White Light High Power Led Long Life and Low Power Consumption Technology

As for the service life of LED, for example, using silicon sealing material and ceramic packaging material can increase the service life of led by 10%. In particular, the luminous spectrum of white LED contains short wavelength light with wavelength lower than 450nm. The traditional epoxy resin sealing material is very easy to be damaged by short wavelength light, and the large amount of light of high-power white LED accelerates the deterioration of sealing material, According to the test results of the manufacturer, the brightness of high-power white LED has been reduced by more than half in less than 10000 hours of continuous lighting, which can not meet the basic requirements of long-life lighting source.

As for the luminous efficiency of LEDs, improving the chip structure and packaging structure can reach the same level as low-power white LEDs. The main reason is that when the current density is increased by more than twice, it is not easy to take out light from large chips. As a result, the luminous efficiency will be inferior to low-power white LEDs. If the electrode structure of the chip is improved, In theory, the above light taking problem can be solved.

With regard to the uniformity of luminous characteristics, it is generally believed that the above problems can be overcome as long as the concentration uniformity of phosphor material of white LED is improved.

As mentioned above, while increasing the applied power, it is necessary to try to reduce the thermal impedance and improve the heat dissipation. The specific contents are as follows:

â‘  Reduce chip to package thermal impedance

â‘¡ Suppress the thermal impedance encapsulated to the printed circuit substrate

â‘¢ Improve the smoothness of heat dissipation of the chip

In order to reduce the thermal impedance, many foreign LED manufacturers set the LED chip on the surface of the heat sink made of copper and ceramic materials, and then connect the heat dissipation wire on the printed circuit board to the heat sink forced by the cooling fan by welding. According to the experimental results of osramopoptosemiconductors GmbH in Germany, The thermal resistance from the LED chip of the above structure to the welding point can be reduced by 9K / W, about 1 / 6 of that of the traditional LED. When the encapsulated led applies 2W power, the bonding temperature of the LED chip is 18K higher than that of the welding point. Even if the temperature of the printed circuit board rises to 500C, the bonding temperature is only about 700C at most; In contrast, once the thermal impedance is reduced in the past, the bonding temperature of LED chip will be affected by the temperature of printed circuit board. Therefore, it is necessary to try to reduce the temperature of LED chip, in other words, reducing the thermal impedance from LED chip to welding point can effectively reduce the burden of LED chip cooling operation. Conversely, even if the white LED has the structure of restraining thermal impedance, if the heat cannot be transmitted from the package to the printed circuit board, the luminous efficiency will drop sharply as a result of the rise of LED temperature. Therefore, Panasonic electric developed the integrated technology of printed circuit board and packaging. The company encapsulated 1mm square blue LED on the ceramic substrate in the form of flip chip, Then paste the ceramic substrate on the surface of the copper printed circuit board. According to Panasonic, the thermal impedance of the whole module including the printed circuit board is about 15K / W.

Because the adhesion between the cooling fin and the printed circuit board directly affects the heat conduction effect, the design of the printed circuit board becomes very complex. In view of this, Lumileds in the United States and lighting equipment and LED packaging manufacturers such as citizen in Japan have successively developed simple heat dissipation technology for high-power LED. Citizen began to ship white LED packaging in 2004, The company said that although the 30K / W thermal impedance from the junction point of LED chip to the cooling fin is greater than 9K / W of OSRAM, and the room temperature will increase the thermal impedance by about 1W in general environment, even under the forced air cooling state of traditional printed circuit board without cooling fan, The white LED module can also be used for continuous lighting.

For the high-power LED chips shipped by Lumileds in 2005, the allowable bonding temperature is as high as 1850c, which is 600C higher than the products of other companies at the same level. When packaged with traditional rf4 printed circuit board, the current equivalent to 1.5W power (about 400mA) can be input within 400C of the ambient temperature.

