Analysis of relevant ways to improve LED heat dissipation performance in the industry

Stopping the use of resin encapsulation can completely eliminate the deterioration factor, because the light generated by the LED is reflected in the encapsulated resin. If a resin reflector is used to change the direction of light from the side of the chip, the reflector will absorb the light, causing the light extraction to drop sharply. For this reason, it is necessary to find a way to reduce the temperature of the LED chip. In other words, reducing the thermal impedance from the LED chip to the soldering point can effectively reduce the burden of the LED chip's temperature drop.

Regarding the lifespan of LEDs, for example, the use of silicon packaging materials and ceramic packaging materials can increase the lifespan of LEDs by a single digit. In particular, the flash spectrum of white LEDs includes short-wavelength light with a wavelength below 450nm. Traditional epoxy resin packaging materials are easily damaged by short-wavelength light. The large amount of light from high-power white LEDs accelerates the degradation of packaging materials. According to the final results of industry tests, the brightness of high-power white LEDs has been reduced by more than half after less than 10,000 hours of continuous lighting, which is fundamentally unable to meet the basic requirements of long lifespan of lighting sources. So far, there are two countermeasures to extend the lifespan of components, namely, controlling the temperature rise of white LED groups and stopping the use of resin packaging.

However, the calorific value of high-power LED is dozens of times higher than that of low-power LED, and the temperature rise will also greatly reduce the flash rate. The specific internal significance is: reduce the thermal impedance from chip to package, limit the thermal impedance from package to printed circuit board, and increase the heat dissipation smoothness of chip.

Find ways to reduce thermal impedance and improve heat dissipation

The flash rate of related LEDs, improved chip structure and packaging structure can all reach the same level as low-power white light LEDs. In view of this, lighting facilities and LED packaging manufacturers such as Lumileds and CITIZEN in the United States have been developing simple heat dissipation technology for high-power LEDs one after another. CITIZEN started to produce white light LED sample packaging in 2004. Without special combination technology, the heat of the heat dissipation device with a thickness of about 2~3mm can be directly discharged to the outside. According to the CITIZEN report, although the 30K/W thermal impedance from the junction of the LED chip to the heat dissipation device is larger than the 9K/W of OSRAM, and the room temperature under normal background will increase the thermal impedance by about 1W, even if the traditional printed circuit board is forced to air-cool without cooling fans, the white light LED panel can continue to light up and use.

Regarding the uniformity of the special properties of the flash, it is generally believed that as long as the uniformity of the liquid concentration of the phosphor material of the white light LED and the manufacturing technology of the phosphor are improved, the above-mentioned difficulties and interferences should be overcome.

Because increasing the power will cause the thermal impedance of the package to drop rapidly to below 10K/W, this overseas company previously developed high-temperature resistant white light LEDs and intended to use this to improve the above-mentioned problems.

Although silicon packaging materials can guarantee the LED's 40,000-hour service life, lighting equipment manufacturers have expressed different opinions. The main debate is that the service life of traditional light bulbs and fluorescent lamps is defined as "the brightness drops below 30%." The time it takes for the brightness of an LED to be halved is 40,000 hours. If converted to a brightness drop below 30%, it will only be around 20,000 hours.

It is generally believed that if the above two life extension measures are fully implemented, the requirement of 40,000 hours at 30% brightness can be achieved. For this reason, Panasonic Electric has developed a technology for integrating printed circuit boards and packages. The company packages 1mm square blue light LEDs on ceramic substrates in the form of flip chips, and then adheres the ceramic substrates to the surface of copper printed circuit boards. According to Panasonic reports, the thermal impedance of the panel group including the printed circuit board is about 15K/W. Therefore, Lumileds and CITIZEN adopt the method of increasing the allowable temperature of the junction point, while the German OSRAM company sets the LED chip on the surface of the heat sink to achieve an ultra-low thermal impedance record of 9K/W, which is 40% lower than the thermal impedance of OSRAM's previous products of the same level. It is worth mentioning that when the LED panel is packaged, it is considered appropriate to use the same flip chip form as the traditional method. However, when the LED panel is combined with the heat sink, the flash layer closest to the LED chip is selected as the bonding surface, so that the heat of the flash layer can be discharged at the shortest distance.

