Improving heat dissipation structure to increase the service life of white light LEDs

过去LED 业者为了获得充分的白光LED 光束,曾经开发大尺寸LED芯片 试图藉此方式达到预期目标。不过,实际上白光LED的施加电力持续超过1W以上时光束反而会下降,发光效率相对降低20～30%。换句话说,白光LED的亮度如果要比传统LED大数倍,消耗电力特性超越荧光灯的话,就必需克服下列四大课题：抑制温升、确保使用寿命、改善发光效率,以及发光特性均等化。

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 small chips; the way to improve the luminous efficiency of the LED is to improve the chip structure and use small chips; as for the way to uniformize the luminous characteristics, it is to improve the LED packaging method. These methods have been developed one after another.

Solving the heat dissipation problem of packaging is the fundamental solution

由于增加电力反而会造成封装的热阻抗急剧降至10K/W以下,因此国外业者曾经开发耐高温白光LED,试图藉此改善上述问题。然而,实际上大功率LED 的发热量比小功率 LED高数十倍以上,而且温升还会使发光效率大幅下跌。即使封装技术允许高热量,不过LED芯片的接合温度却有可能超过容许值,最后业者终于领悟到解决封装的散热问题才是根本方法。

Regarding the service life of LEDs, for example, the use of silicon packaging materials and ceramic packaging materials can increase the service life of LEDs by single digits, especially since the emission spectrum of white light LEDs contains 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 light LEDs accelerates the degradation of packaging materials. According to industry test results, the brightness of high-power white light LEDs has been reduced by more than half if the lighting is continuously turned on for less than 10,000 hours, which cannot meet the basic requirement of long service life of lighting sources.

Regarding the luminous efficiency of LEDs, improving the chip structure and packaging structure can achieve the same level as low-power white light LEDs. The main reason is that when the current density is increased by more than 2 times, it is not only difficult to extract light from large chips, but the result is that the luminous efficiency is not as good as low-power white light LEDs. If the electrode structure of the chip is improved, the above light extraction problem can be solved in theory.

Try to reduce thermal impedance and improve heat dissipation

Regarding the uniformity of luminous properties, it is generally believed that the above problems can be overcome by improving the uniformity of the concentration of the phosphor material of the white light LED and the manufacturing technology of the phosphor. As mentioned above, while increasing the applied power, it is necessary to find ways to reduce thermal impedance and improve heat dissipation. The specific contents are: reducing the thermal impedance from the chip to the package, suppressing the thermal impedance from the package to the printed circuit board, and improving the heat dissipation smoothness of the chip.

In order to reduce thermal impedance, many foreign LED manufacturers place LED chips on the surface of heat sinks made of copper and ceramic materials, and then use welding to connect the heat dissipation wires of the printed circuit board to the heat sink that uses a cooling fan to force air cooling. According to the experimental results of OSRAM Opto Semiconductors Gmb in Germany, the thermal impedance of the LED chip to the welding point of the above structure can be reduced by 9K/W, which is about 1/6 of the traditional LED. When 2W of power is applied to the packaged LED, the junction temperature of the LED chip is 18K higher than the welding 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. Therefore, 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 welding point can effectively reduce the burden of the cooling effect of the LED chip. On the other hand, even if the white light LED has a structure that suppresses thermal impedance, if the heat cannot be transferred from the package to the printed circuit board, the result of the LED temperature rise will still cause the luminous efficiency to drop sharply. Therefore, Panasonic Electric has developed a technology to integrate printed circuit boards and packaging. The company packaged 1mm square blue light LEDs on a ceramic substrate in a flip chip manner, and then pasted the ceramic substrate on the surface of a copper printed circuit board. According to Panasonic, the thermal impedance of the entire module including the printed circuit board is about 15K/W.

