The principle of LED packaging to enhance the luminous flux

Introduction: There is no doubt that the world needs high brightness light-emitting diodes (HB LEDs), not only high brightness white light LEDs (HB WLEDs), but also high brightness LEDs of various colors. In the future, there will be an active effort and need for ultra-high brightness LEDs (Ultra High Brightness LEDs, referred to as UHD LEDs).

Replacing the original EL backlight and CCFL backlight of handheld devices with LED backlight not only makes the circuit design simpler and easier, but also has higher resistance to external forces. Replacing the original CCFL backlight of LCD TVs with LED backlight is not only more environmentally friendly but also more vivid and bright. Replacing white light lamps, halogen lamps and other lighting with LED lighting is not only brighter and more energy-saving, but also has a longer service life and faster lighting response. When used as brake lights, it can reduce the rear-end collision rate.

Therefore, LED has evolved from being only used as status indicator lights in electronic devices to becoming the backlight of liquid crystal displays, and then expanded to electronic lighting and public displays, such as car lights, traffic lights, billboard message marquees, large video walls, and even lighting inside projectors. Its applications are still continuing to expand.

More importantly, the brightness efficiency of LEDs doubles every 24 months, just like Moore's Law. In the past, it was believed that white light LEDs could only be used to replace incandescent lamps and halogen lamps that consume too much power, that is, the luminous efficiency was within 1030lm/W. However, after white light LEDs broke through 60lm/W and even reached 100lm/W, even fluorescent lamps and high-pressure gas discharge lamps began to feel threatened.

Although LEDs continue to increase in brightness and luminous efficiency, in addition to the core patented technologies such as phosphor quality and light mixing, packaging will also be an increasingly greater challenge, and it is a double challenge. On the one hand, packaging must allow LEDs to have the maximum light absorption rate and the highest luminous flux to minimize light loss. At the same time, attention must also be paid to the light divergence angle, light uniformity, and compatibility with the light guide plate.

另一方面，封装必须让LED有最佳的散热性，特别是HB（高亮度）几乎意味著HP（High Power，高功率、高用电），进出LED的电流值持续在增大，倘若不能良善散热，则不仅会使LED的亮度减弱，还会缩短LED的使用寿命。

Therefore, if the packaging technology used in the continued pursuit of high-brightness LEDs is not enhanced accordingly, the high-brightness performance will be compromised. Therefore, this article will discuss the packaging technology of HB LEDs in more detail, including discussions on light flux and thermal conductivity.

Note 1: Generally speaking, HB LED refers to a luminous efficacy of 8 lm/W (8 lumens per watt) or above.

Note 2: Generally speaking, HP LED refers to LEDs that consume more than 1W (watt). The power consumption is calculated by multiplying the forward voltage by the forward current (Vf×If, f=forward).

■ Bare crystal layer: "Quantum well, multi-quantum well" improves "light conversion efficiency"

Although this article mainly discusses the enhancement of luminous flux by LED packaging, we have to first explain the deeper core of the bare die part. After all, the improvement of the bare die structure can also greatly increase the luminous flux.

The first is to enhance the light-to-energy conversion efficiency, which is also the most fundamental approach. Of every watt of electricity used by existing LEDs, only 15% to 20% is converted into light energy, and the rest is converted into heat energy and dissipated (waste heat). The key to improving this conversion efficiency is on the pn junction, which is the main light-emitting and heat-generating location of the LED. The conversion efficiency can be improved by changing the structural design of the pn junction.