Led Lighting Reliability Test Method and Cost Control

(Last Updated On: May 24, 2020)

In recent years, due to the rapid development of LED technology, the main performance indicators have been greatly improved. At present, the luminous efficiency of LED devices exceeds 200lm/W, and the industrial level reaches 110~120lm/W, which can be used as the light source in the field of lighting. Currently, LED devices have entered the field of outdoor landscape lighting, functional lighting and commercial lighting. In the application process, there are several main technical and cost issues, such as LED lighting lamps of energy efficiency is not high, the LED white light color of the light is not suitable in some lighting situations, the reliability of the LED lamps is not high, some of the product life is very short, and the other LED lamps price is still higher now, these issues to solve and improve further.

一. Led Lighting Reliability

As for the classification, performance index and reliability of LED lighting, there are very specific provisions in “energy star” of the United States, among the reliability index, it is mainly stipulated that LED lighting has a life span of 35,000 hours and the chroma change within 0.007 in CIE1976 (u,v) during its whole life.According to the SSL program of the United States, the lifetime of white light LED devices is 50,000 hours from 2010 to 2015. The life requirements of LED lighting lamp in China are also generally mentioned as 30,000 to 35,000 hours.

The above mentioned indexes of LED lamp life and color retention are very high at present, in fact, many LED lamps cannot meet this requirement. Due to the many technical problems involved in LED lamps, which are very complex, the main problem is the reliability of the system, including LED chip, packaging devices, drive power module, heat dissipation and the reliability of lamps.

These problems are analyzed as follows:

1. Introduction of led lamp reliability

Before analyzing the reliability of Led lamps, the basic content of Led reliability is introduced, which will be helpful for the in-depth analysis of Led lamps reliability.

(1) Essential Failure and Subordinate Failure

Led device failure generally falls into two types: essential failure and dependent failure.Essential failure refers to the failure caused by Led chip, which is divided into electrical drift and ionic thermal diffusion failure.The secondary failure is generally caused by packaging structure materials and processes, that is, packaging structure and epoxy, silica gel, conductive adhesive, phosphor, welding, lead wire, process, temperature and other factors.

(2) Ten degree rule

For certain electronic devices within a certain temperature range, the main technical indicators fall by half (or a quarter) for every 10℃ rise in temperature.Practice has proved that Led device heat sink temperature of 50℃ to 80℃, Led’s life value basically conforms to the rule of ten degrees.Recently, it has been reported that the life of Led devices decreases by 10% for every 2℃ rise in temperature; when the temperature rises from 63℃ to 74℃, the average life decreases by three quarters.Because of the device process is different, this phenomenon is completely possible.

(3) The meaning of life

Led’s life refers to the working time when the optical output power or luminous flux attenuates to 70% of the initial value under specified working conditions, and the chroma change remains within 0.007.

The meaning of Led average life is the average working time of Led products before failure, which is represented by MTTF, which is the most commonly used reliability parameter of electronic devices.

The reliability test includes reliability screening, environmental test and life test (long or short term). All we are talking about here is life test.

(4) Long life test

In order to confirm whether the service life of Led lamps can reach 35,000 hours, a long-term service life test is needed, the current practice basically forms as the following consensus: Since the output light power of gan-based Led devices is not stable at the beginning, according to the provisions of American ASSIST alliance, the measured light power or flux is the initial value after 1,000 hours of electrical aging.After adding the rated current for 3000 hours, the measured luminous flux (or optical power) attenuation should be less than 4%; add the current for another 3000 hours, and the flux decreases by less than 8%; After another 4,000 hours of electrification, for a total of 10,000 hours, the attenuation of luminous flux measured is less than 14%, that is, the luminous flux reaches more than 86% of the initial value. At this point can be proven to ensure the Led’s life up to 35,000 hours.

