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Abstract
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.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Display technology has seen widespread application in people's daily lives over the past decade, particularly in consumer electronics such as smartwatches, laptops, and home televisions. At the same time, there has been a gradual increase in the demand for outdoor advertising displays and industrial displays for commercial purposes. These types of displays have different requirements compared to those used in consumer electronics, as they need to achieve higher brightness levels while covering large areas and maintaining high uniformity and color saturation. Emerging micro light-emitting diode (micro-LED) displays are considered the next-generation ultimate displays due to their advantages, including low power consumption, high contrast ratio, and fast response time [1–4]. However, the commercial viability of micro-LED displays is still hindered by yield and cost issues [5–8]. Additionally, their high resolution makes them more suitable for relatively small-sized applications such as augmented reality (AR), virtual reality (VR), and mixed reality (MR) displays rather than for large outdoor and highly demanding environments [9]. Moreover, the dense arrangement of LED light sources in direct-lit designs can lead to heat dissipation issues [10–12]. Extensive research has been conducted in previous literature to improve uniformity and color gamut, reduce the number of mini-LED light sources, and enhance the optical coupling efficiency of backlight modules. Various studies focusing on edge-lit backlights have aimed to enhance brightness and uniformity. For instance, Z. T. Ye et al. proposed a design with ZOD mini-LED backlight with LGM lens for extra-thin, LCDs [13].
Moreover, G. J. Lee et al. achieved a high color conversion efficiency of QD films by designing a side-lit backlight module that used a diffuser instead of a mirror reflector [14]. S. Komura et al. reduced light source scattering and achieved high uniformity and a wide color gamut in a 17-inch side-lit laser backlight module through a dual light guide plate design [15]. The aforementioned studies have made significant contributions to the development of direct-lit backlight designs, with a focus on improving uniformity and reducing the number of LEDs. Z. He et al. proposed the use of birefringent light-shaping films (BLSFs) in direct-lit mini-LED displays, achieving a reduction in the number of LEDs and improved uniformity by scattering the light emission angles using a passive polymer-dispersed liquid crystal (PDLC) thin film [16]. Y. L. Chen et al. utilized a micro-lens array to enhance the emission angle of mini-LEDs, resulting in a highly uniform design in a 17-inch backlight module (BLM) combined with QDs, achieving a measured average brightness of up to 17,574 cd/m2 [17]. L. Xu et al. presented a first-order optical design for mini-LEDs that incorporates a semi-transparent layer, achieving a bat-wing light field distribution for a wide viewing angle and enhanced uniformity [18]. C. H. Huang et al. introduced chip-level packaged mini-LED arrays with free-form surfaces, exhibiting a bat-wing light field distribution, thus reducing the required number of backlight sources and achieving high light extraction efficiency [19]. Z. T. Ye et al. proposed mini-LED arrays with 3D diffuse reflection cavity arrays (DRCAs). Particularly, the authors adjusted the curvature to manipulate the light path and reduced the mixing distance within the backlight module, resulting in a design with high brightness and uniformity [20].
W. H. Tseng et al. developed a secondary optical design using stearic acid to control the illumination angle and range through curved surfaces, resulting in a wide-angle bat-wing light distribution [21]. Several noteworthy studies have been conducted in the field of high-brightness displays. W.C. Miao et al. proposed the utilization of mini-LEDs in the backlight module of VR displays, which employed multiple scanning passive matrix (PM) driving techniques to enhance the display brightness of the liquid crystal module (LCM) up to 1200 cd/m2 [4]. H. W. Chen et al. introduced the Sony 75-inch X940E LCD TV, achieving a peak luminance of 1200 cd/m2 under typical home indoor lighting conditions of 60 lux [7]. Z. Qin et al. developed a free-form surface lens that effectively deflects light rays, resulting in improved optical extraction efficiency, enabling a brightness enhancement of the BLM module up to 15,234 cd/m2 [10].
