Analysis and Improvement of Main Failure Mode of LED(I)
LED is a direct conversion of electrical energy into visible light and radiant energy devices, with a small power consumption, high luminous efficiency, small size, etc., has now become a new type of energy efficient products, and is widely used in display , Lighting, backlighting and many other areas. In recent years, with the LED technology continues to progress, its luminous efficiency has also been significantly improved, the existing blue LED system efficiency can reach 60%; and white LED light effect has more than 150lm / W, these features are made LED More and more attention.
At present, although the theoretical life of LED can reach 50kh, but in actual use, because of various factors, LED often fail to achieve such a high theoretical life, there premature failure phenomenon, which greatly hindered the LED as a new energy Type of product progress. In order to solve this problem, many scholars have carried out relevant research, and got some important conclusions. This paper is based on this, on the cause of LED failure of the important factors for a systematic analysis, and put forward some improvement measures to expect to improve the actual life of LED.
First, LED failure mode
LED failure modes are: chip failure, package failure, thermal over stress failure, power stress failure and assembly failure, especially in the chip failure and packaging failure is the most common. This article will be a detailed analysis of these major failure modes.
(1) Chip failure
Chip failure refers to the chip itself failure or other causes of chip failure. There are many reasons for this failure: chip cracks are due to the bonding process conditions are not appropriate, resulting in greater stress, with the accumulation of heat generated by the thermal mechanical stress also will be strengthened, resulting in micro-cracks in the chip, work When the injection of the current will further intensify the micro-cracks to continue to expand until the device completely failed. Second, if the chip active area has been damaged, it will lead to the process of gradual degradation in the process until the failure, the same will cause the lamp in the course of light failure until the light is not bright. Moreover, if the chip bonding process is bad, in the course of the use of the chip will lead to the adhesive layer completely separated from the bonding surface and make the sample open circuit failure, the same will cause the LED in the course of the use of "dead light" phenomenon. The reason for the poor bonding process may be due to the use of silver paste or exposure time is too long, the use of silver paste is too small, curing time is too long, solid crystal base surface contamination.
(2) Package failure
Encapsulation failure means that the package design or production process does not cause the device failure. Epoxy resin used in packaging materials, in the course of the occurrence of deterioration problems, resulting in reduced LED life. Such deterioration problems include: light transmittance, refractive index, expansion coefficient, hardness, water permeability, permeability, filler properties, especially in light transmittance is most important. Studies have shown that the shorter the wavelength of light, the more serious degradation of light transmittance, but for green above the wavelength (that is greater than 560nm), this effect is not serious. Lumileds announced in 2003 the luminous LED white devices and φ5 white light device life test curve, 19kh, with silicone encapsulated power devices, luminous flux can still maintain the initial 80%, while the epoxy resin package contrast curve is shown in the 6kh, the luminous flux maintenance rate of only 50%. Experiments show that, in the case of the same luminous efficiency of the chip, the chip near the epoxy resin becomes yellow, and then become brown. This apparent degradation process is mainly due to the deterioration of the light transmittance of the epoxy resin due to light and temperature rise. At the same time, in the LED that emits white light by the blue light to emit white light, the browning of the encapsulating lens affects its reflectivity and makes the emitted blue light insufficient to excite the yellow phosphor, resulting in a change in light efficiency and spectral distribution.
For the package, there is also an important factor affecting the life of LED is corrosion. In the use of LED, the general cause of corrosion is mainly due to water vapor into the packaging material inside, resulting in lead deterioration, PCB copper corrosion; sometimes, with the introduction of water vapor activated conductive ions will reside on the chip surface, resulting in leakage. In addition, the poor quality of the device package, within its package will have a lot of residual bubbles, these residual bubbles will also cause corrosion of the device.
(3) Thermal over stress failure
Temperature has always been an important factor affecting the optical properties of LED, and in the study of LED failure mode, domestic and foreign scholars take into account the working environment temperature as accelerated stress, to carry out LED accelerated life experiment. This is because in the LED system thermal resistance under the premise of the package pin welding point of the temperature rise, the junction temperature will rise, resulting in LED failure in advance.
High power LED model structure
Figure: High power LED model structure as well as the working ambient temperature are respectively
(A) 120 ° C, (b) 100 ° C and (c) radiation power at 80 ° C and acceleration time