What are the key technologies for high-power LED packaging?

High power LED packaging mainly involves aspects such as light, heat, electricity, structure, and process. These factors are both independent of each other and influence each other. Among them, light is the purpose of LED packaging, heat is the key, electricity, structure, and process are the means, and performance is the specific manifestation of packaging level. In terms of process compatibility and reducing production costs, LED packaging design should be carried out simultaneously with chip design, that is, the packaging structure and process should be considered during chip design. Otherwise, after the chip manufacturing is completed, the chip structure may need to be adjusted due to packaging requirements, which may prolong the product development cycle and process costs, and sometimes even be impossible.

Specifically, the key technologies for high-power LED packaging include:

1、 Low thermal resistance packaging process
For the current level of LED light efficiency, due to the fact that about 80% of the input electrical energy is converted into heat and the small area of LED chips, chip heat dissipation is a key issue that must be addressed in LED packaging. Mainly including chip layout, packaging material selection (substrate material, thermal interface material) and process, heat sink design, etc.
The thermal resistance of LED packaging mainly includes the internal thermal resistance and interface thermal resistance of materials (heat dissipation substrate and heat sink structure). The function of the heat dissipation substrate is to absorb the heat generated by the chip and conduct it to the heat sink, achieving heat exchange with the outside world. Common materials for heat dissipation substrates include silicon, metals such as aluminum and copper, ceramics such as AlN and SiC, and composite materials. Nichia’s third-generation LED uses CuW as the substrate and flips 1mm chips onto the CuW substrate, reducing packaging thermal resistance and improving luminous power and efficiency; Lamina Ceramics has developed low-temperature co fired ceramic metal substrates and corresponding LED packaging technology. This technology first prepares high-power LED chips and corresponding ceramic substrates suitable for eutectic soldering, and then directly solder the LED chips to the substrate. Due to the integration of eutectic solder layers, electrostatic protection circuits, driving circuits, and control compensation circuits on the substrate, not only is the structure simple, but also the high thermal conductivity of the material and fewer thermal interfaces greatly improve the heat dissipation performance, providing a solution for high-power LED array packaging. The high thermal conductivity copper-clad ceramic plate developed by the German company Curmilk is sintered from a ceramic substrate (AlN or) and a conductive layer (Cu) at high temperature and high pressure, without the use of adhesives. Therefore, it has good thermal conductivity, high strength, and strong insulation. The thermal conductivity of aluminum nitride (AlN) is 160W/mk, and its coefficient of thermal expansion is (equivalent to that of silicon), thereby reducing the thermal stress of the packaging.
Research has shown that the packaging interface also has a significant impact on thermal resistance. If the interface is not handled correctly, it is difficult to achieve good heat dissipation. For example, interfaces with good contact at room temperature may have interface gaps at high temperatures, and substrate warping may also affect bonding and local heat dissipation. The key to improving LED packaging is to reduce interface and interface contact thermal resistance, and enhance heat dissipation. Therefore, the selection of thermal interface material (TIM) between the chip and the heat dissipation substrate is crucial. The commonly used TIMs for LED packaging are conductive adhesives and thermal conductive adhesives. Due to their low thermal conductivity, which is generally 0, 5-2, and 5W/mK, the interface thermal resistance is very high. Using low-temperature or eutectic solder, solder paste, or conductive adhesive doped with nanoparticles as thermal interface materials can greatly reduce interface thermal resistance.