Z.Y. Yang et al. designed a 15.6-inch mini-LED LCM module specifically for laptop computers, achieving a peak brightness of 997 cd/m2, as demonstrated by photometric measurements [22]. J. Nishimura et al. proposed an 8 K BLM module with a size of 31.5 inches, leveraging IGZO-TFT technology to achieve high resolution, low power consumption, and a peak luminance of 10,000 cd/m2 [23]. H. Akimoto et al. developed a micro-LED BLM module for a 15.6-inch display incorporating a light guide plate with reflectors, resulting in high brightness and contrast, with a measured peak luminance of 5000 cd/m2 [24].
High temperatures in LED packaging can adversely affect chip performance and the aging of packaging materials, leading to reduced reliability. Therefore, effective thermal management is crucial for ensuring chip reliability, performance, maximum optical power output, luminous efficiency, and lifespan [25–27]. Traditional EMC packaging exhibits thermal resistance values ranging from 32 K/W for low-power LEDs to 9.9 K/W for high-power LEDs, which is primarily due to poor vertical heat transfer coefficients and high interface resistances. In a comparative study, C. Chen et al. observed that traditional packaging resulted in spectral redshift and degradation of optoelectronic performance at high temperatures. In contrast, chip-scale package (CSP) white LEDs with shorter heat dissipation paths, ceramic substrates, and optimized conductive region dimensions achieved a thermal resistance of 3.3 K/W, thus improving chip performance and heat dissipation [28]. M. Azarifar et al. proposed an immersion cooling design at the packaging level using liquid cooling to reduce thermal resistance in LED chips and phosphors, resulting in a 15% reduction in LED packaging thermal resistance and enhanced light extraction and electro-optical conversion efficiency [29]. T. Luo et al. introduced a plated copper-on-thick film (PCTF) structure to enhance the optoelectronic performance of high-power LEDs, reducing total thermal resistance by 17.45% compared to thick printed ceramic substrates (TPC) and improving light output power [30]. Based on the aforementioned discussion, various research methods have been proposed to achieve large-area, high uniformity, and a reduced number of light sources. These methods involve primary and secondary optical designs to improve light emission angles and analyze the impact of different materials on light scattering. However, existing studies have not presented a solution that fulfills the requirements of large-area, ultra-high luminance for outdoor advertising displays and industrial displays, while also providing high color saturation and considering low packaging thermal resistance without the use of secondary optical components in a planar light source.
Therefore, our study sought to optimize the mini-LED packaging structure through primary optical design. The LED chip bonding process employs flip-chip technology, directly combining with the substrate through a metal eutectic process, aiming to achieve low packaging thermal resistance and wide-angle light distribution. The total thermal resistance of the packaging was 6.7 K/W. When operating each lamp board at 20.5 V and 5.5 A, the average luminance of the backlight module (BLM) can reach 36,310.4 cd/m2, exhibiting a uniformity of 91.03% and a uniformity merit function (UMF) of 1.53. Compared to previous studies, lower thermal resistance, thinner module thickness, and higher luminance can be achieved. The design parameters meet the required specifications for outdoor advertising displays and industrial displays.
2. Methods
This paper introduces a primary optical design that optimizes the packaging structure to achieve a WHS wide-angle light emission pattern. The packaging structure includes a diffuse layer (DL) and a window layer (WL) to enable the design of wide-angle mini-LEDs (WA mini-LEDs). The diffuse layer allows a portion of the light to transmit and reflect towards the window layer (WL), which then facilitates lateral emission, resulting in a broader light emission angle while maintaining high light output efficiency.
In Eq. (1), the variable ni represents the refractive index of the WL, whereas no represents the refractive index of the surrounding medium. When the width of the window layer is fixed, the thickness of the window layer (WLH) affects the area of total internal reflection (TIR), as illustrated by the magenta region in Fig. 1(a). TIR occurs when the incident angle exceeds this region. Our study defines the width of this region as the TIR region width (Tx), which can be calculated using Eq. (1).
Fig. 1. Schematic of the packaging and module structures. (a) Schematic of the WHS mini-LED packaging 3D structure. (b) Schematic of the WHS mini-LED packaging 2D structure.