(5) Accelerated (short) life test

The accelerated life test of electronic devices can be conducted under increased stress (electrical power or temperature), what is discussed here is the method of temperature stress, the measured and calculated life is the average Led’s life, that is, the average working time before failure.This method will greatly shorten the test time of Led’s life, and is conducive to timely improvement and improvement of Led reliability.Life test method for adding temperature stress,mainly refers to “yamakoshi” (luminescent tube light power slow degradation formula,the life test data of Led under different accelerated stress temperature are obtained by degradation coefficient,the average life of Led under normal stress (room temperature) is obtained by the Arrhenius equation,”Analytic method of degradation coefficient” for short,the method USES three different stress temperatures, namely 165℃, 175℃ and 185℃, to calculate the consistency of the average life at room temperature.This test method is reliable. At present, the standard of “the test method of the life of semiconductor light-emitting diode” has been drafted on this research result, and some Chinese enterprises have also developed equipment and instruments to accelerate the life test.

2. Led device reliability

The reliability of Led devices mainly depends on two parts: the performance and quality of epitaxial chip and device package, these two failure mechanisms are completely different, which are described separately.

(1) Failure of epitaxial chip

The influences on epitaxial chip performance and quality are mainly related to the number and distribution of dislocation and defects of epitaxial layer, especially p-n junction, the quality of metal and semiconductor contact layer, and the number and status of ions caused by contamination of epitaxial layer and chip surface and periphery. Under the condition of heating and electrification, the chip will gradually cause dislocation, defect, surface and periphery to produce electrical drift and ion heat diffusion, making the chip failure, which is exactly the essential failure mentioned above. In order to improve the reliability index of epitaxial chips, dislocation and defects generated during epitaxial growth, as well as contamination on the surface and periphery of epitaxial layer should be fundamentally reduced, so as to improve the quality of metal and semiconductor contact and thus improve the working life. At present, it is reported that the accelerated life test of bare chips is carried out, and the calculation shows that the average life is more than 100,000 hours, or even hundreds of thousands of hours.

(2) Device packaging failure

It is reported that more than 70% of LED device failure is caused by packaging, so packaging technology is the key technology for LED devices.

Led package caused by the failure is secondary failure, the reasons are very complex, the main source of three parts:

Poor packaging materials, such as epoxy, silica gel, phosphor, base, conductive adhesive, solid crystal materials, etc.
Packaging structure design is not reasonable, such as material mismatch, stress, cause fracture, open circuit, and so on.
The packaging process is not appropriate, such as film, pressure welding, dispensing technology, curing temperature and time.

In order to improve the packaging reliability of devices, the quality of materials should be strictly controlled in the selection of raw materials. In terms of packaging structure, besides considering the light output efficiency and heat dissipation, the matching problem of heat rise when various materials are combined should also be considered.In terms of packaging technology, the process flow of each process should be strictly controlled. Automatic equipment should be adopted as far as possible to ensure the consistency and repeatability of the process and ensure the performance and reliability index of Led devices.

3. Led driver

At present, there are many quality problems with Led driver in China. It is reported that more than 70% of Led lamp failure is caused by the drive power, which should arouse the attention of the industry.First, analyze the power module function, which generally consists of four parts:

Power conversion: high voltage to low voltage, ac to dc, stable voltage, steady current.

Drive circuit: a circuit composed of discrete devices or integrated circuits that can output higher power.

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

Protection circuit: the protection circuit content is too much, such as overvoltage protection, overheat protection, short circuit protection, output open circuit protection, low-voltage locking, suppression of electromagnetic interference, conduction noise, anti-static, lightning strike, surge, harmonic oscillation and so on.

As the function of Led drive, power supply transformation and drive circuit must have, control circuit to see the actual demand, protection circuit to be determined according to the actual product reliability needs, to take the protection circuit, need to increase the cost, which is contradictory with the cost of power supply.

Some reports say that if the power costs 2 to 3 yuan per watt on average, the cost performance is still high. In principle, the following measures should be taken to improve the quality of driving power module and ensure the reliability of Led lamps:

The led driver must choose good quality electronic components.
The overall circuit design is reasonable, including power conversion, drive circuit, control circuit and protection circuit.
The appropriate protection circuit can protect the performance and quality of the module without increasing too much cost.

According to the quality level of led driver, it is very difficult to ensure the life of Led lamps to reach 35,000 hours.

