High-power LED packaging technology and development trends

1. Introduction

High-power LED packaging has been a research hotspot in recent years due to its complex structure and process, and directly affects the performance and life of LEDs. In particular, high-power white light LED packaging is a hotspot among research hotspots. The functions of LED packaging mainly include: 1. Mechanical protection to improve reliability; 2. Enhanced heat dissipation to reduce chip junction temperature and improve LED performance; 3. Optical control to improve light output efficiency and optimize beam distribution; 4. Power supply management, including AC/DC conversion and power supply control.

The selection of LED packaging methods, materials, structures and processes is mainly determined by factors such as chip structure, optoelectronic/mechanical characteristics, specific applications and costs. After more than 40 years of development, LED packaging has gone through the development stages of bracket type (Lamp LED), SMD type (SMD LED), power type LED (Power LED) and so on. With the increase of chip power, especially the demand for the development of solid-state lighting technology, new and higher requirements are put forward for the optical, thermal, electrical and mechanical structures of LED packaging. In order to effectively reduce the thermal resistance of the package and improve the light output efficiency, a new technical approach must be adopted for package design.

2. Key technologies for high-power LED packaging

High-power LED packaging mainly involves light, heat, electricity, structure and process, as shown in Figure 1. These factors are 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 means, and performance is the specific embodiment of the packaging level. In terms of process compatibility and reducing production costs, LED packaging design should be carried out at the same time as chip design, that is, the packaging structure and process should be considered when designing the chip. Otherwise, after the chip is manufactured, the chip structure may be adjusted due to the need for packaging, thereby extending the product development cycle and process costs, and sometimes even impossible.

Figure 1 High-power white LED packaging technology

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

1. Low thermal resistance packaging process

For the current LED light efficiency level, since about 80% of the input electrical energy is converted into heat and the LED chip area is small, chip heat dissipation is a key issue that must be solved in LED packaging. It mainly includes 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 of the material (heat dissipation substrate and heat sink structure) and the interface thermal resistance. The function of the heat dissipation substrate is to absorb the heat generated by the chip and conduct it to the heat sink to achieve heat exchange with the outside world. Common heat dissipation substrate materials include silicon, metals (such as aluminum, copper), ceramics (such as Al2O3, AlN, SiC) and composite materials. For example, Nichia's third-generation LED uses CuW as a substrate, and flips the 1mm chip on the CuW substrate, which reduces the thermal resistance of the package and improves the luminous power and efficiency; Lamina Ceramics has developed a low-temperature co-fired ceramic metal substrate, as shown in Figure 2 (a), and developed the corresponding LED packaging technology. This technology first prepares a high-power LED chip and a corresponding ceramic substrate suitable for eutectic welding, and then directly welds the LED chip to the substrate. Since the substrate integrates the eutectic welding layer, electrostatic protection circuit, drive circuit and control compensation circuit, it is not only simple in structure, but also greatly improves the heat dissipation performance due to the high thermal conductivity of the material and the small thermal interface, which provides a solution for high-power LED array packaging. The high thermal conductivity copper-clad ceramic board developed by Curmilk in Germany is made of a ceramic substrate (AlN or Al2O3) and a conductive layer (Cu) sintered under high temperature and high pressure without using an adhesive, so it has good thermal conductivity, high strength and strong insulation, as shown in Figure 2 (b). The thermal conductivity of aluminum nitride (AlN) is 160W/mk, and the thermal expansion coefficient is 4.0×10-6/℃ (equivalent to the thermal expansion coefficient of silicon of 3.2×10-6/℃), thereby reducing the thermal stress of the package.

Figure 2 (a) Low-temperature co-fired ceramic-metal substrate

Figure 2 (b) Schematic diagram of the cross section of a copper-clad ceramic substrate

Studies have shown that the package interface has a great influence on thermal resistance. If the interface is not properly handled, it is difficult to obtain a good heat dissipation effect. For example, an interface with good contact at room temperature may have an interface gap at high temperature, and the warping of the substrate may also affect bonding and local heat dissipation. The key to improving LED packaging is to reduce the 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 very important. The commonly used TIMs for LED packaging are conductive adhesives and thermal conductive adhesives. Due to their low thermal conductivity, generally 0.5-2.5W/mK, the interface thermal resistance is very high. The use of low-temperature or eutectic solder, solder paste or conductive adhesive doped with nanoparticles as thermal interface materials can greatly reduce the interface thermal resistance.

