Let's talk about the differences in power LED packaging substrates.

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Let's talk about the differences in power LED packaging substrates. [Copy link]

As a carrier of heat and air convection, the thermal conductivity of power LED package substrate plays a decisive role in the heat dissipation of LED. With its excellent performance and gradually decreasing price, DPC ceramic substrate has shown strong competitiveness among many electronic packaging materials and is the development trend of power LED packaging in the future. With the development of science and technology and the emergence of new preparation processes, high thermal conductivity ceramic materials have broad application prospects as new electronic packaging substrate materials. As the input power of LED chips continues to increase, the large heat generated by large dissipation power puts forward newer and higher requirements for LED packaging materials. In the LED heat dissipation channel, the package substrate is the key link connecting the internal and external heat dissipation channels, and has the functions of heat dissipation channel, circuit connection and physical support for the chip. For high-power LED products, its package substrate is required to have high electrical insulation, high thermal conductivity, and thermal expansion coefficient matching with the chip. 1. Resin-based packaging substrate: high supporting cost and difficult to popularize EMC and SMC have high requirements for compression molding equipment. The price of a compression molding production line is about 10 million yuan, and it is difficult to popularize on a large scale. The SMD LED brackets that have emerged in recent years generally use high-temperature modified engineering plastics, with PPA (polyphthalamide) resin as raw material. By adding modified fillers to enhance certain physical and chemical properties of PPA raw materials, PPA materials are more suitable for injection molding and SMD LED brackets. PPA plastic has very low thermal conductivity, and its heat dissipation is mainly carried out through the metal lead frame. The heat dissipation capacity is limited and it is only suitable for low-power LED packaging. As the industry pays more attention to LED heat dissipation, two new thermosetting plastic materials - epoxy molding compound (EMC) and sheet molding compound (SMC) have been introduced into SMD LED brackets. EMC is a powder molding compound made of high-performance phenolic resin as a curing agent, silicon micropowder with high thermal conductivity as a filler, and a variety of additives. SMC is mainly composed of about 30% unsaturated resin, about 40% glass fiber, inorganic filler and other additives. The thermal curing temperature of these two thermosetting molding compounds is about 150℃. After modification, the thermal conductivity can reach 4W/(m·K)~7W/(m·K), which is much higher than that of PPA plastic. However, the disadvantage is that it is difficult to balance fluidity and thermal conductivity. The hardness is too high during curing and molding, which is easy to produce cracks and burrs. EMC and SMC have a long curing time and relatively low molding efficiency. They have high requirements for molding equipment, molds and other supporting equipment. The price of a molding and supporting production line is about 10 million yuan, and it is difficult to popularize them on a large scale. 2. Metal core printed circuit board: complex manufacturing process and less practical application The processing and manufacturing process of aluminum substrates is complex and costly. The thermal expansion coefficient of aluminum is quite different from that of chip materials, so it is rarely used in practical applications. As LED packaging develops towards thinness and low cost, chip-on-board (COB) packaging technology has gradually emerged. At present, most COB packaging substrates use metal core printed circuit boards, and most high-power LED packaging uses this type of substrate, and its price is between medium and high prices. The insulation layer of the high-power LED heat dissipation substrate currently in production has a very low thermal conductivity, and due to the existence of the insulation layer, it cannot withstand high-temperature welding, which limits the optimization of the packaging structure and is not conducive to LED heat dissipation. How to improve the thermal conductivity of the epoxy insulation layer has become a research hotspot for aluminum substrates at this stage. At present, a modified epoxy resin or epoxy glass cloth adhesive sheet doped with high thermal conductivity inorganic fillers (such as ceramic powder) is used to bond the copper foil, insulator and aluminum plate together through hot pressing. At present, a "full-glue aluminum substrate" has been developed internationally, and the thermal resistance of the aluminum substrate using full glue can reach 0.05K/W. In addition, a company in Taiwan, my country recently developed a diamond-like carbon material DLC and applied it to the insulation layer of high-brightness LED packaging aluminum substrates. DLC has many superior material properties: high thermal conductivity, thermal uniformity and high material strength. Therefore, replacing the epoxy resin insulation layer of the traditional metal-based printed circuit board (MCPCB) with DLC is expected to greatly improve the thermal conductivity of the MCPCB, but its actual use effect has yet to be tested by the market. A better aluminum substrate is to directly generate an insulation layer on the aluminum plate and then print the circuit. The biggest advantage of this method is strong bonding and a thermal conductivity of up to 2.1W/(m·K). However, the processing and manufacturing process of this aluminum substrate is complicated and costly. In addition, the thermal expansion coefficient of metal aluminum is quite different from that of the chip material. When the device is working, the thermal cycle often generates large stress, which may eventually lead to failure. Therefore, it is rarely used in practical applications. 