Cause analysis of LED chip failure and package failure

LED lighting and backlight technology has made remarkable progress in recent years, recognized as a new generation of green light source, LED light source has appeared in the traditional lighting field, but there are still a lot of LED light source did not solve the problem. Including poor, poor consistency of high cost and reliability, the most important problem is the problem of stability and reliability. Although the LED light source is predicted to exceed 50 thousand hours of life. But this life refers to the theoretical life and the service life of the light source at 25 degrees c.. In the actual use process, will encounter high temperature, high humidity and other harsh environment, enlarge the defects of LED light source, accelerate the aging of materials, and make the LED light source rapidly fail. The failure mode of the physical mechanism of LED lamp is composed of a multi module system. Each part of the failure will cause the failure of LED lamp. From the light emitting chip to the LED chip, there are nearly thirty kinds of failure modes, such as table 1, LED lamp failure mode is shown in the table. Here, LED is divided into two parts: chip and external package. Then, the failure modes and mechanisms of LED can be divided into two types: chip invalidation and package failure. Table 1 LED lamp failure mode caused by factors of LED chip failure mainly includes: static, current and temperature. Electrostatic discharge can release the instant high voltage, great harm to the LED chip, LED chip ESD led to the failure of the soft and hard failure failure is divided into two modes. The high voltage / current brought by the static causes the LED chip to short-circuit and 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 creates a current path in the chip. ESD failure soft slightly lower voltage / current will cause the LED chip. Soft failure is usually accompanied by a decrease in the reverse leakage current of the chip, possibly due to the disappearance of a part of the leakage current path due to the high reverse current. Compared with the vertical LED chip, static electricity has a greater harm to the horizontal LED chip. Because the electrode of the horizontal LED chip is on the same side of the chip, the high voltage is easier to short-circuit the electrode of the chip when the static electricity is generated, thus causing the failure of the LED chip. The failure of large current will also bring LED chip: on the one hand, large current will bring the junction temperature is relatively high; on the other hand, high power electronic has entered the PN junction will make the Mg-H bond and Ga-N bond breaking. The fracture Mg-H bond will further activate the carrier p layer, the LED chip has a light power rise stage in aging, and the cleavage of the Ga-N bond formation of nitrogen vacancies. The nitrogen vacancy increases the possibility of nonradiative recombination, thus explaining the attenuation of the optical power of the device. The formation of nitrogen vacancies is a long process when equilibrium is reached, which is the main reason for the slow aging of LED chips. At the same time, large current will bring the current LED chip LED chip in the crowded, defect density is bigger, the current crowding phenomenon is more serious. Excessive current density results in electromigration of the metal, resulting in the failure of the LED chip. In addition, under the double action of the current and temperature, the InGaN led to a very unstable Mg-H2 complex in the well doped P layer. Effect of temperature on the LED chip is mainly to reduce the internal quantum efficiency and shorter life expectancy LED chip. This is because the internal quantum efficiency is a function of temperature, the temperature is high internal quantum efficiency is lower, at the same time, the temperature will make the performance of ohmic contact and LED chip materials poor aging of materials. In addition, the high junction temperature causes uneven temperature distribution in the chip, resulting in strain, thereby reducing the internal quantum efficiency and chip reliability. The thermal stress is large enough to cause LED chip rupture. Causes LED package failure include: temperature, humidity and voltage. At present, research the most deeply and widely is the influence of temperature on LED package reliability. The temperature LED module and system failure lies in the following aspects: (1) high temperature can make packaging materials, accelerate the degradation of performance; (2) will have a great impact on the performance of the junction temperature of LED. The high junction temperature will make the phosphor layer burn black and carbonized, which will make the LED light effect decrease sharply or cause catastrophic failure. In addition, because the coefficient mismatch between the silica gel phosphor particles and the refractive index and thermal expansion, high temperature will make the phosphor conversion efficiency decreased, and the proportion of phosphor doped with higher light efficiency decreased more; (3) due to the mismatch between the thermal conductivity of packaging materials, temperature gradient and the temperature distribution is not uniform, the material may crack or delamination in the material interface. These cracks and delamination can cause the light effect to decrease. The delamination between the chip and the phosphor layer can reduce the light absorption efficiency, and the highest delamination between the phosphor layer and the encapsulating silica gel can reduce the light absorption efficiency by more than 20%. The delamination between the silica gel and the substrate may even lead to fracture of the gold wire and cause catastrophic failure. By high humidity experiment shows that moisture intrusion not only makes LED light efficiency decline, but also may lead to catastrophic failure of LED. By 85 DEG / 85%RH high temperature and high humidity accelerated reliability experiment shows that moisture plays an important role in the formation of delamination, delamination phenomenon makes the optical efficiency of LED decreased, different chip surface roughness due to the different failure modes of different.