The power LED package substrate acts as a carrier for heat and air convection, and its thermal conductivity plays a decisive role in the heat dissipation of the LED. DPC ceramic substrates have shown strong competitiveness in many electronic packaging materials due to their excellent performance and gradually decreasing price, and are 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 as a new electronic packaging substrate material, the application prospects are very broad.
As the input power of the LED chip continues to increase, the large heat generated by the large dissipated power puts new and higher requirements on the LED packaging material. In the LED heat dissipation channel, the package substrate is the key link connecting the internal and external heat dissipation paths, and has the functions of heat dissipation channel, circuit connection and physical support for the chip. For high-power LED products, the package substrate is required to have high electrical insulation, high thermal conductivity, and thermal expansion coefficient matched with the chip.
Resin-based package substrate: it is difficult to achieve high cost of matching
EMC and SMC have high requirements for molding equipment, and the price of a molding production line is about 10 million yuan. It is still difficult to popularize on a large scale.
In recent years, patch-type LED brackets have been generally used in high-temperature modified engineering plastic materials. PPA (polyphthalamide) resin is used as raw material to enhance certain physical and chemical properties of PPA raw materials by adding modified fillers. Therefore, the PPA material is more suitable for the use of injection molding and SMD LED brackets. PPA plastics have very low thermal conductivity, and their heat dissipation is mainly through metal lead frames. The heat dissipation capability is limited, and it is only suitable for low-power LED packages.
With the industry's emphasis on LED heat dissipation, two new thermoset plastic materials, epoxy molding compound (EMC) and sheet molding compound (SMC), were introduced into the SMD LED holder. EMC is a powder molding compound which is a high-performance phenolic resin as a curing agent, a silicon micropowder with a high thermal conductivity, and a mixture of various additives. SMC is mainly composed of about 30% unsaturated resin, about 40% glass fiber, inorganic filler and other additives. The heat curing temperature of these two thermosetting molding compounds is about 150 °C, and the thermal conductivity after modification can reach 4W/(m·K)～7W/(m·K), which is greatly improved compared with PPA plastic, but the disadvantages It is difficult to balance both fluidity and thermal conductivity, and the hardness is too high during solidification molding, which is prone to cracks and burrs. EMC and SMC have long curing time and relatively low molding efficiency. The requirements for molding equipment, molds and other ancillary equipment are quite high. The price of a molding and supporting production line is about 10 million yuan. It is still difficult to mass-produce.
Metal core printed circuit board: manufacturing process is complex, practical applications are less
The processing and manufacturing process of aluminum substrates is complicated and costly. The thermal expansion coefficient of aluminum differs greatly from that of chip materials, and is rarely used in practical applications.
As LED packages are becoming thinner and less costly, on-chip ( COB ) packaging technology is emerging. At present, most of the COB package substrates use metal core printed circuit boards, and high-power LED packages mostly use such substrates, and the price is between medium and high price.
At present, the high-power LED heat-dissipating substrate commonly used in production has extremely low thermal conductivity of the insulating layer, and because of the existence of the insulating layer, it cannot withstand high-temperature soldering, which limits the optimization of the package structure and is not conducive to LED heat dissipation.
How to improve the thermal conductivity of epoxy insulation layer has become a research hotspot of aluminum substrates at this stage. At present, a modified epoxy resin or epoxy glass cloth bonded with a highly thermally conductive inorganic filler (such as ceramic powder) is used, and the copper foil, the insulator and the aluminum plate are bonded by hot pressing. At present, a "all-glue aluminum substrate" has been developed in the world, and the thermal resistance of the fully-coated aluminum substrate can be 0.05K/W. In addition, a company in Taiwan recently developed a diamond-like carbon material DLC and applied it to the insulating layer of a high-brightness LED package aluminum substrate. 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 practical use effect has yet to be tested in the market.
A better performance aluminum substrate is to directly form an insulating layer on the aluminum plate and then print the circuit. The biggest advantage of this method is the strong bonding force and thermal conductivity of up to 2.1 W / (m · K). However, the processing and manufacturing process of the aluminum substrate is complicated and costly. Moreover, the thermal expansion coefficient of the metal aluminum is greatly different from that of the chip material, and the thermal cycle often generates a large stress during the operation of the device, which may eventually lead to failure, so in practical applications, Less adopted.
Silicon-based package substrate: face the challenge rate of less than 60%
The silicon substrate faces challenges in the preparation of the insulating layer, the metal layer, and the via holes, and the yield is not more than 60%.
Silicon-based materials have been introduced into the LED industry from the semiconductor industry in recent years. The thermal conductivity and thermal expansion properties of the silicon substrate indicate that silicon is a packaging material that matches the LED. The thermal conductivity of silicon is 140W/m·K. When applied to LED package, the thermal resistance is only 0.66K/W. Moreover, silicon-based materials have been widely used in semiconductor manufacturing and related packaging, and related equipment and materials are involved. It is quite mature. Therefore, if silicon is fabricated into an LED package substrate, mass production is likely to occur.
