The LED is a direct electrical energy is converted to visible light and radiation emitting devices with small power consumption, high luminous efficiency, small size, has gradually become a new energy-efficient products, and is widely used in display, lighting, backlighting and other areas. In recent years, as LED Light technology continues to progress, the luminous efficiency has also been significantly improved, the blue LED system efficiency can reach 60%; white LED luminous efficiency of more than 150lm / W, these features have made the LED by more and more attention.
At present, although the theoretical life of the LED can be achieved 50kh in actual use, however, due to various factors, the LED is often amounted to less than such a high theoretical life appears the phenomenon of premature failure, which greatly hindered the LED as a new energy-efficient the pace of the products. To solve this problem, many scholars have carried out relevant research, and a number of important conclusions. Is on this basis, the cause of LED failure carry out a systematic analysis, and proposed some improvement measures, and to expect to be able to improve the actual service life of the LED.
First, led power 50w failure modes
LED failure modes: chip failure, the package fails, the thermal stress failure, electrical overstress failures and assembly failure, especially in chip failure and package failure is most common. This paper will these types of failure modes, a detailed analysis.
(1) chip failure
Chip failure refers to the chip itself to failure or other causes chip failure. Cause is often the reason of this failure are many: the die crack is due to improper bonding process conditions, resulting in a larger stress, with the accumulation of heat generated by the thermo-mechanical stress also will strengthen, causing the chip to produce micro-cracks, work when the injected current will further exacerbate the micro-crack continues to expand, until the device is completely ineffective. Second, if the chip active area already has injury, it will lead to gradual degradation during power-up failure, the same can also cause the lamps in the use of a coherent state of serious decline until does not shine. Furthermore, if the bad chip bonding technology, chip bonding layer is completely out of the bonding surface and the sample to open lapse, the same can also cause the LED Spotlight 30W in the course of "dead light" phenomenon occurs in the course. The bad chip bonding process may be due to the use of silver paste expired or the exposure time is too long, silver paste is too small, the curing time is too long, solid crystal base surface contaminated.
(2) package failure
Package failure is improper package design or production processes lead to device failure. Epoxy materials, packaging occur in the course of the degradation problem, resulting in reduced life of the LED. This deterioration include: light transmittance, refractive index, thermal expansion coefficient, hardness, permeability, air permeability, packing performance, especially in light transmittance of the most important. Studies have shown that the shorter the wavelength of light, the deterioration of the light transmittance is more serious, but for the green light above the wavelength (ie, greater than 560nm), this effect is not serious. Lumileds2003 years has published over power white LED Flood light 10W devices and the φ5 white device life test curve 19kh after silicone encapsulated power devices, the luminous flux can still maintain 80% of the initial, said the contrast curve of the epoxy package 6kh after, the luminous flux maintenance rate was 50%. The experiments show that the chip light-emitting efficiency of the same case, the epoxy resin near the chip was to turn yellow, then turn brown. This apparent degradation process is mainly caused due to the deterioration of the light transmittance of light and temperature rise caused by epoxy resin. At the same time, excitation by blue light yellow phosphor for white light LED, encapsulated lens browning will affect its reflectivity, and issued by the blue is not sufficient to stimulate the yellow phosphor, so that the luminous efficiency and spectral distribution of changed.
In terms of the package, there is a important factor to affect the LED life is corrosion. In the use of led driver 120w, the general corrosion caused mainly due to water vapor infiltrated within the packaging material, resulting in lead deterioration, PCB copper corrosion; sometimes movable conductive ions introduced with the water vapor resides in the chip surface, resulting in leakage. In addition, the package poor quality of the devices, encapsulated within a large number of residual bubbles, these residual bubbles would also cause corrosion of the device.
(3) thermal stress failure
The temperature has been an important factor of the LED optical properties, LED failure modes of the research scholars to consider the working environment temperature accelerated stress to the LED accelerated life test. This is because the LED system thermal resistance of the same premise, the welding point of the package pin temperature, junction temperature will also increase, resulting in premature failure LED.