The performance of LED devices is 50% dependent on the chip, and 50% depends on the package and its materials. The packaging material mainly plays a role in protecting the chip and outputting visible light, and plays a key role in the luminous efficiency, brightness and service life of the LED device. With the advancement of technology, the power, brightness and luminous efficiency of LEDs have been continuously improved, and new requirements have been placed on packaging materials. For the packaging process, high bonding strength, good heat resistance and suitable viscosity before curing are required. LED performance is required to have high refractive index, high light transmittance, heat aging resistance, UV aging resistance, low stress, low moisture absorption, etc. LED packaging materials have become a key issue in the current development of power LEDs.
At present, the commonly used packaging materials for LEDs are epoxy resins and silicone materials. Epoxy resin has become a mainstream material for low-power LED packaging because of its excellent adhesion, electrical insulation, adhesion and dielectric properties, low cost, flexible formulation, easy molding, and high production efficiency. For power LEDs, congenital defects such as strong hygroscopicity, aging, and poor heat resistance directly affect the life of the LED; and it is easy to change color under high temperature and short-wave illumination, which affects the luminous efficiency; and it has a certain degree before curing. Disadvantages such as toxicity are far from meeting the requirements of high refractive index, low stress, high thermal conductivity, high UV resistance and high temperature aging resistance of packaging materials, so it is not suitable for packaging materials as power LEDs. Silicone materials have excellent heat aging resistance and ultraviolet aging resistance, and have high light transmittance and low internal stress. They are considered to be the best matrix resin for high refractive index silicone materials for LED packaging, and have also become Research hotspots for materials for power LED packaging.
Development of silicone materials for packaging
The main chain of the silicone material is Si—O—Si bond, the side chain is connected with different functional groups, and the entire molecular chain is spiral. This special hetero chain molecular structure gives it many excellent properties: low temperature resistance, heat resistance Excellent stability and weather resistance, wide operating temperature range (-50-250 ° C), good hydrophobicity and very weak hygroscopicity (<0.2%), can effectively prevent solution and moisture from invading the interior, thus improving LED The service life. In addition to the above characteristics, the silicone material has the advantages of high light transmittance, high ultraviolet light resistance, and the light transmittance and refractive index can be adjusted by the ratio of the phenyl group to the organic group, and its performance is significantly better than that of the epoxy resin. It is an ideal LED packaging material.
With the development of power LEDs, epoxy resins can no longer meet the requirements, but they have good bonding properties and dielectric properties as LED packaging materials, and are inexpensive and easy to operate, in view of the advantages and disadvantages of silicone materials. In terms of cost, silicone-modified epoxy resin has become a research direction through physical blending and chemical copolymerization. Toughening modified epoxy resin through silicone material can improve the flexibility of its molecular chain, reduce its internal stress, and improve the cracking problem. It can be modified by the good heat resistance and UV resistance of silicone. The resin has problems such as aging resistance, poor heat resistance, and resistance to ultraviolet light.
However, the epoxy resin contains an aromatic ring that absorbs ultraviolet rays. After absorbing ultraviolet rays, it will oxidize to form a carbonyl group and form a luminescent chromophore to discolor the resin, and will also change color after preheating, thereby causing the epoxy resin to be in the near ultraviolet wavelength range. The light transmittance decreases, which has a great influence on the luminous intensity of the LED. The outdoor use of LEDs contains a lot of ultraviolet light. Indoor use, a small amount of ultraviolet light will also make it yellow, and the yellowing of epoxy resin is the main reason for the decrease of LED output light intensity. At the same time, the epoxy resin has high crosslink density after curing. The internal stress is large, the brittleness is large, and the impact resistance is poor. Therefore, the silicone modified epoxy resin is not the best choice for the packaging materials for power LEDs.
