LED is a kind of light-emitting device that directly converts electric energy into visible light and radiant energy. It has the advantages of low power consumption, high luminous efficiency and small volume. It has gradually become a new type of high-efficiency energy-saving product and is widely used in display. , lighting, backlight and many other fields. In recent years, with the continuous advancement of LED technology , its luminous efficiency has also been significantly improved, the existing blue LED system efficiency can reach 60%; and the efficacy of white LED has exceeded 150lm / W, these characteristics make the LED More and more attention.
At present, although the theoretical life of LED can reach 50kh, in actual use, because of various factors, LED often does not reach such a high theoretical life, and there is premature failure, which greatly hinders LED as a new energy saving. The pace of progress of the product. In order to solve this problem, many scholars have carried out related research and got some important conclusions. Based on this, this paper systematically analyzes the important factors that cause LED failure, and proposes some improvement measures in order to improve the actual service life of LEDs.
First, the LED failure mode
LED failure modes are mainly: chip failure, package failure, thermal overstress failure, electrical overstress failure and assembly failure, especially chip failure and package failure are the most common. This article will conduct a detailed analysis of these major failure modes.
(1) Chip failure
Chip failure refers to chip failure or other reasons that cause chip failure. There are many reasons for this failure: the chip crack is due to the improper bonding process conditions, causing large stresses, and the thermo-mechanical stress generated by the heat accumulation is also strengthened, resulting in microcracks in the chip, working. The current injected will further exacerbate the microcracks and continue to expand until the device fails completely. Secondly, if the active area of ​​the chip is inherently damaged, it will gradually deteriorate to the failure during the power-on process, and it will also cause the light to be severely degraded until it is not illuminated during use. Moreover, if the bonding process of the chip is poor, the bonding layer of the chip may completely separate from the bonding surface during use, and the open circuit failure of the sample may also cause a "dead light" phenomenon in the LED during use. The reason for the poor bonding process of the chip may be due to the expiration of the used silver paste or the excessive exposure time, the excessive use of the silver paste, the long curing time, and the contamination of the solid crystal base.
(2) Package failure
Package failure refers to device failure due to improper package design or manufacturing process. The epoxy resin material used in the package may deteriorate during use, resulting in a decrease in the life of the LED. Such deterioration problems include: light transmittance, refractive index, expansion coefficient, hardness, water permeability, gas permeability, filler properties, etc., among which light transmittance is most important. Studies have shown that the shorter the wavelength of light, the more severe the degradation of light transmittance, but for wavelengths above green (ie greater than 560 nm), this effect is not serious. Lumileds published the life test curve of power LED white light device and φ5 white light device in 2003. After 19kh, the power flux of the silicon-encapsulated power device can still maintain the initial 80%, and the contrast curve of epoxy resin package indicates After 6kh, the luminous flux maintenance rate is only 50%. Experiments have shown that in the case of the same luminous efficiency of the chip, the epoxy resin close to the chip turns yellow and then turns brown. This apparent degradation process is mainly caused by the deterioration of the light transmittance of the epoxy resin caused by light and temperature rise. At the same time, in an LED that emits white light by blue light-excited yellow phosphor, browning of the encapsulating lens affects its reflectivity, and the emitted blue light is insufficient to excite the yellow phosphor, thereby causing a change in light efficiency and spectral distribution.
For packaging, another important factor affecting LED life is corrosion. In LED use, the main cause of corrosion is that water vapor penetrates into the inside of the package material, causing the lead to deteriorate and the PCB copper wire to rust. Sometimes, the movable conductive ions introduced with the water vapor will reside on the surface of the chip, causing leakage. In addition, devices with poor package quality will have a large amount of residual bubbles inside the package, and these residual bubbles will also cause corrosion of the device.
(3) Thermal overstress failure
Temperature has always been an important factor affecting the optical properties of LEDs. When studying the LED failure mode, domestic and foreign scholars consider the working environment temperature as the acceleration stress to carry out the LED accelerated life test. This is because, under the premise that the thermal resistance of the LED system is constant, the temperature of the solder joint of the package leads to rise, and the junction temperature also rises, resulting in premature failure of the LED.
Figure: The model structure of the high-power LED and the temperature in the working environment are respectively
(a) Relationship between radiated power and acceleration time at 120 ° C, (b) 100 ° C, and (c) 80 ° C
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