Reliability Analysis of LED Lighting Fixtures

The US Energy Star has very specific regulations on the classification, performance indicators and reliability of LED lighting fixtures [1]. Among the reliability indicators, the main requirement is that the life of LED lighting fixtures is 35,000 hours, and the color change during the entire life is within 0.007 in CIE1976 (u, v). The US SSL plan stipulates that the life of white light LED devices is 50,000 hours from 2010 to 2015. The domestic life requirement for LED lighting fixtures is generally 30,000 to 35,000 hours.

The above mentioned indicators of LED lamp life and color retention are very high at present. In fact, many LED lamps cannot meet this requirement because there are many complex technical issues involved in LED lamps, mainly system reliability issues, including LED chips, packaging devices, drive power modules, heat dissipation and lamp reliability. The following analyzes these issues respectively:

1. Introduction to the reliability of LED lamps

Before analyzing the reliability of LED lamps, it is helpful to introduce some basic contents related to LED reliability, which will help the in-depth analysis of the reliability of LED lamps.

(1) Intrinsic failure and dependent failure

LED device failures are generally divided into two categories: intrinsic failure and secondary failure. Intrinsic failure refers to failure caused by the LED chip, which is divided into electrical drift and ion thermal diffusion failure. Secondary failure is generally caused by packaging structure materials and processes, that is, packaging structure and epoxy, silicone, conductive glue, phosphor, welding, lead, process, temperature and other factors.

(2) Rule of Ten Degrees

For some electronic devices, within a certain temperature range, their main technical indicators drop by half (or 1/4) for every 10°C increase in temperature. Practice has shown that when the heat sink temperature of LED devices is between 50°C and 80°C, the LED life value basically conforms to the ten-degree rule. Recently, there have also been media reports that for every 2°C increase in the temperature of LED devices, their lifespan decreases by 10%. When the temperature rises from 63°C to 74°C, the average lifespan decreases by 3/4. This phenomenon is entirely possible due to different device packaging processes.

(3) Meaning of lifespan

LED life refers to the operating time under specified operating conditions when the light output power or luminous flux decays to 70% of the initial value, while the chromaticity change remains within 0.007.

The meaning of the average lifespan of LEDs is the average working time of LED products before failure, which is expressed by MTTF. It 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). Here we are only discussing life testing, and other items 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 tests are required. The current practice has basically formed the following consensus: Because the output optical power of GaN-based LED devices is unstable at the beginning, according to the regulations of the American ASSIST Alliance, the optical power or luminous flux measured after 1,000 hours of electrical aging is the initial value. After that, the rated current is applied for 3,000 hours, and the measured luminous flux (or optical power) attenuation must be less than 4%. The current is applied for another 3,000 hours, and the luminous flux attenuation must be less than 8%. After the power is applied for another 4,000 hours, a total of 10,000 hours, the measured luminous flux attenuation must be less than 14%, that is, the luminous flux reaches more than 86% of the initial value. Only then can it be proved that the LED life reaches 35,000 hours.

(5) Accelerated (short-term) life test

The accelerated life test of electronic devices can be carried out under increased stress (electric power or temperature). What we want to discuss here is the method of using temperature stress. The life measured and calculated is the average life of the LED, that is, the average working time before failure. This method will greatly shorten the test time of the LED life, which is conducive to timely improvement and improvement of LED reliability. The life test method of adding temperature stress has been discussed in detail in the article [2]. It mainly quotes the slow degradation formula of the light-emitting tube light power of "Yamakoshi". The life test data of the LED under different accelerated stress temperatures are obtained through the degradation coefficient. Then, the numerical analysis method of the "Arrhenius" equation is used to obtain the average life of the LED under normal stress (room temperature). This method is referred to as the "degradation coefficient analysis method". This method uses three different stress temperatures, namely 165℃, 175℃ and 185℃, and the measured data is used to calculate the consistency of the average life at room temperature. This test method is reliable. At present, based on this research result, the "Test Method for the Life of Semiconductor Light Emitting Diodes" standard has been drafted and formulated. Some domestic companies are also developing equipment and instruments for accelerated life tests.

2. LED device reliability

The reliability of LED devices mainly depends on two parts: the performance quality of the epitaxial chip and the device packaging. These two failure mechanisms are completely different and are described separately.

