I. Introduction
Because of its many advantages such as green and environmental protection, LED is considered to be a revolutionary solid light source that replaces traditional lighting sources such as incandescent lamps and fluorescent lamps that consume large power and pollute the environment. Conventional LEDs are generally bracket-type, encapsulated with epoxy resin, with low power, low overall luminous flux, and high brightness can only be used as some special lighting. With the development of LED chip technology and packaging technology, in response to the demand for high luminous flux LED products in the lighting field, power LEDs have gradually entered the market. This type of power LED generally puts the light-emitting chip on the heat sink, and is equipped with an optical lens to achieve a certain optical spatial distribution. The lens is filled with low-stress flexible silicone.
There are many problems to be solved for power LEDs to truly enter the field of lighting and realize daily household lighting. The most important one is luminous efficiency. At present, the highest lumen efficiency reported by power LEDs on the market is around 50lm / W, which is still far from the daily lighting requirements of homes. In order to improve the luminous efficiency of power LEDs, on the one hand, the efficiency of its light-emitting chips needs to be improved; on the other hand, the packaging technology of power LEDs also needs to be further improved, starting from many aspects such as structural design, material technology and process technology to improve the product's Package light extraction efficiency.
Second, the packaging elements that affect the light extraction efficiency
1. Cooling technology
For a light-emitting diode composed of a PN junction, when a forward current flows from the PN junction, the PN junction has a heat loss, and this heat is radiated into the air through the adhesive, potting material, heat sink, etc. In this process, each Some materials have thermal resistance that prevents heat flow, that is, thermal resistance, which is a fixed value determined by the size, structure, and material of the device. Let the thermal resistance of the light-emitting diode be Rth (℃ / W) and the heat dissipation power be PD (W). At this time, the temperature rise of the PN junction due to the heat loss of the current is:
△ T (℃) = Rth & TImes; PD.
The junction temperature of the PN junction is:
TJ = TA + Rth & TImes; PD
Where TA is the ambient temperature. As the junction temperature rises, the probability of PN junction light-emitting recombination decreases, and the brightness of the light-emitting diode decreases. At the same time, due to the increased temperature rise caused by heat loss, the brightness of the light-emitting diode will no longer continue to increase proportionally with the current, which shows the phenomenon of thermal saturation. In addition, as the junction temperature rises, the peak wavelength of the light emission will also drift toward the long wave direction, about 0.2-0.3nm / ℃, which is the white light of the blue LED mixed by the YAG phosphor coated by the blue chip. Drift will cause a mismatch with the excitation wavelength of the phosphor, thereby reducing the overall luminous efficiency of the white LED and causing a change in the color temperature of the white light.
For power light-emitting diodes, the drive current is generally more than a few hundred milliamperes, the current density of the PN junction is very large, so the temperature rise of the PN junction is very obvious. For packaging and applications, how to reduce the thermal resistance of the product, so that the heat generated by the PN junction can be dissipated as soon as possible, not only can increase the saturation current of the product, improve the luminous efficiency of the product, but also improve the reliability and life of the product . In order to reduce the thermal resistance of the product, the choice of packaging materials is especially important, including heat sinks, adhesives, etc. The thermal resistance of each material should be low, that is, good thermal conductivity is required. Secondly, the structural design should be reasonable, the thermal conductivity between the materials should be continuously matched, and the thermal conductivity between the materials should be well connected to avoid the heat dissipation bottleneck in the thermal conduction channel and ensure that the heat is dissipated from the inner layer to the outer layer. At the same time, we must ensure from the process that the heat is dissipated in a timely manner according to the pre-designed heat dissipation channel.
2. The choice of filler
According to the law of refraction, when light enters a dense medium from a dense medium, when the angle of incidence reaches a certain value, that is, greater than or equal to the critical angle, full emission occurs. For the GaN blue chip, the refractive index of the GaN material is 2.3. When light is emitted from the crystal to the air, according to the law of refraction, the critical angle θ0 = sin-1 (n2 / n1)
Among them, n2 is equal to 1, which is the refractive index of air, and n1 is the refractive index of GaN, and the critical angle θ0 is calculated to be about 25.8 degrees. In this case, the only light that can be emitted is the light within the spatial solid angle of incidence angle ≤ 25.8 degrees. According to reports, the external quantum efficiency of GaN chips is currently around 30% -40%. Therefore, due to the internal absorption of the chip crystal , The proportion of light that can be emitted outside the crystal is very small. According to reports, the external quantum efficiency of GaN chips is currently around 30% -40%. Similarly, the light emitted by the chip must be transmitted through the packaging material to the space, and the influence of the material on the light extraction efficiency must also be considered.
Therefore, in order to improve the light extraction efficiency of LED product packaging, the value of n2 must be increased, that is, the refractive index of the packaging material must be increased to increase the critical angle of the product, thereby improving the luminous efficiency of the product packaging. At the same time, the absorption of light by the packaging material should be small. In order to increase the proportion of outgoing light, the outer shape of the package is preferably arched or hemispherical. In this way, when the light is emitted from the packaging material to the air, it almost hits the interface almost vertically, so total reflection is no longer generated.
3. Reflection treatment
There are two main aspects of reflection processing, one is the internal reflection processing of the chip, and the other is the reflection of the packaging material on the light. Through the internal and external reflection processing, the ratio of the light flux emitted from the inside of the chip is increased to reduce the internal absorption of the chip. Improve the luminous efficiency of power LED products. In terms of packaging, power LEDs are usually equipped with power chips on a metal bracket or substrate with a reflective cavity. The bracket-type reflective cavity is generally plated to improve the reflective effect, and the substrate-type reflective cavity is generally polished. Methods, electroplating treatment will also be carried out if possible, but the above two treatment methods are affected by the accuracy of the mold and the process. The reflective cavity after treatment has a certain reflection effect, but it is not ideal. At present, the substrate-type reflective cavity produced in China is poorly reflected due to insufficient polishing precision or oxidation of the metal coating. This results in a lot of light being absorbed after reaching the reflective area, and cannot be reflected to the light exit surface according to the expected target, resulting in the final The light extraction efficiency after packaging is low.
After many researches and experiments, we have developed a reflection treatment process with independent intellectual property rights that uses organic material coatings. Through this process, the light reflected into the cavity of the slide is absorbed very little, and most of the light can be absorbed. The light incident on it is reflected to the light exit surface. In this way, the light extraction efficiency of the processed product can be increased by 30% -50% compared with that before the treatment. Our current 1W white light power LED's light efficiency can reach 40-50lm / W (test results on a remote PMS-50 spectrum analysis test instrument), and a good packaging effect is obtained.
4. Phosphor selection and coating
For white power LEDs, the improvement in luminous efficiency is also related to the choice of phosphors and process. In order to improve the efficiency of the phosphor to excite the blue chip, first of all, the choice of the phosphor should be appropriate, including the excitation wavelength, particle size, excitation efficiency, etc., which needs to be comprehensively evaluated and take into account each performance. Secondly, the coating of the phosphor should be uniform, preferably the thickness of the glue layer on each light-emitting surface of the light-emitting chip is uniform, so as to prevent the local light from being emitted due to the uneven thickness, and also improve the quality of the light spot.
3. Conclusion
Good heat dissipation design plays a significant role in improving the luminous efficiency of power LED products, and it is also a prerequisite for ensuring product life and reliability. For a well-designed light exit channel, the emphasis is on the structural design, material selection, and process treatment of the reflective cavity, filler, etc., which can effectively improve the light extraction efficiency of the power LED. For power-type white LEDs, the choice of phosphor and process design are also crucial to the improvement of light spot and the improvement of luminous efficiency.
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