method is more complex and not suitable Production. Zhan Xi Bing and other non-hydrolytic sol-viscose prepared transparent titanium hybrid silicon resin, refractive index can reach 1.62, and has good transparency and photoelectric properties.
2.2 high thermal conductivity
LED chip conversion efficiency of about 15%, the remaining 85% converted to heat, the chip size is small, the power density is large, not timely cooling LED operating temperature will increase, the main impact of diminished brightness, the decay of life, the brightness The impact is linear and has an exponential relationship with life expectancy. Damage to the chip and packaging materials, affecting the life of the LED, reliability and luminous efficiency and other properties. Therefore, the packaging material is required to have good thermal conductivity, while the thermal conductivity of the silicone material is very low. The thermal conductivity of the pure silicone material is only 0.168 W / m · K. Therefore, it is very important to improve the thermal conductivity of the silicone material , Is also the main mode of power LED cooling.
The majority of polymer materials for thermal insulation materials, molecular structure alone to improve their thermal conductivity is very difficult to modify, the current method is often used to add matrix resin matrix filler filled with high thermal conductivity, such as alumina, aluminum nitride , Boron nitride, silicon carbide and the like. The thermal conductivity of the composite material is determined by the polymer itself and the high thermal conductivity filler. The thermal conductivity, shape, particle size and dosage of the thermal conductivity filler all affect the thermal conductivity of the final product. In addition, the compatibility between the thermal conductive filler and the resin matrix interface is poor, and the filler tends to agglomerate in the matrix resin, resulting in uneven dispersion and a difference in surface tension between the two, resulting in porosity in the interface and increased thermal resistance Therefore, it is necessary to modify the surface of the thermal filler.
Chen Jinghua and so on with different viscosity of vinyl silicone oil compound system as the base rubber, hydrogen silicone oil as a crosslinking agent to KH-570 treated silicon micropowder for the thermal filler, prepared a thermal conductivity of 0.63 W / m · K Silicone encapsulant was prepared by using aluminum oxide as thermal conductive filler and aluminum hydroxide as flame retardant to prepare a room temperature curing silicone electronic potting agent with thermal conductivity of 0.72 W / m · K. Zhao Nian and other cetyltrimethoxysilane modified alumina as a thermal conductive filler, diethyl aluminum phosphinate (ADP) as a flame retardant, vinyl silicone oil, hydrogen silicone oil-based glue, the thermal resistance Burning insulation silicone electronic potting, thermal conductivity after curing of 2.12 W / m · K, tensile strength of 1.72 MPa, elongation at break of 62%, the volume resistivity of 3.9 x 10Ω · cm. Hi-roshi and other spherical alumina filler for the thermal conductivity, and three siloxane-based single-terminated silicone resin mixed to prepare a high temperature vulcanized silicone rubber, thermal conductivity up to 3 W / m · K.
2.3 high transmittance
The light transmittance of silicone resin is better than that of epoxy resin. The larger the light transmittance is, the higher the luminous intensity and efficiency of LED are. The power LED requires the light transmittance of packaging material not less than 98% (wavelength is 400-800 nm, sample thickness 1 cm). Shiobara et al. Synthesized a variety of polymerization degree of vinyl silicone oil and hydrogen-containing silicone resin, to cross-link, curing, the resulting packaging material after aging at 200 ℃ for a long time after the light transmittance still reached 94%. Maneesh and other branched vinyl phenyl silicone resin and vinyl silicone oil, hydrogen silicone oil mixed curing LED packaging materials, the refractive index> 1.40, 200 ℃ aging 14d, the transmittance can still reach 98% (wavelength of 400 nm).
Power LED is the future direction of the development of light sources, with the support of national industrial policy, LED technology and products have been rapid development. LED package plays a key role in the performance of LED, which determines the product's luminous efficiency, service life, reliability and many years of research has made some achievements, developed a high refractive index, high thermal conductivity, high transmittance Silicone packaging materials, but there are still some technical barriers to overcome.
(1) Performance of Power LED Packaging Materials There is no outstanding and reliable performance of foreign products. Such products are basically monopolized by foreign countries.
(2) By modifying the organic silicon material can improve the performance of some aspects, but the overall performance is poor;
(3) Power LED has poor heat dissipation, adding filler can improve the thermal conductivity of the silicone encapsulation material, but leads to the decrease of the light transmittance of the encapsulation material, thus affecting the luminous efficiency;
(4) Silicone materials are expensive. Believe that as researchers continue to explore and experiment, we will be able to develop a comprehensive high performance, reliability, price-friendly silicone packaging materials.
Source: Guangdong LED