Cause analysis of the most popular LED chip failur

Analysis on the causes of LED chip failure and packaging failure

LED lighting and backlight technology have made significant progress in terms of chain in recent ten years. As a recognized new next-generation green light source, LED light sources have appeared in traditional lighting and other fields, but there are still many unsolved problems in LED light sources

including poor consistency, high cost and poor reliability, among which the most important problems are stability and reliability. Although the service life of LED light source is currently predicted to exceed 50000 hours. But this life refers to the theoretical life, the service life of the light source at 25 ℃. In the process of actual use, we will encounter harsh environments such as high temperature and humidity, amplify the defects of LED light source, accelerate material aging, and make LED light source fail quickly

physical mechanism of failure mode

LED beads are a system composed of multiple modules. The failure of each component will cause the LED bulb to fail. From light-emitting chips to LED beads, there are nearly 30 failure modes, as shown in Table 1, failure mode table of LED beads. Here, the LED is divided into two parts: chip and external packaging. Then, the LED failure mode and physical mechanism are also divided into chip failure and package failure

electrostatic discharge can release instantaneous ultra-high voltage, which brings great harm to LED chips. LED chip failure caused by ESD can be divided into two modes: soft failure and hard failure. The high voltage/current caused by static electricity causes the LED chip to short circuit and become a hard failure mode. The reason for the short circuit of LED chip is that too high voltage breaks the electrolyte, or too high current density is the current path in the chip

slightly lower voltage/current of electrostatic discharge will lead to soft failure of LED chip. Soft failure is usually accompanied by the reduction of chip reverse leakage current, which may be caused by the disappearance of a part of the leakage current path due to high reverse current. Compared with vertical LED chips, static electricity is more harmful to horizontal LED chips. Because the electrodes of the horizontal LED chip are on the same side of the chip, the instantaneous high voltage generated by static electricity is more likely to short circuit the electrodes on the chip, resulting in the failure of the LED chip

high current will also lead to the failure of LED chip. Inverted metallographic microscope is a high-precision optical instrument: on the one hand, high current will bring relatively high junction temperature; On the other hand, electrons with high kinetic energy entering the PN junction will break the MG-H bond and ga-n bond

the breaking of MG-H bond will further activate the carrier of p layer, so that the LED chip has a stage of optical power rise at the beginning of aging, and the breaking of ga-n bond will form nitrogen vacancy. Nitrogen vacancy increases the possibility of non radiative recombination, which explains the attenuation of optical power of the device. It is a long process for the formation of nitrogen vacancies to reach equilibrium, which is the main reason for the slow aging of LED chips

at the same time, high current will bring current congestion inside the LED chip. The greater the defect density in the LED chip, the more serious the current congestion. Excessive current density will cause metal electromigration, making LED chips invalid. In addition, in has set up a professional reinforcement processing and distribution center and prefabrication processing center. Under the dual action of current and temperature, GaN LED will also appear very unstable mg-h2 complex in the effectively doped p layer

the effect of temperature on LED chips is mainly to reduce the internal quantum efficiency and shorten the life of LED chips. This is because the internal quantum efficiency is a function of temperature. The higher the temperature, the lower the internal quantum efficiency. At the same time, the aging effect of temperature on materials will deteriorate the performance of ohmic contacts and LED chip internal materials. In addition, the high junction temperature makes the temperature distribution in the chip uneven, resulting in strain, which reduces the internal quantum efficiency and the reliability of the chip. When the thermal stress is large to a certain extent, it may also cause the LED chip to crack

at present, the most in-depth and extensive research is the influence of temperature on the reliability of LED packaging. The reasons for the failure of LED modules and systems due to temperature are as follows:

(1) high temperature will accelerate the degradation of packaging materials and reduce the performance

(2) junction temperature will have a great impact on the performance of LED. Too high junction temperature will cause the phosphor layer to burn black and carbonize, which will sharply reduce the luminous efficiency of LED or cause catastrophic failure. In addition, due to the mismatch of refractive index and thermal expansion coefficient between silica gel and phosphor particles, too high temperature will reduce the conversion efficiency of phosphor, and the higher the proportion of doped phosphor, the worse the reduction of luminous efficiency

(3) due to the mismatch of thermal conductivity between packaging materials and the uneven temperature gradient and temperature distribution, cracks may occur inside the materials or delamination may occur at the interface between materials. These cracks and delamination will cause the light efficiency to decline. The delamination between the chip and the phosphor layer can reduce the light efficiency. The delamination between the phosphor layer and the potted silica gel can reduce the light efficiency by more than 20%. The delamination between silica gel and substrate may even lead to the fracture of gold wire, resulting in catastrophic failure

through the experimental research on high humidity environment, the sample slips during the experimental process. The invasion of moisture not only reduces the luminous efficiency of LED, but also may lead to catastrophic failure of LED. Through the accelerated reliability test of 85 ℃/85% RH high temperature and high humidity, it is found that moisture plays an important role in the formation of delamination defects. The generation of delamination reduces the luminous efficiency of LED, and different chip surface roughness leads to different failure modes