What is affecting the light extraction efficiency in LED packaging?

LED is called the fourth-generation illumination 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 fields such as indication, display, decoration, backlight, general illumination and urban night scene. According to the different functions, it can be divided into five categories: information display, signal light, vehicle light, LCD backlight, and general illumination.

 

Conventional LED lights have shortcomings such as insufficient brightness, which leads to insufficient penetration. The power LED lamp has the advantages of long life and long life, but the power LED has technical difficulties such as packaging. The following is a simple analysis of the factors affecting the light extraction efficiency of the power LED package.

 

Package elements that affect light extraction efficiency

 

1, cooling technology

 

For a light-emitting diode composed of a PN junction, when a forward current flows from the PN junction, the PN junction has a heat loss, which is radiated into the air via a bonding glue, a potting material, a heat sink, etc., in the process. Some materials have thermal impedance that blocks heat flow, that is, thermal resistance, which is a fixed value determined by the size, structure, and material of the device.

 

Let the thermal resistance of the LED be Rth (°C/W) and the heat dissipation power be PD(W). At this time, the PN junction temperature rise due to the heat loss of the current is:

 

T(°C)=Rth&TImes;PD

 

The PN junction temperature is:

 

TJ=TA+Rth&TImes;PD

 

Where TA is the ambient temperature. As the junction temperature rises, the probability of PN junction luminescence recombination decreases, and the brightness of the LED decreases. At the same time, due to the increase in temperature rise due to heat loss, the brightness of the LED will no longer continue to increase proportionally with the current, indicating thermal saturation. In addition, as the junction temperature rises, the peak wavelength of the luminescence will also drift toward the long wavelength, about 0.2-0.3 nm/°C, which is for the white LED obtained by mixing the YAG phosphor coated with the blue chip. Drift causes 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 white color temperature.

 

For power LEDs, the drive current is generally several hundred milliamperes 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 dissipated as soon as possible, not only can improve 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 is low, that is, the thermal conductivity is required to be good. Secondly, the structural design should be reasonable, the thermal conductivity between the materials should be continuously matched, and the thermal connection between the materials is good, avoiding the heat dissipation bottleneck in the heat conduction channel and ensuring the heat is dissipated 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, the choice of filling glue

 

According to the law of refraction, when light is incident from the optically dense medium to the light-diffusing medium, when the incident angle reaches a certain value, that is, greater than or equal to the critical angle, full emission occurs. In the case of a GaN blue chip, the refractive index of the GaN material is 2.3, and when light is emitted from the inside 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 approximately 25.8 degrees. In this case, the light that can be emitted is only the light within the solid angle of the space where the incident angle is ≤ 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 small. It is reported that the external quantum efficiency of GaN chips is currently around 30%-40%. Similarly, the light emitted by the chip is transmitted through the encapsulating material to the space, and the effect of the material on the light extraction efficiency is also considered.

 

Therefore, in order to improve the light extraction efficiency of the LED product package, 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 package luminous efficiency of the product. At the same time, the encapsulation material absorbs light less. 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 directed from the encapsulating material to the air, it is almost perpendicularly incident on the interface, so that no total reflection is produced.

 

3, reflection processing

 

There are two main aspects of reflection treatment. One is the reflection treatment inside the chip, and the other is the reflection of light by the encapsulation material. Through the reflection treatment of the inner and outer sides, the proportion of the light flux 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 usually mount power chips on metal brackets or substrates with reflective cavities. Bracket-type reflective cavities generally use electroplating to improve reflection, while substrate-type reflective cavities are generally polished. In the mode, the plating treatment is carried out under conditions, but the above two treatment methods are affected by the precision and process of the mold, and the reflective cavity after the treatment has a certain reflection effect, but it is not ideal. At present, the substrate-type reflective cavity is made in China, because the polishing precision is insufficient or the metal plating layer is oxidized, the reflection effect is poor, which causes a lot of light to be absorbed after being incident on the reflection area, and cannot be reflected to the light-emitting surface according to the intended target, thereby resulting in the final The light extraction efficiency 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 phosphor and process. In order to improve the efficiency of the phosphor to stimulate the blue chip, firstly, the selection of the phosphor should be appropriate, including the excitation wavelength, the particle size, the excitation efficiency, etc., and comprehensive evaluation is required, taking into account various performances. Secondly, the coating of the phosphor should be uniform, preferably the thickness of the glue layer of each light-emitting surface of the light-emitting chip is uniform, so as to avoid local light being unable to be emitted due to uneven thickness, and the quality of the spot can be improved.

 

Summary:

 

Good thermal 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 well-designed light exit channel, which focuses on the structural design, material selection and process processing of the reflective cavity, the filling glue, etc., can effectively improve the light extraction efficiency of the power LED. For power-type white LEDs, the choice of phosphor and process design are also critical to the improvement of the spot and the improvement of luminous efficiency.