3. Silicon-based packaging substrate: facing challenges with a yield rate of less than 60% Silicon substrates face challenges in the preparation of insulation layers, metal layers, and vias, and the yield rate does not exceed 60%. In recent years, the technology of using silicon-based materials as LED packaging substrates has been gradually introduced from the semiconductor industry to the LED industry. The thermal conductivity and thermal expansion properties of silicon substrates both indicate that silicon is a packaging material that is more compatible with LEDs. The thermal conductivity of silicon is 140W/m·K. When used in LED packaging, the thermal resistance caused is only 0.66K/W; and silicon-based materials have been widely used in semiconductor processes and related packaging fields, and the related equipment and materials involved are quite mature. Therefore, if silicon is made into an LED packaging substrate, it is easy to form mass production. However, there are still many technical problems in LED silicon substrate packaging. For example, in terms of materials, silicon materials are easy to break and have problems with structural strength. In terms of structure, although silicon is an excellent thermal conductor, it has poor insulation and must be treated with oxidation insulation. In addition, its metal layer needs to be prepared by sputtering combined with electroplating, and the conductive holes need to be made by etching. In general, the preparation of the insulating layer, metal layer, and vias are all challenging, and the yield rate is not high. Although some Taiwanese companies have developed LED silicon substrates and mass-produced them, the yield rate does not exceed 60%. 4.Ceramic packaging substrate: Improve heat dissipation efficiency to meet the needs of high-power LEDs With a high thermal conductivity ceramic substrate, DPC significantly improves heat dissipation efficiency and is the most suitable product for the development needs of high-power, small-size LEDs. Ceramic heat dissipation substrates have new thermal conductive materials and new internal structures, which make up for the defects of aluminum metal substrates, thereby improving the overall heat dissipation effect of the substrate. Among the ceramic materials that can be used as heat dissipation substrates, although BeO has high thermal conductivity, its linear expansion coefficient is very different from that of silicon (Si), and it is toxic during manufacturing, which limits its application; BN has good comprehensive performance, but as a substrate material, it has no outstanding advantages and is expensive. It is currently only under research and promotion; Silicon carbide (SiC) has high strength and high thermal conductivity, but its resistance and insulation withstand voltage values are low, and the bonding is unstable after metallization, which will cause changes in thermal conductivity and dielectric constant, and it is not suitable as an insulating packaging substrate material. Although Al2O3 ceramic substrates are currently the most produced and widely used ceramic substrates, due to their relatively high thermal expansion coefficient relative to Si single crystals, Al2O3 ceramic substrates are not very suitable for use in high-frequency, high-power, and ultra-large-scale integrated circuits. A1N crystals have high thermal conductivity and are considered to be ideal materials for new-generation semiconductor substrates and packaging. AlN ceramic materials have been widely studied and gradually developed since the 1990s. They are currently generally considered to be electronic ceramic packaging materials with great development prospects. The heat dissipation efficiency of AlN ceramic substrates is 7 times that of Al2O3 substrates. The heat dissipation benefits of AlN substrates applied to high-power LEDs are significant, which greatly increases the service life of LEDs. The disadvantage of AlN substrates is that even if there is a very thin oxide layer on the surface, it will have a great impact on the thermal conductivity. Only by strictly controlling the materials and processes can AlN substrates with good consistency be manufactured. At present, large-scale production of AlN is not mature. Compared with the currently widely used Al2O3 substrates, the cost of AlN substrates is about 3 to 5 times that of Al2O3 substrates. However, if mass production can be achieved in the future, the cost of AlN substrates can drop rapidly, and then AlN substrates with strong heat dissipation benefits will have the opportunity to replace Al2O3 substrates. At present, ceramic substrates used in LED packaging can be divided into HTCC, LTCC, DBC, and DPC according to the preparation technology. HTCC, also known as high-temperature co-fired multilayer ceramics, is mainly made of metals such as tungsten, molybdenum, and manganese with high melting points but poor conductivity. It is expensive to make and is rarely used now. LTCC, also known as low-temperature co-fired multilayer ceramic substrate, has a thermal conductivity of about 2W/(m·K) to 3W/(m·K), which is not much better than the existing aluminum substrate. In addition, since LTCC uses thick film printing technology to complete circuit production, the circuit surface is relatively rough and the alignment is not precise. Moreover, the multilayer ceramic stacking and sintering process also has the problem of shrinkage ratio, which limits its process resolution and poses great challenges to the promotion and application of LTCC ceramic substrates. Direct copper-clad ceramic board (DBC), which was developed based on on-board packaging technology, is also a ceramic substrate with excellent thermal conductivity. No adhesive is used in the preparation process of DBC substrate, so it has good thermal conductivity, high strength, strong insulation, and a thermal expansion coefficient that matches semiconductor materials such as Si. However, ceramic substrates have low reactivity with metal materials and poor wettability, making metallization difficult. It is also not easy to solve the problem of micropores between Al2O3 and the copper plate, which poses great challenges to the mass production and yield rate of this product. It is still the focus of research by domestic and foreign researchers. DPC ceramic substrates are also called direct copper-plated ceramic substrates. DPC products have the characteristics of high circuit accuracy and high surface flatness. They are very suitable for LED flip chip/eutectic processes. With a high thermal conductivity ceramic matrix, the heat dissipation efficiency is significantly improved. It is the most suitable ceramic heat dissipation substrate for the development needs of high-power, small-size LEDs.