Four major factors affecting the light collection efficiency of LED packaging

　　Conventional LEDs are generally bracket-type, encapsulated with epoxy resin, with low power and low overall luminous flux. High-brightness LEDs can only be used for 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 places the light-emitting chip on a heat sink, assembles an optical lens on it to achieve a certain optical spatial distribution, and fills the lens with low-stress flexible silicone.

　　There are still many problems to be solved for power LEDs to enter the lighting field and realize daily household lighting, the most important of which is luminous efficiency. The highest lumen efficiency reported for power LEDs on the market is about 50lm/W, which is far from meeting the requirements of daily household lighting. In order to improve the luminous efficiency of power LEDs, on the one hand, the efficiency of its light-emitting chip needs to be improved; on the other hand, the packaging technology of power LEDs also needs to be further improved, starting from structural design, material technology and process technology, etc., to improve the packaging and light collection efficiency of products.

　　Packaging factors that affect light extraction efficiency

　　1. Heat dissipation technology

　　For a light-emitting diode composed of a PN junction, when the forward current flows through the PN junction, the PN junction will have heat loss. This heat is radiated into the air through adhesive, potting material, heat sink, etc. In this process, each part of the material has a thermal impedance to prevent heat flow, that is, thermal resistance. Thermal resistance is a fixed value determined by the size, structure and material of the device. Assuming that the thermal resistance of the light-emitting diode is Rth (℃/W), and the heat dissipation power is PD (W), the PN junction temperature rise caused by the heat loss of the current is:

　　T(℃)=Rth×PD.

　　PN junction temperature is:

　　TJ=TA+Rth×PD

　　Where TA is the ambient temperature. As the junction temperature rises, the probability of PN junction light recombination decreases, and the brightness of the LED decreases. At the same time, due to the increase in temperature caused by heat loss, the brightness of the LED will no longer continue to increase in proportion to the current, that is, it will show thermal saturation. In addition, as the junction temperature rises, the peak wavelength of the light will also drift toward the long wave direction, about 0.2-0.3nm/℃. For white LEDs obtained by mixing blue light chips with YAG phosphors , the drift of the blue light wavelength will cause a mismatch with the phosphor excitation wavelength, thereby reducing the overall luminous efficiency of the white light LED and causing a change in the color temperature of the white light.

　　For power light-emitting diodes, the driving current is generally more than several hundred milliamperes, and the current density of the PN junction is very large, so the temperature rise of the PN junction is very obvious. For packaging and application, 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 can not only increase the saturation current of the product and 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 selection of packaging materials is particularly important, including heat sinks, adhesives, etc. The thermal resistance of each material must be low, that is, good thermal conductivity is required. Secondly, the structural design should be reasonable, the thermal conductivity of each material should be continuously matched, and the thermal connection between the materials should be good to avoid the generation of heat dissipation bottlenecks in the heat conduction channel and ensure that the heat is dissipated from the inside to the outside. At the same time, it is necessary to ensure from the process that the heat is dissipated in time according to the pre-designed heat dissipation channel.

　　2. Selection of filling glue

　　According to the law of refraction, when light is incident from a dense medium to a sparse medium, total emission will occur when the incident angle reaches a certain value, that is, greater than or equal to the critical angle. For GaN blue chips, the refractive index of GaN material is 2.3. When light is emitted from the inside of the crystal to the air, according to the law of refraction, the critical angle θ0=sin-1(n2/n1)

　　Where n2 is equal to 1, which is the refractive index of air, and n1 is the refractive index of GaN. 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 with an incident angle ≤ 25.8 degrees. It is reported that 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. It is reported that the external quantum efficiency of GaN chips is currently around 30%-40%. Similarly, the light emitted by the chip must pass through the packaging material and be transmitted 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 collection efficiency of LED product packaging, it is necessary to increase the value of n2, that is, to increase the refractive index of the packaging material, so as to increase the critical angle of the product, thereby improving the packaging luminous efficiency of the product. At the same time, the packaging material should absorb less light. In order to increase the proportion of emitted light, the shape of the package is preferably arched or hemispherical, so that when the light is emitted from the packaging material to the air, it is almost vertical to the interface, and no total reflection occurs.

　　3. Reflection processing

　　There are two main aspects of reflection processing: one is the reflection processing inside the chip, and the other is the reflection of light by the packaging material. Through the reflection processing of both internal and external aspects, the proportion of light flux emitted from the inside of the chip can be increased, the absorption inside the chip can be reduced, and the luminous efficiency of the power LED finished product can be improved. From the perspective of packaging, power LEDs usually assemble power chips on metal brackets or substrates with reflection cavities. The bracket-type reflection cavity generally adopts electroplating to improve the reflection effect, while the substrate-type reflection cavity generally adopts polishing. If conditions permit, electroplating will also be performed. However, the above two processing methods are affected by the mold precision and process. The processed reflection cavity has a certain reflection effect, but it is not ideal. At present, the reflection cavity made in China has a poor reflection effect due to insufficient polishing precision or oxidation of the metal coating. This causes a lot of light to be absorbed after hitting the reflection area, and cannot be reflected to the light-emitting surface as expected, resulting in low light collection efficiency after the final packaging.

　　After many aspects of research and experiments, we have developed a reflective processing technology with independent intellectual property rights using organic material coating. Through this process, the light reflected into the carrier cavity is absorbed very little, and most of the light incident on it can be reflected to the light-emitting surface. The light-collecting efficiency of the product after such treatment can be increased by 30%-50% compared with that before treatment. At present, the luminous efficiency of our 1W white light power LED can reach 40-50lm/W (test results on the Yuanfang PMS-50 spectrum analysis test instrument), and a good packaging effect has been achieved.

　　4. Phosphor selection and coating

　　For white power LEDs, the improvement of luminous efficiency is also related to the selection and process of phosphors. In order to improve the efficiency of phosphors in stimulating blue chips, first of all, the selection of phosphors should be appropriate, including the excitation wavelength, particle size, excitation efficiency, etc., which need to be comprehensively evaluated and all performances should be taken into account. Secondly, the coating of phosphors should be uniform, and it is best to have a uniform thickness of the glue layer relative to each luminous surface of the luminous chip to avoid the failure of local light to be emitted due to uneven thickness, and at the same time, the quality of the light spot can also be improved.

　　Good heat dissipation design plays a significant role in improving the luminous efficiency of power LED products, and is also a prerequisite for ensuring product life and reliability. A well-designed light-emitting channel, which focuses on the structural design, material selection and process treatment of the reflective cavity and filling glue, can effectively improve the light-collecting efficiency of power LEDs. For power white light LEDs, the selection and process design of phosphors are also crucial to improving the light spot and luminous efficiency.