In recent years, people's research hotspots have gradually shifted to silicone packaging materials with high refractive index, high thermal conductivity and high light transmittance. At present, the chips of power LEDs are mostly gallium nitride (GaN), and their refractive index is high, about 2.2, while the refractive index of silicone packaging materials is relatively low, about 1.4, and the difference in refractive index between them is taken. Light rate has a big impact. When the chip emits light through the package material, a total reflection effect occurs on the interface, causing most of the light to be reflected back to the inside, which cannot be effectively exported, and the brightness performance is directly impaired. In order to reduce the light loss caused by the interface refraction more effectively, and to improve the light extraction efficiency as much as possible, the refractive index of the silicone and the lens material is required to be as high as possible. If the refractive index is increased from 1.5 to 1.6, the light extraction efficiency can be improved by about 20%. . The refractive index of an ideal packaging material should be as close as possible to the refractive index of GaN. Therefore, a silicone material for a high refractive index transparent LED package is critical for reducing the refractive index difference between the chip and the package material.
As the LED power continues to increase, the heat dissipation problem of the LED becomes more and more prominent. The larger the input power, the greater the heating effect. The excessive temperature directly leads to the performance degradation or attenuation of the LED device, which seriously affects the photoelectric performance of the LED and even disables the LED. .
Key technologies for packaging silicone materials
2.1 High refractive index
The biggest challenge of LED packaging technology is to increase the light extraction rate of LED chips to air, according to the Snell equation:
Where i is the optical critical angle of the interface between the chip and the package material, n1 is the refractive index of the package material, n2 is the refractive index of the LED chip, and η0 is the light extraction rate. It can be seen from equations (1) and (2) that only the smaller the difference between n1 and n2, the closer i is to 180? The greater the light extraction rate. Therefore, the power LED device packaging material has a high refractive index requirement of >1.5.
The refractive index nd can be expressed by the Lorentz-Lorentz equation:
Where nd is the refractive index, RLL is the molar refraction, and V is the molar volume. It can be seen from the formula (3) that the refractive index is proportional to the molar refraction and the molecular molar volume is inversely proportional. The molar refraction has an additive property. Therefore, the introduction of an atom or a group having a larger molar refractive index and a larger molecular volume ratio in a molecular chain can increase the refractive index of the polymer, and the refractive index of a common atom and the degree of refraction when a chemical bond is formed are increased. The amount is shown in Table 1.
It can be seen from Table 1 that the increase in the refractive index of halogen is large, but the introduction of halogen causes the density of the silicone material to increase, the weather resistance is poor, and the yellowing is easy, so that the silicone can be increased by introducing a group such as benzene, sulfur, nitrogen or the like. The refractive index of the material, however, Liu Jingang et al. pointed out that the introduction of an aromatic group, a sulfur atom, a halogen atom other than fluorine, and a metal organic compound have a maximum refractive index of hardly exceeding 1.8. Because the benzene ring has a high molar refraction and a relatively small molecular volume, the high refractive index encapsulating material is mainly composed of a phenyl type silicone material, and the refractive index varies from 1.40 to 1.7, which is the most mature research at present. One of the methods. Studies have shown that the higher the phenyl mass fraction, the higher the refractive index of the silicone encapsulating material, the lower the shrinkage of the material, the higher the thermal shock resistance, and the refraction of the silicon material with a phenyl mass fraction of 40%. The ratio is 1.51, the refractive index is >1.54 when the phenyl content is 50%, and the refractive index is 1.57 when the phenyl group is too high; however, when the phenyl content is too high (more than 50%), the transmittance of the encapsulating material is lowered, and the thermoplasticity is lowered. Too large to make the product useless, when W phenyl = 20% -40%, the overall performance of the product is relatively best.
Dow Corning's OE-645O series is a high refractive index two-component addition silicone packaging material with a refractive index of 1.54. The 0E-6630 series is also a high refractive index addition molding material. After curing, it is a resin with a refractive index of 1.54. D hardness is 33-52 degrees, and elongation at break is 75%-100%. Miyoshi K et al. synthesized vinyl phenyl silicone resin by hydrolysis polycondensation method, and cross-linked with phenyl hydrogen silicone oil under the action of platinum catalyst to obtain a package material with refractive index of 1.51, and its Shore D hardness was 75- The 85 degree, the bending strength is 95-135 MPa, the tensile strength is 5.4 MPa, and the light transmittance is reduced from 95% to 92% after 500 h of ultraviolet irradiation. Joon-Soo Kim et al. used a sol-gel method to synthesize phenylvinylpolysiloxane from vinyltrimethoxysilane and diphenyldihydroxysilane, and cross-linked with a silicon hydride compound under a platinum catalyst. The refractive index is 1.56 and maintains good thermal stability at around 440 °C.
