LED the main failure mode analysis

LED the main failure mode analysis

Discovery
LED light is a direct energy conversion for visible and radiant energy of light, and has little power consumption device, high luminous efficiency, the advantages of small size, has gradually become a new high efficiency and energy saving products, and is widely used in display, lighting, backlit and so on many domains. In recent years, with the development of technology of LED, the luminous efficiency also had markedly improved, the existing blue LED the efficiency of the system can reach 60%; And white LED the efficacy is more than 150 lm/W, these characteristics make LED by more and more attention.

At present, although the theory of LED live for up to 50 kh, however in be used actually, because been all factor, the LED often hit such high theoretical service life, the emergence of a premature failure phenomenon, this greatly hindered the new energy-saving products LED as pace. In order to solve this problem, many scholars have carried out the related research, and get some important conclusions. This paper is on the basis of the important factors LED failure caused by systematic analysis, and put forward some improvement measures, expecting to be able to perfect the actual use of the LED life.

A, LED failure mode

The main failure modes have LED chip failure, encapsulation: failure, thermal stress failure, electricity lead stress failure and assembly failure, especially with chip failure and packaging failure is most common. This paper will this several main failure modes, a detailed analysis of the.

(1) chip failure

Chip failure is refers to the chip itself failure or other causes chip failure. Cause the failure reason often have very DuoZhong: chip crack is due to bonding process conditions are not appropriate, to cause major stress, along with the heat accumulation the heat produced in the mechanical stress also will strengthen, produced in chip micro cracks, when the current work into worsen the micro cracks is expanding constantly, until device failure [4]. Second, if chips have source area had a damage, so may result in plus electric process until the failure, the steady deterioration will also cause lamps and lanterns is in use process until no bright light serious decline. Furthermore, if bonding process is bad, the chip in use process will lead to chip bond completely off the surface and bonding layer that samples, and also happened to open failure can cause LED in use process happen "dead lamp" phenomenon. In chip bonding process undesirable reasons, may be due to the use of silver pulp exposure time expired or too long, silver plasma usage too little, curing time is too long, solid crystal face is pollution,.

(2) encapsulation failure

Failure is refers to the packaging design packages or manufacturing process to device failure improper. Packaging materials used in the epoxy resin, in use process will occur, the degradation of the lower life LED. The degradation problems including: the light through rate, refractive index, expansion coefficient, hardness, water permeability, permeability and packing performance, especially with light through rate is most important. Studies show that the shorter the wavelength of light, light through rate of degradation, but for the more serious the wavelength of green light above (that is greater than 560 nm) speaking, this influence is not serious. Lumileds2003 years have revealed power LED the white light devices and ¦Õ 5 white light device of life the experimental curves, 19, use silicon resin after kh encapsulates the power components, flux can still maintain the initial 80%, but with epoxy resin packaging contrast curve, said in June after kh, flux maintain rate is only 50%. Experimental results show that the chip luminous efficiency under the same, near the chip epoxy resin obvious yellow, then to brown. This apparent degeneration process, and is mainly due to the light and temperature rise of epoxy resin by light through rate of caused by the degradation. Meanwhile, in blue light yellow phosphor issued by stimulate the white light of the lens, encapsulation LED Browning will affect its reflection sex, and makes the blue light enough to inspire a yellow phosphor, thus make the efficacy and spectral distribution change. As for packaging, and a LED the life of the important influence factor is corrosion. In use, the general LED by corrosion is the main reason of the water vapor infiltration package material internal, leading to lead metamorphism, PCB copper wire corrosion; Sometimes, with the introduction of the water vapor can move conductive ion will stay in the chip surface, and cause leakage. In addition, packaging quality is bad, the device in the package there are a large number of department of the body, the residue of the residual air bubbles bubble will also cause the corrosion of the device.

(3) thermal stress failure

The temperature has been the effect of the important factors LED optical properties, and in the research of the failure modes, LED scholars both at home and abroad to work environment temperature will be considered as accelerating stress, to LED accelerated life experiment [8, 9]. This is because in the LED system thermal resistance of the same premise, encapsulation pins raise the temperature of welding points, the junction temperature will increase, resulting in failure LED early.



Figure 1 high power LED structure of the model and the work environment temperature respectively for (a) 120 ¡æ, (b) 100 ¡æ and 80 ¡æ (c) under the radiation power and speed up the time relationship chart

Hsu, people of different manufacturers to provide the LED samples accelerated life experiment, this experiment will LED samples are set on 80, 100, 120 ¡æ, use 3.2 V voltage driver, and provisions of light power when the sample dropped to 50% of the initial value, which is judged to be a failure.

Figure 1 experimental results show that: the life of high power LED with the increase of temperature accelerated life experiment and the increase of the accelerating time reduced. In the accelerated life experiment, and high temperature rise will make LED epoxy resin material alteration happened to increase the thermal resistance, make system chip and packaging of heated happened between surface degradation, and eventually lead to encapsulate failure [9].

(4) electricity lead stress failure

LED in the current if use (EOS) or electrostatic impact injury (ESD) the chips, can create chip), formed electricity lead stress failure. For example, GaN is wide forbidden band material. Resistance rate is higher. If using this chip, in the process of production because of the electrostatic generation induced charge is not easy to disappear, when its accumulated to a degree, can produce high the electrostatic voltage, the voltage once more than material bear ability, will take place breakdown phenomenon and discharge, makes the device failure.

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