In this study, the WHS mini-LEDs utilized GaN-based flip-chip with a wavelength of 455 nm. The packaging structure of the WHS mini-LEDs is shown in Fig. 1(b) and consists of the flip-chip, the DL, and the WL. The WL enables lateral light emission, providing control over optical efficiency and the light emission pattern. On the other hand, the DL is used to regulate the balance between transmitted and reflected light, thus allowing for adjustments to the light emission pattern. The reflectivity of the diffuse layer plays a crucial role in influencing the percentage of light emitted from the front. Increased reflectivity or greater thickness leads to a higher amount of light being emitted to the rear. The window layer consists of transparent and highly transmissive silicone. This layer serves to facilitate Total Internal Reflection (TIR) or refraction of light. A thicker window layer makes it easier for light to emerge in the forward direction. While this results in a smaller Full Width at Half Maximum (FWHM) for the relative light, the light extraction efficiency is improved.
The schematic diagram shows the uniformity factor, with the pitch representing the spacing between each mini-LED and the optical distance (OD) representing the distance from the light board surface to the diffusion plate. The uniformity factor is calculated using Eq. (2) to evaluate the module thickness and the uniformity of the planar light source. Uniformity is determined by dividing the minimum luminance by the maximum luminance.
The uniformity merit function (UMF) formula (Eq. (2)) is defined below:
3. Results and discussion
In the same backlight area, increasing peak intensity angles and reducing the central intensity can help reduce the number of light sources used. A 2D structure of the WHS mini-LEDs was designed using the SolidWorks 2019 software. The overall package size of the mini-LEDs was 1700µm × 1700µm, with a height of 400 µm. The dimensions of the flip chip (FS) are 1030 µm × 1030 µm, and the chip thickness (FH) is 150 µm. In Fig. 2(a), it is shown that by optimizing the light extraction efficiency (LEE) of the WL and DL to be 0.3 and 0.1, respectively, better results can be achieved. The luminous intensity distribution curve of WHS mini-LEDs is depicted in Fig. 2(b), revealing peak intensity angles of 48° and a FWHM angle of 168°. Compared to traditional lambertian light sources (black line), WHS mini-LEDs (blue line) offer a broader irradiation range and achieve lower central intensity.
Fig. 2. Optimizing the WHS mini-LED packaging structure, (a) Optimizing the light extraction efficiency of the WL and DL. (b) Illustrates the luminous intensity distribution curve of WHS mini-LEDs.
The large-area, ultra-high luminance planar light source was designed using the 3D modeling software SolidWorks 2019 (Dassault Systèmes, Vélizy-Villacoublay, France) and the SPEOS 2020 optical simulation software (Ansys, Canonsburg, Pennsylvania, USA) for optimization. The light source module comprises various components, including the lightbox, WHS mini-LEDs, light board, QD film, diffusion plate, and light collecting film. The width (WB) and length (LB) of the lightbox are 680 mm and 1210 mm respectively, and the overall thickness is 30 mm. The backlight source consists of 8 light boards, each with dimensions of 302 mm × 338 mm and a thickness of 1.5 mm. The light boards were placed at the bottom of the lightbox. Each light board was composed of 7 mini-LEDs on the short side and 8 mini-LEDs on the long side, totaling 56 mini-LEDs. We performed simulations to optimize different mini-LED pitch designs to identify the design with the best uniformity.
Simulations were conducted on the 55-inch WHS mini-LEDs large-area light source module using the Ansys SPEOS 2020 software. We simulated different LED pitch sizes to determine the optimal spacing arrangement. The parameters were set as follows: input power of 1 W, center wavelength of 455 nm, and 50,000,000 rays. The materials of the lightbox and light board in the light source module were modeled as Lambertian diffuse surfaces with a reflectance of 90%. The QD film was also modeled as a Lambertian diffuse surface with a reflectance and transmittance of 50%. The QD film used in the simulations was provided by Wah Hong Corporation (Wah Hong Industrial Corporation, Kaohsiung City, Taiwan). The film had a thickness of 150 µm and was suitable for excitation wavelengths ranging from 448 to 455 nm. The green QDs had a peak wavelength of 535.5 nm and a full width at half maximum (FWHM) of 23 nm, whereas the red QDs had a peak wavelength of 630 nm and an FWHM of 23 nm. The refractive index of the brightness enhancement film (BEF) was set to 1.56.