4. Heat dissipation

The reliability (life span) of Led lighting fixtures largely depends on the level of heat dissipation, so improving the level of heat dissipation is one of the key technologies.Mainly to solve the chip generated excess heat through heat sink, heat dissipation body, which is a very complex technical problem.The following are described respectively:

(1) Definition of power LED

Consider heat dissipation, power Led needs heat dissipation.Power Led refers to the LED with operating current above 100mA. Some people may have different views, but practice has proved that to improve the reliability (life) of power Led, it is necessary to consider the heat dissipation problem of power Led.

(2) Heat dissipation parameters

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

A, Thermal resistance

Thermal resistance is the quotient of the difference between the effective temperature of the device and the temperature at the external reference point divided by the steady state power dissipation in the device.It is the most important parameter to indicate the heat dissipation degree of the device.The thermal resistance of Led with good front heat dissipation is no more than 10℃/W, the best reported thermal resistance in China is no more than 5℃/W, and the best reported thermal resistance abroad is no more than 3℃/W.

B, The junction temperature

Junction temperature refers to the temperature of semiconductor junction in the main heating part of Led device.It is to reflect the Led device in working conditions, which can withstand the temperature value.

C, Temperature rise

There are several different temperature rises, and what we are talking about here is the shell-environment temperature rise.It refers to the difference between the temperature of the shell of the Led device (the hottest spot that can be measured by the Led lamp) and the temperature of the environment (on the luminescent plane of the lamp, 0.5 meters away from the lamp).It is a temperature can be measured directly, and can directly reflect the LED device peripheral cooling degree, practice has proved that in the ambient temperature of 30 ℃, if measured LED tube shell for 60 ℃, the temperature rise shall be the 30 ℃, this basically ensures the service life of LED device value, such as temperature rise too high, LED light source, maintain rate will decrease greatly.

D. New problems of heat dissipation

With the development of Led lighting products, there are two new technologies:

In order to increase the luminous flux of a single tube, more current density is injected, as mentioned below, so that the chip generates more heat and requires cooling.
Encapsulate the new structure, with the increase of the Led light source power, need more power Led chip set packaging together, such as lamps of COB structure, modular, will produce more heat, the need for more effective heat dissipation structure and measures, which in turn to put forward new task heat dissipation, otherwise it will greatly affect the performance and life of Led lamps.

To sum up, the level of heat dissipation must be improved. However, some people recently proposed that “with the improvement of Led light efficiency, heat dissipation is not important”, which is wrong, because Led lamps are well made, and their total energy efficiency is only 50%, and there is still a lot of electricity to be turned into heat.Secondly, Led high current density and modular lamps will generate more concentrated waste heat, which requires good heat dissipation.Put forward several principles for improving heat dissipation level:

From the perspective of Led chip, new structure and new technology should be adopted to improve the heat resistance of Led chip junction temperature, as well as the heat resistance of other materials, so as to reduce the requirements for heat dissipation.
To reduce the thermal resistance of Led devices, adopt new packaging structure and new process, choose new materials with better thermal conductivity and heat resistance, including bonding materials between metals, mixed adhesive of phosphor powder, etc., and make the thermal resistance ≤10℃/W or lower.
Reduce the temperature rise, as far as possible to use heat conduction of good heat dissipation materials, in the design requirements of a better ventilation channel, so that waste heat as soon as possible to spread out, the temperature rise should be less than 30℃.
There are many ways to dissipate heat, such as the use of thermal catheter, of course very good, but to consider the cost factor, in the design should consider the cost performance problem.

In addition, the design of Led lamps and lanterns in addition to improving the efficiency of lamps and lanterns and light distribution requirements, good appearance, to improve the level of heat dissipation, USES the material with good thermal conductivity, it is reported that the cooling body coated with some nano material, its thermal performance is increased by 30%. In addition, want to have good mechanical properties and sealing, heat dissipation and dustproof, the rise of temperature of the requirements of Led lamps and lanterns should be 30 ℃.