2、 High light extraction rate packaging structure and process
The loss of photons generated by radiation recombination during the use of LEDs mainly includes three aspects: internal structural defects of the chip and material absorption; Photon reflection loss caused by refractive index difference at the exit interface; And the total reflection loss caused by the incident angle being greater than the critical angle of total reflection. Therefore, many light rays cannot be emitted from the chip to the outside. By coating a relatively high refractive index transparent adhesive layer (potting adhesive) on the surface of the chip, the adhesive layer is located between the chip and air, effectively reducing the loss of photons at the interface and improving the light harvesting efficiency. In addition, the function of the potting adhesive also includes mechanical protection of the chip, stress relief, and serving as a light guiding structure. Therefore, it is required to have high transmittance, high refractive index, good thermal stability, good fluidity, and easy spraying. To improve the reliability of LED packaging, it is also required that the potting adhesive has low moisture absorption, low stress, and aging resistance characteristics. The commonly used potting adhesives currently include epoxy resin and silicone. Silicone, due to its high transmittance, high refractive index, good thermal stability, low stress, and low moisture absorption, is significantly superior to epoxy resin and has been widely used in high-power LED packaging, but at a higher cost. Research has shown that increasing the refractive index of silicone can effectively reduce photon loss caused by refractive index physical barriers and improve external quantum efficiency, but the performance of silicone is greatly affected by environmental temperature. As the temperature increases, the thermal stress inside the silicone increases, resulting in a decrease in the refractive index of the silicone, which affects the LED light efficiency and intensity distribution.
The function of fluorescent powder is to combine light and color, forming white light. Its characteristics mainly include particle size, shape, luminous efficiency, conversion efficiency, stability (thermal and chemical), among which luminous efficiency and conversion efficiency are key. Research has shown that as the temperature increases, the quantum efficiency of the fluorescent powder decreases, the light output decreases, and the radiation wavelength also changes, causing changes in the color temperature and chromaticity of white LED. Higher temperatures can also accelerate the aging of the fluorescent powder. The reason is that the fluorescent powder coating is made by mixing epoxy or silicone with fluorescent powder, which has poor heat dissipation performance. When exposed to purple or ultraviolet radiation, it is prone to temperature quenching and aging, resulting in a decrease in luminous efficiency. In addition, there are also issues with the thermal stability of potting adhesive and fluorescent powder at high temperatures. Due to the commonly used fluorescent powder size being above 1um and having a refractive index greater than or equal to 1.85, while the refractive index of silica gel is generally around 1-5. Due to the mismatch in refractive index between the two and the fact that the size of the fluorescent powder particles is much larger than the light scattering limit (30nm), there is light scattering on the surface of the fluorescent powder particles, which reduces the light output efficiency. By incorporating nano fluorescent powder into silicone gel, the refractive index can be increased to above 1 or 8, reducing light scattering, improving LED light output efficiency (10% -20%), and effectively improving light color quality.

The traditional method of coating fluorescent powder is to mix the fluorescent powder with potting adhesive and then apply it onto the chip. Due to the inability to precisely control the thickness and shape of the fluorescent powder coating, the emitted light color is inconsistent, resulting in a bias towards blue or yellow light. The conformal coating technology developed by Lumileds can achieve uniform coating of fluorescent powder, ensuring the uniformity of light color. However, research has shown that when the fluorescent powder is directly coated on the surface of the chip, the light emission efficiency is lower due to the presence of light scattering. In view of this, the Rensselaer Institute in the United States has proposed a Scattered Photon Extraction method (SPE), which involves placing a focusing lens on the surface of the chip and placing a glass sheet containing fluorescent powder at a certain distance from the chip. This not only improves the reliability of the device, but also greatly enhances the light efficiency (60%).
Overall, in order to improve the light output efficiency and reliability of LEDs, there is a trend for the encapsulation adhesive layer to be gradually replaced by high refractive index transparent glass or microcrystalline glass. By doping or coating fluorescent powder on the glass surface, not only does it improve the uniformity of the fluorescent powder, but it also enhances the encapsulation efficiency. In addition, reducing the number of optical interfaces in the direction of LED light output is also an effective measure to improve light output efficiency.

3、 Array Packaging and System Integration Technology
After more than 40 years of development, LED packaging technology and structure have gone through four stages.

