The process of high-power LED package is complex and the structure is cumbersome. LED package technology is influencing the performance and life of LED. In recent years, LED packaging technology has been a hot research project. Many large companies have invested a lot of human and material resources in research. LED packaging technology has been greatly improved, especially the research on high-power white LED packaging has been more in-depth, and a variety of LED packaging processes have been obtained.
The functions of LED packaging mainly include:
- Strengthen the strength and stability of LED and improve the reliability of LED
- Strengthen heat dissipation to reduce chip junction temperature and improve led performance;
- Strengthen the light output control, improve the light output efficiency and optimize the beam distribution;
- Strengthen power supply management, optimize the control of power supply, and improve the conversion efficiency of DC / AC;
The selection of LED package methods, materials, structures and processes mainly depends on many factors such as wafer structure, optoelectronic/mechanical characteristics, specific applications, and cost. After more than 40 years of development, LED packaging has experienced development stages such as bracket type (Lamp LED), patch type (SMD LED), and power LED (Power LED). With the increase of chip power, especially the demand for the development of solid-state lighting technology, new and higher requirements are put forward on the optical, thermal, electrical and mechanical structures of LED package. In order to effectively reduce the thermal resistance of the package and improve the efficiency of light extraction, we should adopt new technical ideas in the package design.
Key technology of high-power LED package
High power LED packaging involves optical, thermal, electrical and other fields, as well as design structure and manufacturing technology. These factors are independent and influence each other. LED package reduces the refractive index of light, improves the passing rate of light, reduces the resistance of materials, reduces the heating of LED and reduces the power consumption of LED. To achieve these purposes, we need better manufacturing process and better design. The improvement of these properties is the embodiment of the level of LED package. Chip design is closely related to LED packaging design. We should consider what packaging process and packaging structure to use in chip design. Otherwise, after the chip manufacturing is completed, the chip structure may be adjusted due to the needs of packaging, which prolongs the product R & D cycle and process cost, and sometimes even leads to the mismatch between the LED packaging process and the chip and the failure of LED package.
Low thermal resistance packaging process
The theoretical light efficiency of LED is very high, but the existing LED lighting efficiency is only about 20%, and 80% of electric energy will be converted into heat. The LED lamp has small volume and small chip area. The heat dissipation of LED lamp is the key consideration of LED package. LED heat dissipation mode, chip heat dissipation layout, packaging material selection, strong heat dissipation, heat sink design, etc.
The thermal resistance of LED package mainly includes the internal thermal resistance and interface thermal resistance of materials (heat dissipation substrate and heat sink structure). The heat dissipation substrate absorbs the heat generated by the wafer and transfers the heat to the heat sink. The heat sink transfers the heat to the air. The LED transfers the heat of the wafer through the heat dissipation substrate and heat sink. The materials used for heat dissipation substrates are generally materials with strong thermal conductivity, including ceramics (such as Al2O3, ain, SiC), metals (such as aluminum, copper), silicon and composites.
In order to solve the heat dissipation of high-power lamp, the manufacturing process of high-power LED is relatively complex. We need to weld the LED chip and the corresponding heat dissipation substrate together. It integrates driving circuit, control compensation circuit, scenic spot protection circuit and eutectic welding layer on the substrate. The heat dissipation substrate has simple structure and high material thermal conductivity, which greatly improves the heat dissipation performance and solves the heat dissipation problem of high-power LED packaging.
The processing of packaging interface is another important factor to solve the heat dissipation problem. Correctly handling the interface will greatly improve the heat dissipation effect, and the packaging interface has a great impact on the thermal resistance. The packaging interface is in good contact at room temperature. At high temperature, the substrate will bend slightly with the high temperature, resulting in the gap between the interface and the interface, which will affect bonding and local heat dissipation.
An optimization point of LED packaging is to reduce the contact thermal resistance between interfaces and enhance heat dissipation. Thermal interface (TIM) is the interlayer between wafer and thermal substrate. The material selection of TIM is very important. TIM commonly used materials are conductive adhesive and thermally conductive adhesive, which have poor thermal conductivity, generally 0.5-2.5w/mk. The conductive adhesive of TIM will incorporate thermally conductive nanoparticles to reduce the thermal resistance of the interface.
