LED (Light Emitting Diode) is widely used in our daily life. Both light and heat are generated in the LED chip and then transmitted or conducted through different packaging materials and interfaces. Some of the transmitted light is converted to heat as the light propagates. In return, the build-up of heat lowers the light output. Thermal management is important in LED packaging and applications as accumulated heat negatively affects the reliability and lifespan of LEDs.
On the other hand, in the LED packaging process, there are many fluid flow problems such as phosphor coating, silicon injection, chip bonding, solder reflow, and the like. Among them, phosphor coating is the most important process essential for LED performance. Phosphor gels are a kind of non-Newtonian fluid, and the coating process is a typical fluid flow problem. Overall, LED packaging and applications present many thermal and fluid flow challenges, so a full understanding of these issues can advance LED process and design development. This review focuses on heat generation in chips, heat flow in packages and applications, and fluid flow in phosphor coating processes. This is an area where significant progress has been made over the past decade and these advances will facilitate state-of-the-art LED packaging and application technologies.
In LEDs, electricity is converted into light. It is well known that LEDs offer the following benefits Energy saving: LEDs require less energy to produce equivalent light compared to other light sources. Long lifespan: LEDs last longer than other lamps due to their compact physical properties. Incandescent lamps tend to last for 1000 hours, as the heat destroys the filament, while fluorescent lamps tend to last for 10,000 hours. LEDs can theoretically last over 50,000 hours. Eco-friendly characteristics: Unlike fluorescent lamps, LEDs do not contain mercury, so whenever LEDs are discarded, they are eco-friendly. Wide color temperature: LEDs offer a wider color temperature range (4500 K–12,000 K) and a wider operating temperature (-20 °C to 85 °C).
Fast start-up: LEDs do not have cold start-up problems, unlike many other light sources such as metal halide lamps. Based on these advantages, LED has been considered as the 4th generation light source so far , . It is widely applied in our daily life, such as street lamps, backlights, automobile headlamps, and general lighting. It is believed that LEDs will bring even more widespread and profound benefits to the world in the future. For the contribution of LEDs to human society as a whole, the 2014 Nobel Prize in Physics was awarded to three scientists for inventing an efficient blue LED based on gallium nitride (GaN) .
For high-power LED chips, the key part is a “PN junction” where a quantum well or multiple quantum well (MQW) layer is sandwiched between a p-GaN layer and an n-GaN layer .In a PN junction, the “P” material contains an excess of positive charge (also known as holes) because it has no electrons. “N” materials contain an excess of negative charge due to the presence of electrons. To understand the lighting principle, we can consider an unbiased PN junction. shows the PN energy band diagram. The depletion region mainly extends to the p-side. There is a potential barrier from Ec on the n side to Ec on the p side, which is called the built-in voltage V0. This potential barrier prevents excessive free electrons from the n side from diffusing to the p side. Applying a voltage V across the junction reduces the built-in potential from V0 to V0-V. Through this, electrons on the n side are injected into the p side and recombine with the holes there, resulting in spontaneous emission of photons (light).