Yang Xiongfa et al. co-hydrolyzed methylphenyldichlorosilane with dimethyldichlorosilane, methylvinyldichlorosilane and phenyltrichlorosilane, copolymerized under KOH catalysis, and sealed with trimethylchlorosilane. The terminal agent prepares a methylphenyl vinyl resin containing a methylphenylsiloxane chain and is vulcanized with a methylphenyl hydrogen silicone oil in a certain ratio under a platinum catalyst to obtain an LED packaged silicone resin. The product is 400. The light transmittance at nm is >90% and the refractive index is 1.52. Chen Zhidong et al. used methyl, vinyl and phenylchlorosilane as raw materials to prepare high refractive index silicone resin by hydrolysis-polycondensation method. The refractive index was 1.542 1, the transmittance was >99%, and different process pairs were discussed. The effect of silicone resin properties. Kesong vinyl vinyl polymer (composed of vinyl silicone resin, vinyl terminated polysiloxane), solid catalyst, hydrogen-containing polymer (from polyhydrogensiloxane, vinyl silicone) Or a vinyl hydrogen-based silicone resin composition, an inhibitor to synthesize a high refractive index silicone resin, a refractive index of 1.53, a light transmittance of 99%, a cure shrinkage of 2%, and good UV resistance test and moisture resistance.
The above studies generally use II platinum catalysts. Studies have shown that when 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, and the refractive index of the platinum catalyst. The rate will also have an impact on the system. Kato et al. synthesized a platinum complex of 1,3-dimethyl-1,3-diphenyl-1,3-divinylsiloxane by introducing a ligand containing a phenyl group to form a catalyst and a packaged raw material. The difference in refractive index is reduced, and the refractive index of the encapsulating material synthesized by the catalyst is higher than 1.50, and the transmittance is higher than 92%.
In recent years, many scholars have begun to pay attention to nanocomposite silicone packaging materials with high refractive index, strong ultraviolet radiation resistance, high light transmittance and good comprehensive performance. For example, the refractive index of TiO2 and ZrO2 is in the range of 2.0 to 2.4, which is close to the refractive index of the LED chip. The refractive index range greatly exceeds the modification of the phenyl group to the silicone material, and is an ideal material for modifying the silicone material. Wen-Chang Chen et al. used hydrolytic condensation to obtain phenyl silsesquioxane from phenyltrimethoxysilane, which was added to n-butyl titanate to undergo condensation reaction, and finally an optical film was obtained. The Ti content varies from 0 to 54.8%, and the refractive index can be increased from 1.527 to 1.759 (corresponding wavelength is 277-322 nm). Taskar Nikhil R et al. used nano-TiO2 particles to prepare nano-TiO2 particles, and the outer layer was coated with magnesium compound. At the same time, it was made into a core-shell structure coated with aluminum oxide or titanium oxide. The surface was modified and added to the silicone package. Among the materials, a nano-modified LED packaging material with a refractive index of about 1.7 is obtained, which has less optical absorption, can slow the light attenuation of the LED, increase the light-emitting efficiency of the LED, and prolong the service life, but the preparation method is complicated and unsuitable. Mass production. Zhan Xibing et al. prepared a transparent titanium hybrid silicone resin by non-hydrolyzed cosolvent method with a refractive index of 1.62, and had good transparency and photoelectric properties.
