In recent years, due to the rapid development of LED technology, the main performance indicators have been greatly improved. At present, the luminous efficacy of LED devices exceeds 200lm/W, and the industrialization level reaches 110~120lm/W, which can be used as a light source in the field of lighting. Enter outdoor landscape lighting, functional lighting, commercial lighting and other fields. In the application process, there are several major technical and cost issues. For example, the energy efficiency of LED lighting fixtures is not high. The color of LED white light is not suitable in some lighting occasions. The reliability of LED lamps is not high, and some products have a long life. Very short, and the price of LED lamps is generally high, and these problems need to be further resolved and improved. Industry peers pay more attention to the reliability and cost of LED light sources, and are working hard to solve them. This article will also focus on the more detailed description and analysis of these two issues.
First, the reliability of LED lighting
Regarding the classification, performance indicators and reliability of LED lighting fixtures, there are already very specific regulations in the US "Energy Star" [1]. In the reliability index, the main life of LED lighting fixtures is 35,000 hours, during the whole life. The internal chromaticity change is within 0.007 of CIE1976(u,v). The US SSL program stipulates that the lifetime of white LED devices will be 50,000 hours in 2010-2015. Domestic life expectancy for LED lighting fixtures is also generally mentioned 3 to 35,000 hours. The above mentioned indicators of LED lamp life and color retention are very high from the current point of view. In fact, many LED lamps do not meet this requirement, because the technical problems involved in LED lamps are many and complex, mainly System reliability issues, including the reliability of LED chips, packaged devices, drive power modules, heat sinks, and luminaires. These issues are analyzed separately below:
1. LED lamp reliability related content introduction
Before analyzing the reliability of LED lamps, first introduce some basic contents related to LED reliability, which will help to analyze the reliability of LED lamps.
(1) Essential failure, subordinate failure
LED device failures are generally divided into two types: intrinsic failure and dependent failure. Intrinsic failure refers to the failure caused by the LED chip, which is divided into electrical drift and ion thermal diffusion failure. Subordinate failure is generally caused by the package structure material and process, that is, the package structure and the epoxy, silica gel, conductive paste, phosphor, solder, lead, process, temperature and other factors.
(2) Ten degree rule
Some electronic devices are reduced by half (or down 1/4) for every 10 °C increase in temperature over a range of temperatures. Practice has proved that when the temperature of the LED device heat sink is between 50 ° C and 80 ° C, the LED lifetime value basically conforms to the ten-degree rule. Recently, there have been reports in the media that the life of LED devices has dropped by 10% for every 2 °C rise. When the temperature rises from 63 °C to 74 °C, the average lifetime is reduced by 3/4. This phenomenon is entirely possible due to the different packaging process of the device.
(3) meaning of life
LED lifetime is the operating time at which the optical output power or luminous flux decays to 70% of the initial value under specified operating conditions, while the chromaticity change remains within 0.007.
The meaning of the average life of the LED is the average of the working time before the LED product fails. It is expressed by MTTF, which is the most commonly used reliability parameter for electronic devices.
Reliability testing includes reliability screening, environmental testing, and life testing (long-term or short-term). What we are discussing here is only the life test, and other projects are not considered for the time being.
(4) Long-term life test
In order to confirm whether the life of LED lamps reaches 35,000 hours, long-term life test is required. The current practice basically forms the following consensus: Since the output optical power of GaN-based LED devices is unstable, electrical aging is required according to the regulations of the American ASSIST Alliance. After 1000 hours, the measured optical power or luminous flux is an initial value. After adding the rated current for 3000 hours, the measured luminous flux (or optical power) attenuation is less than 4%, and the current is added for 3000 hours. The luminous flux attenuation is less than 8%, and then the power is applied for 4000 hours for 10,000 hours. The measured luminous flux attenuation is less than 14 %, that is, the luminous flux reaches 86% or more of the initial value. At this point, it can be proved that the LED life is up to 35,000 hours.
(5) Accelerated (short-term) life test
The accelerated life test of electronic devices can be tested under increased stress (electric power or temperature). The method of temperature stress is used here. The measured lifetime is the average life of the LED, that is, the average working time before failure. Adopting this method will greatly shorten the test time of LED life, which is beneficial to timely improve and improve LED reliability. The life test method with temperature stress has been discussed in detail in the article [2], mainly referring to the slow degradation formula of the light power of the illuminating tube of yamakoshi, and the life of the LED under different accelerated stress temperatures is obtained by the degradation coefficient. Test data, and then use the numerical analysis method of "Arrhenius" equation to obtain the average life of LED under normal stress (room temperature), referred to as "degradation coefficient analytical method", which adopts three different stress temperatures, namely 165 °C. At 175 ° C and 185 ° C, the measured data calculated the uniformity of the average life at room temperature. The test method is reliable. At present, the standard of "test method for life of semiconductor light-emitting diodes" has been drafted in this research. Some domestic enterprises have also developed equipments for accelerated life test.