(1) Failure of epitaxial chips

The factors that affect the performance and quality of epitaxial chips are mainly related to the number and distribution of dislocations and defects in the epitaxial layer, especially the Pn junction, the quality of the metal-semiconductor contact layer, and the number and condition of ions caused by contamination in the epitaxial layer and the surface and periphery of the chip. When the chip is heated and powered on, dislocations, defects, electrical drift and ion thermal diffusion on the surface and periphery will gradually occur, causing the chip to fail, which is the essential failure mentioned above. To improve the reliability index of epitaxial chips, it is fundamentally necessary to reduce the dislocations and defects generated during the epitaxial growth process and the contamination on the surface and periphery of the epitaxial layer, improve the contact quality between the metal and the semiconductor, and thus increase the working life. At present, it is reported that the accelerated life test of bare chips is carried out and the life is generally calculated to be more than 100,000 hours, or even hundreds of thousands of hours.

(2) Failure of device packaging

It is 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 articles [3] and [4], so it will not be introduced here. Only a brief analysis of the reliability issues related to LED device packaging is given. The failure caused by LED packaging is a secondary failure, and its causes are very complex. There are three main sources:

First, it is caused by poor packaging materials, such as epoxy, silicone, phosphor, base, conductive glue, and die-bonding materials.

Second, the packaging structure design is unreasonable, such as material mismatch, stress generation, breakage, open circuit, etc.

Third, the packaging process is not suitable, such as chip mounting, pressure welding, glue dispensing process, curing temperature and time, etc.

In order to improve the reliability of device packaging, first of all, the quality of raw materials should be strictly controlled. In addition to considering light output efficiency and heat dissipation in the packaging structure, the thermal expansion matching problem when multiple materials are combined together should also be considered. In the packaging process, the process flow of each process should be strictly controlled, and automated equipment should be used as much as possible to ensure the consistency and repeatability of the process, so as to guarantee the performance and reliability indicators of LED devices.

3. LED driver power module

At present, there are many quality problems in domestic LED driver power supplies. According to reports, more than 70% of LED lamp failures are caused by driver power supplies. This problem should attract the attention of industry insiders. First, let's analyze the function of the power module, which generally consists of four parts:

Power conversion: high voltage to low voltage, AC to DC, voltage stabilization, current stabilization.

Driving circuit: A circuit composed of discrete devices or integrated circuits that can output higher power.

Control circuit: control luminous flux, light tone, timing switch and intelligent control, etc.

Protection circuit: There are too many contents in the protection circuit, such as overvoltage protection, overheat protection, short circuit protection, output open circuit protection, low voltage latch, electromagnetic interference suppression, conducted noise, anti-static, lightning protection, surge protection, and harmonic oscillation protection.

As the function of LED driver module, power conversion and drive circuit must be included. Control circuit depends on actual demand. Protection circuit should be determined according to the actual product reliability. Adopting protection circuit needs to increase the cost, which is inconsistent with the cost of power supply. It is reported that if the power cost is 2~3 yuan per watt on average, its cost performance is still high. How to improve the quality of driving power module and ensure the reliability of LED lamps, in principle, the following measures should be taken:

First, the power module must use high-quality electronic components.

Second, the overall circuit design is reasonable, including power conversion, drive circuit, control circuit and protection circuit.

Third, choosing a suitable protection circuit can protect the performance and quality of the module without increasing too much cost.

According to the quality level of existing power drive modules, it is very difficult to ensure that the life of LED lamps reaches 35,000 hours.

4. Heat dissipation problem

The reliability (lifespan) of LED lighting fixtures depends largely on the heat dissipation level, so improving the heat dissipation level is one of the key technologies. It mainly solves the problem of excess heat generated by the chip being transferred through the heat sink and heat sink, which is a very complex technical problem. The following will describe them separately:

(1) Definition of power LED

Which LEDs need to consider heat dissipation? Power LEDs need heat dissipation. Power LEDs refer to light-emitting diodes with an operating current of more than 100mA. This is defined by the ASSIST Alliance in the United States. According to the typical forward voltage values ​​of the two existing LEDs, 2.1V and 3.3V, that is, LEDs with input powers of more than 210mw and 330mw are all power LEDs. All of them need to consider the heat dissipation of the device. Some people may have different opinions, but practice has proved that to improve the reliability (lifespan) of power LEDs, it is necessary to consider the heat dissipation of power LEDs.

