High-power LED thermal and conductive silver paste and its packaging technology and trends

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 LED . 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 properties, 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 idea must be adopted for the package design.

1. High-power LED high thermal conductivity silver glue

Uninwell International BQ-6886 series conductive silver glue is an indispensable glue in IED production and packaging. The requirements for conductive silver glue are good electrical and thermal conductivity, high shear strength, and strong bonding force.

As the world's leading brand of high-end electronic adhesives, Uninwell International's conductive silver glue, conductive silver paste, patch red glue, bottom filling glue, TUFFY glue, LCM sealant, UV glue, anisotropic conductive glue ACP, solar cell conductive paste and other series of electronic adhesives have the highest product cost performance. Uninwell International conductive silver glue has good conductivity, strong shear force, good rheology, and low moisture absorption. It is particularly suitable for the packaging of high-power and high-brightness LEDs .

2. Key technologies for high-power LED packaging

High-power LED packaging mainly involves light, heat, electricity, structure and process, etc. These factors are both independent of each other and affect 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 simultaneously with 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.

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 and developed corresponding LED packaging technology. This technology first prepares a high-power LED chip suitable for eutectic welding and a corresponding ceramic substrate, and then directly welds the LED chip to the substrate. Since the substrate integrates a eutectic solder layer, an electrostatic protection circuit, a drive circuit and a control compensation circuit, it is not only simple in structure, but also has a high thermal conductivity of the material and few thermal interfaces, which greatly improves the heat dissipation performance and provides a solution for high-power LED array packaging.

During the use of LED, the loss of photons generated by radiation recombination when emitted outward mainly includes three aspects: internal structural defects of the chip and absorption of materials; reflection loss of photons at the exit interface due to refractive index difference; and 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 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), there is light scattering 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 tendency to be blue or yellow. The conformal coating technology developed by Lumileds can achieve uniform coating of the phosphor and ensure the uniformity of the light color. However, studies have shown that when the phosphor is directly coated on the chip surface, the light output efficiency is low due to the existence of light scattering.

1. Pin-type (Lamp) LED package

Pin-type package is a commonly used 3-5mm 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 them 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 pastes LED chips onto PCB boards through Uninwell International -6886 series 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 integration method developed in recent years based on the system on chip System on Chip to meet the requirements of the development of portable whole machines and system miniaturization. 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 , and short development cycle. 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, high-brightness LED devices must increase the total luminous flux , or the luminous flux that can be used, to replace incandescent lamps and high-pressure mercury lamps. The increase in luminous flux can be achieved by increasing integration, increasing current density, and using large-size chips. All of these will increase the power density of the LED. 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, reduced life, etc.). 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 adopted. 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 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 Xinqiang Optoelectronics Co., Ltd. uses system packaging technology (SiP) and uses fins + heat pipes with high-efficiency heat dissipation modules to develop 72W and 80W high- brightness white light LED light sources. 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.

(II) Packaging mass production technology

Wafer bonding technology refers to the production and packaging of chip structure and circuit on the wafer, and then cutting to form a single chip after packaging is completed; the corresponding chip bonding refers to the chip structure and circuit on the wafer, which is cut to form a chip, and then the single chip is packaged (similar to the current LED packaging process). Obviously, the efficiency and quality of wafer bonding packaging are higher. Since the packaging cost accounts 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 by the dicing and slicing process before bonding, and improve the packaging yield and reliability, so it is an effective means to reduce the packaging cost.

In addition, for high-power LED packaging, it is necessary to use a packaging form with less process (Package-less Packaging) as much as possible during the chip design and packaging design process, simplify the packaging structure, and reduce the number of thermal and optical interfaces as much as possible to reduce the thermal resistance of the package and improve the light output efficiency.