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Reliability Testing Method and Cost Control of LED Lights


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, such as the energy efficiency of LED lighting fixtures is not high, LED white light color is not suitable in some lighting occasions, the reliability of LED lamps is not high, some product 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 complicated, mainly System reliability issues, including the reliability of LED chips, packaged devices, drive power modules, heat dissipation, and luminaires. These issues are analyzed separately below:

  1. LED lamp reliability related content introduction

Before analyzing the reliability of LED lamps, we will 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: essential 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 also been reports in the media that the life of LED devices is reduced 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 significance of LED average life is the average of the working time before the LED product fails, 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 US ASSIST alliance regulations. 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 improve and improve LED reliability in time. 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 for the test method of semiconductor light-emitting diode life 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 dislocation, defects, surface and peripheral electrical drift and ion thermal diffusion, which will cause the chip to fail. It is the above-mentioned essential failure. To improve the reliability index of the epitaxialchip, 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, and 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 the LED device package. The failure caused by the LED package is a subordinate failure, and 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 more than 70% of them are caused by the driving power supply. This problem should be paid attention to by the industry. 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 current.

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 consider 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. It is defined by the American ASSIST Alliance. According to the current two kinds of LEDs, the typical forward voltage is 2.1V and 3.3V, that is, the LEDs with input power above 210mw and 330mw are 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.

a. Thermal resistance

Thermal resistance is the quotient of the difference between the effective temperature of the device and the external specified reference point temperature divided by the steady state power dissipation in the device. It is the most important parameter that indicates the degree of heat dissipation from the device. At present, the heat dissipation of the LED with good heat dissipation is ≤10°C/W, the best thermal resistance reported in China is ≤5°C/W, and the thermal resistance of foreign countries is ≤3°C/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 heating portion of the LED device. It is the temperature value that the LED device can withstand under working conditions.

The heat resistance of the chip and the phosphor is still very high. At present, the junction temperature of the chip is 150 ° C, and the phosphor at 130 ° C basically has no effect on the lifetime of the device. It 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 in the temperature rise. What we are discussing here is: the shell-environment temperature rise. It refers to the difference between the temperature of the LED device package (the most hot spot that can be measured by the LED luminaire) and the temperature of the environment (on the light-emitting plane of the luminaire, 0.5 meters away from the luminaire). It is a temperature value that can be directly measured, and can directly reflect the heat dissipation degree of the LED device. It has been proved that when the ambient temperature is 30 °C, if the LED tube is 60 °C, the temperature rise should be 30 °C. At this time, the lifetime value of the LED device can be basically ensured. 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 heat dissipation. Second, the new structure is packaged. As the power of the LED light source increases, multiple power LED chips are required to be packaged together, such as a COB structure or a modular lamp, which generates more heat and requires a more efficient heat dissipation structure. Measures, which in turn raise new issues for heat dissipation, otherwise it will greatly affect the performance and life of LED lamps.

In summary, we must improve the level of heat dissipation, but recently it has been suggested that “with the improvement of LED light efficiency, heat dissipation is not important”, I think this is not true, because LED lamps are well done, and their total energy efficiency is only 50. %, there is still a lot of power to be hot. Secondly, LEDs with high current density and modular luminaires will generate more concentrated heat and require good heat dissipation. Several principled comments are proposed to improve the level of heat dissipation:

First, from the LED chip, it is necessary to adopt new structures and new processes, improve the heat resistance of the junction temperature of the LED chip, and the heat resistance of other materials, so that the requirements for heat dissipation conditions are lowered.

Second, reduce the thermal resistance of the LED device, adopt a new structure and a new process of packaging, and select a new material with better thermal conductivity and heat resistance, including a bonding material between the metal and a mixed rubber of the phosphor, so that the thermal resistance ≤ 10 ° C / W or lower.

Third, reduce the temperature rise, try to use heat-dissipating materials with good thermal conductivity, and design a better ventilation hole, so that the residual heat is dissipated as soon as possible, and the temperature rise should be less than 30 °C. In addition, increasing the level of heat dissipation of modular luminaires should be mentioned on the agenda.

Fourth, there are many ways to dissipate heat. For example, the use of heat pipes is of course good, but cost factors should be considered. Cost-effectiveness should be considered in design.

In addition, in addition to improving the efficiency of the luminaire, the light distribution requirements, and the beautiful appearance, the design of the LED luminaire should improve the heat dissipation level and use a material with good heat conductivity. It has been reported that the heat sink is coated with some nano materials and its thermal conductivity is increased by 30. %. In addition, to have better mechanical properties and sealing, the heat sink should be dustproof, and the temperature rise of the LED lamp should be 30 °C.

