LED is a light-emitting device that directly converts electrical energy into visible light and radiant energy. It has the advantages of low power consumption, high luminous efficiency and small size. It has gradually become a new type of high-efficiency energy-saving product and is widely used in display. , lighting, backlighting and many other fields. In recent years, with the continuous progress of LED technology, its luminous efficiency has also been significantly improved. The efficiency of the existing blue LED system can reach 60%; and the luminous efficiency of white LED has exceeded 150lm/W. more and more attention.
At present, although the theoretical life of LED can reach 50kh, in actual use, due to the constraints of various factors, LED often cannot reach such a high theoretical life, and premature failure occurs, which greatly hinders the use of LED as a new type of energy-saving. The pace of advancement of type products. In order to solve this problem, many scholars have carried out relevant research and obtained some important conclusions. On this basis, 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 LED.
1. LED failure mode
LED failure modes mainly include: chip failure, package failure, thermal overstress failure, electrical overstress failure and assembly failure, among which chip failure and package failure are the most common. This article will analyze these main failure modes in detail.
(1) Chip failure
Chip failure refers to the failure of the chip itself or the failure of the chip due to other reasons. There are often many reasons for this failure: chip cracks are caused by unsuitable bonding process conditions, resulting in large stress, and the thermomechanical stress generated by heat accumulation is also strengthened, resulting in micro-cracks in the chip, and the work The injected current will further aggravate the microcracks and make them continue to expand until the device fails completely. Secondly, if the active area of the chip is already damaged, it will gradually degrade until it fails during the power-on process, and it will also cause the light to decay seriously until it does not light up during use. In addition, if the die bonding process is not good, the die bonding layer will be completely separated from the bonding surface during use, which will cause the sample to have open circuit failure. It will also cause the "dead light" phenomenon of the LED during use. The reasons for the poor die bonding process may be due to the expired silver paste used or the exposure time is too long, the amount of silver paste used is too small, the curing time is too long, the solid crystal base surface is contaminated, etc.
(2) Package failure
Package failure refers to device failure caused by improper package design or production process. The epoxy resin material used in the encapsulation will deteriorate during use, resulting in a reduction in the life of the LED. Such deterioration problems include: light transmittance, refractive index, expansion coefficient, hardness, water permeability, air permeability, filler properties, etc., among which light transmittance is the most important. Studies have shown that the shorter the wavelength of light, the more serious the deterioration of light transmittance, but for wavelengths above green light (that is, greater than 560nm), 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 luminous flux of the power device encapsulated with silicone resin can still maintain 80% of the initial luminous flux, while the contrast curve encapsulated with epoxy resin is expressed in After 6kh, the luminous flux maintenance rate is only 50%. Experiments show that in
When the luminous efficiency of the chip is the same, the epoxy resin close to the chip obviously turns yellow and then brown. This obvious degradation process is mainly caused by the deterioration of the light transmittance of epoxy resin caused by light and temperature rise. At the same time, in LEDs that emit white light from a yellow phosphor excited by blue light, the browning of the encapsulating lens affects its reflectivity and makes the emitted blue light insufficient to excite the yellow phosphor, resulting in changes in efficacy and spectral distribution.
For packaging, another important factor that affects the life of LEDs is corrosion. In the use of LEDs, the main cause of corrosion is generally that water vapor penetrates into the packaging material, resulting in deterioration of leads and corrosion of PCB copper wires; sometimes, movable conductive ions introduced with water vapor will reside on the surface of the chip, resulting in leakage. In addition, a device with poor packaging quality will have a large number of residual air bubbles inside the package body, and these residual air 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 failure mode of LEDs, scholars at home and abroad consider the working environment temperature as the accelerated stress to conduct accelerated life experiments of LEDs. This is because under the premise that the thermal resistance of the LED system remains unchanged, the temperature of the solder joints of the package leads will increase, and the junction temperature will also increase, resulting in premature failure of the LED.
Figure: The model structure diagram of the high-power LED and the operating ambient temperature are
(a) Relationship between radiation power and acceleration time at 120°C, (b) 100°C and (c) 80°C. Hsu et al. conducted accelerated lifetime experiments on LED samples provided by different manufacturers.