High light extraction rate packaging structure and process
The light generated by LED will cause great losses when sent to the outside. These losses are mainly in three aspects: the internal structure defects of the wafer and the absorption of some photons by the wafer material; The reflection loss caused by the large refractive index of the transparent adhesive layer when the photon is out of color; The total reflection loss caused by the thermal emission angle of photons entering the transparent adhesive layer is greater than the total reflection angle of total reflection. It is a pity that many photons are lost when emitted from the chip to the outside.
The potting adhesive coated on the surface of the wafer has a relatively high refractive index, which will reduce the loss of photons at the interface and increase the transmittance. As the potting glue wraps the wafer, the potting glue should have a mechanical protection effect on the wafer. Therefore, the potting adhesive should not only have high refractive index and light transmittance, but also have good stability and fluidity. In order to improve the service life of LED and reduce the light decay of LED, the potting adhesive should preferably have the characteristics of low moisture absorption and aging resistance.
At present, the commonly used potting adhesives include epoxy resin and silicone adhesive. Silicone adhesive has low hygroscopicity, low stress, good thermal stability, high transmittance and high refractive index. It has obvious advantages over epoxy resin. Most high-power LED package use silicone adhesive, but the cost is much higher than epoxy resin. Although silica gel has good stability and light transmittance, its performance is related to the temperature of the external environment. With the increase of temperature, the thermal stress inside the silica gel will increase and the refractive index will decrease, which will increase the loss of photon refraction and total reflection and affect the uniform distribution of light intensity.
The role of phosphors is to combine light and color to form white light. The properties of phosphors include size, shape, stability, luminous rate and conversion rate. The main performance indexes are luminous rate and conversion rate. Studies have shown that as the temperature rises, the quantum efficiency of the phosphor decreases, the light output decreases, and the radiation wavelength will also change, which will cause the color temperature and chromaticity of the white LED to change. Higher temperatures will also accelerate the aging of the phosphor. The coating of phosphor generally utilizes phosphor and silicone or epoxy. The prominent problem is poor heat dissipation. When exposed to high temperature radiation or purple and ultraviolet radiation, it will absorb too much heat and age, reducing the luminous efficiency of phosphor.
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 1μm, the refractive index is greater than or equal to 1.85, while the refractive index of silicone is generally around 1.5. The refractive index of the two materials is different. When light passes through fluorescent powder particles, refraction and total reflection will occur, which forms light scattering at the macro level, reduces the light output efficiency and affects the light intensity distribution. By incorporating nano-phosphor into the silicon gel, the refractive index can be increased to over 1.8, the light scattering can be reduced, the LED light output efficiency (10%-20%) can be improved, and the light color quality can be effectively improved.
It is difficult to accurately control the coating thickness of phosphor, resulting in uneven LED lighting, light spots, and the color of some irradiation areas is blue or yellow. In order to solve this problem, many companies have made different attempts. The conformal coating technology developed by MOKOLight company can solve the problem of uniform coverage of phosphor and uniform distribution of LED light. However, there is a problem of light scattering in the process of specific use, and the light output rate will decrease. Rensselaer Research Institute of USA has proposed a photon scattering extraction method-Scattered Photon Extraction method (SPE). Placing a focusing lens on the surface of the wafer, and placing the glass sheet containing phosphor near the focus of the lens can improve the light transmission efficiency.
In general, in order to improve the efficiency and reliability of LED light, the encapsulant layer has a tendency to be gradually replaced by transparent glass or glass-ceramic with high refractive index. By adding or coating phosphor powder on the glass surface, it will not only improve The uniformity of the phosphor is improved, and the packaging efficiency is improved.
Array packaging and system integration technology
After years of development, led mechanism design and packaging technology have experienced countless changes, and the others have four subversive technology iterations.
- Lead type (Lamp) LED package
The pin-type package is the A3-5mm package structure of common use. Generally used in 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 during large-scale integration. The disadvantage is that the package has a large thermal resistance (generally higher than 100K/W) and a short life.
