Conventional LEDs are generally bracket type, encapsulated in epoxy resin, with low power, and the overall luminous flux is not large, and the 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 LEDs have gradually entered the market. The power type LED generally has a light emitting chip placed on a heat sink heat sink, and an optical lens is assembled thereon to achieve a certain optical spatial distribution, and the lens is filled with a low stress flexible silicone.
Power LEDs have to enter the field of lighting to achieve daily lighting in the home. There are still many problems to be solved, the most important of which is luminous efficiency. At present, the highest lumen efficiency reported by power LEDs on the market is around 50 lm/W, which is far from the requirements of household daily lighting. In order to improve the luminous efficiency of 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 power LED needs to be further improved, starting from structural design, material technology and process technology, and improving the product. Package light extraction efficiency.
First, the package elements that affect the efficiency of light extraction
1. Heat dissipation 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(/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() = Rth x PD.
The PN junction temperature is:
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/, which is the drift of the blue wavelength for the white LED obtained by mixing the YAG phosphor coated with the blue chip. It 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 color temperature of the white light.
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 should be good, avoiding the heat dissipation bottleneck in the heat conduction channel and ensuring 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. Filling glue selection
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 incident angle ≤ 25.8 degrees. It is reported that the external quantum efficiency of the current GaN chip is about 30%-40%, and 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 encapsulating 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. The reflection treatment of the inside and the outside improves the proportion of the light emitted from the inside of the chip and reduces the internal absorption of the chip. Improve the luminous efficiency of power LED products. From the perspective 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 also carried out under conditions, but the above two treatment methods are affected by the precision of the mold and the process, 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. Due to insufficient polishing precision or oxidation of the metal plating layer, 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 result. The light extraction efficiency after packaging is low.
Through various researches and experiments, we have developed a reflection treatment 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 them can be used. The light hitting it is reflected to the light exiting surface. The light extraction efficiency of the product thus treated can be increased by 30% to 50% compared with that before the treatment. Our current 1W white light power LEDs have a luminous efficacy of 40-50lm/W (test results on a remote PMS-50 spectral analysis tester) and have achieved good packaging results.
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, particle size, excitation efficiency, etc., and comprehensive evaluation is required, taking into account various properties. 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.
Second, the conclusion
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, here refers to the structural design, material selection and process treatment of the reflective cavity, the filling glue, etc., which can effectively improve the light extraction efficiency of the power LED. For power-type white LEDs, phosphor selection and process design are critical to improved spot and improved luminous efficiency.