Conventional LED lamp beads are generally stent type, using epoxy resin encapsulation. The power is small, 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. Conform to the demand for LED lamp beads in Gaoguangtong. Power LED gradually entered the market. This kind of power LED light-emitting diode generally puts the light-emitting chip on a heat sink, and is equipped with an optical mirror 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 field of lighting and realize daily household lighting. Among them, the most important is luminous efficiency. At present, the highest lumen efficiency of power LED light-emitting diodes and reported on the market is around 50MmW, which is far from meeting the requirements of daily household lighting. In order to improve the luminous efficiency of power LEDs, on the one hand, the efficiency of its light-emitting chips needs to be improved on the other hand. The packaging technology of power LEDs also needs to be further improved, starting from many aspects such as structural design, material technology and process technology to improve the efficiency of product packaging and light extraction.
—、Packaging elements that affect light extraction efficiency
1. Heat dissipation technology
For light-emitting diodes composed of PN junction, when the forward current flows through the PN junction, the PN junction has heat loss, and the heat is radiated into the air through the adhesive, potting material, heat sink, etc., in the process Each part of the material has a thermal resistance 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. Suppose the thermal resistance of the light-emitting diode is Rt(CW), and the heat dissipation power is PD(W). At this time, the temperature rise of the PN junction caused by the heat loss of the current is: T(RC)=Rth>PD. The junction temperature of PN junction is: TJ=TA+ Rth×PD
Where TA is the ambient temperature. As the junction temperature rises, the probability of PN junction luminescence recombination decreases. 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 in proportion to the current, that is, it shows a phenomenon of thermal saturation. In addition, as the junction temperature rises, the peak straight wavelength of the light will also shift to the long-wave direction, about 0.2-0.3nml*°C. This is for white LEDs obtained by mixing blue chips with YAG phosphors. The drift of the blue wavelength will cause a mismatch with the excitation wavelength of the phosphor, thereby reducing the overall luminous efficiency of the white light LED and leading to a change in the color temperature of the white light.
For power LEDs, 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 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 increase the saturation current of the product, improve the luminous efficiency of the product, but also improve the reliability and reliability of the product. life. In order to reduce the thermal resistance of the product, the choice of packaging materials is particularly important, including heat sinks and adhesives. The thermal resistance of each material sample 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. The heat conduction connection between the materials is good, to avoid the heat dissipation bottleneck in the heat conduction channel, and to ensure that the heat is dissipated from the inner to the outer layer. At the same time, it is necessary to ensure from the process that the heat is emitted in time according to the pre-designed heat dissipation channel.