The test places the LED samples at 80, 100, and 120 °C, respectively, and drives them with a voltage of 3.2V. It is stipulated that when the optical power of the sample drops to 50% of the initial value, it is judged as failure. The experimental results in Fig. 1 show that the lifetime of the high-power LED decreases with the increase of the accelerated lifetime experimental temperature and the increase of the acceleration time. In the accelerated life experiment, the rise of the LED junction temperature will cause the epoxy resin material to mutate, thereby increasing the thermal resistance of the system, causing degradation of the heated surface between the chip and the package, and eventually leading to the failure of the package.
(4) Electrical overstress failure
If the LED is used in the case of overcurrent (EOS) or the chip is damaged by electrostatic shock (ESD), the chip will be opened, resulting in electrical overstress failure. For example, GaN is a wide bandgap material. Resistivity is higher. If this type of chip is used, the induction caused by static electricity during the production process
The generated charge is not easy to disappear. When it accumulates to a considerable extent, a high electrostatic voltage can be generated. Once this voltage exceeds the bearing capacity of the material, a breakdown phenomenon will occur and discharge will occur, causing the device to fail.
2. Improvement measures
Through the analysis of the main failure modes of LEDs introduced above, we can learn technical methods to improve the actual service life of LEDs.
(1) Heat dissipation technology
Heat dissipation technology has always been an important link affecting LED applications. If the LED device cannot be dissipated in time, the junction temperature of the chip will be seriously increased, and then the luminous efficiency will drop sharply, and the reliability (such as life, color shift, etc.) will deteriorate; At the same time, high temperature and high heat will generate mechanical stress inside the LED package structure, which may further lead to a series of reliability problems . Therefore, in the manufacturing process, a base with good thermal conductivity can be selected, and the heat dissipation area of the LED can be made as large as possible, thereby increasing the heat dissipation performance of the device.
(2) Anti-static technology
A big problem in the use of LEDs using GaN as a chip is the electrostatic effect. If this problem is not handled properly, it will seriously affect the life of the device. Therefore, in the LED design, the anti-static design should be fully considered to avoid device failures such as breakdown due to high electrostatic voltage.
(3) Packaging technology
The epoxy resin material used for encapsulation will cause the deterioration of its light transmittance due to light and temperature rise. In use, the original transparent epoxy resin material will brown, which will affect the original spectral power distribution of the device. Therefore, when encapsulating LEDs, we must strictly control the curing temperature to avoid premature aging of the epoxy resin during encapsulation.
On the other hand, in order to prevent the corrosion of the device, while selecting the packaging material with good transparency, it is necessary to pay attention to the air bubbles inside the material as much as possible during the injection molding process, so as to reduce the residual amount of water vapor and reduce the corrosion of the device. probability.
(4) Optimize the manufacturing process
Appropriate bonding conditions are required in the LED manufacturing process. If the bonding is too large, the chip will be crushed. Otherwise, the bonding strength of the device will be insufficient, and the device will be easily loosened. Therefore, while ensuring the bonding strength of the device, it is necessary to minimize the damage caused by the bonding process to the chip, so as to achieve the purpose of optimizing the bonding process.
When bonding chips, it is required to control the temperature and time within an appropriate range, so that the solder can meet the process requirements such as dense, void-free, and small residual stress.
(5) Reasonable screening
Before the LED leaves the factory, a screening process can be added, that is, a reasonable aging and screening test is carried out on some of the samples, and some devices that may fail prematurely are eliminated, so as to reduce the premature failure of the LED in actual use.
To sum up, although LEDs have a high theoretical life, in actual use, due to factors such as chips, packaging, and stress, the use time is far from the expected theoretical value. In order to actually improve the life of LEDs, further research, exploration and practice are required, both in terms of manufacturing process and application level. With the continuous development of LED technology, new problems are bound to emerge. However, as long as the root cause of LED failure can be grasped, the performance of LED devices can be improved in practice, and this new light source can be promoted to the front end of the application field to better serve production and life.
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