Second, the cost of LED lighting

Whether the LED light source can fully enter the lighting field, the cost of the light source is the most critical. From the current point of view, the cost difference between different LED products is 5-1 times higher than that of the traditional products, and the main technical indicators are further improved. The introduction of new structures, new materials, new technologies, and new processes will undoubtedly bring new pressure on LED costs. According to the requirements of the US SSL plan, the integration price reached $2/klm in 2015. From the current cost price, the required cost is reduced by an average of 20% per year, which can basically reach the above-mentioned indicators. This is a very difficult task. Here are two ways to reduce costs from two levels for discussion.

1. Large-scale production and improved yield

Mass production using automated equipment can significantly increase production efficiency, save costs, and reduce costs. In addition, the use of process measures and quality management system management methods to improve the yield is also a good way to reduce costs.

    2. Technology innovation reduces costs

To reduce costs, it is necessary to innovate technically, adopt new structures, new technologies, new materials, and new processes, which can not only improve LED performance indicators, but also effectively reduce costs significantly. This is the direction of efforts. Method.

(1) Epitaxial chip cost reduction method

From the current stage, the cost of LED chips accounts for a higher proportion of LED light sources. It is necessary to focus on reducing costs from epitaxial chips and introduce four specific methods.

First, increase the area of the epitaxial wafer: the area of the epitaxially grown wafer, from the current 2 inch and part of the 4 inch target has been aimed at 6 inches. Although the area of the epitaxial chip is increased, it is technically necessary to overcome the uniformity of the film. New problems with cracks, deformations, etc., but the cost reduction is very significant. In addition, manufacturers of MOCVD are currently developing equipment for 8-inch wafers.

Second, improve epitaxial growth: Veeco Asia President Wang Keyang introduced the MOCVD quality, process and architecture, specifically the mean time between failure (MTBF), mean cleaning interval (MTBC) and mean time to repair (MTTR). Time improvements, as well as inter-equipment matching and on-line process control, can increase throughput and reduce the cost of epitaxial wafers from $1/cm2 in 2009 to $0.2/cm2 in 2014.

Third, increase the current density: several major foreign companies are developing and increasing the current density of the LED forward current to increase the luminous flux of a single power LED, to reduce the number of LEDs when the same illumination is achieved, of course, at the expense of some of the light effects. If the current forward current is increased from 350 mA to 2 A, the luminous flux can be increased by 4 to 5 times, and the cost will be greatly reduced. Of course, new problems such as junction temperature heat resistance, heat resistance of packaging materials, and heat dissipation are also required.

Fourth, lowering the turn-on voltage VF: The current value of the GaN turn-on voltage VF is 3.3V. The VF value is being developed abroad. If it is within 2.8V, the same luminous flux (light effect) can be obtained when the input power is reduced. Improve energy efficiency and save costs.

(2) LED package

Improving the LED packaging process, adopting new structures, new materials, and new processes, improving the LED package yield and reducing the cost of LED packaging are the goals that packaging companies have always strived for. Here are some ways to reduce the cost of packaging.

First, the encapsulating material is separated from the chip, and the encapsulating material lens, the phosphor film, and the like are formed, and the package is separated from the chip. The method is simple in process, good in heat dissipation, good in performance stability and uniformity, and the package device can be improved. The reliability and low cost of the package are one of the main directions of device packaging.

Second, the COB package is used, that is, the LED multi-chip integrated package. It has been reported that the COB package can reduce the packaging cost by 30%, but it must solve the light-emitting efficiency and heat dissipation of the package.

(3) LED lamps

The cost of LED luminaires including heat sinks is also higher than that of LED lighting products. In order to reduce the cost of lamps, it can be considered from two aspects.

First, the design of the luminaire containing the heat sink is reasonable, reducing material waste. The selection of suitable materials has both good mechanical properties and heat dissipation performance, as well as reasonable cost performance.

Second, modular luminaires, that is, LED chips, drive power and heat sinks are packaged together into modular units for standardized production. According to different luminaire requirements, modular unit assembly can be used. This modular assembly method can greatly reduce manufacturing costs. But to solve the new problems of energy efficiency and heat dissipation.

In order to fully enter the LED light source into the field of illumination, it is urgent to solve the two problems of reliability and cost reduction of the LED lighting system. Through the above analysis, from the aspects of the components of the LED lamps, namely the epitaxial chip, the device package, the driving power supply, the heat dissipation, the lamps, etc., it is necessary to separately carry out reliability design and test, and each achieves the reliability index to ensure the system reliability. To make the life of LED lamps up to 35,000 hours. In terms of cost reduction, we must conscientiously improve the yield and large-scale production, while focusing on technological innovation, especially in epitaxial chips, device packaging, and luminaire design, we must continuously improve and innovate, so that the cost of LED lamps can be greatly reduced. Before 2015, the sales price of LED lighting fixtures was within 15 RMB/klm.

Yiwu Fuwei e-commerce firm (IACS Co., Limited) is a very professional Led manufacturer who produces high quality outdoor led light. Our popular products are solar led lights, UFO lights, underwater lights and so on. If you have any question about the led product, please feel free to contact us.