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Improve Thermal Control to Enhance The Lifetime of White LEDs

28 Sep

LED industry has ever tried to develop large-size LED chips in the past in order to achieve the desired goal of getting full white LED light beam, but actually when the imposed power of white LEDs exceeds 1w, the light beam will be weakened, and the luminous efficiency will reduce by 20 ~ 30%, in other words the white LED is several times bright as conventional LED, if we want the power consumption characteristic of white LED to surpass fluorescent lights, then we must first overcome the following four topics including inhibition of temperature increase, ensuring lifetime, improving light efficiency, and equalization of luminescence properties etc.

The detailed solution to isuues related to temperature increase is to lower the package thermal impedance; the detailed methods to maintain the lifetime of LEDs is to improve chip shape, utilize small size chips; the detailed methods to improve the luminous efficiency of LEDs is to improve the chip structure, utilize small size chips; as to the equalization of luminescence, we can achieve by improving LED package method, and all these methods have already been developed in succession.

Solving the thermal control issue of package is the fundamental method

Since increasing power will result that the thermal impedance of package rapidly decrease to 10K/W, the foreign LED industry have developed high temperature resistant white LEDs to improve the above mentioned problems, however, the heat generation of high-power LEDs is actually tens of times higher than low power LEDs, and temperature increase can also sharply decrease the luminous efficiency, even if the package technology allows high heat generation, but the junction temperature of LED chip is likely to exceed the allowable value, the industry has finally realized that the fundamental method is to solve the thermal control issue of package.

As for the lifetime of LEDs, for instance, switching current package materials to silicon and ceramic package materials can improve the lifetime of LEDs by ten times, especially the white LED’s light spectrum includes short wavelength light whose wavelength is below 450nm, traditional epoxy resin package material is easy to be destroyed by short wavelength light, large quantity light output of high-power white LEDs light is easier to accelerate the deterioration of package materials, industry tests showed that if the light has been lit continuously for less than ten thousand hours,  brightness of high-power white LED has been degraded by more than 50%, which totally can not meet the basic requirements of long life for illumination light source.

As for LED luminous efficiency, improving both LED chip structure and package structure, can achieve the same level as low power white LED, which is mainly because that when the current density increases by more than 2 times, it will not be easy to extract light from the large size LED chips , whereas it will cause the luminous efficiency of high power white LEDs is even lower than low-power white LEDs, if we improve the electrode structure of the LED chip, theoretically the light extraction issue can be solved.

Try to reduce thermal resistance and improve thermal control

As for the uniformity of luminescence properties, it is generally believed that as long as we improve the intensity and uniformity of fluorescent material of white LEDs and the manufacture technology of fluorescent material, the above mentioned difficulties can be solved. As noted above, while increasing exerting power, we must try to reduce thermal resistance, improve heat dissipation issue, the specific content is: reducing thermal resistance from LED chip to package, inhibiting thermal resistance from package to the printed circuit board, improving cooling smoothness of LED chips.

In order to reduce the thermal resistance, many foreign LED manufacturers locate LEDs in the surface of the heat sink which is made from copper and ceramic material, and then use metal wire to connect the thermal control system of printed circuit board to the heat sink which uses cooling fan to achieve forced air cooling through welding, according to the German OSRAM Opto Semiconductors Gmb experiment results, the thermal resistance from LEDs with the above mentioned structure to the soldering point can reduce 9K/W, which is only 1/6 as the traditional LED’s, when imposing 2W power to the packaged LEDs, LED chip junction temperature is 18K higher than the temperature of the soldering point, even if the printed circuit board temperature rises to 500 degree Celsius, junction temperature is at most 700 degree Celsius; in contrast, in the past once the thermal resistance reduces, LED chip junction temperature will be affected by the printed circuit board temperature, therefore we must try to reduce the temperature of LED chips, in other words reducing the thermal resistance of LED chip to the the same level as sodering point can effectively reduce the burden of LED chip cooling operation. Conversely, even if the white LED has the structure which can inhibit the thermal resistance, once the heat can not be conducted from the package to the printed circuit board, temperature rise of LEDs will cause the rapid decrease of luminous efficiency, so Panasonic Electric developed the integration technology of printed circuit board and package, the company packaged 1mm square blue LED chip on a ceramic substrate by the way of flip chip, then stuck the ceramic substrate onto the surface of the copper printed circuit board, the thermal resistance of the entire module including printed circuit board is around 15K/W.

