Development of Deep Ultraviolet LED Packaging

Abstract

Deep UV LED package is a hotspot of growing concern for research scholars, it is through the LED semiconductor light-emitting devices emit UVC band ultraviolet (typical wavelength 260 ~ 280nm), is a new type of healthy artificial light source, compared with the traditional UV light source mercury lamps, the deep UV LED has a wavelength of accurate and controllable, green, and so on many advantages. Due to the high-energy deep UV radiation capability, it has a strong bactericidal and inactivation effect on bacteria, viruses and other microorganisms. In recent years, with the continuous progress of deep packaging technology, the optical efficiency and reliability of deep UV LEDs have been significantly improved. This paper summarizes the key technologies of deep-ultraviolet packaging, the performance of deep-ultraviolet LEDs and the application of deep-ultraviolet LEDs; through the understanding of the packaging materials, packaging structure, packaging process and so on to improve the performance of deep-ultraviolet LEDs, so that it is better to apply to the market, which can be seen that the deep-ultraviolet LEDs have a great prospect for development.

Overall, encapsulation technology and material selection for deep-ultraviolet light-emitting diodes (DUVs) are critical for improving light output power and optical performance. Amorphous fluoro resin film encapsulation and fluoro resin-filled encapsulation are two promising encapsulation methods to improve light extraction and optical performance. However, further research and improvements are still needed to solve the problems faced by fluorosis, such as decay and high cost.

2.2. Optimization of packaging materials

Fluoropolymers are a candidate encapsulation material where amorphous fluoropolymers have high UV transparency and UV resistance. However, pure fluoropolymers reduce the light extraction efficiency of DUV-LEDs. To improve the light extraction efficiency, the researchers proposed a new approach based on fluoropolymer encapsulation layers doped with aluminum nitride. And results were obtained to show that the method can indeed achieve improved light extraction efficiency and long-term optical stability of DUV-LED COB modules. [11]. Another researcher proposed an aluminum nitride doped silica-filled chip side (ASFCS) encapsulation method, which was experimentally shown to reduce the side light loss and improve the radiation efficiency of deep ultraviolet light emitting diodes (DUV-LEDs) [12].

In addition, for the packaging of AlGaN-based deep-ultraviolet LEDs, the researchers also studied the main chain structure of optically isotropic amorphous fluorine resins. It was found that for resins with two oxygen atoms in the ring, the visible damage to the electrode and the significant increase in leakage current were thought to be caused by photolysis of the ring induced by deep UV light irradiation. In contrast, in the case of resins with a single oxygen ring, no electrode damage and increased leakage were observed. This suggests that the monooxygenated resins are more suitable for encapsulation of DUV-LEDs. [13].

What's more, some research scholars proposed a micromachining method for quartz lenses, which can simultaneously design the inner and outer surfaces of the lenses for better control of the optical path and uniform illumination of light-emitting diodes (LEDs). They used the Monte Carlo method to track the UV light and performed numerical simulations to analyze the effect of different array microprocessing structures on the UV light using a rigorous coupled wave analysis method [14].

The results of the study show that the intensity of 265 nm UVC-LEDs can be significantly enhanced by using micromachined arrays. As the size of the internal quartz micromachining increases, the enhancement effect of the lens on the UV light increases. In addition, micromachining can enhance the concentration of light and effectively increase the intensity of UV light. Simulation and computational results show that the illumination uniformity can be greatly improved and the minimum Fresnel loss as low as 7.67% can be achieved by this free-form lens design [14].

It can be concluded that the researchers have achieved better light path control and uniform illumination effect through the micromachining method of free-form lenses. This is of great significance for the development of high-quality UV LED lighting and brings new possibilities for the lighting industry. However, this research is only numerical simulation and simulation results, and further experimental verification and engineering applications are needed. As shown in Figure 2.

Figure 2 (a) Silicon dioxide glass structure for external microprocessor arrays (b) Internal microprocessor arrays

3.1. Package structure design improves deep UV thermal performance

Despite the significant progress and commercialization of deep UV LEDs, many challenges remain [24]. For example, the light extraction efficiency (LEE) of DUV LEDs is quite low and the efficiency is still much lower than that of blue LEDs [25]. Light extraction efficiency is a key challenge in the development of high-power DUV-LEDs, while thermal performance also needs to be enhanced [26]. This has been a catalyst for ongoing research.