- Surface mount (SMD) type package(SMT LED package)
SMT LED package technology is a packaging technology that can weld electronic components to the designated position of the electronic circuit board. Specifically, use a specific tool or equipment to align the chip pins to the land pattern pre-coated with adhesive and solder paste, and then directly mount it on the PCB surface without drilling the mounting holes. Next, we can firmly fix the electronic components on the electronic circuit board through reflow soldering or wave soldering, and in this process we can make full use of the characteristics of electronic components. 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.
- Chip-on-board (COB) LED package
COB is the English abbreviation of Chip On Board (Chip On Board). It is a kind of LED chip that is directly pasted to the PCB board through adhesive or solder, and using wire bonding packaging technology can help realize the electrical interconnection between the chip and the PCB board. The material selection of PCB depends on the use of LED. If it is used in important places, it can accept high cost. PCB should use high-cost technical materials or ceramic materials with good thermal conductivity (such as aluminum substrate or copper-clad ceramic substrate, etc.); PCB can use FR-4 (glass fiber reinforced epoxy resin) as the material if it is used in unimportant places that have low performance requirements and are sensitive to cost.
For wire bonding, it can use thermosonic bonding at high temperature (gold wire ball bonding) and ultrasonic bonding at normal temperature (aluminum wedge bonding). COB technology can not only effectively reduce the thermal resistance of packaging, but also improve the power density of LED packaging. Now the packaging of high-power multi chip array LED mainly uses COB technology.
- System packaged (SiP) LED package
In recent years, chip packaging is a new way to meet the needs of chip packaging system. SIP LED packaging technology can not only assemble multiple light-emitting chips in one package, but also integrate various electronic components together to assemble a complete and multifunctional system. Compared with other packaging structures, SIP is more inclined to complex integrated circuit packaging, not just a simple LED packaging. SIP packaging process has high flexibility, low cost, separate testing and short development cycle. SIP packaging can fall into four types according to technology: three-dimensional (3D) packaging type, wafer stacking type, module type and MCM type.
At present, replacing incandescent lamps and high-pressure mercury lamps with high-brightness LED devices requires an increase in the total luminous flux or the available luminous flux. And we can increase the luminous flux through measures such as increasing the integration, increasing the current density, and using large-size chips.
To realize high brightness LED lamps, we need to increase the power of LED, which means that led will produce more heat. The heat dissipation performance of led directly affects the luminous efficiency and service life of LED. If the heat dissipation is poor, the temperature of LED chip will increase due to heat accumulation, which may reduce the luminous efficiency, accelerate the aging of relevant components and reduce the service life of LED. Multi chip array is the best way to obtain high throughput, but the array packaging has many problems, such as heat dissipation, electronic component connection, available space and so on, which limits its development, especially heat dissipation.
The high-density integration of LED chips produces a lot of heat, which can not go away in a short time, and the temperature of the heat dissipation substrate rises rapidly. It is very necessary to adopt efficient packaging process and heat sink structure. There are two types of heat sink structures, one is active heat dissipation, and the other is passive heat dissipation. Passive heat dissipation generally selects fins with a high ribbed coefficient, and dissipates heat to 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 heat transfer coefficient of natural convection, it is only suitable for low power density and low integration. For high-power LEDs (packages), it should adopt active heat dissipation, such as fins + fans, heat pipes, forced convection of liquids, microchannel refrigeration, phase change refrigeration, and so on.
COB packaging technology and SMD packaging technology comparison
COB packaging has gradually matured in the field of LED display applications, especially in the field of outdoor small pitches with its unique technical advantages. Especially in the last two years, with the improvement of production technology and production process, COB packaging technology has made a qualitative breakthrough. Some factors that restricted development in the past have also been solved in the process of technological innovation.
So, what are the advantages of COB packaging technology? What is the difference between it and traditional SMD packaging? Will it replace SMD and become the mainstream of LED display in the future?
Technology comparison
COB packaging is to directly fix the LED chip with conductive glue and insulating glue on the soldering pad of the lamp bead on the PCB board, and then solder the LED chip for conduction performance. After completing the test, we need to encapsulate the chip with epoxy resin.
