Light extraction efficiency of LED package

Conventional LEDs are generally bracket-type, encapsulated with epoxy resin, with low power and low overall luminous flux. LEDs with high brightness 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, on which an optical lens is installed to achieve a certain optical spatial distribution, and the lens is filled with low-stress flexible silicone.

There are still many problems to be solved for power LEDs to truly enter the lighting field and achieve daily home lighting, the most important of which is luminous efficiency. At present, the highest lumen efficiency reported by power LEDs on the market is about 50lm/W, which is far from meeting the requirements of daily home 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-collecting 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 has 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 that prevents heat flow, that is, thermal resistance. The 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.
The junction temperature of the PN junction is:
TJ=TA+Rth×PD,

where TA is the ambient temperature. Since the increase in junction temperature will reduce the probability of PN junction light recombination, 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 proportionally with 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 the white LED obtained by coating the blue light chip with YAG phosphor, 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 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 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, 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 should 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 heat dissipation bottleneck 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, when the incident angle reaches a certain value, that is, greater than or equal to the critical angle, full emission will occur. 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, that 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 of the 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 collection 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 is generated.

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 internal and external reflection processing, the proportion of light flux emitted from the inside of the chip is increased, the internal absorption of the chip is reduced, and the luminous efficiency of the power LED product is improved. In terms of packaging, power LEDs usually mount power chips on metal brackets or substrates with reflective cavities. Bracket-type reflective cavities generally use electroplating to improve the reflection effect, while substrate-type reflective cavities generally use polishing. If conditions permit, electroplating will also be performed. However, the above two treatment methods are affected by mold precision and process. The treated reflective cavities have a certain reflection effect, but it is not ideal. At present, the reflection effect of substrate-type reflective cavities produced in China is poor due to insufficient polishing precision or oxidation of metal plating. 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 extraction efficiency after final packaging. After many

aspects of research and experiments, we have developed a reflective processing process using organic material coatings with independent intellectual property rights. Through this process, the light reflected into the carrier cavity is absorbed very little, and most of the light emitted on it can be reflected to the light-emitting surface. The light extraction 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 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. Selection and coating of phosphors

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 to excite blue chips, first of all, the selection of phosphors should be appropriate, including excitation wavelength, particle size, excitation efficiency, etc., which need to be comprehensively evaluated and take into account various performances. 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 light-emitting chip, so as to avoid the inability of local light to be emitted due to uneven thickness, and at the same time, it can also improve the quality of the light spot.

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