Analysis of LED Chip Failure and Packaging Failure
LED lighting and backlight technology in the past decade has made significant progress, as a recognized new generation of green light source, LED light source has appeared in the traditional lighting and other fields, but there are still many LED light source does not solve the problem.
Including poor consistency, high cost and poor reliability, the most important of which is the stability and reliability issues. Although the current forecast LED light source life of more than 50,000 hours. But this life refers to the theoretical life, light source at 25 ℃ service life. In the actual use of the process, will encounter high temperature, high humidity and other harsh environments, zoom LED light source defects, accelerate the aging of the material, the LED light source quickly failed.
1. Failure mode of the physical mechanism
LED lamp beads 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, the failure mode has nearly 30 kinds, as shown in Table 1, LED lamp beads failure mode table shown. Here the LED from the composition of the structure is divided into two parts of the chip and external packaging. Then, LED failure mode and physical mechanism is also divided into chip failure and packaging failure to discuss the two.
2. Table 1 LED lamp beads failure mode
LED chip failure factors include: static electricity, current and temperature.
3. Electrostatic discharge can release the moment 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 static electricity causes the LED chip to short-circuit into a hard failure mode. The reason why the LED chip is short-circuited is that the high voltage causes the electrolyte to break, or too high the current density is the current path in the chip.
Electrostatic discharge A slightly lower voltage / current can cause soft failure of the LED chip. Soft failure is usually accompanied by a decrease in the chip back leakage current, which may be due to the high reverse current caused by the disappearance of a portion of the leakage current path. Compared to the vertical LED chip, static on the level of LED chip damage. Because the level of the LED chip electrode on the same side of the chip, the instantaneous high voltage generated by static electricity easier to short the chip on the chip, causing LED chip failure.
High current will bring the failure of LED chip: on the one hand a large current will bring a relatively high junction temperature; the other hand, with high power of the electronic into the PN junction will make Mg-H bond and Ga-N bond break The
The rupture of the Mg-H bond will further activate the carrier of the p-layer, so that the LED chip has an optical power rise stage at the beginning of the aging, and the Ga-N bond will form a nitrogen vacancy. The nitrogen vacancy increases the likelihood of nonradiative recombination, thereby explaining the attenuation of the optical power of the device. The formation of nitrogen vacancies to achieve a balance when a very long process, which is the main reason for the slow aging LED chip.
At the same time, high current will bring the current inside the LED chip crowded, LED chip defects in the larger density, the more serious the phenomenon of current congestion. Excessive current density can cause the phenomenon of metal electromigration, making the LED chip failure. In addition, InGaN light-emitting diodes in the dual role of current and temperature, in the effective doping p layer will appear very unstable Mg-H2 complex.
The effect of temperature on the LED chip is mainly to reduce the internal quantum efficiency and LED chip life shorter. This is because the internal quantum efficiency is the temperature function, the higher the temperature the lower the quantum efficiency, while the temperature of the aging of the material will make ohmic contact and LED chip internal material performance deterioration. In addition, the high junction temperature makes the temperature distribution within the chip 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.
4. LED packaging failure caused by the main factors include: temperature, humidity and voltage.
At present, the most thorough and extensive study of the impact of temperature on the reliability of LED packaging. Temperature causes LED module and system failure because of the following aspects:
(1) high temperature will slow down the degradation of packaging materials, performance degradation;
(2) junction temperature on the LED performance will have a great impact. Excessive junction temperature will blacken the phosphor layer black, making the LED light effect is reduced or causing catastrophic failure. In addition, because the refractive index and thermal expansion coefficient between the silica gel and the phosphor particles do not match, the high temperature will reduce the conversion efficiency of the phosphor, and the higher the proportion of the phosphor doped, the worse the luminous efficiency decreases;
(3) Due to the mismatch of the thermal conductivity between the encapsulating materials, the temperature gradient and the temperature distribution are uneven, cracks may be generated inside the material or delaminated at the interface between the materials. These cracks and delamination will cause the decline in luminous efficiency, chip, phosphor layer between the delamination can reduce the efficiency of light extraction, phosphor layer and potting silica gel between the highest level of extraction efficiency can be reduced by more than 20% The The delamination between the silica gel and the substrate may even lead to breakage of the gold wire, resulting in catastrophic failure.
Through the experimental study of high humidity environment found that moisture intrusion not only makes the LED luminous efficiency decreased, and may lead to catastrophic failure of LED. It is found that moisture plays an important role in the formation of stratified defects by the high temperature and high humidity reliability of 85 ℃ / 85% RH. The result of stratification caused the decrease of the luminous efficiency of LED and the difference of roughness of different chips Different failure modes.