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As the carrier of heat and air convection, the thermal conductivity of power package substrate plays a decisive role in heat dissipation. DPC ceramic substrate with its excellent performance and gradually reduced price, in many electronic packaging materials show a strong competitiveness, is the future development trend of packaging.
With the development of science and technology and the emergence of new preparation technology, the application prospect of high thermal conductivity ceramic material as a new type of electronic packaging substrate material is very broad.
With the continuous increase of chip input power, the large heat generated by large dissipation power puts forward new and higher requirements for packaging materials. In the heat dissipation channel, the package substrate is the key link to connect 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 products, the packaging substrate requires characteristics such as high electrical insulation, high thermal conductivity, and thermal expansion coefficient matching the chip.
Resin-based packaging substrate: It is difficult to popularize with high supporting cost
EMC and SMC have high requirements for molding equipment, and the price of a molding production line is about 10 million yuan, which is still difficult to popularize on a large scale.
In recent years, the rise of the mount-type bracket generally uses high-temperature modified engineering plastic material, with PPA(polyphthalamide) resin as the raw material, by adding modified fillers to enhance some of the physical and chemical properties of PPA raw materials, so that PPA materials are more suitable for injection molding and the use of mount-type brackets. PPA plastic has very low thermal conductivity, its heat dissipation is mainly carried out through the metal lead frame, the heat dissipation capacity is limited, and it is only suitable for small power packages.
With the industry's attention to heat dissipation, two new thermosetting plastic materials - epoxy plastic sealer (EMC) and sheet molding plastic (SMC) were introduced into the mount-type bracket.
EMC is a powdery molding plastic made of high performance phenolic resin as curing agent, silicon powder with high thermal conductivity as filler and a variety of additives. SMC is mainly composed of about 30% unsaturated resin, about 40% glass fiber, inorganic fillers and other additives.
The thermal curing temperature of these two thermosetting molding plastics 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 plastics, but the disadvantage is that the fluidity and thermal conductivity are difficult to take into account, and the hardness of curing molding is too high, and it is easy to produce cracks and burrs. EMC and SMC curing time is long, the molding efficiency is relatively low, the requirements for molding equipment, molds and other supporting equipment is quite high, a molding and supporting production line price of about 10 million yuan, large-scale popularization is still difficult.
Metal core printed circuit board: the manufacturing process is complex and the practical application is less
Aluminum substrate manufacturing process is complex, high cost, the coefficient of thermal expansion of aluminum and chip material is very different, the practical application is rarely used.
With the development of packaging in the direction of thin and low cost, chip on board (COB) packaging technology is gradually emerging. At present, most COB packaging substrates use metal core printed circuit boards, and high-power packaging mostly uses such substrates, and its price is between middle and high prices.
At present, the high power cooling substrate commonly used in production has a very low thermal conductivity of the insulation layer, and because of the existence of the insulation layer, it can not withstand high temperature welding, which limits the optimization of the package structure and is not conducive to heat dissipation.
How to improve the thermal conductivity of epoxy insulation layer has become the research hotspot of aluminum substrate at present. At present, a modified epoxy resin or epoxy glass cloth binder with a high thermal conductivity inorganic filler (such as ceramic powder) is used to bind the copper foil, insulator and aluminum sheet by hot pressing.
At present, a "all-rubber aluminum substrate" has been developed internationally, and the thermal resistance of the aluminum substrate using all rubber can be 0.05K/W. In addition, a company in Taiwan recently developed a kind of drilling carbon material DLC, and applied it to the insulation layer of high-brightness LED packaging aluminum substrate.
DLC has many superior material properties: high thermal conductivity, thermal uniformity and high material strength. Therefore, replacing the epoxy insulation layer of the traditional metal-based printed circuit board (MCPCB) with DLC is expected to greatly improve the thermal conductivity of MCPCB, but its actual use effect has yet to be tested by the market.
A better performing aluminum substrate is to create an insulating layer directly on the aluminum plate and then print the circuit. The biggest advantage of this method is that the binding force is strong and the thermal conductivity is as high as 2.1W/(m·K). However, the processing and manufacturing process of this aluminum substrate is complex and costly, and the thermal expansion coefficient of metal aluminum is greatly different from the chip material, and the thermal cycle of the device often produces a large stress, which may eventually lead to failure, so it is less used in practical applications.