Various industry players demonstrate their thermal design skills

Since the compactness between the heat sink and the printed circuit board directly affects the heat conduction effect, the design of the printed circuit board becomes very complicated. In view of this, lighting equipment and LED packaging manufacturers such as Lumileds in the United States and CITIZEN in Japan have successively developed simple heat dissipation technology for high-power LEDs. CITIZEN began to manufacture white light LED sample packaging in 2004, which can directly discharge the heat of the heat sink of about 2 to 3mm thick to the outside without special bonding technology. According to the CITIZEN report, although the 30K/W thermal impedance from the junction of the LED chip to the heat sink is larger than the 9K/W of OSRAM, and the room temperature in a general environment will increase the thermal impedance by about 1W, even if the traditional printed circuit board is forced to cool without a cooling fan, the white light LED module can be used continuously.

Lumileds于2005年开始制造的高功率LED芯片,接合容许温度更高达+185℃,比其它公司同级产品高60℃,利用传统RF 4印刷电路板封装时,周围环境温度40℃范围内可以输入相当于1.5W电力的电流(大约是400mA)。所以Lumileds与CITIZEN是采取提高接合点容许温度,德国OSRAM公司则是将LED芯片设置在散热器表面,达到9K/W超低热阻抗记录,该记录比OSRAM过去开发同级产品的热阻抗减少40%。值得一提的是该LED模块 封装时,采用与传统方法相同的flip chip方式,不过LED模块与散热器接合时,则选择最接近LED芯片发光层作为接合面,藉此使发光层的热量能够以最短距离传导排放。

In 2003, Toshiba Lighting once laid white light LEDs with a luminous efficiency of 60lm/W and low thermal impedance on a 400mm square aluminum alloy surface, and trial-produced LED modules with a beam of 300lm without special heat dissipation components such as cooling fans. As Toshiba Lighting has rich trial production experience, the company said that due to the advancement of simulation analysis technology, white light LEDs with a beam of more than 60lm/W after 2006 can easily use lamps and frames to improve thermal conductivity, or use cooling fans to force air cooling to design the heat dissipation of lighting equipment, and module structures that do not require special heat dissipation technology can also use white light LEDs.

Changing the packaging material to reduce the rate of material degradation and light transmittance reduction

Regarding the longevity of LEDs, the current countermeasures taken by LED manufacturers are to change the packaging materials and disperse the fluorescent materials in the packaging materials. In particular, silicon packaging materials can more effectively suppress the degradation of materials and the speed of light transmittance reduction than the epoxy resin packaging materials on the traditional blue light and near-ultraviolet light LED chips. Since 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 1%, the time for epoxy resin to halve the brightness is less than 10,000 hours, while the silicon packaging material can be extended to about 40,000 hours, which is almost the same as the design life of the lighting equipment. This means that the white light LED does not need to be replaced during the use of the lighting equipment. However, silicon resin is a highly elastic and soft material, and the processing must use a manufacturing technology that will not scratch the surface of the silicon resin. In addition, silicon resin is very easy to adhere to powder during processing, so it is necessary to develop technology that can improve surface properties in the future.

Although silicon packaging materials can ensure the service life of LEDs for 40,000 hours, lighting equipment manufacturers have different opinions. The main debate is that the service life of traditional incandescent lamps and fluorescent lamps is defined as "the brightness drops below 30%. The time for LEDs to halve their brightness is 40,000 hours. If the brightness drops below 30%, there are only about 20,000 hours left. There are currently two countermeasures to extend the service life of components, namely, suppressing the overall temperature rise of white light LEDs and stopping the use of resin packaging.

It is generally believed that if the above two life extension measures are thoroughly implemented, the requirement of 40,000 hours at 30% brightness can be achieved. The temperature rise of white light LEDs can be suppressed by cooling the LED package printed circuit board. The main reason is that the package resin will deteriorate rapidly when it is exposed to strong light under high temperature. According to the Arrhenius law, the life will be extended by 2 times if the temperature is reduced by 10°C. Stopping the use of resin packaging can completely eliminate the degradation factor, because the light generated by the LED is reflected in the package resin. If a resin reflector that can change the direction of light on the side of the chip is used, the reflector will absorb the light, causing the amount of light to drop sharply. This is also the main reason why LED manufacturers consistently use ceramic and metal packaging materials.