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LED temperature problem solutions - (AddTime : 2013-01-29)

In the past LED industry in order to take full advantage of the white 200W led Flood light beam, ever developed large size LED chip attempts to make use of the means to achieve goals. But, in fact, the white light led on the power for more than 1 w above beam it will drop, luminous efficiency relative reduce twenty to 30%. In other words, if the white led brightness than the traditional led tarsus times, consumption power characteristic beyond the fluorescent lamp, it is necessary to overcome the following four topics: inhibition of temperature rise, to ensure that the service life, improve the luminous efficiency, and equal access to the luminescence properties.

Temperature rise the solution to the problem is to reduce packaging thermal impedance; Maintain LED life method is to improve chip shape, using small chip; Improve the LED luminous efficiency method is to improve chip structure, using small chip; As for the luminescence properties homogenization method is to improve the LED encapsulation method, these methods have been have been developing.

To solve the problem is the basic package cooling method

Due to the increasing power but can cause encapsulated thermal impedance sharp drop to below 10 k/W, so the foreign investors ever developed high temperature resistant white light LED, trying to improve by the above problems. In practice, however, the high power LED calorific value than small high power LED several times above, and the temperature rise will make luminous efficiency plummets. Even if the package technology allows high quantity of heat, but LED chip joint temperature could more than permissible value, and finally industry finally understood to solve encapsulated heat dissipation problems is the basic method.

The service life of the LED, such as convert siliceous packaging materials and ceramic packaging material, can make the LED the working life is raised a digit, especially the white 200W led Flood light spectrum contains less than 450 nm wavelength of short wavelength light, traditional epoxy resin encapsulation materials can be short wavelength light damage, high power white LED large quantity of light more accelerated packaging material degradation, according to industry test results show that less than ten thousand hours of continuous light, high power white LED brightness has reduced more than half, cannot satisfy the lighting light source long life of the basic requirements.

The LED luminous efficiency, improve chip structure and encapsulation structure, can achieve and low power white LED the same level. The main reason is that current density increase more than two times, not only not easy to take out from a large chip light, it will cause the luminous efficiency as low power white LED dilemma. If improve chip electrode structure, theory can solve the above problem take light.

Try to reduce thermal impedance, improve heat dissipation problems

The luminescence properties uniformity, it is generally thought that as long as the improvement of white LED fluorescent body material density uniformity and phosphor production technology, should be able to overcome the above problems. As mentioned above to improve on the power at the same time, must try to reduce thermal impedance, improve heat dissipation problems. Specific content respectively is: reduce the chip to the packaging of the thermal impedance, inhibition of packaging to printed circuit board thermal impedance, improve chip heat flow.

In order to reduce thermal impedance, many foreign LED manufacturers will LED chip installed in copper and ceramic material made of radiator (heat sink) surface, and then use the welding way of printed circuit board heat with wire connected to the use of cooling fan forced air cooling on the radiator. According to Germany OSRAM Opto Semi conductors Gmb experimental results confirmed that the structure of the LED chip to weld thermal impedance can reduce 9 k/W, is about 1/6 of the traditional LED around package after 2 W LED applied power, LED chip joint temperature than welding point high and k, even if printed circuit board temperature rise to fifty ℃, joint temperature at most only 70 ℃ or so; Compared with previous thermal impedance once reduce words, LED chip joint temperature will be the influence of the temperature of printed circuit boards. Therefore, it is necessary to try to reduce the temperature of the LED chip, in other words, to reduce the LED chip to weld thermal impedance, can effectively reduce the burden of the LED chip cooling effect. On the contrary if white light LED have inhibition thermal impedance structure, if the heat from the package cannot be transmitted to printed circuit board, it LED temperature rise results still can make the luminous efficiency drop sharply. Therefore, matsushita electric development printed circuit board and packaging integration technology, the company will 1 mm square blue light LED to flip chip package way in ceramic substrate, then will ceramic substrate copper paste in printed circuit board surface, according to the panasonic reports include printed circuit board, module overall thermal impedance is about 15 k/W or so.