As described above, Lumileds and citizen have increased the allowable temperature of the junction, while German OSRAM has set the LED chip on the surface of the heat dissipation fin to achieve 9K / W ultra-low thermal impedance record, which is 40% lower than the thermal impedance of the same product developed by OSRAM in the past. It is worth mentioning that the same flip chip method as the traditional method is used for the packaging of the LED module, However, when the LED module is connected with the thermal fin, the light-emitting layer closest to the LED chip is selected as the joint surface, so that the heat of the light-emitting layer can be conducted and discharged in the shortest distance.

In 2003, Toshiba lighting company tried to build a 300lm LED module on a 400mm square aluminum alloy surface with a low thermal impedance white LED with a luminous efficiency of 60lm / W and no cooling fan and other special heat dissipation components. Because Toshiba lighting company has rich trial experience, the company said that due to the progress of simulation analysis technology, After 2006, more than 60lm / w White LEDs can easily use lamps and frames to improve thermal conductivity, or use cooling fans to design the heat dissipation of lighting equipment by forced air cooling. White LEDs can also be used without the module structure of special heat dissipation technology.

With regard to the longevity of LED, the current countermeasures taken by LED manufacturers are to change the sealing materials and disperse the fluorescent materials in the sealing materials, especially the silicone sealing materials, which can more effectively inhibit the deterioration of materials and the reduction of light penetration than the epoxy sealing materials above traditional blue and near ultraviolet LED chips.

As the percentage of light with the wavelength of 400 450nm absorbed by epoxy resin is as high as 45%, while that of silicon sealing material is less than 1%. The brightness reduction time of epoxy resin is less than 10000 hours, and the silicon sealing material can be extended to about 40000 hours, which is almost the same as the design life of lighting equipment, which means that there is no need to replace white LED during the service life of lighting equipment. However, siliceous resin is a kind of soft material with high elasticity. In processing, it is necessary to use the manufacturing technology that will not scratch the surface of siliceous resin. In addition, siliceous resin is easy to adhere to powder chips in the manufacturing process. Therefore, it is necessary to develop technologies that can improve the surface characteristics in the future.

Although the silicon sealing material can ensure the service life of LED for 40000 hours, the lighting equipment industry has different views. The main debate is that the service life of traditional incandescent lamp and fluorescent lamp is defined as "the brightness is reduced to less than 30%. If the brightness is reduced by half to 40000 hours, there are only about 20000 hours left if the brightness is reduced to less than 30%. At present, there are two countermeasures to prolong the service life of components, namely:

1. Inhibit the overall temperature rise of white LED;

2. Stop using resin packaging.

It is generally believed that if the above two life extension measures are fully implemented, the requirement of 30% 40000 hours of brightness can be achieved. To suppress the temperature rise of white LED, the method of cooling LED packaging printed circuit board can be adopted. The main reason is that the packaging resin will deteriorate rapidly under high temperature and strong light irradiation. According to Arrhenius law, the temperature will be reduced by 100C and the service life will be prolonged by 2 times.

Stopping the use of resin packaging can completely eliminate the deterioration factors, because the light generated by LED is reflected in the packaging resin. If a resin reflector that can change the direction of light travel on the side of the chip is used, the extraction amount of light will decrease sharply because the reflector will absorb light, which is also the main reason why LED manufacturers unanimously adopt ceramic and metal packaging materials.

There are two ways to improve the luminous efficiency of white LED chips. One is to use a large LED chip with an area 10 times larger than a small chip (about 1mm2); Another way is to use a plurality of small LED chips with high luminous efficiency to form a single module. Although large LED chips can obtain large beams, increasing the chip area will have disadvantages, such as unequal electrical boundary of the light-emitting layer in the chip, limited light-emitting parts, serious attenuation of the light emitted from the chip to the outside, and so on. To solve the above problems, LED manufacturers have achieved a luminous efficiency of 50lm / W through the improvement of electrode structure, the use of flip chip packaging and the integration of chip surface processing skills.