However, LED silicon substrate packages still have many technical problems. For example, in terms of materials, silicon materials are easily broken and the strength of the mechanism is also problematic. In terms of structure, although silicon is an excellent heat conductor, it has poor insulation and must be treated with oxidative insulation. In addition, the metal layer needs to be prepared by sputtering and electroplating, and the conductive holes are required to be corroded. In general, the preparation of insulating layers, metal layers, and via holes is challenging, and the yield is not high. At present, although some Taiwanese companies have developed LED silicon substrates and mass production, the yield rate does not exceed 60%.
Ceramic package substrate: Improve heat dissipation efficiency to meet high power LED requirements
With the high thermal conductivity ceramic substrate, DPC significantly improves the heat dissipation efficiency, and is the most suitable product for high power and small size LED development.
The ceramic heat-dissipating substrate has a new heat-conducting material and a new internal structure, which compensates for the defects of the aluminum metal substrate, thereby improving the overall heat dissipation effect of the substrate. Among the ceramic materials currently used as heat-dissipating substrates, BeO has a high thermal conductivity, but its linear expansion coefficient is very different from that of silicon (Si), and it is toxic during manufacturing, which limits its application; BN has better comprehensive performance. However, as a substrate material, there are no outstanding advantages, and it is expensive. It is currently only in research and promotion; silicon carbide (SiC) has high strength and high thermal conductivity, but its resistance and insulation withstand voltage are low, and metallized keys Unstable, it will cause changes in thermal conductivity and dielectric constant, and should not be used as an insulating package substrate material. Although Al2O3 ceramic substrate is the most widely used and widely used ceramic substrate, its thermal expansion coefficient is higher than that of Si single crystal, which makes the Al2O3 ceramic substrate not suitable for high frequency, high power and very large scale integrated circuits. Used in. A1N crystals have high thermal conductivity and are considered to be ideal materials for next-generation semiconductor substrates and packages.
AlN ceramic materials have been extensively studied since the 1990s and gradually developed. It is an electronic ceramic packaging material that is generally considered to be promising. The heat dissipation efficiency of the AlN ceramic substrate is 7 times that of the Al2O3 substrate, and the heat dissipation effect of the AlN substrate applied to the high-power LED is remarkable, thereby greatly increasing the service life of the LED. The disadvantage of the AlN substrate is that even a very thin oxide layer on the surface has a large influence on the thermal conductivity, and only a strict control of the materials and processes can produce a uniform AlN substrate. At present, the mass production of AlN is still immature. Compared with the commonly used Al2O3 substrate, the cost of the AlN substrate is about 3-5 times that of the Al2O3 substrate. However, if energy is produced in the future, the cost of the AlN substrate can be rapidly reduced, and the AlN substrate with strong heat dissipation will have an opportunity to replace the Al2O3 substrate.
At present, ceramic substrates used in LED packaging can be classified into four types: HTCC, LTCC, DBC, and DPC according to the preparation technology. HTCC is also known as high-temperature co-fired multi-layer ceramics. Its main materials are tungsten, molybdenum, manganese and other metals with higher melting point but poor conductivity. The production cost is high and it is rarely used now. LTCC is also known as low-temperature co-fired multilayer ceramic substrate, and its thermal conductivity is about 2W/(m·K) to 3W/(m·K), which is not much superior to existing aluminum substrates. In addition, LTCC uses thick film printing technology to complete the circuit production, the surface of the circuit is rough, and the alignment is not accurate. Moreover, the multilayer ceramic laminate sintering process also has the problem of shrinkage ratio, which limits the process resolution, and the popularization and application of LTCC ceramic substrates is greatly challenged.
A direct copper-clad ceramic plate (DBC) developed based on on-board packaging technology is also a ceramic substrate with excellent thermal conductivity. The DBC substrate does not use a binder during the preparation process, so the thermal conductivity is good, the strength is high, the insulation is strong, and the thermal expansion coefficient is matched with a semiconductor material such as Si. However, the ceramic substrate has low reactivity with metal materials and poor wettability. It is difficult to carry out metallization, and it is difficult to solve the problem of micropores between Al2O3 and copper plates, which makes the mass production and yield of the product more challenging. It is still the focus of research by researchers at home and abroad.
DPC ceramic substrate is also called direct copper-plated ceramic plate. DPC products have the characteristics of high line precision and high surface flatness. They are very suitable for LED flip-chip/eutectic process. With high thermal conductivity ceramic substrate, the heat dissipation efficiency is significantly improved. It is the most suitable ceramic heat sink substrate for high power and small size LED development.
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