Cause of the failure of important factors LED analysis

Published: 06th June 2012
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At present, although the theory of led strip live for up to 50 kh, but in actual use, because they are a variety of factors, 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 this paper, the cause of the failure of important factors LED systematic analysis, and put forward some improvement measures, expecting to be able to perfect LED the actual use of the life.

A, LED failure mode

Flexible LED Strip Lights mainly has: chip failure, encapsulation failure, thermal stress failure, electricity lead failure and assembly stress failure, especially the 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 any other reason chip failure. Cause the failure reason often have a lot of kinds: chip crack is due to the bonding process conditions are not appropriate, 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 the device failure [4]. Second, if chips have source area had a damage, so may result in the degradation process, electric gradually until failure, also can cause lamps and lanterns in use process until no bright light serious decline. Furthermore, if chip bonding process bad, in use process will lead to chip bond completely off the surface layer and make sample bond failure occurred open, also can cause LED in use process happens "dead lamp" phenomenon. In chip bonding process undesirable reasons, may be due to the use of the silver plasma expired or exposure time is too long, silver plasma usage too little, curing time is too long, solid crystal face is pollution, etc,.

(2) encapsulation failure

Failure is refers to the packaging design packaging or manufacturing process to improper device failure. The use of the epoxy resin packaging materials, in use process will occur degradation problems, the life of the LED reduced. The degradation problems include: the light through rate, refractive index, expansion coefficient, hardness, water permeability, permeability and packing performance, especially the light through rate is most important. Research has shown that the shorter the wavelength of light, light through rate of the more serious degradation, but for green light above wavelength (that is greater than 560 nm) speaking, this influence is not serious. Lumileds2003 years have revealed power LED the white light devices and φ 5 white light devices life the experimental curves, 19 kh, use silicon resin encapsulates the power devices, flux can still maintain the initial 80%, but with epoxy resin packaging contrast curve said it in 6 kh, flux maintain rate is only 50%. Experimental results show that the chip luminous efficiency under the same, near the chip epoxy resin, then obviously yellow brown. This apparent degeneration process, and is mainly due to the light and temperature rise of epoxy resin by light through rate caused by the degradation. Meanwhile, in blue light yellow phosphor issued by stimulate the white LED, polymorphism of lens Browning affects their reflective, and makes a blue light enough to stimulate the yellow phosphor, thus make the efficacy and spectral distribution change.

As for packaging, and a Outdoor LED Lighting the important factor of influence life is corrosion. In the use of LED, generally cause corrosion is the main reason of the water vapor penetrated into the packaging materials causing lead metamorphism, PCB copper corrosion; Sometimes, with the introduction of the water moving conductive ion will stay in the chip surface, and cause leakage. In addition, packaging quality is bad devices, in its packaging body would have a lot of residual bubbles, the residual bubbles will also cause corrosion of the device.

(3) thermal stress failure

Temperature effect is always the important factors LED optical properties, and in the research of the failure mode when LED scholars at home and abroad to work environment temperature will be considered as accelerating stress, LED to accelerated life experiment [8, 9]. This is because in the LED system thermal resistance of the same premise, encapsulation pin point welding temperature increases, the junction temperature will increase, resulting in failure LED in advance.

Hsu on different manufacturers to provide the LED samples accelerated life experiment, this experiment will LED respectively in the samples in 80, 100, and 120 ℃, the use of 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 speed up the time increases and decreases. In the accelerated life experiments, and high temperature rise will be LED the epoxy resin material happen variation, thus increasing the thermal resistance of the system, making chips and packaging of heated happened between surface degradation, and eventually lead to packaging failure [9].

(4) electricity lead stress failure

LED in the current if use (EOS) or static shock damage (ESD) chip, can create open chip, form 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 electrostatic voltage, the voltage once more than material bear ability, will take place breakdown phenomenon and discharge, makes the device failure.

Second, improving measures

Through the above-mentioned LED the analysis of the main failure modes, which learned that improve the service life of the actual use in LED technology.

(1) thermal technology

Cooling technology has been LED the important link of the application effect, if not in time to heat LED devices, will cause the chip junction temperature rise serious, and then fell sharply luminous efficiency, reliability (such as life, color shift etc) will deteriorate; At the same time, high temperature heat will make LED encapsulation structure internal produce mechanical stress, may be further cause a series of reliability problem [5]. Therefore, in the manufacturing process, can choose good thermal conductivity of the base, and make the cooling area LED as far as possible big, thus increasing the heat dissipating performance of devices.

(2) anti-static technology

In the first section 2.4 already mentioned, as the chip to GaN LED, in the use of existing in a very big problem is electrostatic effect, if don't deal with this, it will be seriously affected the life span of the device. Therefore, in LED design should be taken into consideration when to antistatic design, to avoid devices because high electrostatic voltage cause breakdown and failure.

(3) packaging technology

Encapsulation of epoxy resin material used for illumination and temperature fields and cause its light through rate of degradation, in use is behaved for originally transparent epoxy resin material happen Browning, affect device originally spectrum power distribution. Therefore, in the packaging LED, we will strictly control the curing temperature, avoid encapsulate in, has caused the epoxy resin early ageing.

On the other hand, in order to prevent device corrosion phenomena occurred in selecting the transparency good packaging materials, at the same time to pay attention to the process of plastic injection, try to row material internal bubble clean, in order to reduce the residues of moisture, reduce the rate of corrosion occurred device.

(4) to optimize manufacturing process

In the process of producing LED to the appropriate bonding conditions, if the bonding general will have crushed chip, and will cause device the bonding strength is insufficient, make devices easy to take off the loose. Therefore, in order to ensure bonding strength of the device at the same time, need to try to reduce the bonding process of chip of the damage, in order to optimize the purpose of the bonding process.

In the chip adhesive, control temperature and time requirements in the appropriate range, making the solder to density, no empty, a small residual stress process requirements.

(5) reasonable screening

In LED before they leave the factory, can increase a screening process, is to some of these samples of reasonable aging and screening tests, eliminate some possible failure in advance of the device, in order to reduce the actual use in LED early failure phenomenon.

conclusion

To sum up, although LED has the very high theoretical life, but in actual process, the chip, packaging, stress the influence of factors, use the time far not reach the expectations of the theoretical value. In order to really improve the life of LED, whether in the manufacturing process, or in the application layer surface, the need for further research, exploration and practice. As long as can grasp the root cause of the failure LED can in the practice does, indeed, improve the performance of the LED devices, will the new light source of the application of generalized to front, better service to the production and the life.


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