2. High light extraction rate packaging structure and process

During the use of LEDs, the losses of photons generated by radiation recombination when they are emitted outward mainly include three aspects: internal structural defects of the chip and absorption of materials; reflection losses of photons at the exit interface due to refractive index differences; and total reflection losses caused by the incident angle being greater than the critical angle of total reflection. Therefore, many rays of light cannot be emitted from the chip to the outside. By coating a layer of transparent glue (potting glue) with a relatively high refractive index on the surface of the chip, since the glue layer is between the chip and the air, the loss of photons at the interface is effectively reduced, and the light extraction efficiency is improved. In addition, the role of potting glue also includes mechanical protection of the chip, stress release, and as a light guide structure. Therefore, it is required to have high light transmittance, high refractive index, good thermal stability, good fluidity, and easy spraying. In order to improve the reliability of LED packaging, the potting glue is also required to have low hygroscopicity, low stress, and aging resistance. Currently commonly used potting glues include epoxy resin and silicone. Silicone is obviously superior to epoxy resin because of its high light transmittance, high refractive index, good thermal stability, low stress, and low hygroscopicity. It is widely used in high-power LED packaging, but the cost is relatively high. Studies have shown that increasing the refractive index of silicone can effectively reduce the photon loss caused by the physical barrier of the refractive index and improve the external quantum efficiency, but the performance of silicone is greatly affected by the ambient temperature. As the temperature rises, 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 light intensity distribution.

The function of phosphor is to compound light and color to form white light. Its characteristics mainly include particle size, shape, luminous efficiency, conversion efficiency, stability (thermal and chemical), etc. Among them, luminous efficiency and conversion efficiency are the key. Studies have shown that as the temperature rises, the quantum efficiency of phosphor decreases, the light output decreases, and the radiation wavelength also changes, which causes the color temperature and chromaticity of white light LEDs to change. Higher temperatures will also accelerate the aging of phosphors. The reason is that the phosphor coating is made of epoxy or silicone and phosphors, and has poor heat dissipation performance. When irradiated by purple or ultraviolet light, it is prone to temperature quenching and aging, which reduces the luminous efficiency. In addition, there are also problems with the thermal stability of potting glue and phosphors at high temperatures. Since the size of commonly used phosphors is above 1um, the refractive index is greater than or equal to 1.85, while the refractive index of silicone is generally around 1.5. Due to the mismatch of the refractive index between the two and the fact that the size of phosphor particles is much larger than the light scattering limit (30nm), light scattering occurs on the surface of phosphor particles, which reduces the light output efficiency. By adding nano-phosphor to silica gel, the refractive index can be increased to above 1.8, reducing light scattering, improving LED light output efficiency (10%-20%), and effectively improving light color quality.

The traditional phosphor coating method is to mix the phosphor with the potting compound and then apply it on the chip. Since the coating thickness and shape of the phosphor cannot be precisely controlled, the color of the emitted light is inconsistent, with a blue or yellowish color. The conformal coating technology developed by Lumileds can achieve uniform coating of the phosphor and ensure the uniformity of the light color, as shown in Figure 3 (b). However, studies have shown that when the phosphor is directly coated on the chip surface, the light output efficiency is low due to the presence of light scattering. In view of this, the Rensselaer Institute in the United States proposed a photon scattering extraction process (Scattered Photon Extraction method, SPE), which arranges a focusing lens on the chip surface and places a glass sheet containing the phosphor at a certain distance from the chip, which not only improves the reliability of the device, but also greatly improves the light efficiency (60%), as shown in Figure 3 (c).

Figure 3 High-power white light LED packaging structure

In general, in order to improve the light extraction efficiency and reliability of LEDs, the encapsulation layer has a tendency to be gradually replaced by high-refractive index transparent glass or microcrystalline glass. By internally doping or externally coating the phosphor on the glass surface, not only the uniformity of the phosphor is improved, but also the encapsulation efficiency is improved. In addition, reducing the number of optical interfaces in the light extraction direction of LEDs is also an effective measure to improve light extraction 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, as shown in Figure 4.