LED manufacturers demonstrate their thermal design capability

Since the adhesion between heat sink and printed circuit board directly impacts the heat conduction effects, the printed circuit board design becomes very complicated, under such circumstance, lighting device and LED package manufacturers such as Lumileds in the United States and CITIZEN in Japan, have developed simple cooling technology for high-power LEDs, CITIZEN’s white LED package which started sample shipments in 2004 can emit the heat of heat sink with a thickness of 2-3mm out requiring no special bonding technique, although the thermal resistance of 30K/W from LED chip junction to the heat sink is higher than the OSRAM’s 9K/W, and under normal environment the room temperature will increase the thermal resistance by about 1W, however even under the condition that traditional printed circuit board has no cooling fan to achieve forced air cooling, the white LED module can be lit continuously.

The high power LED chips that Lumileds began sample shipments in 2005 allow a junction temperature as high as +1850 degree celsius, which is 600 degree Celsius higher than the same level products of other companies, when using traditional RF4 printed circuit board to package, ambient temperature range within 400 degree Celsius we can input the current equivalent to 1.5W power. So Lumileds and CITIZEN utilized the method of improving allowable junction temperature, while OSRAM located LED chips in the heat sink surface, and reached 9K/W ultra-low thermal impedance record, the record is 40% less than the thermal resistance of same level products developed in the past, it is worth mentioning that when packaging the LED module, they use the method of flip chip which is the same as traditional method, however when bonding LED module and heat sink, they choose the surface which is nearest to light-emitting layer of LED chips as the joint face, which can conduct and emit the heat of light-emitting layer in the shortest distance.

In 2003, Toshiba Lighting ever built low thermal resistance white LEDs with a luminous efficacy of 60lm/W on the surface of 400mm square aluminum alloy, there were no special cooling components such as cooling fan, they tried to make LED module with a light beam of 300lm, due to rich trial manufacturing experience of Toshiba Lighting, the company indicated that as a result of advances in simulation analysis technology, since 2006 the white LEDs with luminous efficacy of more than 60lm/W, can easily use light fittings, framework to improve thermal conductivity, or use cooling fan to achieve forced air and design the thermal control of lighting devices, the module structure without special heat dissipation technology can also use white LEDs.

Change package materials to inhibit material deterioration and the decreasing speed of light transmittance

As for the longevity of LEDs, LED manufacturers now utilize the method of changing package materials, meanwhile distribute fluorescent material within the package materials, especially silicon package material can more effectively inhibit material deterioration and decrease speed of light transmittance than the traditional blue and near-UV LED chip package material-epoxy resin material. Since the absorption percentage of epoxy resin to light with wavelength of 400 ~ 450nm light is as high as 45%, which of silicon package material is as low as 1%, the time for epoxy resin to degrade LED brightness by 50% is less than ten thousand hours, while silicon material can extend the time to around forty thousand hours, which is almost the same as the designed lifetime of lighting devices, which means there is need to replace white LEDs during the using period of lighting devices. However, silicon resin material is a kind of high elastic soft materials, when processing we must use the production technology which will not scratch the surface of the silicon resin, in addition during processing silicon resin is easy to attach dust, so in the future technology of improving the surface property must be developed.

Although silicon package material can ensure forty thousand hours lifetime of LEDs, the lighting device manufacturers in the industry have different views, the main argument is that the lifetime of traditional incandescent and fluorescent lamps is defined as “the brightness degrading by less than 30%”, the time for LEDs to degrade brightness by 50% is around forty thousand hours, if it is converted into the time to degrade brightness by 30%, it will be only about twenty thousand hours. There are two measures currently to extend the service life of components, inhibiting the overall temperature rise of white LEDs and stopping using the resin package method.

It is believed that if the above two life extension measures can be fully implemented, the time for LEDs to degrade brightness by 30% can achieve forty thousand hours. We can utilize the method of cooling LED package printed circuit board to inhibit white LED temperature rise, which is mainly because that package resin under high temperature and strong light will quickly deteriorate, when the temperature decreases by 100 degree Celsius, the lifetime of LEDs will extend to 2 times as original life. Stopping using resin package  can completely eliminate deterioration factor, because the light generated by LEDs will be reflected within the packaging resin, if we use resin material reflector which can change the light travelling direction of LED chip side, the reflector will absorb light, so the quantity of light extraction will rapidly decrease, which is also the reason that LED manufacturers are more willing to utilize ceramic and metal package materials.

 

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