Analysis of failure and encapsulation failure of LED chip

LED lighting and backlight technology have made remarkable progress in the past decade. As a new generation of the next generation green light source, LED light source has appeared in the traditional lighting and other fields, but there are still many problems that have not been solved in the LED light source. It includes poor consistency, high cost and poor reliability, among which the most important problem is stability and reliability. Although it is now predicted that the life span of the LED light source is over 50 thousand hours. But the life span refers to the life of the theoretical life and the light source at 25. In the actual use process, high temperature, high humidity and other bad environment will be encountered, enlarging the defects of the LED light source, accelerating the aging of the material, and making the LED light source invalid quickly. The physical mechanism of the failure mode LED lamp is a system composed of multiple modules. Failure of each component causes LED bead failure. From the luminescent chip to the LED lamp, there are nearly thirty failure modes, as shown in Table 1, the failure mode table of the LED lamp. Here, the LED is divided into two parts from the composition structure to the chip and the external encapsulation. Then, the mode and physical mechanism of LED failure are also divided into two types: chip failure and encapsulation failure. Table 1 LED lamp failure modes caused by the failure mode of LED chip mainly include: static electricity, current and temperature. Electrostatic discharge can release instantaneous high voltage, which brings great harm to LED chip. The failure of LED chip caused by ESD can be divided into two modes: soft failure and hard failure. The high voltage / current caused by static electricity causes the short circuit of the LED chip to become a hard failure mode. The reason for the short circuit of the LED chip is that the high voltage causes the electrolyte to break, or the high current density is the current path in the chip. The release of a slightly lower voltage / current from the static electricity will lead to the soft failure of the LED chip. The soft failure is usually accompanied by the decrease of the reverse leakage current of the chip, which may be caused by the disappearance of the path of a part of the leakage current due to the high reverse current. Compared with the vertical LED chip, static electricity has a great harm to the horizontal LED chip. Because the electrode of the horizontal LED chip is on the same side of the chip, the instantaneous high voltage produced by the static electricity is easier to short the electrode on the chip, thus causing the failure of the LED chip. Large current also causes failure of LED chips: on one hand, large current brings a higher junction temperature; on the other hand, the entry of high power energy into PN junction will break the Mg-H and Ga-N bonds. The fracture of the Mg-H bond will further activate the carrier of the p layer, so that the LED chip has a light power rise phase at the beginning of aging, while the fracture of the Ga-N bond will form a nitrogen vacancy. The nitrogen vacancy increases the possibility of non radiation recombination, which explains the attenuation of the optical power of the device. The formation of nitrogen vacancies is a long process to reach equilibrium, which is the main reason for the slow aging of LED chips. At the same time, large current will bring the current congestion in the LED chip, the larger the defect density in the LED chip, the more serious the current congestion is. The excessive current density will cause the electromigration of metal, which makes the LED chip fail. In addition, under the dual action of current and temperature, the InGaN luminescent diode will also have a very unstable Mg-H2 complex in the effectively doped p layer. The effect of temperature on the LED chip is mainly to reduce the internal quantum efficiency and to shorten the life of the LED chip. This is because the internal quantum efficiency is a temperature function, and the higher the temperature is, the lower the quantum efficiency. At the same time, the aging of the material will make the properties of the ohm contact and the internal material of the LED chip become worse. In addition, the high junction temperature makes the temperature distribution inhomogeneous in the chip and produces strain, thus reducing the internal quantum efficiency and the reliability of the chip. The thermal stress to a certain extent may also cause the rupture of the LED chip. The main factors that cause the failure of LED package include temperature, humidity and voltage. At present, the most in-depth and extensive research is the effect of temperature on the reliability of LED packaging. The reasons for the failure of the LED module and the system are as follows: (1) high temperature will accelerate the degradation of the packaging materials and decrease the performance of the packaging materials; (2) the junction temperature will have a great effect on the performance of LED. High junction temperature can make the phosphor layer burn black carbon, which reduces the LED light efficiency rapidly or causes catastrophic failure. In addition, due to the mismatch between the refractive index and the thermal expansion coefficient between the silica gel and the phosphor particles, the high temperature will reduce the conversion efficiency of the phosphor, and the higher the proportion of the phosphor, the worse the light effect is. (3) the temperature gradient and temperature are divided because of the mismatch of the thermal conductivity between the encapsulated materials. The uneven distribution of the cloth may produce cracks within the material or delayer at the interface between the materials. These cracks and delamination will cause the decrease of the light efficiency, and the delamination between the chip and the phosphor layer can decrease the efficiency of the light extraction. The delamination between the phosphor layer and the encapsulated silica gel can decrease the efficiency of the light extraction by more than 20%. The delamination between the silica gel and the substrate may even lead to the fracture of the gold wire, causing catastrophic failure. Through the experimental study of high humidity environment, it is found that the invasion of wet gas not only reduces the light effect of LED, but also may lead to the catastrophic failure of LED. Through the experimental study of high temperature and high humidity reliability at 85 C / 85%RH, it is found that wet gas plays an important role in the formation of delaminating defects. The formation of stratification makes the light effect of LED decrease, and the different surface roughness of different chips leads to different failure modes.