2. LED device reliability
The reliability of LED devices depends mainly on two parts: the performance quality of epitaxial chips and device packages. These two failure mechanisms are completely different and are described separately.
(1) Failure of the epitaxial chip
The performance and quality of the epitaxial chip are mainly related to the number and distribution of dislocations and defects in the epitaxial layer, especially the Pn junction portion, the quality of the metal and semiconductor contact layer, and the number of ions caused by the epitaxial layer and the surface of the chip and the surrounding contamination. The situation is related. Under the condition of heating and power-on, the chip will gradually cause dislocations, defects, surface and peripheral electrical drift and ion thermal diffusion, which will cause the chip to fail. In order to improve the reliability index of the epitaxial chip, it is fundamental to reduce the dislocations and defects generated during the epitaxial growth process and the contamination of the surface and the periphery of the epitaxial layer, and improve the contact quality between the metal and the semiconductor, thereby increasing the working life time. At present, it has been reported that the accelerated life test of the bare chip is carried out and the calculation is carried out. The general life span is more than 100,000 hours or even hundreds of thousands of hours.
(2) Failure of the device package
It has been reported that more than 70% of LED device failures are caused by packaging, so packaging technology is a key technology for LED devices. The LED device packaging technology is discussed in detail in [3], [4], so I will not introduce it here, but only briefly analyze the reliability of LED device packaging. The failure caused by LED package is subordinate failure, the reason is very complicated, the main source has three parts:
First, the packaging material is not good, such as epoxy, silica gel, phosphor, susceptor, conductive adhesive, solid crystal material.
Second, the package structure design is unreasonable, such as material mismatch, stress, crack, open circuit, and the like.
Third, the packaging process is not suitable, such as loading, pressure welding, dispensing process, curing temperature and time.
In order to improve the reliability of the device package, firstly, the quality of the material should be strictly controlled in terms of raw material selection. In addition to considering the light efficiency and heat dissipation in the package structure, it is also necessary to consider the heat rise matching problem when a plurality of materials are combined. In the packaging process, it is necessary to strictly control the process flow of each process, try to use automation equipment, ensure the consistency and repeatability of the process, and ensure the performance and reliability indicators of LED devices.
3. LED drive power module
At present, domestic LED driver power supply has more quality problems. According to reports, LED lamps fail, and about 70% of them are caused by the driving power supply. This problem should be paid attention to by the industry insiders. First of all, to analyze the power module function, generally consists of four parts:
Power conversion: high voltage to low voltage, AC to DC, voltage regulation, steady flow.
Drive circuit: A discrete device or integrated circuit can output a circuit composed of a larger power.
Control circuit: control luminous flux, light tone, timing switch and intelligent control.
Protection circuit: protection circuit content is too much, such as overvoltage protection, overheat protection, short circuit protection, output open circuit protection, low voltage latch, suppression of electromagnetic interference, conducted noise, anti-static, lightning protection, anti-surge, anti-harmonic oscillation Wait.
As the function of the LED driver module, the power conversion and drive circuit must be there. The control circuit depends on the actual needs. The protection circuit should be determined according to the needs of the actual product reliability. To take the protection circuit, it is necessary to increase the cost. The cost is contradictory. It has been reported that if the power cost is 2-3 yuan per watt, the price/performance ratio is still high. How to improve the quality of the drive power module and ensure the reliability of the LED luminaire, in principle, the following measures should be taken:
First, the power module must use good quality electronic components.
Second, the overall circuit design is reasonable, including power conversion, drive circuit, control circuit and protection circuit.
Third, the selection of a suitable protection circuit can not only protect the performance quality of the module, but also increase the cost.
According to the quality level of the existing power supply drive module, it is difficult to ensure that the life of the LED lamp reaches 35,000 hours.
4. Heat dissipation problem
The reliability (life) of LED lighting fixtures depends largely on the level of heat dissipation, so increasing the level of heat dissipation is one of the key technologies. It is mainly to solve the problem that the chip generates excess heat through the heat sink and the heat sink. This is a very complicated technical problem. The following will be described separately:
(1) Power LED definition
Which LEDs need to be considered for heat dissipation, power LEDs need to dissipate heat. The power LED refers to a light-emitting diode with an operating current of 100 mA or more. According to the definition of the current ASSIST alliance in the United States, the typical forward voltages of the two types of LEDs are 2.1V and 3.3V, that is, the LEDs with input powers above 210mw and 330mw are all power LEDs. Some people may have different opinions, but practice has proved that to improve the reliability (life) of power LEDs, it is necessary to consider the heat dissipation problem of power LEDs.
(2) Heat dissipation related parameters
The main parameters related to LED heat dissipation are thermal resistance, junction temperature and temperature rise.