In the past, in order to obtain sufficient white light LED beams, LED manufacturers developed large-size LED chips in order to achieve their expected goals. While increasing the power supply as above, it is necessary to find ways to reduce thermal impedance and improve heat dissipation. However, in fact, when the power supply of white light LEDs exceeds 1W, the beam will decrease, and the flash rate will be relatively reduced by 20~30%. In other words, if the brightness of white light LEDs is to be several times greater than that of traditional LEDs and the power consumption characteristics exceed those of fluorescent lamps, it is necessary to overcome the following four major issues: limiting temperature rise, ensuring the service life, improving the flash rate, and averaging the flash characteristics. On the other hand, even if the white light LED has a structure that limits thermal impedance, if the heat cannot be transferred from the package to the printed circuit board, the final result of the LED temperature rise will definitely cause the flash rate to drop rapidly.

Solving the heat dissipation problem of packaging is the fundamental solution

The solution to the temperature rise problem is to reduce the thermal impedance of the package; the way to maintain the service life of the LED is to improve the chip shape and use a small chip as appropriate; the way to improve the flash rate of the LED is to improve the chip structure and use a small chip as appropriate; as for the way to average the flash characteristics, it is to improve the LED packaging method, and these methods have been developed one after another. Because the percentage of epoxy resin absorbing light with a wavelength of 400~450nm is as high as 45%, while the silicon packaging material is less than 100%, the time for epoxy resin to halve the brightness is less than 10,000 hours, and the silicon packaging material can be extended to about 40,000 hours, which is almost the same as the expected service life of the lighting device, which means that the lighting device does not need to change the white light LED during its service life. However, silicon resin is a highly elastic and soft material, and the processing technology that will not scratch the surface of silicon resin must be used during processing. In addition, silicon resin is very easy to adhere to dust during processing, so it is necessary to develop technology that can improve the surface characteristics in the future.

To extend the life of related LEDs, the countermeasures taken by LED manufacturers so far are to change the packaging materials and disperse fluorescent materials inside the packaging materials. In particular, silicon packaging materials can be more effective in controlling the speed of material degradation and the reduction of light penetration rate than the epoxy natural resin packaging materials on traditional blue light and near-ultraviolet LED chips.

Changing packaging materials to limit material degradation and the rate at which light penetration decreases

In 2003, Toshiba Lighting tried to make a LED panel with a light beam of 300lm on a 400mm square aluminum alloy surface, with a low thermal impedance white light LED with a flash rate of 60lm/W, without special heat dissipation components such as cooling fans. The main reason is that when the current density increases by more than 2 times, it is not only difficult to extract light from the large chip, but the final result will lead to the dilemma of a flash rate that is not as good as a low-power white light LED. According to the final results of the experiment of OSRAM Opto Semiconductors Gmb in Germany, the thermal impedance of the LED chip to the soldering point of the above-mentioned structure can be reduced by 9K/W, which is about 1/6 of that of traditional LEDs. When the packaged LED is given 2W of power, the junction temperature of the LED chip is 18K higher than the soldering point. Even if the temperature of the printed circuit board rises to 50℃, the junction temperature is only about 70℃ at most; in contrast, once the thermal impedance is reduced, the junction temperature of the LED chip will be affected by the temperature of the printed circuit board. To control the temperature rise of white light LEDs, it is appropriate to use cooling methods for LED package printed circuit boards. The main reason is that the package natural resin will degrade rapidly under high temperature conditions and strong light reflection. According to the Arrhenius law, the life span will be extended by 2 times if the temperature decreases by 10℃.

Because the delicate precision between the heat sink and the printed circuit board directly affects the thermal conductivity, the design of this printed circuit board becomes very complicated.

In order to reduce thermal impedance, many overseas LED manufacturers place LED chips on the surface of heat sinks made of copper and ceramic materials, and then use soldering to connect the heat sink wires of the printed circuit board to the heat sink that uses cooling fans to force air cooling. Because Toshiba Lighting has a vast experience in trial manufacturing, the company said that due to the advancement of simulation technology, white light LEDs with more than 60lm/W can be easily used in lamps and frames to increase thermal conductivity, or use cooling fans to force air cooling to preset the heat dissipation of lighting facilities. White light LEDs can also be used without special heat dissipation technology.

Lumileds started to produce high-power LED chips in 2005, with a combined allowable temperature of up to +185℃, 60℃ higher than other companies' similar products. When using traditional RF 4 printed circuit board packaging, the input current equivalent to 1.5W power (about 400mA) can be within the surrounding background temperature of 40℃. This is also the main reason why LED manufacturers think it is appropriate to use ceramic and metal packaging materials. Even though the packaging technology allows high calories, the combined temperature of the LED chip may exceed the allowable value. In the end, the industry finally realized that solving the heat dissipation problem of the package is the fundamental solution.