Each industry show thermal design capability

Due to the radiator and printed circuit board between the compactness about direct heat conduction effect, therefore the printed circuit board design become very complex. In view of the United States and Japan Lumileds CITIZEN and lighting, LED packaging manufacturers, one after another, to the development of the high power LED with a simple cooling technology, CITIZEN started in 2004 began to make white light LED sample package, do not need special joint technology can also will be thick about 2 ~ 3 mm the quantity of heat of radiator direct emissions to the outside, according to the CITIZEN reported although LED chip joint point to radiator 30 k/W heat resistance than OSRAM 9 k/W, and in general environment under room temperature can make the thermal impedance increase 1 W or so, even if is the traditional printed circuit board without cooling fan forced air cooling condition, the white light LED module can be used for lighting.

Lumileds in 2005 began to manufacture of high power LED chip, joint permissible temperature up to + 185 ℃ higher than other company, the product high sixty ℃, the use of traditional RF 4 printed circuit board packaging, the ambient temperature and ℃ range can enter the equivalent of 1.5 W power current (about 400 ma). So Lumileds and CITIZEN is taken to improve joint permissible temperature, Germany OSRAM company is LED chip installed in the radiator surface to 9 k/W low thermal impedance record, the record in the past than OSRAM products development at the same level of thermal impedance reduced by 40%. Be worth what carry is the LED module encapsulation, the traditional method of the same flip chip mode, but LED module and radiator joint, then select the most close to the LED chip luminous layer as the joint surface, thereby make luminous layer heat can in the most short distance conduction emissions. In 2003, Toshiba Lighting once in 400 mm square aluminum alloy surface, laid luminous efficiency is sixty lm/W low thermal impedance white light LED, no cooling fan and other special cooling module premise, trial-produce beam for 300 lm LED module. Due to the Toshiba Lighting has rich development experience, so the company that the simulation analysis technology progress, after 2006 more than sixty lm/W white LED, can easily use of lamps and lanterns, frame body improve thermal conductivity, or use of cooling fan forced air cooling mode design Lighting equipment cooling, do not need special cooling technology module structure can also use white light LED.

Change packaging material suppression material degradation and light penetration rate reduce speed

The LED longevity change, at present, the LED manufacturers, the countermeasure is to change the packaging materials, at the same time will be scattered fluorescent material in packaging material inside, especially siliceous packaging material than traditional blue and black light LED chip above epoxy resin encapsulation materials, can be more effective suppression material degradation and light penetration rate reduce speed. Due to the epoxy resin absorption wavelength for 400 ~ 450 nm light percentage reaches as high as 45%, the siliceous packaging material is lower than 1%, luminance half time epoxy resin is less than ten thousand hours, siliceous packaging materials can be extended to forty thousand hours, almost and lighting equipment design life the same, this means that the lighting equipment use period without the need to change the white light LED. But siliceous resin belong to high elastic soft materials, the processing required to use will not scratch the surface of silicon resin production technology, in addition the processing siliceous resin easily attached cuttings, therefore the future must development can improve the surface characteristics of the technology.

Although the siliceous packaging material can ensure that LED forty thousand hours of service life, however lighting equipment industry there are different views, the main argument is the traditional incandescent lamp and fluorescent lamp life, has been defined as a "brightness drop to below 30%". Brightness by half time for forty thousand hours of LED, if the conversion into brightness down to 30% of the words, only about twenty thousand hours. There are two prolong the service life of components countermeasures, it is respectively, inhibition of white LED the rise of temperature of the whole, and stop using resin encapsulation.

Generally think that if thoroughly carry out above two countermeasures to prolong life, can achieve 30% brightness when the requirement of forty thousand hours. Inhibition of white LED temperature rise can use cooling LED packaging printing circuit board method, the main reason is the packaging resin high temperature condition, plus glare will rapid degradation, in accordance with the law, "ray new temperature by 10 ℃ life will extend 2 times. Stop using resin packaging can eliminate degradation factors, because of the light from the LED in packaging resin internal reflection, if use can change the chip side light course of resin material reflection plate, the reflection plate can absorb the light, make the light out of the quantity decrease sharply. This is also LED manufacturers in the ceramic system and metal packaging materials is the m