2.2 High thermal conductivity
The electro-optical conversion efficiency of the LED chip is about 15%, and the remaining 85% is converted into thermal energy. Due to the small chip size and high power density, the heat dissipation of the LED will increase due to the heat dissipation, which mainly affects the decrease of the brightness of the light, the decay of the service life, and the brightness. The impact is linear and has an exponential effect on life. Damage to the chip and packaging materials, affecting the life of the LED, reliability and luminous efficiency. Therefore, the packaging material is required to have good thermal conductivity, and the thermal conductivity of the silicone material is very low. The thermal conductivity of the pure organic silicon material is only 0.168 W/m·K, so it is important to improve the thermal conductivity of the silicone material. It is also the main way of cooling the current power LED.
Most of the polymer materials are thermal insulation materials. It is very difficult to improve the thermal conductivity by modifying the molecular structure itself. The current method is to add high thermal conductivity fillers to the matrix resin for filling modification, such as alumina and 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 conductive filler will affect the thermal conductivity of the final product. In addition, the compatibility between the thermally conductive filler and the resin matrix interface is poor, the filler is liable to agglomerate in the matrix resin, resulting in uneven dispersion, and the surface tension of the two is also different, which may cause pores between the interfaces and increase the thermal resistance of the material. Therefore, the surface of the thermally conductive filler needs to be modified.
Chen Jing et al. used a different viscosity end vinyl silicone oil compounding system as the base rubber, and hydrogenated silicone oil as the crosslinking agent. The silicon micropowder treated with KH-570 was used as a heat conductive filler to prepare a thermal conductivity of 0.63 W/m·K. Silicone potting compound with alumina as heat conductive filler and aluminum hydroxide as flame retardant prepared room temperature curing silicone electronic potting compound with thermal conductivity of 0.72 W/m·K. Zhao Nian et al. used cetyltrimethoxysilane modified alumina as a thermally conductive filler, diethylaphosphonate (ADP) as a flame retardant, vinyl silicone oil and hydrogenated silicone oil as a base rubber to obtain thermal resistance. Insulating and insulating silicone electronic potting compound, the thermal conductivity after vulcanization is 2.12 W/m·K, the tensile strength is 1.72 MPa, the elongation at break is 62%, and the volume resistivity is 3.9 x 10 Ω·cm. Hi-roshi et al. used spherical alumina as a thermally conductive filler and mixed with a trisiloxane-based single-end silicone resin to prepare a high-temperature vulcanized silicone rubber with a thermal conductivity of up to 3 W/m·K.
2.3 High transmittance
The transmittance of silicone resin is better than epoxy resin. The higher the light transmittance, the higher the luminous intensity and efficiency of LED. The power LED requires the transmittance of packaging material to be no less than 98% (wavelength is 400-800). Nm, sample thickness 1 cm). Shiobara et al. synthesized a variety of polymerization degree vinyl silicone oils and hydrogen-containing silicone resins to crosslink and cure them, and the obtained packaging materials still had a light transmittance of 94% after prolonged aging at 200 °C. Maneesh et al. used branched vinyl phenyl silicone resin mixed with vinyl silicone oil and hydrogen-containing silicone oil to obtain LED packaging materials with refractive index >1.40, aging at 200 °C for 14 days, and light transmittance of 98% (wavelength is 400 nm).
Conclusion
Power LED is the development direction of future light sources. With the support of national industrial policies, LED technology and products have developed rapidly. LED packaging plays a key role in the performance of LEDs. It has determined many years of research on the luminous efficiency, service life and reliability of the products, and has developed high refractive index, high thermal conductivity and high transmittance. Silicone packaging materials, but there are still some technical barriers to overcome.
(1) The performance of materials for power LED packaging is not as excellent and reliable as that of foreign products, and such products are basically monopolized by foreign countries;
(2) The performance of a certain aspect can be improved by modifying the silicone material, but the overall performance is not good;
(3) The heat dissipation of the power LED is poor, and the addition of the filler can improve the thermal conductivity of the silicone packaging material, but causes the light transmittance of the packaging material to decrease, thereby affecting the luminous efficiency;
(4) Silicone materials are expensive. It is believed that with the in-depth discussion and experimentation by researchers, it is certain to develop a silicone packaging material with excellent comprehensive performance, good reliability and affordable price.
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