Silicon-based packaging substrate: Facing challenges the yield rate is less than 60%
The silicon substrate faces challenges in the preparation of insulation layer, metal layer and through hole, and the yield is not more than 60%.
Silicon-based materials as packaging substrate technology has gradually been introduced from the semiconductor industry to the industry in recent years. The thermal conductivity and thermal expansion properties of silicon substrate indicate that silicon is a suitable packaging material.
The thermal conductivity of silicon is 140W/m·K, and the thermal resistance caused by it is only 0.66K/W when applied to packaging. Moreover, silicon-based materials have been widely used in semiconductor processes and related packaging fields, and the related equipment and materials involved have been quite mature. Therefore, if the silicon is made into a package substrate, it is easy to form mass production.
However, there are still many technical problems with silicon substrate packaging. For example, in terms of materials, silicon is prone to fragmentation, 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 oxidation insulation.
In addition, the metal layer needs to be prepared by sputtering combined with electroplating, and the conductive hole needs to be corroded. In general, the preparation of insulation layer, metal layer and through-hole faces challenges, and the yield is not high. At present, although some Taiwan enterprises have developed LED silicon substrate and mass production, but the yield is not more than 60%.
Ceramic encapsulated substrate: Improves heat dissipation efficiency to meet high power requirements
Combined with high thermal conductivity ceramic matrix, DPC significantly improves heat dissipation efficiency and is the most suitable product for high power and small size development needs.
The ceramic cooling substrate has a new thermal conductivity material and a new internal structure, which makes up for the defects of the aluminum metal substrate, thereby improving the overall heat dissipation effect of the substrate.
Al2O3 ceramic substrate is currently the largest production, the most widely used ceramic substrate, but due to its thermal expansion coefficient relative to Si single crystal is high, resulting in Al2O3 ceramic substrate is not suitable for use in high frequency, high power, very large scale integrated circuits. A1N crystals have high thermal conductivity and are considered ideal materials for next-generation semiconductor substrates and packages.
AlN ceramic materials have been widely studied and gradually developed since the 1990s, and it is generally considered to be a promising electronic ceramic packaging material. The heat dissipation efficiency of AlN ceramic substrate is 7 times that of Al2O3 substrate, and the heat dissipation efficiency of AlN substrate applied to high-power LED is significant, which greatly improves the service life of LED.
The disadvantage of AlN substrate is that even if there is a very thin oxide layer on the surface, it will have a greater impact on the thermal conductivity, and only by strictly controlling the material and process can the AlN substrate be manufactured with good consistency.
At present, ceramic substrates used in packaging can be divided into HTCC, LTCC, DBC and DPC4 according to the preparation technology. HTCC is also known as high temperature co-fired multi-layer ceramics, its main material is high melting point but poor conductivity of tungsten, molybdenum, manganese and other metals, high production costs, now less used.
LTCC is also known as low-temperature co-fired multilayer ceramic substrate, and its thermal conductivity is about 2W/(m·K) ~ 3W/(m·K), which is not much advantage compared with the existing aluminum substrate. In addition, LTCC uses thick film printing technology to complete the line production, the line surface is rough and the alignment is not accurate. In addition, the shrinkage ratio of multi-layer ceramic sintering process is also a problem, which limits the process resolution and challenges the popularization and application of LTCC ceramic substrates.
Direct copper-coated ceramic plate (DBC) developed based on on-board packaging technology is also a ceramic substrate with excellent thermal conductivity. No binder is used in the preparation process of DBC substrate, so the thermal conductivity is good, the strength is high, the insulation is strong, and the thermal expansion coefficient is matched with semiconductor materials such as Si.
However, the reaction ability of ceramic substrate and metal materials is low, poor wettability, the implementation of metallization is quite difficult, and it is not easy to solve the problem of micro-porosity between Al2O3 and copper plate, which makes the mass production and yield of the product subject to greater challenges, and is still the focus of research by domestic and foreign researchers.
DPC ceramic package substrate is also known as direct copper-plated ceramic plate, DPC products have high line accuracy and high surface flatness characteristics, very suitable for LED coating/eutectic process, with high thermal conductivity ceramic matrix, significantly improve the heat efficiency, is the most suitable for high power, small size LED development needs of ceramic cooling substrate.
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