Light-emitting diodes (LEDs) are widely used in our daily lives. Both light and heat are generated from LED chips and then transmitted or conducted through multiple packaging materials and interfaces. Part of the transmitted light converts into heat along the light propagation; in return, the accumulation of heat leads to the degradation of light output. The accumulated heat negatively influences the reliability and longevity of LEDs, and thus thermal management is critical for LED packaging and applications. On the other hand, in LED packaging processes, many fluid flow problems exist, such as phosphor coating, silicone injection, chip bonding, solder reflow, etc. Amongst them, phosphor coating is the most important process which is essential for LED performance. Phosphor gel is a kind of non-Newton fluid and its coating process is a typical fluid-flow problem. Overall, since LED packaging and applications present many heat and fluid flow problems, obtaining a full understanding of these problems enables advancements in the development of LED processes and designs. In this review, the emphasis is placed on heat generation in chips, heat flow in packages and application products, fluid flow in phosphor coating process, etc. This is a domain in which significant progress has been achieved in the last decade, and reporting on these advances will facilitate state-of-the-art LED packaging and application technologies.
Light-emitting diode (LED), as a type of solid-state lighting (SSL), is a lighting technology that arguably constitutes the greatest advancement in the lighting industry in the previous century [1-3]. In an LED, electricity is converted into light. It is well recognized that LEDs offer the following advantages. 1) Energy savings: LED requires less energy to emit equivalent light compared to other light sources. 2) Long lifetime: Due to their compact physical characteristics, LEDs are also more long-lasting than other lamps. Incandescent bulbs tend to last 1,000 hours as heat destroys the filament, and fluorescent lamps tend to last 10,000 hours. While LEDs can last over 50,000 hours or more in theory. 3) Environment-friendly characteristics: Unlike fluorescent lamps, there is no mercury in LEDs, which is environment-friendly whenever the LEDs are discarded. 4) Wide color temperatures: LEDs provide a wider range of color temperature (4500 K-12,000 K) and a wider operation temperature (-20°C to 85°C). 5) Quick startup: LEDs do not have low-temperature startup problems, which is different from many other lighting sources, such as metal halogen lamps. Based on the above advantages, so far, LEDs have been considered as the fourth generation of light sources. They have been extensively applied in our daily lives, including street lamps, backlighting, automobile headlamps, and general lighting. It is believed that LEDs will benefit the whole world even more extensively and profoundly in the future [5, 6]. Due to LEDs’ contribution to the whole of human society, the 2014 Nobel Prize in Physics was awarded to three scientists for the invention of efficient blue LEDs based on gallium nitride (GaN).
For high-power LED chips, the key part is the “PN junction” where a quantum well or multiple quantum well (MQW) layers are sandwiched with a p-GaN layer and an n-GaN layer. In a PN junction, the “P” material contains an excess of positive charges (also called holes) due to the absence of electrons. The “N” material contains an excess of negative charges due to the presence of electrons. To understand the lighting principle, we can consider an unbiased PN junction. Fig. 1 shows the PN energy band diagram. The depletion region extends mainly into the p-side. There is a potential barrier from Ec on the n-side to the Ec on the p-side, which is called the built-in voltage, V0. This potential barrier prevents the excess free electrons on the n-side from diffusing into the p-side. When a voltage V is applied to the junction, the built-in potential is reduced from V0 to V0-V. This allows electrons from the n-side to get injected into the p-side and recombine with the holes there, resulting in spontaneous emission of photons (light).
Fig1
Blue LED chips cannot be directly used in application products, and it is necessary that they are packaged and emit white light. Currently, the most common packaging method of producing white light is shown in Fig. 3. Under the blue light emission, yellow phosphors (such as cerium-doped yttrium aluminum garnet, YAG:Ce3+) could emit yellow light, and the mixture of transmitted blue and yellow light generate white light. By controlling the energy ratio of blue light and yellow light, the white light can be obtained at different color temperatures. This method is easy and simple to handle with high luminous efficiency. Thus, it has been widely adopted in industry.
Fig. 4 shows the LED industry chain. It consists of three categories: the upstream, the midstream, and the downstream. Epitaxy growth is the first step in the upstream, in which the emitting layer, cladding layer, buffer layer, reflector, etc., are accomplished in this process. The metal organic chemical vapor deposition (MOCVD) is the main method to produce blue, green, and ultraviolet epitaxy materials, such as GaN or GaAs. Different colors of LEDs can be made by employing different types of epiwafer. After the fabrication of LED wafers in the MOCVD chamber, the chip manufacturers will dice the wafer into LED chips with positive and negative electrodes. The chip manufacturing process includes film plating, lithography, chemical or iron etching, scribing, etc. In Fig. 4, the midstream industry is referred to as packaging. Through the upstream, we can obtain blue light LED chips. Before the application, we should convert the blue light emitted from the blue light LED chips into white light LEDs by packaging. The typical way to convert blue light into white light is shown in Fig. 3. Currently, there are many types of LED packages. According to the number of LED chips, it can be categorized as single-chip package, multiple-chip package, or chip-on-board (COB) package. The downstream industry refers to the integration of LED modules into lighting luminaries and systems aimed at different applications, such as street lamps, backlighting units, general lighting, etc.
