LED is known as the fourth generation lighting source or green light source. It has the characteristics of energy saving, environmental protection, long service life and small volume. It is widely used in various fields such as indication, display, decoration, backlight, general lighting and urban night scene. According to different functions, it can be divided into five categories: information display, signal lamp, vehicle lamps, LCD backlight and general lighting.

Conventional LED lamps have shortcomings such as insufficient brightness, which leads to insufficient penetration. Power LED lamp has the advantages of sufficient brightness and long service life, but power LED has technical difficulties such as packaging. Here is a brief analysis of the factors affecting the light extraction efficiency of power LED packaging.

Packaging factors affecting light extraction efficiency

1. Heat dissipation technology

For the light-emitting diode composed of PN junction, when the forward current flows out of the PN junction, the PN junction has heat loss. These 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. The thermal resistance is a fixed value determined by the size, structure and material of the device.

Let the thermal resistance of the LED be rth (℃ / W) and the thermal dissipation power be PD (W). At this time, the PN junction temperature caused by the thermal loss of the current rises to:

T(℃)=Rth&TImes; PD

PN junction temperature:

TJ=TA+Rth&TImes; PD

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

For power LED, the driving current is generally more than hundreds of Ma, and the current density of PN junction is very large, so the temperature rise of PN junction is very obvious. For packaging and application, how to reduce the thermal resistance of the product and make the heat generated by PN junction dissipate as soon as possible can not only improve the saturation current of the product and improve the luminous efficiency of the product, but also improve the reliability and service life of the product. In order to reduce the thermal resistance of products, firstly, the selection of packaging materials is particularly important, including heat sink, adhesive, etc. the thermal resistance of each material should be low, that is, it is required to have good thermal conductivity. Secondly, the structural design should be reasonable, the thermal conductivity between materials should be continuously matched, and the thermal conductivity between materials should be well connected, so as to avoid the heat dissipation bottleneck in the heat conduction channel and ensure the heat dissipation from the inner to the outer layer. At the same time, it is necessary to ensure that the heat is dissipated in time according to the pre-designed heat dissipation channel.

2. Selection of filler

According to the refraction law, when light is incident from light dense medium to light 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 chip, 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 refraction law, the critical angle θ 0=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, from which the critical angle is calculated θ 0 is about 25.8 degrees. In this case, the only light that can be emitted is the light within the spatial solid angle with the incident angle ≤ 25.8 degrees. It is reported that the external quantum efficiency of Gan chip is about 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 chip is about 30% – 40%. Similarly, the light emitted by the chip should be transmitted to the space through the packaging material, and the influence of the material on the light extraction efficiency should 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 packaging material must be increased to improve the critical angle of the product, so as to improve the packaging luminous efficiency of the product. At the same time, the light absorption of packaging materials should be small. In order to improve the proportion of outgoing light, the package shape is preferably arched or hemispherical, so that when the light is emitted from the packaging material to the air, it is almost perpendicular to the interface, so there is no total reflection.

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 packaging materials. Through the internal and external reflection processing, the light flux ratio emitted from the chip can be improved, the internal absorption of the chip can be reduced, and the luminous efficiency of power LED products can be improved. In terms of packaging, the power LED usually assembles the power chip on the metal support or substrate with reflection cavity. The support type reflection cavity generally adopts electroplating to improve the reflection effect, while the base plate reflection cavity generally adopts polishing. If possible, electroplating treatment will be carried out, but the above two treatment methods are affected by mold accuracy and process, The processed reflection cavity has a certain reflection effect, but it is not ideal. At present, due to insufficient polishing accuracy or oxidation of metal coating, the reflection effect of substrate type reflection cavity made in China is poor, which leads to a lot of light being absorbed after shooting into the reflection area and unable to be reflected to the light emitting surface according to the expected target, resulting in low light extraction efficiency after final packaging.

4. Phosphor selection and coating

For white power LED, the improvement of luminous efficiency is also related to the selection of phosphor and process treatment. In order to improve the efficiency of phosphor excitation of blue chip, firstly, the selection of phosphor should be appropriate, including excitation wavelength, particle size, excitation efficiency, etc., which need to be comprehensively evaluated and take into account all performance. Secondly, the coating of the phosphor should be uniform, preferably the thickness of the adhesive layer on each light-emitting surface of the light-emitting chip should be uniform, so as not to prevent local light from being emitted due to uneven thickness, but also improve the quality of the light spot.

overview:

Good heat dissipation design plays a significant role in improving the luminous efficiency of power LED products, and it is also the premise to ensure the service life and reliability of products. The well-designed light outlet channel here focuses on the structural design, material selection and process treatment of reflection cavity and filling glue, which can effectively improve the light extraction efficiency of power LED. For power white LED, the selection of phosphor and process design are also very important to improve the spot and luminous efficiency.