Experimental Discussion on Improving Temperature Resistance of LED Light Sources (II)

Experimental Discussion on Improving the Temperature Resistance of LED Light Sources (II) Introduction Semiconductor lighting is a technological revolution in this century. It is still a baby from the perspective of technology maturity. Although LED high-power white light technology develops rapidly, LED light decays and heats up. The cost of these three innate ailments is still a roadblock for the development of LED lighting. LED light decay and heat dissipation have penetrated the entire industrial chain from chip manufacturing, packaging process, material selection, and lamp development. At present, the industry has insufficient understanding of the concept of light decay, the causes and how to solve it. There is no authoritative explanation for the theory. It is difficult to introduce it, so that there are people who are clouded, and you copy me into a strange technical chaos. For a long time, people have been desperately trying to reduce the LED light decay by using heat dissipation methods, but the effect is very small. Solving the problem of LED light decay has become a common concern and eagerly awaited technical problem in the industry. People can't help but ask: Why do designers not find another way to find the cause of LED light decay from the source? The author has made in-depth discussion on the improvement of LED light source's temperature resistance by reducing the LED light decay from theory and practice, and will successively launch related articles and experimental reports.

Key words

LED light effect, LED thermal resistance, LED light decay, WFCOB light source, LED module lighting

table of Contents

Chapter One

First, LED light effect: the meaning of distinguishing between transient light effect and steady light effect

Second, LED thermal resistance: the meaning of distinguishing the internal thermal resistance of the light source and the external thermal resistance of the light source

Third, LED light decay: the decrease of luminous flux is not equal to light decay, irreversible damage of LED = light decay

Chapter two

Fourth, the cause of LED light decay

Fifth, improve the temperature resistance of LED light source can reduce LED light decay

Why do you want to improve the temperature resistance of LED light sources?

Seven, how to make the LED light source to withstand high temperature

third chapter

Seven, aluminum substrate is completely unnecessary to achieve 3750V withstand voltage

Eight, how to solve the short circuit of the drive power

Chapter Four

Nine, introduce a WFCOB light source

chapter Five

X. Introducing an LED module lighting



Chapter two

Fourth, the cause of LED light decay

1, LED light decay is the irreversible attenuation failure caused by the damage of the light source material

LED light decay means that after a period of illumination of the LED, its light intensity will be lower than the original light intensity, and the low part is the light decay of the LED. At present, China has not formulated the LED light decay standard, and the industry internally stipulates 5000H. The hourly luminous flux maintenance rate is ≥70%, which is considered to be invalid.

2, the decrease in luminous flux is not equal to the light decay

It is well known that after the LED works, its light intensity will decrease as the junction temperature of the chip rises, and the light efficiency will decrease. This is the inherent physical property of the semiconductor as it changes with temperature. As long as a certain component of the LED light source does not exceed the temperature limit and the LED is stopped, the light intensity will be restored as soon as the temperature is restored. That is to say, no matter how long the LED works, as long as the initial light intensity is constant, it cannot be regarded as light decay. The author believes that LED light decay refers to the failure phenomenon that the LED light source is no longer recovered due to certain material damage, that is, the LED light source has no loss of luminous flux (initial light intensity) and lossy luminous flux (attenuated unrecoverable light intensity) within the specified time. The ratio.

3, the main cause of LED light source light decay is that the gel is not resistant to temperature

As we all know, the chip (including phosphor) is an inorganic material, and experiments have proved that the chip and the phosphor are not a problem in the principle of high-temperature operation of two or three Baidu. From the light source system, the main cause of LED light decay is that the gel is not resistant to temperature. At present, the best package rubber has a temperature resistance of only one hundred degrees. The test proves that a 50W integrated light source is often used when the radiator is large enough. Up to more than 200 degrees, whether it is potting glue or PPA in the long-term high temperature operation will inevitably cause colloid cracking, carbonization, and separation from the chip and cause light decay.



From the luminaire system, LED light decay is related to system thermal resistance, including heat dissipation channels, heat dissipation materials, heat dissipation methods, and temperature-related components.

The light decay of the LED light source is determined by the structure of the support, the chip, the quality of the phosphor, the temperature resistance of the gel, and the process of the packaging process. These conditions are selected by the packaging factory, and the preferred is the weather resistance of the package bracket and the gel. In a sense, the core technology of LED packaging should be the development and manufacturing technology of package bracket, which determines the purpose, function and cost performance of LED light source.

Fifth, improve the temperature resistance of LED light source can reduce LED light decay

1. Why do you want to improve the temperature resistance of LED light sources?

It is well known that LEDs belong to semiconductor low-temperature heating devices. Low-temperature heat sources have low heat dissipation efficiency under natural heat dissipation conditions. LEDs transfer heat to the air through convection and radiation. If the temperature difference between the LED heat sink and the ambient temperature is small, heat dissipation is further increased. The area of ​​the device has little change in heat dissipation. Theoretical studies have shown that the amount of radiative heat dissipation is proportional to the fourth power of temperature: Q = εσ S (T w 4 -T0 4 ), the greater the temperature difference, the more heat is dissipated. That is, the higher the temperature of the heat sink at the same ambient temperature, the more heat is dissipated. Therefore, the operating temperature of the heat sink is appropriately increased, and the principle that the steady-state light effect does not change greatly after the LED light source is operated for a long time without light decay is controlled. This design idea of ​​improving the high temperature resistance of the LED light source is not only based on balanced heat dissipation and cost considerations, but also mainly allows the LED light source to work safely at a higher temperature without occurrence of light decay. This not only reduces the amount and cost of the heat sink, but also increases the load carrying capacity of the working current of the chip, and at the same time achieves the purpose of reducing the LED light decay and prolonging the service life, and is a design revolution with multiple achievements.

