Why reduce the white LED light decay?

Compared with incandescent lamps and fluorescent lamps, white LEDs have the advantages of small size, low heat generation, low power consumption, long life, fast response, environmental protection, flat packaging, easy development into thin and light products, and no incandescent lamps. Power consumption, fragile and fluorescent lamp waste contain mercury pollution, etc. At the same time, the spectral distribution of LED also determines that its color purity and saturation are unmatched by traditional light sources.

However, at present, the light-induced degradation of white LEDs, especially low-power white LEDs, has become more and more recognized by people. Because the price of white LEDs is still very high, they want to stand on the lighting market and let users In order to save electricity and save money, it is necessary to make up for the high price of LED by the long-life and saved electricity cost. Therefore, the light decay of white LED is a problem that LED has to face, and it is also one of the primary problems for its entry into civilian lighting. .

LED light attenuation factors

In order to improve the reliability and reduce the cost of white LEDs, scientists from various countries have conducted in-depth research on the aging mechanism of white LEDs. It is believed that the light decay of white LEDs is affected by the increase of deep and non-radiative composite centers and the electromigration of contact electrode metals. And the effects of slow or catastrophic failure caused by degradation and poor heat dissipation, and the influence of factors such as aging of the packaging materials, phosphor aging, and static electricity are not negligible.

From the normal use of LEDs, the most important factor causing white LED light decay is heat. For example, the light fading of small power LEDs is significantly higher than that of high-power LEDs . Because high-power LEDs have a special heat dissipation structure in addition to the two electrodes, they are used to improve the heat dissipation effect, while the small power LEDs are due to the volume. And the cost reason, there is almost no special heat dissipation structure, only two electrodes and external connection, the heat dissipation ability is poor.

The main sources of heat are: the Joule heat generated by the normal resistance of the material during energization, the heat generated by the PN junction, the Joule heat generated by the parasitic resistance brought about by the process, and the heat generated by the absorption of light. The generated heat cannot be dissipated in time to increase the temperature, and the continuous high temperature accelerates the degradation of the performance of the various components of the white LED, thereby affecting the service life.

The effect of various materials on the light decay of white LED

The rapid decay of the blue LED chip itself causes the attenuation of the light output of the device

By studying the structure and materials of LED chips, it is found that the increase of deep level defects and non-radiative recombination centers in chip materials has an important influence on the aging of LEDs. In the current accelerated life experiment, the unstable compounds in the chip are decomposed into shallow acceptors, which constitute a new energy level and reduce the luminous efficiency. Moreover, it is found through experiments that the defects and misalignment on the chip increase the reverse leakage current of the device, which reduces the luminous efficiency and reliability, while the LED with optimized process, fewer defects and dislocations has less light decay.

Therefore, scientists have improved the substrate material and epitaxial fabrication process of the chip, reducing defects and dislocations, and effectively improving the quality of the chip.

The high temperature generated by the energization of the chip causes the LED's luminous efficiency to drastically decrease. Therefore, using a substrate with good heat dissipation, such as replacing the sapphire substrate with a copper substrate, expands the heat dissipation area, and improves the heat dissipation, thereby increasing the luminous flux. Improve reliability.

Thermal degradation of the potting material causes attenuation of light output

There are many types of electronic potting glues. According to the type of materials, the most common ones are the three most commonly used, namely epoxy resin potting glue, ordinary refractive index silicone potting glue, high refractive index silicone potting glue. .

Epoxy potting compound

The epoxy resin has two or more epoxy groups in its molecular structure and can form a three-dimensional network cured product in the presence of a suitable chemical agent.

Epoxy resin and its cured product have the following excellent performance characteristics:

(1) High mechanical properties: epoxy resin and its cured product have strong cohesive force and tight molecular structure, so its mechanical properties are high;

(2) Strong adhesion: the curing system contains extremely active epoxy groups, hydroxyl groups, and polar groups such as ether bonds, amine bonds, and ester bonds, and imparts epoxy cured materials to metals, ceramics, glass, concrete, and wood. The adhesion of the polar substrate is excellent, and the shrinkage rate of the epoxy resin is low, and the internal stress is small, which also contributes to the improvement of adhesion;

(3) Good processability: basically no low molecular volatiles are generated during curing, and different curing agents are selected, and the epoxy resin system can be cured in the range of 0 to 180 degrees;

(4) Good stability, excellent chemical resistance, excellent electrical insulation, and small cure shrinkage: generally 1% to 2%;

But epoxy also has some disadvantages:

(1) The heat resistance does not meet the requirements of the LED: the heat resistance of the epoxy cured product is generally 80 to 100 degrees. This is because the CO in the epoxy group and the ether is easily oxidized to an aldehyde or a ketone under heating.

(2) Poor weather resistance: epoxy resins generally contain aromatic ether bonds, which absorb ultraviolet rays and oxidize to form hydroxyl groups and form chromophores to discolor the resin;

These two disadvantages of epoxy potting will result in severe light decay of the potted LED.

Ordinary refractive index silicone encapsulant

Ordinary refractive index silicone encapsulant: based on branched methyl vinyl siloxane based resin, linear methyl vinyl siloxane as reactive diluent, methyl hydrogen siloxane as crosslinking agent A two-component addition molding silicone electronic potting material prepared by combining platinum catalyst, temperature sensitive inhibitor and various additives.

Methyl silicone potting materials have better performance than epoxy resins:

(1) Better heat resistance: The silicone material is a semi-organic semi-inorganic material whose main chain Si-O is an inorganic segment with a bond energy of up to 422.5 KJ/mol, while the CC bond and CO bond of the epoxy resin The bond energy is only 347KJ/mol and 351KJ/mol respectively. Therefore, the silicone material is not easily broken down at high temperature. It does not decompose or discolor at 200~250°C for a long time. It can withstand 350~500°C in short time after cross-linking curing. . And the methyl silicone also imparts good thermal stability, release property, water repellency and arc resistance to the material, so the thermal stability of the siloxane is quite good.

