Packaged structure optimization for enhanced light extraction efficiency and reduced thermal resistance of ultraviolet B LEDs.

Ultraviolet B light-emitting diodes (UVB LEDs) hold promise in medical and agricultural applications. The commonly used sapphire substrate for their epitaxy growth possesses a high refractive index and excellent UV light absorption characteristics. However, this high refractive index can induce total internal reflection (TIR) within the substrate, leading to decreased Light Extraction Efficiency (LEE) due to light absorption within the material. In this study, UVB LED chips were detached from the sub-mount substrate and directly affixed onto an aluminum nitride (AlN) substrate with superior heat dissipation using a eutectic process. This was undertaken to diminish packaged thermal resistance (PTR). Simultaneously, optimization of the UVB LED packaging structure was employed to alleviate LEE losses caused by the TIR phenomenon, with the overarching goal of enhancin external quantum efficiency (EQE). The final experimental findings suggest that optimal LEE is achieved with packaging dimensions, including a length (ELL) of 2 mm, a width (ELW) of 1.62 mm, and a height (ELH) of 0.52 mm. At an input current of 200 mA, the output power reaches 50 mW, resulting in an EQE of 6.3%. Furthermore, the packaged thermal resistance from the chip to the substrate surface can be reduced to 4.615 K/W.

Full-angle chip scale package of mini LEDs with a V-shape packaging structure.

The light distribution of light-emitting diodes (LEDs) generally resembles that of a Lambertian light source. When used as large-area light sources, the light distribution angle of LEDs must be modified through secondary optics design to achieve uniformity and minimize the number of light sources. However, secondary optical components pose several challenges such as demanding alignment accuracy, material aging, detachment, and lower reliability. Therefore, this paper proposes a primary optical design approach to achieve full-angle emission in LEDs without the need for lenses. The design employs a flip-chip as the light source and incorporates a V-shaped packaged structure, including a white wall layer, optical structure layers, and a V-shaped diffuse structure. With this design, the LEDs achieve full-angle emission without relying on lenses. Our experimental results demonstrated a peak intensity angle of 77.7°, a 20.3% decrease in the intensity of the central point ratio, and a full width at half maximum (FWHM) of the light distribution of 175.5°. This design is particularly suitable for thin, large-area, and flexible backlight light sources. Moreover, the absence of secondary optical components allows for a thinner light source module.

Wide heart-shaped mini-LEDs without a second lens as a large area, ultra-high luminance, and flat light source.

In recent years, the demand for outdoor advertising and industrial display applications has been steadily increasing. Outdoor environments require higher brightness levels, thus requiring a reduction in the thermal resistance of the light source package. However, using secondary optical lenses to decrease the number of light sources is not a suitable solution because it may lead to the issue of lens detachment. Therefore, this paper proposes a packaging structure for wide heart-shaped angular light distribution mini-light emitting diodes (WHS mini-LEDs) with a primary optical design to enhance the light-emitting angle. The chips are directly bonded to an aluminum substrate using the metal eutectic process to minimize thermal resistance in the packaging. The experimental results indicated that the WHS mini-LED package had a total thermal resistance of 6.7 K/W. In a 55-inch backlight module (BLM), only 448 WHS mini-LEDs coupled with a quantum dot (QD) film and a brightness enhancement film (BEF) were required. Each lamp board was operated at 20.5 V and 5.5 A. The average luminance of the liquid crystal module (LCM) can reach 2234.2 cd/m2 with a uniformity of 90% and an NTSC value of 119.3%. This design offers a competitive advantage for outdoor advertising displays and industrial displays that require large areas, high brightness, and high color saturation.