LED industry in the past in order to get the full white LED beam, once the development of large-size LED chips trying to take this way to achieve the desired goals. But, in fact, exert power white LED lasts for more than 1W beam but will drop 20 ~ 30% reduction in the relative luminous efficiency. In other words, if the brightness of the white LED is several times larger than traditional LED power consumption characteristics beyond the fluorescent , you need to overcome the following four topics: inhibit the temperature rise to ensure that the service life, improve the luminous efficiency, and Luminescence Properties of equalization.
Temperature rise solution to the problem is to reduce the package thermal impedance; to maintain the service life of LED is to improve the chip shape, a small chip; improve the LED luminous efficiency of the method is to improve the chip architecture, small chip; The luminescence properties homogenization The method is to improve the LED packaging methods, these methods have been developed one after another.
To solve the package heat dissipation problem is the fundamental method
Due to the increased power but will result in the package thermal impedance is sharply down to below 10K / W, foreign companies have developed high-temperature white LED, and an attempt to improve the problem. In practice, however, high-power LED heat than low-power LED a few times more, and the temperature rise will make the light-emitting efficiency fell sharply. Packaging technology allows high-calorie, but the LED chip junction temperature there is likely to exceed the allowable value of the last industry finally realized that the heat dissipation problem is the fundamental method to solve the package.
high power led life, for example, to switch the the siliceous packaging materials and ceramic packaging materials, make the life of the LED's to improve a number of white LED light spectrum contains wavelengths below 450nm short wave length light, traditional epoxy encapsulated materials can easily be short-wavelength light damage, high-power white LED light to accelerate the degradation of packaging materials, less than one million hours of continuous lighting, according to the test results of the industry, high-power white LED brightness has been reduced by more than half simply can not meet the basic requirements for lighting long life.
For the LED luminous efficiency, improve chip package structure, can achieve the same level with the low-power white LED. The main reason is the current density increased by more than 2 times, not only does not easily removed from large chip light, the results may even lead to the dilemma of the luminous efficiency than the low-power white LED. If improving the electrode structure of the chip can theoretically solve these light extraction problem.
Managed to reduce the thermal resistance to improve the heat dissipation problem
For light-emitting characteristics of uniformity, is generally believed to improve white LED uniform concentration of fluorescent material and the phosphor production technology should be able to overcome this trouble. As described above to improve the applied power at the same time, it is necessary to try to reduce the thermal resistance, improved heat dissipation problem. The specific contents are: to reduce the chip to the package thermal impedance, inhibiting the thermal resistance of the package to the printed circuit board, chip cooling smooth.
In order to reduce thermal resistance, and many foreign LED Blue 150W manufacturers will set the LED chip surface of the radiator made of copper and ceramic materials (the heat the sink), and then using the welding and the heat of the printed circuit board with a wire connected to the use of forced air cooling fan radiator. Germany OSRAM Opto Semi conductors Gmb experimental results confirmed the structure of the LED chip can reduce the thermal impedance of the welding point of 9K / W, about 1/6 of the traditional LED LED package is applied to 2W power LED chips bonding temperature than welding points 18K, even if the printed circuit board temperature rise to 50 ° C junction temperature at most, only about 70 ℃; thermal impedance contrast in the past reduced the LED chip junction temperature will be a printed circuit board temperature. Therefore, it is necessary to reduce the temperature of the LED chip, in other words, reducing the LED chip to the thermal impedance of the welding point, can effectively reduce the burden of the cooling effect of the LED chip. On the other hand even if the LED strip lights is able to inhibit the thermal impedance of the structure, if the heat conduction from the package to the printed circuit board, the results still make light-emitting efficiency of LED temperature rise fell sharply. , Matsushita Electric Works, the development of printed circuit board and package integration technologies, the company will be 1mm square blue LED flip chip package on the ceramic substrate, followed by and then paste the ceramic substrate in the copper printed circuit board surface, according to Panasonic reported contains the modules of the printed circuit board, including the overall thermal impedance is about 15K / W or so.
The industry to demonstrate the thermal design skill
Due to the compactness of the heat sink and printed circuit board between about heat conduction effect, the printed circuit board design is very complicated. In view of the United States Lumileds Japan CITIZEN lighting, LED 200W packaging manufacturers, have developed a high-power LEDs with a simple heat dissipation technology, CITIZEN in 2004, began manufacturing the white LED samples package does not require special bonding technique can also be a thickness of 2 ~ 3mm radiator heat directly emitted into the external, the CITIZEN reports the junction of the LED chip to the heat sink 30K / W thermal resistance than that of OSRAM 9K / W, and in the general environment at room temperature makes thermal impedance increases about 1W Even the traditional printed circuit board cooling fan to force air-cooled state, the white LED module can be continuous lighting use.
Lumileds in 2005 began to manufacture high power LED chip junction to allow the temperature to as much as to +185 ° C, high 60 ° C at the same level than any other company products, the use of traditional RF 4 printed circuit board package, you can enter the ambient temperature range of 40 ℃ equivalent to 1.5W of power current (about 400mA). Lumileds and CITIZEN is taken to improve the junction to allow the temperature, Germany OSRAM the sucked LED chip set in the radiator surface to reach the 9K / W low thermal impedance record, the record from OSRAM in the past to develop the same level of thermal impedance by 40% It is worth mentioning that the LED 400W module package, with the traditional method using the same flip chip, but the LED module and heat sink bonding, select the closest to the LED chip emitting layer as the joints to make the heat of the light-emitting layer to the shortest distance between conductive emissions.