(2) Better weather resistance: methyl silicone material absorbs almost no ultraviolet light, and phenyl only absorbs light below 280 nm, so the light emitted by LED itself or sunlight is more affected by methyl silicone material. Small, even under the strong ultraviolet light, the silicone resin is resistant to yellowing, does not cause a radical reaction, and is not susceptible to an oxidation reaction.

(3) Better electrical insulation performance: The silicone resin has an electric shock intensity of 90 to 98 KV/mm2. Its dielectric constant is small and decreases with increasing temperature. This property makes silicone resin use as a high voltage insulation. At the same time, the arc resistance and corona resistance of silicone resin are also outstanding, and its arc resistance (180s) is twice that of epoxy resin (90s).

But methyl-based silicone materials also have areas where performance needs to be improved:

(1) Mechanical properties: determined by the molecular structure and the molecular bond force, the methyl silicone encapsulant has poor bending, tensile, impact and scratch resistance.

(2) Chemical resistance: Strong alkali can break Si-O-Si bond. Moreover, the solvent resistance of the silicone resin is poor, and the resin film is completely destroyed in a few minutes.

(3) Anti-moisture and oxygen permeability: Although the methyl silicone potting compound has excellent hydrophobic properties due to the arrangement of organic groups in the side groups of the silicon oxide main chain, the molecular segment is soft and the methyl group is small. There will be some gaps between the segments, so the moisture permeability is relatively large. It has been found through experiments that the methyl silicone potting compound has strong resistance to cold water and weak resistance to boiling water. In particular, high-pressure water vapor has poor resistance, which also leads to oxidation of the silver-plated layer and acceleration of chip aging, thereby accelerating LED light decay.

Compared with other types of potting compounds, methyl silicone materials have excellent thermal stability and UV stability, and can be applied to indoor lighting fields with low humidity and low heat dissipation requirements.

High refractive index silicone encapsulant

Compared with epoxy resin potting compound and ordinary refractive index silicone potting compound, high refractive index silicone potting compound has superior performance:

(1) Higher refractive index: The refractive index of methyl silicone potting compound is usually only about 1.41, while the refractive index of phenyl silicone potting material is higher, and the refractive index can reach 1.5 or more. The difference between the refractive index of the methyl silicone potting compound and the refractive index of the chip is not conducive to the light output of the chip. Studies have shown that the use of high refractive index, high UV resistance, high heat aging resistance, low stress phenyl silicone potting material can significantly improve the light output and service life of LED.

(2) Better mechanical properties and moisture permeability: Since the high refractive index silicone potting material mainly increases the refractive index of the material by introducing a benzene ring into the molecular structure, the benzene ring is bulky and generally The high refractive index potting glue on the market is mostly a three-dimensional silicone resin, rather than a linear silicone oil, which greatly reduces the moisture permeability and linear expansion coefficient of the potting material, and effectively improves its hardness, impact resistance and resistance. After scratching and other properties, and the moisture permeability is reduced, the chip is not easily corroded, and the silver plating layer on the surface of the stent is not easily oxidized, vulcanized and blackened, and it is not easy to reduce the reflective ability of the surface of the stent, thereby delaying light decay and extending the LED. The service life.

But now high refractive index silicone encapsulants are not perfect, such as:

(1) Weather resistance: Due to the presence of the benzene ring, the conjugated large π bond is easily oxidized and discolored under the attack of free oxygen or under ultraviolet irradiation, which affects the light transmittance of the potting material. Studying the surface, in the synthesis of phenyl silicone resin, by adjusting the monomer feed ratio, reducing the residual of silanol and silicon methoxy, can increase the stability of the material, on the other hand, can also optimize the purification process of silicone materials Reducing the residue of oligomers, monomers and by-products, and greatly improving the stability of materials.

(2) Refractive index is difficult to increase: From the perspective of chemical structure and composition of organic silicon, due to the large volume of benzene ring, it is impossible to introduce it in large quantities. Pursuing higher refractive index is another major problem in the research and development field. It is achieved by using ultra-fine modified nanoparticles with high refractive index. The technical team of Hong Kong Junma Technology Group has conducted this research under the leadership of Professor Wang Zhengping, an academician of the American Academy of Engineering, and has made good progress.

Attenuation of phosphor performance causes attenuation of light output

other

In addition to the above causes of light decay by the LED itself, there are external factors that cause the attenuation of the LED light output. LEDs are afraid of heat. The higher the temperature, the shorter the LED life and the greater the light decay. Therefore, when designing the luminaire, we should minimize the generation of heat, accelerate the heat dissipation, improve the thermal stability of the material, thereby reducing the reduction of luminous flux and prolonging the service life of the LED lamp bead .

In addition, the user does not understand the performance parameters of the LED, and improperly supplies power to the LED. Less than the rated current will cause the light intensity of the lamp be not up to the required level, and exceed the rated current, the light intensity of the lamp bead is up, but a large number The heat, beyond the heat transfer capacity of the material, leads to an acceleration of the aging of the material, resulting in attenuation of the light output of the luminaire.

Conclusion

In summary, whether it is the aging of the chip, the aging of the silica gel, the decline of the performance of the phosphor, the improper supply of electricity, the harsh environment of the use, etc., the factors of the light decay of the LED lamp have the shadow of heat, so the largest LED lamp The enemy is actually hot! Therefore, reducing the generation of heat, accelerating the derivation of heat, and improving the heat resistance of materials are three research directions for LED lamps to reduce light decay and improve their service life.