Package elements that affect the efficiency of LED light extraction -TV LED lens strips

The general LED package uses a bracket type, epoxy resin package, power is small, the overall luminous flux is not large, 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-light flux LED products in the lighting field, power-based LEDs have gradually entered the market. This kind of power type LED generally puts the light emitting chip on the heat sink and the heat sink, and the optical lens is assembled on it to achieve a certain optical spatial distribution. The inside of the lens is filled with low-stress flexible silicone.

Power LED to enter the lighting field, to achieve the family's daily lighting, there are many problems to be solved, the most important of which is the luminous efficiency. Currently, the highest lumen efficiency reported by power LEDs in the market is around 50lm/W, which is far from the requirements of household daily lighting. In order to improve the luminous efficiency of the power LED, on the one hand, the efficiency of the light-emitting chip needs to be improved; on the other hand, the packaging technology of the power-type LED needs to be further improved, starting from various aspects such as structural design, material technology, and process technology to improve the product's Encapsulation light efficiency.

What are the packaging elements that affect light extraction efficiency?

Heat dissipation technology

For light emitting diodes composed of PN junctions, when the forward current flows from the PN junction, the PN junction has a heat loss, and this heat is radiated to the air via adhesives, potting materials, heat sinks, etc., in the process. Some materials have a thermal impedance that blocks 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 light emitting diode be Rth(°C/W) and the heat dissipation power be PD(W). At this time, the temperature rise of the PN junction due to the current heat loss is:

T (°C) = Rth × PD.

The 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-emitting recombination decreases, and the brightness of the light-emitting diode 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 proportionally with the current, ie, it will show thermal saturation. In addition, as the junction temperature rises, the peak wavelength of the light will also shift to the long-wave direction, about 0.2-0.3nm/°C, which is the blue wavelength of the white LED obtained by mixing the YAG phosphor coated with a 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 the power LED, the driving current is generally several hundred milliamps or more, 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 emitted as soon as possible, not only can improve the product's saturation current, 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, first of all, the choice of packaging materials is particularly important, including heat sinks, adhesives, etc. The thermal resistance of the materials is 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 heat-conducting connection between the materials should be good so as to avoid the heat-dissipating bottleneck in the heat-conducting channel and ensure that the heat is dissipated from the inner layer to the outer layer. At the same time, it must be ensured from the process that heat is dissipated in a timely manner according to the pre-designed heat dissipation channels.

2. Selection of filling glue

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

Where n2 is equal to 1, ie, the refractive index of air, and n1 is the refractive index of GaN, from which the critical angle θ0 is calculated to be about 25.8 degrees. In this case, only the light within the spatial solid angle with the incident angle ≤ 25.8 degrees can be emitted. According to reports, the external quantum efficiency of the GaN chip is currently about 30% to 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 impact of the material on the light extraction efficiency must also be considered.

Therefore, in order to improve the light extraction efficiency of the LED product package, the value of n2 must be increased, that is, the refractive index of the package material is increased to improve the critical angle of the product, thereby improving the package luminous efficiency of the product. At the same time, the absorption of light by the encapsulating material is small. In order to increase the proportion of outgoing light, the shape of the package is preferably arched or hemispherical, so that when light is emitted from the encapsulating material to the air, it is almost perpendicularly directed to the interface and no longer generates total reflection.

3. Reflection processing

There are two main aspects of the reflection process. One is the reflection process inside the chip. The second is the reflection of light from the package material. Through both the internal and external reflection processes, the proportion of light emitted from the chip is increased to reduce the internal absorption of the chip. Improve the luminous efficiency of power LED products. From the perspective of packaging, power-type LED is usually assembled on a metal stent or substrate with a reflective cavity. The stent-type reflective cavity is generally used to enhance the reflective effect of plating, while the substrate-based reflective cavity is generally polished. The methods will be subject to electroplating treatment, but the above two methods are affected by the precision and process of the mold. The reflective cavity after processing has a certain reflection effect, but it is not ideal. At present, in the domestic production of substrate-type reflective cavities, the reflective effect is poor due to insufficient polishing accuracy or oxidation of the metal plating. This causes many light rays to be absorbed after hitting the reflective region and cannot be reflected to the light-emitting surface according to the intended target, resulting in the eventual The efficiency of light extraction after packaging is low.

4. Phosphor selection and coating

For white power LEDs, the increase in luminous efficiency is also related to the choice of phosphors and the processing process. In order to improve the efficiency of the phosphor to excite the blue chip, the first choice of phosphor should be appropriate, including the excitation wavelength, particle size, and excitation efficiency, etc. It needs to be comprehensively assessed to take into account various properties. Secondly, the phosphor coating should be uniform, and it is preferable that the thickness of the adhesive layer on each light emitting surface of the light emitting chip is uniform so as to prevent local light from being emitted due to uneven thickness, and the quality of the light spot can also be improved.

A good heat dissipation design has a significant effect on 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-emitting channel, the structural design, material selection, and process treatment of the reflective cavity, filled glue, etc. are emphasized here, and the light-emitting efficiency of the power-type LED can be effectively improved. For power-type white LEDs, the choice of phosphors and process design are also crucial for improvement of the spot and improvement of luminous efficiency.