(2) Heat dissipation related parameters

The main parameters related to LED heat dissipation include thermal resistance, junction temperature and temperature rise.

a. Thermal resistance

Thermal resistance refers to the quotient obtained by dividing the difference between the effective temperature of the device and the temperature of the external reference point by the steady-state power dissipation in the device. It is the most important parameter to indicate the degree of heat dissipation of the device. At present, the thermal resistance of power LEDs with good heat dissipation is ≤10℃/W, the best thermal resistance reported in China is ≤5℃/W, and the thermal resistance abroad can reach ≤3℃/W. If this level is achieved, the life of the power LED can be ensured.

b. Junction temperature

Junction temperature refers to the temperature of the semiconductor junction of the main heat-generating part of the LED device. It reflects the temperature value that the LED device can withstand under working conditions. For this reason, the US SSL program has set a goal to improve heat resistance, as shown in Table 1:

Table 1 US SSL plan sets goals for improving heat resistance

The table shows that the heat resistance of the chip and phosphor is still very high. At present, the chip junction temperature has reached 150℃ and the phosphor temperature has reached 130℃, which basically has no effect on the life of the device. This shows that the higher the heat resistance of the chip phosphor, the lower the requirement for heat dissipation.

c. Temperature rise

There are several different temperature rises. What we are discussing here is: tube shell - ambient temperature rise. It refers to the difference between the temperature of the LED device tube shell (the hottest point that can be measured by the LED lamp) and the ambient temperature (on the light-emitting plane of the lamp, 0.5 meters away from the lamp). It is a temperature value that can be directly measured and can directly reflect the degree of heat dissipation of the LED device. Practice has proved that when the ambient temperature is 30℃, if the LED tube shell is measured to be 60℃, its temperature rise should be 30℃, at which time the life value of the LED device can be basically guaranteed. If the temperature rise is too high, the maintenance rate of the LED light source will be greatly reduced.

d. New heat dissipation problem

With the development of LED lighting products, there are two new technologies: First, in order to increase the luminous flux of a single tube, a larger current density is injected, as mentioned below, so that the chip generates more heat and needs to be dissipated. Second, the new packaging structure, as the power of LED light sources increases, multiple power LED chips need to be packaged together, such as COB structure, modular lamps, etc., which will generate more heat and require more effective heat dissipation structures and measures, which in turn raises new issues for heat dissipation, otherwise it will greatly affect the performance and life of LED lamps.

In summary, it is necessary to improve the heat dissipation level. However, some people have recently proposed that "as the LED light efficiency increases, heat dissipation is not important". I think this is wrong, because even if the LED lamps are well made, their total energy efficiency is only 50%, and a lot of electrical energy needs to be converted into heat. Secondly, LED high current density and modular lamps will generate more concentrated waste heat, which needs to be well dissipated. To improve the heat dissipation level, several principled suggestions are put forward:

First, from the perspective of LED chips, new structures and new processes should be adopted to improve the heat resistance of the LED chip junction temperature and the heat resistance of other materials, so as to reduce the requirements for heat dissipation conditions.

Second, reduce the thermal resistance of LED devices, adopt new packaging structures and new processes, and select new materials with good thermal conductivity and heat resistance, including bonding materials between metals and mixed glue for phosphors, so that the thermal resistance is ≤10℃/W or lower.

Third, reduce the temperature rise, try to use heat dissipation materials with good thermal conductivity, and require good ventilation holes in the design to dissipate the waste heat as quickly as possible. The temperature rise should be less than 30°C. In addition, improving the heat dissipation level of modular lamps should be put on the agenda.

Fourthly, there are many ways to dissipate heat, such as using heat pipes, which is of course good, but cost factors must be considered, and cost-effectiveness should be considered during design.

In addition, the design of LED lamps should not only improve lamp efficiency, light distribution requirements, and beautiful appearance, but also improve heat dissipation and use materials with good thermal conductivity. It is reported that the heat dissipation body coated with certain nanomaterials can increase its thermal conductivity by 30%. In addition, it must have good mechanical properties and sealing, and the heat dissipation body must be dustproof, requiring the temperature rise of LED lamps to be less than 30℃.