Figure 4 LED packaging technology and structure development

1. Pin-type (Lamp) LED package

Pin-type package is a commonly used Æ3-5mm package structure. It is generally used for LED packages with low current (20-30mA) and low power (less than 0.1W). It is mainly used for instrument display or indication, and can also be used as a display screen for large-scale integration. Its disadvantage is that the package thermal resistance is large (generally higher than 100K/W) and the life is short.

2. Surface Mount (SMT-LED) Packaging

Surface mount technology (SMT) is a packaging technology that can directly attach and solder packaged devices to designated locations on the PCB surface. Specifically, it is to use specific tools or equipment to align the chip pins with the pad pattern pre-coated with adhesive and solder paste, and then directly attach it to the PCB surface without drilling mounting holes. After wave soldering or reflow soldering, a reliable mechanical and electrical connection is established between the device and the circuit. SMT technology has the advantages of high reliability, good high-frequency characteristics, and easy automation. It is the most popular packaging technology and process in the electronics industry.

3. Chip-on-Board (COB) LED packaging

COB is the abbreviation of Chip On Board. It is a packaging technology that directly attaches LED chips to PCB boards through adhesives or solders, and then realizes electrical interconnection between chips and PCB boards through wire bonding. 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, etc.). Wire bonding can be achieved by thermosonic bonding at high temperatures (gold wire ball welding) and ultrasonic bonding at room temperature (aluminum chopper welding). 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/mK).

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 the whole machine. For SiP-LED, not only can multiple light-emitting chips be assembled in one package, but also various types of devices (such as power supply, control circuit, optical microstructure, sensor, etc.) can be integrated together to build a more complex and complete system. Compared with other packaging structures, SiP has good process compatibility (existing electronic packaging materials and processes can be used), high integration, low cost, can provide more new functions, easy block testing, short development cycle, etc. According to different technology 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, the total luminous flux, or the available luminous flux, must be increased. The increase in luminous flux can be achieved by increasing integration, increasing current density, and using large-size chips. These will increase the power density of LEDs. If the heat dissipation is poor, the junction temperature of the LED chip will increase, which will directly affect the performance of the LED device (such as reduced luminous efficiency, red shift of the emitted light, and reduced life). Multi-chip array packaging is currently the most feasible solution to obtain high luminous flux, but the density of LED array packaging is limited by price, available space, electrical connection, 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 an effective heat sink structure and a suitable packaging process must be used. Commonly used heat sink structures are divided into passive and active heat dissipation. Passive heat dissipation generally uses fins with a high rib coefficient to dissipate heat into the environment through natural convection between the fins and the air. This solution 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 must be adopted, such as fins + fans, heat pipes, liquid forced convection, microchannel cooling, phase change cooling, etc.

In terms of system integration, Taiwan's Xinqiang Optoelectronics Company uses system packaging technology (SiP) and uses fins + heat pipes with high-efficiency heat dissipation modules to develop 72W and 80W high-brightness white LED light sources, as shown in Figure 5 (a). Due to the low thermal resistance of the package (4.38℃/W), when the ambient temperature is 25℃, the LED junction temperature is controlled below 60℃, thus ensuring the service life and good luminous performance of the LED. Huazhong University of Science and Technology uses COB packaging and micro-spray active heat dissipation technology to package 220W and 1500W ultra-high-power LED white light sources, as shown in Figure 5 (b).

(IV) Packaging mass production technology

Wafer bonding technology refers to the process of making and packaging chip structures and circuits on wafers, and then cutting them to form individual chips after packaging. The corresponding die bonding refers to the process of cutting the chip structures and circuits on wafers to form chips, and then packaging individual chips (similar to the current LED packaging process), as shown in Figure 6. Obviously, wafer bonding packaging has higher efficiency and quality. Since packaging costs account for a large proportion of the manufacturing cost of LED devices, changing the existing LED packaging form (from chip bonding to wafer bonding) will greatly reduce the packaging manufacturing cost. In addition, wafer bonding packaging can also improve the cleanliness of LED device production, prevent the damage of the device structure caused by the dicing and slicing processes before bonding, and improve the packaging yield and reliability, so it is an effective means to reduce packaging costs.