二. The cost of Led lighting

Whether Led light source can fully enter the lighting field, the cost of its light source is the most critical. At present, the cost difference of different Led products is 5~10 times than that of traditional products.Moreover, due to the further improvement of the main technical indicators, new structures, new materials, new technologies and new processes are proposed constantly, which undoubtedly brings new pressure to the cost of Led.Below put forward a few methods that reduce cost from two level below, offer everybody discussion.

1. Large-scale production and increase yield

Large-scale production with automatic equipment can greatly improve production efficiency, save costs and reduce costs.In addition, the use of process measures and quality assurance system management methods to improve yield, is also a good way to reduce costs.

2. Technological innovation reduces costs

In order to reduce the cost, we should focus on technological innovation. Adopting new structure, new technology, new material and new process can not only improve the performance index of LED, but also effectively reduce the cost, this is the direction of efforts.Here are some ideas;

(1) Cost reduction method for epitaxial chips

At the present stage, the cost of Led chip takes up a relatively high proportion of Led light source. It is necessary to focus on reducing the cost from the epitaxial chip. Four specific methods are introduced:

Increase the epitaxial chip area: the epitaxial chip area has been expanded from 2 “and 4” to 6 “. Although the epitaxial chip area has been increased, new problems such as chip uniformity, crack and deformation must be overcome technically, but the cost reduction is very significant.
Second, improve epitaxial growth:In terms of quality, process and structure, the average time between failures (MTBF), average time between cleanings (MTBC) and average time between repairs (MTTR), as well as the compatibility between equipment and online process control, can improve the output.
Increase the current density: several major foreign companies are developing to increase the current density of the positive current of Led so as to improve the luminous flux of single power Led, and reduce the number of Led when the same illumination is reached. Of course, some light efficiency will be sacrificed.If the current forward current 350mA is increased to 2A, the luminous flux can be increased by 4 to 5 times, and the cost will be greatly reduced. Of course to solve the junction temperature heat resistance, packaging material heat resistance and heat dissipation and other new problems.
Reduce the starting voltage VF: currently, the typical value of GaN starting voltage VF is 3.3v, and the VF value is being developed abroad. If the value reaches within 2.8v, the same luminous flux (luminous efficiency) can be obtained when the input power is reduced, that is, the energy efficiency is improved and the cost is saved.

(2) Led packaging

To improve LED packaging process, adopt new structure, new material and new process, improve LED packaging yield, reduce LED packaging cost, is the packaging enterprise’s goal all the time.Some methods to reduce packaging cost are also introduced:

Packaging materials and chip separately, lens, phosphor film production of packaging materials, etc., with chip off encapsulation, this approach is simple, good heat dissipation process stability of product performance, good uniformity can improve the reliability of packaging components, and packaging costs are low, is one of the main orientation device packaging.
It adopts the COB package, i.e. Led multi-chip integrated package.It is reported that the COB package can reduce the packaging cost by 30%, but the light output efficiency and heat dissipation of the package should be solved.

(3) Led lamps

The cost of Led lamps including radiator also accounts for a relatively high proportion of Led lighting products. In order to reduce the cost of Led lamps, it can be considered from two aspects:

The design of lamps and lanterns containing heat dissipation body is reasonable to reduce material waste.Choose the right material, both good mechanical performance and heat dissipation performance, and reasonable cost performance.
Modular lamps, namely, Led chip, drive power and radiator are packaged together into modular units for standardized production.According to different lighting requirements, modular unit assembly can be used.This modular assembly method can greatly reduce the manufacturing cost.But there are new problems with energy efficiency and cooling.

In order to make Led light source into the lighting field, it is urgent to solve the two major problems of Led lighting system reliability and cost reduction.Through the above analysis, from the aspects of each component of Led lamps, namely epitaxial chip, device packaging, drive power supply, heat dissipation, lamps, etc., the reliability design and test must be carried out respectively to reach the reliability index, so as to guarantee the reliability of the system and make the life of Led lamps reach 35,000 hours.In terms of cost reduction, while earnestly improving yield and large-scale production, we should focus on technological innovation, especially in epitaxial chip, device packaging and lamp design, so as to greatly reduce the cost of Led lamps.