Simulations were conducted using optical software to analyze the optical light efficiency (LE) of the 55-inch WHS mini-LED light source module with different LED pitches. The results are presented in Fig. 3(a). The LED pitches were set as 39 mm, 41 mm, 43 mm, and 45 mm, corresponding to LE values of 82.1%, 82.1%, 82.0%, and 81.7%, respectively. Interestingly, the different pitch arrangements had a negligible impact on the light efficiency. Figure 3(b) shows the simulation results for uniformity (UM). Using a 9-point uniformity calculation, the values for the different LED pitches (39 mm, 41 mm, 43 mm, and 45 mm) were 79.6%, 86.1%, 93.1%, and 85.7%, respectively.
Fig. 3. Simulation results. (a) Optical light efficiency (LE) of a 55-inch WHS mini-LED light source module with different pitches. (b) Simulated distribution of uniformity for a 55-inch WHS mini-LED light source module with different pitches.
To validate the results obtained from optical simulations, we conducted measurements and analysis on WHS mini-LED samples and integrated them into an optical backlight module to create physical prototypes. Figure 4(a) presents the electroluminescence (EL) spectrum of WHS mini-LEDs obtained through actual measurements. Chip-scale packaged WHS mini-LEDs were used as the backlight source for the large-area, high-luminance light source module, with a peak wavelength of 455 nm. Figure 4(b) displays the distribution of luminous intensity for WHS mini-LEDs, with a measured peak angle of 52° and an FWHM of 164°. The light distribution exhibited a characteristic wide-heart shape. Figures 4(c)-(d) show the 3D polar distribution and irradiance plot, with Figs. 4(c) representing WHS mini-LEDs and Fig. 4(d) representing Lambertian LEDs. From the results, it can be observed that the illumination range of WHS mini-LEDs is 1.45 times that of Lambertian LEDs.
Fig. 4. Experimental characterization of the light emitted by the WHS mini-LED BLM. (a) Measured spectrum of the WHS mini-LED electroluminescence. (b) Distribution of luminous intensity. (c) Show the 3D polar distribution and, (d) Irradiance plot.
Figure 5(a) shows the luminance-current-voltage (LIV) curve of WHS mini-LEDs, revealing a driving voltage of 3.15 V and an output radiant flux of 1.01 W at a current of 1.2 A. Figure 5(b) presents the layered thermal resistance values of WHS mini-LEDs at 300 mA using the T3STER Thermal Transient Tester. The layered thermal resistances were determined by analyzing the differential curve and differentiating the LED-to-chip junction, die-attach, and substrate layers. The respective thermal resistance values were 0.5 K/W, 2.2 K/W, and 4.0 K/W, whereas the total packaging thermal resistance was 6.7 K/W. The packaging thermal resistance can be reduced by employing a metal eutectic process to directly bond the LED chips to an aluminum substrate. Figure 5(c) illustrates the 55-inch WHS mini-LED backlight module, which consists of 448 WHS mini-LEDs with a 43 mm pitch. Figure 5(d) presents the actual illumination image of the 55-inch WHS mini-LED backlight module. We employ a luminance meter (Topcon BM-7) utilizing a multi-point method (9 points) for measurements. The lightbox dimensions include a width (WB) of 680 mm, a length (LB) of 1210 mm, and an overall thickness of 30 mm. The active area (AA) is represented by the red box, serving as both the measurement range and the active area. The measurement range is defined as 0.9 times LB and 0.9 times WB, ensuring a uniformity of 90% within the AA zone. The uniformity across measurement points, encompassing the four corners C1-4, is recorded at 82%.