In addition, for high-power LED packaging, it is necessary to use packaging forms with fewer processes (Package-less Packaging) as much as possible during chip design and packaging design, while simplifying the packaging structure and reducing the number of thermal and optical interfaces as much as possible to reduce packaging thermal resistance and improve light output efficiency.

(V) 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 potting glue caused by high temperature, degradation of optical performance, etc.) and mechanical failure (such as lead breakage, desoldering, etc.), and these factors 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 use time when the output luminous flux of the LED decays to 70% of the initial value (generally defined as 50% of the initial value for display purposes). Due to the long life of LED, accelerated environmental testing methods are usually used for reliability testing and evaluation. 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 cycle (85℃～-55℃), thermal shock, corrosion resistance, solubility resistance, mechanical shock, etc. However, accelerated environmental testing is only one aspect of the problem. The research on the prediction mechanism and method of LED life is still a difficult problem to be studied.

3. Solid-state lighting requirements for high-power LED packaging

Compared with traditional lighting fixtures, LED lamps do not need to use filters to produce colored light. They are not only highly efficient and pure in color, but also can achieve dynamic or gradual color changes. They can maintain a high color rendering index while changing the color temperature to meet the needs of different applications. However, new requirements are also put forward for its packaging, which are specifically reflected in:

1. Modularity

By connecting multiple LED lamps (or modules) to each other, good lumen output superposition can be achieved to meet the requirements of high-brightness lighting. Through modular technology, multiple point light sources or LED modules can be combined in any shape to meet the lighting requirements of different fields.

2. Maximizing system efficiency

In order to improve the light output efficiency of LED lamps, in addition to a suitable LED power supply, it is also necessary to adopt an efficient heat dissipation structure and process, as well as optimize the internal/external optical design to improve the efficiency of the entire system.

3. Low cost

For LED lamps to enter the market, they must have a competitive advantage in cost (mainly referring to the initial installation cost), and packaging accounts for a large part of the entire LED lamp production cost. Therefore, adopting new packaging structures and technologies to improve the light efficiency/cost ratio is the key to realizing the commercialization of LED lamps.

4. Easy to replace and maintain

Since LED light sources have a long life and low maintenance costs, higher requirements are placed on the reliability of LED lamp packaging. LED lamp designs must be easy to improve to accommodate future requirements for more efficient LED chip packaging, and LED chips must be interchangeable so that lamp manufacturers can choose which chips to use.

LED lamp light source can be composed of multiple distributed point light sources. Due to the small chip size, the packaged lamp is light in weight, delicate in structure, and can meet the needs of various shapes and different integration levels. The only drawback is that there is no ready-made design standard, but at the same time it provides ample imagination space for design. In addition, the primary goal of LED lighting control is power supply. Since the general mains power supply is high voltage alternating current (220V, AC), and LED requires constant current or current limiting power supply, conversion circuits or embedded control circuits (ASICs) must be used to achieve advanced calibration and closed-loop feedback control systems. In addition, through digital lighting control technology, the use and control of solid-state light sources mainly rely on intelligent control and management software, thus establishing a new relationship between users, information and light sources, and giving full play to the imagination of designers and consumers.

IV. Conclusion

LED packaging is a research topic involving multiple disciplines (such as optics, thermal, mechanical, electrical, mechanical, material, semiconductor, etc.). From a certain perspective, LED packaging is not only a manufacturing technology (Technology), but also a basic science (Science). Good packaging requires the understanding and application of physical essences such as thermal, optics, material and process mechanics. LED packaging design should be carried out simultaneously with chip design, and it is necessary to consider the performance of light, heat, electricity, structure, etc. in a unified manner. In the packaging process, although the selection of materials (heat dissipation substrate, phosphor, potting glue) is very important, the packaging structure (such as thermal interface, optical interface) also has a great impact on the light efficiency and reliability of LED. High-power white light LED packaging must adopt new materials, new processes, and new ideas. For LED lamps, it is even more necessary to consider the light source, heat dissipation, power supply and lamps in an integrated manner.