LED lighting and backlighting technology in the last decade has made significant progress, as recognized by the new next-generation green light source, LED light source has appeared in traditional areas such as lighting, LED light source, but there are still many unresolved issues.
These include poor consistency, high cost and poor reliability, the most important problem is the stability and reliability issues. Although the predicted life of the LED light source more than 50,000 hours. But this means that life theoretical life, lamp life at 25 deg.] C of. In actual use, you will encounter high temperatures, humidity and other harsh environment, the LED light source amplifying defects, accelerated aging of the material, rapid failure of the LED light source.
Physical mechanism failure mode
LED lamp system is composed of a plurality of modules. Each component can cause failure of the LED lamp failure. From the light emitting chip to the LED lamp, nearly thirty kinds of failure modes, as shown in Table 1, LED lamp failure modes are shown in Table beads. Here the LED chip and the outside of the package is divided into two parts from the compositional structure. Then, LED failure modes and physical mechanisms are also divided into two kinds of chip failure and failure of the package to be discussed.
Table 1 LED lamp beads Failure Mode
LED chip failure factors include: electrostatic, current and temperature.
Electrostatic discharge can be released instantaneously ultrahigh voltage, the LED chip to a great deal of harm, leading to the ESD failure LED chips are divided into soft and hard failures failure modes. Static electricity from the high-voltage / short-circuit current causes the LED chips becomes hard failure mode. Causes of the LED chip is a short-circuit high voltage electrolyte rupture, or too high current densities generated in the chip is the current path.
Electrostatic discharge slightly lower voltage / current causes the LED chips soft errors. Soft failure is usually accompanied by a decrease in the reverse leakage current chip, which may be due to the high reverse current path so that a portion of the leakage current due to the disappearance. Compared to a vertical LED chip, the LED chip level of electrostatic hazards greater. Because the horizontal LED chip electrodes on the same side of the chip, transient high voltage static electricity more easily short-circuited electrodes on the chip, thereby causing failure of the LED chip.
It will bring a large current failure LED chip: one large current would bring a higher junction temperature; on the other hand, having a high power electronic energy into the PN junction causes Mg-H bonds and Ga-N bond cleavage .
Breaking Mg-H bonds will further activate the carrier of the p-layer, the LED chip has a light power rising stage at the beginning of aging, would be broken Ga-N bond formation of nitrogen vacancies. Nitrogen vacancies increases the possibility of non-radiative recombination, thus explaining the attenuation of light power of the device. Nitrogen vacancies formed to reach equilibrium when a very long process, which is the main LED chip slow aging.
At the same time, the large current brings the current congestion inside the LED chip. The larger the defect density in the LED chip, the more serious the current congestion. Excessive current density causes electromigration phenomena of metal, making the LED chip fail. Further, the INGAN LEDs are also unstable Mg-H2 complexes in the effective doped P layer under the dual effect of current and temperature.
Effect of temperature on the LED chip and primarily to enable LED chips to reduce the quantum efficiency of a short life. This is because the internal quantum efficiency is a function of the temperature, the lower the temperature the higher the internal quantum efficiency, at the same time, the temperature of the material properties of the effects of aging and make ohmic contacts of the LED chip inside the material deteriorates. Further, the high junction temperature such that an uneven temperature distribution within the chip, strained, thereby reducing internal quantum efficiency and the reliability of the chip. Thermal stress to a certain extent, and may cause breakage of the LED chip.
LED package failure caused by factors including: temperature, humidity, and voltage.
Currently, the most in-depth and extensive study of the influence of temperature on the reliability of the LED package. And the temperature of the LED module system failure due to the following aspects:
(1) A high temperature will accelerate the degradation of the encapsulating material, performance degradation;
(2) the performance of the LED junction temperature will have a great impact. High junction temperature will burn black carbonized phosphor layer, so that the LED light efficiency drastically reduced or catastrophic failure. Further, since the refractive index mismatch and the thermal expansion coefficient between the phosphor particles and the silica, high conversion efficiency of the phosphor temperature will drop, and the higher the proportion of phosphors doped, the more powerful the droop;
(3) heat transfer coefficient due to a mismatch between the encapsulating material, the temperature gradient and non-uniform temperature distribution within the material might crack or delamination at the interface between the materials. These can cause cracks and delamination droop, delamination between the chip, the phosphor layer can decrease the light extraction efficiency, delamination between the phosphor layer and the silicone potting maximum light extraction efficiency can drop by more than 20% . Delamination between the substrate and the silica gel may even lead to the gold wire breakage, resulting in catastrophic failure.
For high-humidity environment by experimental study we found that not only by moisture intrusion LED droop, and may lead to catastrophic failure of the LED. Study on reliability acceleration test RH high-temperature high-humidity found 85 ℃ / 85%, moisture plays an important role in the formation of defects in the delamination, delamination phenomenon of the LED luminous efficiency drops, resulting in different surface roughness different chips different failure modes. Technology area
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