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LED light decay, this innate dysentery, its "long life" crown makes people feel disappointed, LED light decay caused product defects and heat dissipation costs remain high, becoming the roadblock for the development of LED lighting. LED light decay runs through the entire industrial chain from chip manufacturing, packaging process, material selection, and lamp development. For a long time, people have been desperately trying to reduce the LED light decay by using heat dissipation methods. However, the effect is very small, resulting in randomness of theory and practice. A variety of whimsical technical chaos. The author's improvement of the temperature resistance of LED light source can reduce the mechanism of LED light decay from the theory and practice, and will launch related articles and experimental reports.
First, LED light effect
The luminous flux (lm) emitted by the LED light source divided by the electrical power consumed by the light source (w) is called the LED luminous efficacy in lm/w. LED light effects are divided into transient light effects and steady-state light effects.
LED transient light efficiency refers to the initial luminous efficiency when the LED light source starts to work, also called the initial cold light effect.
The steady-state light effect refers to the ratio of the luminous flux measured by the LED light source to the electric power after the thermal operation of the LED light source is not stable and the light intensity is no longer changed.
Transient light effects are fundamentally independent of temperature. Steady-state light effect is related to many variables of system light, electricity and heat. At present, the luminous efficacy values ​​of all the chip factories and packaging factories are transient light effect values, and the user acceptance test is also a transient light effect value.
Second, LED light decay
The LED light decay is due to the irreversible failure phenomenon of the LED light source due to insufficient temperature resistance of certain materials. After the LED light source is illuminated for a period of time, its light intensity will be lower than the original light intensity. This is an inherent physical property of a semiconductor as a function of temperature. As long as a component of the LED system does not exceed the temperature limit and the LED is stopped, the LED will stop working and its light intensity will return to its original value after the temperature is restored. This reduction in light intensity can also be restored without being called light decay. The decrease in luminous flux is not equal to light decay. A decrease in light intensity is not equal to a failure.
The relationship between temperature and luminous flux/optical power indicated by CREE or other company's light source specification indicates that the LED junction temperature should not exceed a certain range to ensure a certain luminous flux and cause light decay. This is not a single light decay curve of the chip material. Whether it is Ta=25° or Ta=85°C, it is a transient measurement of the chip or other components of the LED at a certain temperature. It does not indicate that it is The steady state value of the actual system. The steady-state value is only measured after the system is thermally stable, and the steady-state luminous flux of temperature and luminous flux/optical power is measured, that is, the system efficacy is affected by many comprehensive factors.
LED chips and phosphors are inorganic materials. The final manufacturing process is usually in the order of several hundred degrees, Ta=85°C. It is by no means the LED chip and the phosphor temperature limit. Our experimental high temperature WF COB light source does not use any lighting at 30W. When the heat sink is used, the temperature of the light source housing is as high as 165 degrees, and it can work continuously for 24 hours or even longer, and the luminous flux still maintains the initial value.
Third, the defect of the traditional integrated light source
The industry generally believes that COB packaging is an inevitable trend in the future, because it has many advantages in heat dissipation, light distribution and cost. Compared with many small power arrays, because of the parallel connection between light intensity and heat dissipation, it can effectively increase the light intensity/thermal resistance ratio. . At present, the widely popular integrated light source and aluminum (copper) substrate COB light source have the following disadvantages: the plastic injection molding integrated light source bracket, because the plastic PPA will turn yellow and pulverize under high temperature and ultraviolet irradiation, causing ventilation and water ingress, failure rate Very high. The aluminum or copper substrate COB bracket relies on the PCB for the line connection and is made by the FR4 fiber pressing process. The interlayer contains insulating fibers and viscose, which not only increases the thermal resistance of the light source, but also is difficult to weld gold wire and lead wire, and the skin will fall off at high temperature. The surface of the substrate coated with white oil has a poor reflective effect, which not only affects the decrease in light efficiency, but also causes yellow cracking and damage under long-term exposure to ultraviolet light.
Fourth, a WFCOB light source
A distinct advantage of a WFCOB source compared to existing COBs and integrated sources is:
* With a metal integrated structure, the injection molding process with integrated light source is completely eliminated. It eliminates the trouble of product failure caused by the use of PPA due to high temperature and ultraviolet radiation, which will turn yellow, powder and breathe.
* Using special electrode design, the COB light source is completely removed from the aluminum (copper) substrate bonding process, which avoids many disadvantages such as the electrode lead is not easy to wire, solder, high temperature running surface fading, peeling off and so on.
* Highly reflective mirrored aluminum for increased reflectance. Not only does it provide high light efficiency, it completely overcomes the drawbacks of the integrated light source substrate, which is easy to cause vulcanization and carbonization due to the silver plating process.
* With groove technology, there is no need to set up a dam. The plastic dam is completely eliminated to cause cracking and venting, and the amount of phosphor and encapsulant is also reduced.
A WFCOB light source adopts the above technical measures to improve the temperature resistance of the light source and reduce the LED light decay. Of course, this is the most basic technology, and we further design and manage from the photoelectric thermal synthesis method.
Conclusion
In a sense, the core technology of LED packaging should be the development and manufacturing technology of packaged brackets, which determines the purpose, function and cost performance of LED light sources. At present, the development of packaged brackets lags behind the market, and the design of simple, compact, reliable, practical and cost-effective package brackets is imperative. The WFC0B light source bracket can withstand higher temperatures under the same heat dissipation conditions. The cost of the bracket is only 1/3 of the normal COB and 1/5 of the integrated bracket. The amount of fluorescence and encapsulant can be reduced by 50-70%. Because the product is resistant to high temperatures, not only can the amount of heat sink be reduced, but also the chip current can be increased to increase the power of the light, which greatly reduces the system manufacturing cost and prolongs the service life of the light source. We have conducted extensive experiments to make LED chips more resistant to conventional temperatures and have a longer life.