Fig. 5. Experimental characterization of the light emitted by the WHS mini-LED BLM. (a) Luminance-Current-Voltage curve. (b) Analysis of the layered thermal resistance of the WHS mini-LEDs’ packaging. (c) 55-inch WHS mini-LED light source module. (d) Actual illumination image of the 55-inch WHS mini-LED BLM.
Table 1 provides the measurement results of luminance and CIE x, y color coordinates for the direct emission of the 55-inch WHS mini-LED backlight module. The uniformity was calculated using from the minimum luminance value of 34,615 cd/m2 was divided by the maximum luminance value of 38,024 cd/m2, resulting in a uniformity value of 91%. The average luminance measurement of BLM was 36,310 cd/m2, with CIE x coordinates of 0.3077 and CIE y coordinates of 0.2626 and the average luminance measurement of LCM was 2234 cd/m2, with CIE x coordinates of 0.3096 and CIE y coordinates of 0.2930.
Table 1. Measurements of luminance, uniformity, and CIE x, y color coordinates for 55-inch WHS mini-LED BLM and LCM.
Measurements and demonstrations were conducted to analyze the optical characteristics and examine the performance of the actual samples. Figure 6(a) illustrates the actual illumination image of the 55-inch WHS mini-LED LCD module. The sample image depicts the light source module with optical films, liquid crystals, color conversion layers, and surface glass. As illustrated in Fig. 6(b), the 55-inch WHS mini-LED LCD module in this study achieved a wide color gamut, surpassing 119.3% of the NTSC 1931 color gamut. This was indicative of high color saturation and advanced display design. Figure 6(c) displays the EL spectrum of the 55-inch WHS mini-LED LCD module combined with QDs film. The peak emission wavelengths for red, green, and blue were 455, 535.5, and 630 nm, respectively, with an FWHM of 23 nm. These results highlight the advantage of combining WHS mini-LEDs with QD film for large-area light sources, providing high color purity and enabling designs with high brightness and uniformity.
Fig. 6. Experimental results of WHS mini-LED LCM. (a) Actual illumination image of the 55-inch WHS mini-LED liquid crystal module (LCM). (b) CIE 1931 color space. (c) Electroluminescence (EL) spectrum.
4. Conclusions
Outdoor displays require high brightness for visibility in bright environments. In harsh conditions, it is not suitable to apply secondary optical lenses to adjust the emission angle of the light source, as it may lead to detachment issues and require precise alignment. We propose the use of a WHS mini-LED packaging structure to achieve a wide-angle light distribution in a 55-inch direct-lit backlight module. This approach aims to improve the light field angle, reduce the number of light sources, and address heat dissipation issues caused by high density. Our experimental results demonstrated that the WHS mini-LED packaging had a total thermal resistance of 6.7 K/W. The 55-inch BLM requires only 448 WHS mini-LEDs, combined with QD film and a sheet of BEF to achieve an average luminance of 36,310.4 cd/m2 with a uniformity of 91%. The uniformity merit function (UMF) reached a value of 1.53 and the average luminance of the liquid crystal module (LCM) was 2,234 cd/m2 with a uniformity rate of 90%. The NTSC color gamut coverage was 119.3%. Collectively, our findings hold potential for applications in outdoor advertising and industrial display panels that require large areas, high brightness, and high color saturation, compared to previous studies, lower thermal resistance, thinner module thickness, and higher luminance can be achieved. The design parameters meet the required specifications for outdoor advertising displays and industrial displays. Thus offering a competitive advantage over commercially available alternatives.
Funding
National Science and Technology Council (NTSC 112-2622-E-194-004).
Acknowledgment
This work was supported by the National Science and Technology Council of Taiwan (NTSC 112-2622-E-194-004).
Disclosures
The authors have no conflicts to disclose
Authorship contribution. Zhi Ting Ye and Chia Chun Hu are responsible for the structure and conception of the entire article. Zhi Ting Ye, Zheng Yang Jun, and Chia Chun Hu is responsible for the simulation data of the article. Zhi Ting Ye, Yu Fu Hsu, and Zheng Yang Jun is responsible for the initial writing of the manuscript.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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