2. How to make the LED light source withstand high temperature

Since the advent of LEDs, the industry has invested heavily in researching LED light decay. LED cooling affects every nerve engaged in LED people, and has tried many ways, among which:

*Flip-chip technology: Flip-chip technology as early as 10 years ago, foreign large companies invested heavily in research, aiming to avoid the use of substrate glue die welding technology, not only eliminating the fatal injury of the surface of the flip chip on the surface, but also The direct soldering technology can effectively reduce the thermal resistance of the package and allow the light source to withstand high temperatures. The industry is generally considered to be the leading edge technology of LED packaging. But people have to ask: After years of research and experimentation, why is the flip-chip technology unable to replace the positive-loading chip technology and become the mainstream? The root cause is that the early process of flip-chip relies not only on the ceramic substrate, but also on the aluminum (copper) substrate. Since the (twice) over-weld is subjected to a high temperature of 280 degrees, it will cause damage to materials and components. Compared with the metal substrate, the ceramic substrate has a low reflectance, and it is difficult to improve the transient light effect. The thermal conductivity of the ceramic substrate and the limited contact area of ​​the chip also determine that the steady-state light effect is difficult to improve. The processing and installation are not as simple and convenient as the metal substrate. The flip-chip process twice soldering and the ceramic substrate + aluminum substrate double thermal resistance is enough to offset the above advantages. In addition, the equipment of hot-press welding and reflow soldering of the flip-chip process is expensive and immature, and the future of the flip-chip technology is still LM\yuan value, that is, the light effect is the first, and the price is king. Therefore, it caused a small factory to wait and see, and the promotion of large-scale products was difficult. The future development direction of flip chip technology is to marry the mirror aluminum COM, and the second is to graft with the fluorescent film.

*, phosphor away from chip technology

The phosphor and the potting compound tightly wrap the chip, which seriously affects heat dissipation. Keeping the phosphor away from the chip can reduce the thermal resistance of the light source package. Internationally, there are as many as two or three hundred patents for phosphor and chip separation technology, but so far the market has not seen such products successfully, because the difficulty of this technology is how to change the interlayer dielectric refraction matching after the phosphor is far away from the chip, and the chip How to get protection with gold wire bare is also a technical difficulty. At present, some factories use fluorescent films in small-scale power source experiments, but the manufacturing process and material weather resistance are not yet mature. The phosphors away from the chip technology can not only reduce the thermal resistance of the light source package, but more importantly, it can remove the glue and pump in the packaging factory. Vacuum, baking and other complicated processes. Once the breakthrough is undoubtedly a technological revolution that changes the packaging landscape.

*, liquid cooling technology

The method of dissipating the chip into the cooling liquid is to let the LED be immersed in the light-transmitting and heat-conducting liquid. The heat is quickly transferred and dissipated due to the heat exchange of the liquid. As long as there is a temperature difference between the chip and the thermal fluid, its heat flow exchange never stops. It can greatly reduce the thermal resistance of the package and is a very good design idea. Many experts at home and abroad have proposed this method, and there are countless patents. However, in the actual design, the liquid is cooled in a small package space, and subjected to high and low temperature repeated changes without leaking, and the structural design is very difficult.

*. In an integrated (COB) package, for multiple low power arrays, the light intensity/thermal resistance ratio can be increased because the light intensity is in parallel with the heat dissipation.

*. Leather base rubber, leather de-filling glue, leather high temperature and vulnerable materials to improve the temperature resistance of the light source.

*. Reduce the total reflection of the light path, optimize the light angle, improve the light efficiency of the LED light source, and reduce system heating.

*. The aluminum substrate is removed to improve the steady-state light effect and transient light efficiency ratio.

*. Reduce the lens light distribution loss.

*. Reduce the light blocking loss of the protective member.

Conclusion

LED is a low-temperature semiconductor heating device. The low-temperature heat source has low heat dissipation efficiency under natural heat dissipation conditions. The greater the temperature difference at the same ambient temperature, the more heat is dissipated. The higher the temperature of the heat sink, the more heat it dissipates. Therefore, the operating temperature of the heat exchanger is appropriately increased, and the principle that light decay does not occur after the LED light source is operated for a long time is adopted. Can effectively reduce LED light decay. This design idea can not only reduce the amount and cost of the heat sink, but also increase the carrying capacity of the working current of the chip, and at the same time achieve the purpose of reducing the LED light decay and prolonging the service life, which is a design revolution with multiple wins.

about the author

Wang Feng, male born in 1941, Tianjin native, university culture, engineer. He has worked as a technician, chief engineer, factory manager, and company manager. Has deep electronic and mechanical structure technology, more than 10 years of experience in LED system design, has a number of LED invention patents. Strong technical R&D and project management capabilities, certain industry relations and social resources.

Contact number: 13430533163 QQ
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