1. Lamp LED package
   Pin type packaging is a commonly used 3-5mm packaging structure. Generally used for LED packaging with low current (20-30mA) and low power (less than 0, 1W). Mainly used for instrument display or indication, it can also be used as a display screen when integrated on a large scale. Its disadvantage is that the packaging thermal resistance is relatively high (generally higher than 100K/W) and the lifespan is short.
2. Surface mount (SMT) – LED packaging
   Surface mount technology (SMT) is a packaging technique that allows packaged components to be directly attached and soldered to designated positions on the surface of a PCB. Specifically, it is to align the chip pins with the pre coated solder pad pattern with adhesive and solder paste using specific tools or equipment, and then directly mount them onto the surface of the PCB without drilling mounting holes. After wave soldering or reflow soldering, reliable mechanical and electrical connections are established between the device and the circuit. SMT technology has the advantages of high reliability, good high-frequency characteristics, and easy automation, and is the most popular packaging technology and process in the electronics industry.
3. On board direct mount (COB) LED package
   COB stands for Chip On Board, which is a packaging technology that uses adhesives or solder to directly attach LED chips to a PCB board, and then uses wire bonding to achieve electrical interconnection between the chip and the PCB board. PCB boards can be low-cost FR-4 materials (glass fiber reinforced epoxy resin) or high thermal conductivity metal based or ceramic based composite materials (such as aluminum substrates or copper-clad ceramic substrates). Wire bonding can be achieved through high-temperature hot ultrasonic bonding (gold wire ball bonding) and room temperature ultrasonic bonding (aluminum chopper bonding). COB technology is mainly used for high-power multi chip array LED packaging. Compared with SMT, it not only greatly improves the packaging power density, but also reduces the packaging thermal resistance (generally 6-12W/m, K).
4. System in Package (SiP) LED packaging
   SiP (System in Package) is a new type of packaging and integration method developed in recent years based on System on Chip (SOC) to meet the requirements of portable development and system miniaturization of whole machines. For SiP LED, not only can multiple light-emitting chips be assembled in one package, but various types of devices such as power supplies, control circuits, optical microstructures, sensors, etc. can also be integrated together to build a more complex and complete system. Compared with other packaging structures, SiP has the advantages of good process compatibility (utilizing existing electronic packaging materials and processes), high integration, low cost, providing more new functions, easy block testing, and short development cycle. According to different technical types, SiP can be divided into four types: chip stacking type, module type, MCM type, and three-dimensional (3D) packaging type.
   At present, in order for high brightness LED devices to replace incandescent lamps and high-pressure mercury lamps, it is necessary to increase the total luminous flux, or the usable luminous flux. The increase in luminous flux can be achieved through measures such as improving integration, increasing current density, and using large-sized chips. And all of these will increase the power density of LED, such as poor heat dissipation, which will lead to an increase in the junction temperature of LED chips, directly affecting the performance of LED devices (such as reduced luminous efficiency, red shift of emitted light, reduced lifespan, etc.). Multi chip array packaging is currently the most feasible solution for achieving high light flux, but the density of LED array packaging is limited by factors such as price, available space, electrical connections, and especially heat dissipation. Due to the high-density integration of light-emitting chips, the temperature on the heat dissipation substrate is very high, and effective heat sink structures and suitable packaging processes must be adopted. The commonly used heat sink structures are divided into passive and active heat dissipation. Passive heat dissipation generally uses fins with high rib coefficients, which dissipate heat to the environment through natural convection between the fins and the air. This scheme has a simple structure and high reliability, but due to the low natural convection heat transfer coefficient, it is only suitable for situations with low power density and low integration. For high-power LED packaging, active heat dissipation methods such as fins+fans, heat pipes, liquid forced convection, microchannel cooling, phase change cooling, etc. must be used.
   In terms of system integration, Taiwan’s Xinqiang Optoelectronics Company has adopted System Packaging Technology (SiP) and developed 72W and 80W high brightness white LED light sources by combining fins and heat pipes with high-efficiency heat dissipation modules. Due to the low thermal resistance of the packaging (4, 38 ℃/W), when the ambient temperature is 25 ℃, the junction temperature of the LED is controlled below 60 ℃, ensuring the service life and good luminous performance of the LED. Huazhong University of Science and Technology has adopted COB packaging and micro spray active heat dissipation technology to package ultra high power LED white light sources of 220W and 1500W.

4、 Packaging production technology
Wafer bonding technology refers to the process of manufacturing and packaging chip structures and circuits on a wafer, followed by cutting to form a single chip after packaging is completed; Die bonding, which corresponds to it, refers to the process of cutting the chip structure and circuit onto the chip to form a die, and then packaging individual chips (similar to the current LED packaging process). It is evident that the efficiency and quality of chip bonding packaging are higher. Due to the significant proportion of packaging costs in the manufacturing cost of LED devices, changing the existing LED packaging form (from chip bonding to chip bonding) will greatly reduce packaging manufacturing costs. In addition, chip bonding packaging can also improve the cleanliness of LED device production, prevent damage to device structure caused by pre bonding wafer cutting and slicing processes, and improve packaging yield and reliability. Therefore, it is an effective means to reduce packaging costs.
In addition, for high-power LED packaging, it is necessary to adopt packaging forms with fewer processes (Package less Packaging) as much as possible in the chip design and packaging design process, while simplifying the packaging structure and minimizing the number of thermal and optical interfaces to reduce packaging thermal resistance and improve light output efficiency.

5、 Packaging reliability testing and evaluation
The failure modes of LED devices mainly include electrical failure (such as short circuit or open circuit), optical failure (such as yellowing of encapsulation adhesive caused by high temperature, deterioration of optical performance, etc.), and mechanical failure (such as lead breakage, desoldering, etc.), all of which are related to the packaging structure and process. The service life of LED is defined by the mean time to failure (MTTF). For lighting purposes, it generally refers to the time during which the output luminous flux of LED decays to 70% of its initial value (for display purposes, it is generally defined as 50% of its initial value). Due to the long lifespan of LEDs, accelerated environmental testing is commonly used for reliability testing and estimation. The test content mainly includes high temperature storage (100 ℃, 1000h), low temperature storage (-55 ℃, 1000h), high temperature and high humidity (85 ℃/85%, 1000h), high and low temperature cycling (85 ℃~-55 ℃), thermal shock, corrosion resistance, solubility, mechanical shock, etc. However, accelerated environmental testing is only one aspect of the problem, and the research on the prediction mechanism and methods of LED lifespan is still a difficult problem to be studied.