To enhance the heat dissipation characteristics of LED packages, several researchers have taken various approaches. One of the studies introduced ceramic barrier ribs between the LED chips, which were measured by a FLIR T-250 infrared camera and were found to reduce the top surface temperature of the package and the junction temperature, thus improving the heat dissipation [27]. Another study explored the use of liquid encapsulation structures to improve light extraction efficiency and reduce thermal resistance. The study used silicone oil as the encapsulation material and conducted experiments on planar and lensed 281 nm deep-ultraviolet LEDs, and the results showed that liquid encapsulation can significantly increase the optical power and applies to deep-ultraviolet LEDs with different wavelengths. In addition, the liquid encapsulation structure can reduce the thermal resistance and further improve the performance of the LEDs [28]. However, due to the immaturity of the current technology, liquid encapsulation may have a certain impact on the reliability of device sealing. For high-power light sources, their lifetime may also be affected to some extent.

In addition to improving thermal performance, many efforts have been made to enhance LEE over the past decades, among which nanostructure patterning is one of the most widely studied and applied techniques [29]. A specific study has proposed to enhance the light extraction efficiency of DUV LEDs by utilizing a double-layer nanopatterned array (NPA) packaging method. This method modulates the interfacial light field to significantly enhance the light extraction of LEDs by combining high-quality polymer materials and nanoarray structures. The physical mechanism was verified by theoretical finite element analysis simulations, and the method was demonstrated to have the advantages of low cost, direct process, and effective enhancement of light extraction efficiency [30]. Some researchers and scholars have also found that Aluminum (Aluminum) is a material with good reflective properties to improve the light extraction efficiency of deep UV LED, and the reflectivity in the whole UV spectral range is about 0.92. By optimizing the characteristic dimensions of the reflector such as the angle, the height, and the inner radius, the light extraction effect of the sidewall emission of the DUV-LEDs can be enhanced. The researchers established an optical model of the DUV-LEDs and fabricated optimized reflectors with different reflectivities, which were then applied to the packaging of the DUV-LEDs. As shown in Figure 4 [31]

Figure 5 A schematic representation of a typical flip-chip CSP arrangement showing (a) isometric and (b) planar views, with the different components identified on the planar view

3.3. Resolving voids

When SMT was first developed, vapor phase soldering was the preferred reflow soldering technique because of its excellent heat transfer capability. vPS is not a newly developed soldering method; it was invented in the 1970s. Since then, the process and equipment have improved. In the past, VPS used hazardous chemicals as heat transfer gases, but with the introduction of Galden fluid (i.e., perfluoropolyether, PFPE), which is also considered a non-hazardous, inert material that is non-corrosive, non-toxic, and non-flammable, VPS is now recognized as an environmentally friendly reflow soldering solution that is also highly efficient from the standpoint of energy consumption, and the chemicals used (e.g., perfluoropolyether) are not as hazardous as they could be. and the chemicals used (Galden) are harmless and environmentally friendly [40].

Unique to the VPS method is the use of vapor phase encapsulation of a special heat transfer fluid to complete the soldering of printed circuit boards (PCBs). The heat generation and transfer of the reflow process is accomplished through the latent heat of the condensate of this fluid. The main advantages of this method are the elimination of overheating, the prevention of shadowing effects (especially in large components), and the limitation of the incidence of voids in solder joints [41].

Today's vapor phase welding uses an oxygen-free environment and eliminates the possibility of overheating and vacuum application, a method that ensures higher quality joints and minimizes voids within the solder [42]. Vacuum vapor phase welding is accomplished using the principle of condensation heat transfer. The first step is to place the prepared component in the oven and then fill the bottom with a vapor liquid. At this point, the heater starts working and the water vapor liquid can be found evaporating and rising. Until the water vapor comes into contact with the cold PCB components and the cooling tubes, a lot of water vapor is produced, which is the phenomenon of condensation. The solder melts because of the release of latent heat. A schematic diagram of vacuum vapor phase soldering is shown in Figure 6 [43].

4. Deep UV LED Application Market

Deep UV LEDs (UVC LEDs) are LEDs that operate at wavelengths in the range of 200 to 280 nm. They can emit high-energy deep UV radiation, which has a strong bactericidal and inactivating effect on bacteria, viruses ,and other microorganisms. The following is the application history of deep UV LEDs [48]:

Sterilization of air, water, and surfaces remains a prominent challenge in addressing current crises, combating future epidemics ,and improving overall health [49]. Water and air are the most efficient media for bacteria dispersal, and the ability to reduce bacterial levels is critical for safe drinking for millions of people worldwide [50]. Deep UV LEDs are widely used in water and air purification systems. They are used to kill bacteria, viruses, and other microorganisms to provide cleaner water and air [51].DUV-LEDs are the key enabling technology for innovative disinfection systems that promise to be miniaturized, easily modulated, and not based on contaminant low-pressure mercury lamps as shown in Figure 7 which shows a first-generation stainless steel DUV LED water disinfection chamber[52].

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