Analysis of the four factors affecting the light extraction efficiency of power LED packaging

LED is known as the fourth-generation lighting source or green light source. It has the characteristics of energy saving, environmental protection, long life, and small size. It is widely used in various indications, displays, decorations, backlight sources, general lighting and urban night scenes. According to the different functions, it can be divided into five categories: information display, signal light, car lamps, LCD backlight, and general lighting.

Conventional LED lights have shortcomings such as insufficient brightness, which leads to insufficient penetration. Power LED lamps have the advantages of sufficient brightness and long service life, but power LEDs have technical difficulties such as packaging. The following briefly analyzes the factors that affect the light extraction efficiency of power LED packages.

Packaging elements that affect light extraction efficiency

1. Heat dissipation technology

For the light emitting diode composed of PN junction, when the forward current flows through the PN junction, the PN junction has heat loss. This heat is radiated into the air through the adhesive, potting material, heat sink, etc. Some materials have thermal resistance that prevents heat flow, that is, thermal resistance. Thermal resistance is a fixed value determined by the size, structure, and material of the device.

Suppose the thermal resistance of the light-emitting diode is Rth (℃/W), and the heat dissipation power is PD (W). At this time, the temperature rise of the PN junction caused by the heat loss of the current is:

T(℃)=Rth&TImes;PD

The junction temperature of PN junction is:

TJ=TA+Rth&TImes;PD

Where TA is the ambient temperature. As the junction temperature rises, the probability of PN junction luminescence recombination will decrease, and the brightness of the light-emitting diode will decrease. At the same time, due to the increase in temperature caused by heat loss, the brightness of the light-emitting diode will no longer continue to increase in proportion to the current, that is, it shows a phenomenon of thermal saturation. In addition, as the junction temperature rises, the peak wavelength of light emission will also shift to the long-wave direction, about 0.2-0.3nm/℃. For the white LED obtained by mixing the blue chip with YAG phosphor, the blue wavelength is Drift will cause a mismatch with the excitation wavelength of the phosphor, thereby reducing the overall luminous efficiency of the white light LED and leading to a change in the color temperature of the white light.

For power LEDs, the driving current is generally more than a few 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 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 particularly 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 of the materials should be continuously matched, and the thermal connection between the materials should be good to avoid the heat dissipation bottleneck in the heat conduction channel and ensure that the heat is dissipated from the inner to the outer layer. 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. The choice of filler

According to the law of refraction, when light is incident from a dense medium to a light thin medium, when the incident angle reaches a certain value, that is, when the incident angle is greater than or equal to the critical angle, full emission will occur. For the GaN blue chip, the refractive index of the 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).

Among them, n2 is equal to 1, which is the refractive index of air, and n1 is the refractive index of GaN. From this calculation, the critical angle θ0 is approximately 25.8 degrees. In this case, the only light that can be emitted is the light within the solid angle of the space ≤ 25.8 degrees. According to reports, the current external quantum efficiency of GaN chips is about 30%-40%. Therefore, due to the internal absorption of the chip crystal, the proportion of light that can be emitted to the outside of the crystal is very small. According to reports, the current external quantum efficiency of GaN chips is 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 extraction efficiency of LED product packaging, the value of n2 must be increased, that is, to increase the refractive index of the packaging material to increase the critical angle of the product, thereby improving the luminous efficiency of the product packaging. At the same time, the light absorption of the packaging material should be small. In order to increase the proportion of the emitted light, the shape of the package is preferably arched or hemispherical, so that when the light is emitted from the encapsulating material to the air, it is almost perpendicular to the interface, so total reflection is no longer generated.

3. Reflection treatment

The reflection treatment mainly has two aspects. One is the reflection treatment inside the chip, and the other is the reflection of light by the packaging material. Through internal and external reflection treatment, the proportion of light emitted from the inside of the chip is increased, and the internal absorption of the chip is reduced. Improve the luminous efficiency of power LED products. In terms of packaging, power LEDs are usually assembled with power chips on a metal bracket or substrate with a reflective cavity. The bracket-type reflective cavity is generally plated to improve the reflection effect, while the substrate-type reflective cavity is generally polished. Method, if possible, electroplating treatment will be carried out, but the above two treatment methods are affected by the accuracy of the mold and the process, and the reflection cavity after the treatment has a certain reflection effect, but it is not ideal. At present, due to insufficient polishing precision or oxidation of the metal coating, the reflection effect of the substrate-type reflective cavity produced in China is poor. This causes a lot of light to be absorbed after hitting the reflective area and cannot be reflected to the light-emitting surface as expected, resulting in the final The light extraction efficiency after packaging is low.

4. Phosphor powder selection and coating

For white power LEDs, the improvement of luminous efficiency is also related to the selection and processing of phosphors. In order to improve the efficiency of phosphors to excite the blue chip, firstly, the selection of phosphors should be appropriate, including excitation wavelength, particle size, excitation efficiency, etc., which need to be comprehensively assessed, taking into account each performance. Secondly, the coating of the phosphor should be uniform, and it is best to have a uniform thickness of the glue layer on each light-emitting surface of the light-emitting chip, so as to avoid local light cannot be emitted due to uneven thickness, and at the same time can improve the quality of the light spot.

Summary:

Good heat dissipation design has a significant effect on improving the luminous efficiency of power LED products, and it is also a prerequisite to ensure product life and reliability. The well-designed light output channel, here focuses on the structural design, material selection and process treatment of the reflective cavity, filling glue, etc., which can effectively improve the light extraction efficiency of the power LED. For power white LEDs, the selection of phosphors and process design are also crucial to the improvement of light spot and the improvement of luminous efficiency.

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