The SMD package is to fix the LED chip on the pad of the lamp bead holder with conductive glue and insulating glue, and then use the same conduction performance as the COB package to solder. After the performance test, it is encapsulated with epoxy resin, and then subjected to light splitting, cutting and taping, and transported to the screen factory.
Comparison of advantages and disadvantages
There is no doubt that the SMD packaging factory can produce high-quality lamp beads, but the production process is too much and the cost will be relatively high. It will also increase the cost of transportation, material storage and quality control from the lamp bead packaging plant to the screen plant.
SMD believes that the COB packaging technology is too complicated, and the one-time pass rate of the product is not as easy to control as a single lamp, and it is even an insurmountable obstacle. The failure point cannot be repaired, and the yield is low.
SMD packaging, a single lamp bead monomer packaging technology, has accumulated many years of practical experience. Each company has unique skills, scales, and mature technology. It is relatively easy to implement.
COB packaging is a new packaging technology integrating multiple lamp beads. In practice, many technical experience in production equipment, production process equipment and testing methods is accumulated and verified in continuous innovation practice. It requires a high technical threshold and great difficulty. The biggest difficulty currently facing is how to improve the first pass rate of the product. COB packaging is facing a technological peak, which is not insurmountable but is relatively difficult to implement.
The four-corner or hexagonal bracket used in the SMD package brings technical difficulties and reliability risks to the subsequent production links. For example, the reflow soldering process of the lamp bead surface needs to solve the problem of the soldering yield of a large number of bracket pins. If SMD is to be applied outdoors, we must solve the outdoor protection yield problem of the bracket pins.
The COB technology is precisely because this bracket is omitted, there will hardly be too much technical difficulties and reliability hidden dangers in the subsequent production links. There are only two technical hills: one is how to ensure that the lamp bead surface does not show failure points when the IC driver chip surface is reflowed, and the other is how to solve the problem of module ink color consistency.
Comprehensive comparison
COB packaging technology:
From packaging to the completion of display manufacturing, COB packaging technology integrates the middle and downstream links of the LED display industry chain, and all production is completed in one factory. This kind of production organization is simple, the process is compact, the production efficiency is higher, and it is more conducive to the fully automated production layout. This organizational form is also more conducive to the quality control of the entire product process. This form of organization is still an organic whole. In the product development stage, it is necessary to consider the problems that you may encounter in each production link, and comprehensively evaluate and formulate a technical implementation plan. This type of organization can also better assume quality responsibility for end customers.
COB packaging only faces a technological peak on the road of multi-bead integrated packaging technology in the LED display field, which appears in the lamp bead packaging link. And this kind of technology is not insurmountable, but not everyone can climb over it. It is a manifestation of comprehensive technology. This requires countless failures and lessons learned as well as years of technical accumulation and precipitation. It needs a craftsman spirit that is firm, down-to-earth, not afraid of difficulties and innovative. Once you cross this technological peak, it’s like a carp jumping over a dragon gate, and the road behind the mountain will be smooth. There will be no more technical difficulties in the entire production process.
The red product reliability curve shows that once the lamp bead is sealed by the COB package, the subsequent production process has little effect on its reliability. After one year of client application, the reliability index is almost the same as that of packaging.
SMD packaging technology:
In the SMD display industry chain, packaging companies and display companies are two independent types of companies, and these two types of companies share industrial profits with each other. Although the cake is big, there are many enterprises, fierce competition and thin profits. This kind of complex production organization will waste part of the industrial profits and efficiency, and it is relatively difficult to control product quality. Since the packaging link and the display factory link are independent of each other, it is difficult to effectively cooperate and coordinate to tackle the technical difficulties in the production process. Once the end customer uses a product with quality problems, it is difficult to hold accountable with many links involved.
From the perspective of the difficulty of technical implementation in the entire production process and the impact on product reliability, the color and meaning of the curve in the figure below are the same as those in the previous figure. From the figure, we can see that there is a double-humped technology peak in the SMD display packaging industry chain. Both of these technical difficulties appear in the screen factory link, and the packaging link is relatively low due to the mature and stable technology. Therefore, the technical difficulty of SMD display will exceed the difficulty of COB packaging technology when added together.
