analysis of led chip failure and package failure
Led fluorescent tube light repacements fixtures and backlight technology in the last decade has made significant progress, as a recognized new generation of green light source, LED light source has been in the traditional areas such as lighting, LED light source, but there are still many unresolved issues.led fluorescent tube light repacements fixtures and backlight technology in the past decade has made significant progress, as recognized as a new generation of green light source, LED light source has been in the traditional areas such as lighting, LED light source, but there are still many unresolved issues.Including the poor consistency, high cost and poor reliability, the most important problem is stability and reliability issues. Although the current LED light source life expectancy of more than 50,000 hours. But this life refers to the theoretical life, light source at 25 ℃ under the life. In the actual use of the process, will encounter high temperature, high humidity and other harsh environments, amplification LED light source defects, accelerated aging of the material, the LED light source failure quickly.1Physical Mechanism of Failure ModeLED lamp bead is a system composed of multiple modules. Failure of each component will cause LED lamp beads failure. From the light-emitting chip to the LED lamp beads, failure mode has nearly 30, as shown in Table 1, LED lamp beads failure mode table. Here the LED from the composition of the structure is divided into two parts of the chip and the external package. Then, LED failure mode and physical mechanisms are divided into two kinds of chip failure and package failure to be discussed.2Table 1 LED lamp beads failure modeLED chip failure caused by the main factors include: static electricity, current and temperature.3Electrostatic discharge can release instant ultra-high voltage, to the LED chip brings great harm, ESD LED chip failure is divided into soft failure and hard failure two modes. The high voltage / current caused by the static electricity causes the LED chip to short-circuit to become the hard failure mode. LED chip short circuit is due to high voltage to electrolyte breakdown, or too high current density is generated in the chip current path.Electrostatic discharge A slightly lower voltage / current results in soft failure of the LED chip. Soft failures are usually accompanied by a decrease in the reverse leakage current of the chip, which may be caused by a high reverse current that causes a portion of the leakage current path to disappear. Compared to the vertical LED chip, static electricity on the level of LED chip damage. Because the level of the LED chip electrode on the same side of the chip, static electricity generated by the instant high voltage is more likely to short-circuit the electrode on the chip, which led to LED chip failure.High current will also bring LED chip failure: on the one hand, high current will bring a relatively high junction temperature; the other hand, with high kinetic energy into the PN junction will make the Mg-H bond and Ga-N bond fracture .The cleavage of the Mg-H bonds further activates the carriers of the p-layer, causing the LED chip to have an increased optical power at the beginning of the aging, while the Ga-N bond will form a nitrogen vacancy. Nitrogen vacancies increase the probability of non-radiative recombination, thus explaining the attenuation of the optical power of the device. The formation of nitrogen vacancies to achieve a very long time to balance the process, which is the slow aging LED chip the main reason.At the same time, high current will bring LED chip internal current crowding, LED chip defect density is bigger, the phenomenon of current crowding is more serious. Excessive current density will cause the phenomenon of metal migration, making the LED chip failure. In addition, InGaN light-emitting diode in the current and temperature under the dual role in the effective doping of the p-layer will also appear very unstable Mg-H2 complex.
The effect of temperature on the LED chip is mainly to reduce the quantum efficiency and LED chip life shorter. This is because the internal quantum efficiency is a function of temperature, the higher the temperature the lower the quantum efficiency, while the aging of the material on the temperature of the ohmic contact and LED chip material properties within the poor. In addition, the high junction temperature makes the chip temperature distribution is uneven, resulting in strain, thereby reducing the internal quantum efficiency and chip reliability. Thermal stress to a certain extent, may also cause LED chip rupture.4LED packaging failure caused by the main factors include: temperature, humidity and voltage.At present, the study of the most in-depth and wide range of temperature on the reliability of LED packaging. Temperature LED module and system failure is due to the following aspects:(1) high temperature will accelerate the degradation of packaging materials, performance degradation;(2) junction temperature on the performance of LED will have a great impact. Excessive junction temperature will make the phosphor layer black carbonization, making the LED light effect dramatically reduced or cause catastrophic failure. In addition, due to the silica gel and the phosphor particles between the refractive index and thermal expansion coefficient does not match, too high temperature will reduce the phosphor conversion efficiency, and the higher the proportion of doped phosphor, the more serious decline in luminous efficiency;(3) due to the heat transfer coefficient between the packaging material does not match, the temperature gradient and temperature distribution is uneven, the material may be cracked or interfacial delamination between the materials. These cracks and delamination will cause the decline in luminous efficiency, chip, phosphor layer delamination between the light extraction efficiency can be reduced, the phosphor layer and the encapsulation of silica gel between the delamination of the highest access efficiency can be reduced by more than 20% . The delamination between the silica gel and the substrate may even lead to fracture of the gold wire, resulting in catastrophic failure.Through the experimental study of high humidity environment, moisture invasion not only makes LED light efficiency, and may lead to catastrophic failure of LED. The experimental results show that moisture plays an important role in the formation of delamination defects, which results in the decrease of the luminous efficiency of LED and the different roughness of the chip surface. Different failure modes.