Active Metal Brazing Ceramic Substrate
According to the 3-dimensional structure of the ceramic substrate, it can be divided into plane and multi-layer. The well-known LTCC and HTCC belong to the multi-layer. And the plane has thin film substrate, thick film substrate, ceramic copper clad substrate and so on.
AMB Active Metal Brazing Ceramic Substrate is actually an improvement of the DBC Direct Bonded Copper Ceramic Substrate technology we introduced earlier.
The difference between DBC and AMB process
The so-called DBC technology refers to heating at a high temperature of about 1063 ° C in oxygen-containing nitrogen, and directly welding a layer of copper foil on the surface of alumina or aluminum nitride ceramics.
The basic principle is: the copper-oxygen eutectic liquid phase formed by copper and oxygen during sintering is used to wet the surfaces of the two materials in contact with each other, that is, the copper foil surface and the ceramic surface, and also react with alumina to form CuAlO2, Cu (AlO2)2 and other composite oxides act as solders for eutectic brazing to achieve a firm bond between copper foil and ceramics.
Aluminum nitride is a non-oxide ceramic. The key to bonding copper foil is to form an oxide transition layer on the surface, and then to achieve the bonding of AlN and Cu foil through the above-mentioned transition layer and Cu foil.
The AMB active copper soldering process is a further development of the DBC process technology. It is a method of using a small amount of active elements contained in the solder to react with the ceramic to form a reaction layer that can be wetted by the liquid solder, so as to realize the bonding of the ceramic and the metal.
First, the ceramic surface is printed with active metal solder, then clamped with oxygen-free copper, and then welded at high temperature in a vacuum brazing furnace. After the substrate is covered, a circuit is made on the surface by a wet etching process similar to a PCB board, and finally the surface is plated to prepare produce reliable products.
AMB substrate is bonded by chemical reaction between ceramic and active metal solder paste at high temperature, so its bonding strength is higher and reliability is better. However, due to the high cost of this method, less suitable solder, and the solder has a great influence on the reliability of soldering.
The above two paragraphs illustrate the difference between AMB and DBC. The key point is that in the AMB process, active metal solder is printed on the ceramic surface, and the welding interface has high strength and better reliability.
DBC technology is only suitable for oxide ceramics such as alumina (Al2O3) and zirconia-doped alumina (also known as HPS).
Non-oxide ceramics must be oxidized before they can be bonded to copper by DBC technology.
Aluminum nitride (AlN) can be made into DBC or AMB substrates, while silicon nitride (Si3N4) can only be used as AMB substrates.
Three materials of AMB copper clad laminate
1 AMB alumina substrate
In contrast, alumina sheet has a wide range of sources and the lowest cost. It is the most cost-effective AMB ceramic substrate, and the process is the most mature. However, due to the low thermal conductivity and limited heat dissipation capacity of alumina ceramics, AMB alumina substrates are mostly used in fields with low power density and no strict requirements on reliability.
2 AMB aluminum nitride based aluminum nitride plate
The AMB substrate has a high heat dissipation capacity, so it is more suitable for some high-power, high-current working environments. However, due to the relatively low mechanical strength, the high and low temperature cycle impact life of the aluminum nitride AMB copper clad substrate is limited, which limits its application range. The aluminum nitride AMB substrate has high heat dissipation capacity, so it is more suitable for some high-power, high-current working environments. However, due to the relatively low mechanical strength, the high and low temperature cycle impact life of the aluminum nitride AMB copper clad substrate is limited, which limits its application range.
3 AMB silicon nitride substrate
Silicon nitride ceramics have two crystal forms, α-Si3N4 and β-Si3N4, in which the α phase is an unstable phase, which is easily transformed into a stable β phase at high temperatures. The content of β phase in high thermal conductivity silicon nitride ceramics is generally greater than 40%. With the excellent characteristics of silicon nitride ceramics, AMB silicon nitride substrates have excellent high temperature resistance, corrosion resistance and oxidation resistance.
Advantages of AMB silicon nitride substrates
1 AMB silicon nitride substrate has high thermal conductivity.
On the one hand, the AMB silicon nitride substrate has high thermal conductivity (>90W/mK), and the thick copper layer (up to 800μm) also has high heat capacity and heat transfer. Therefore, AMB silicon nitride substrates are the preferred substrate material for automobiles, wind turbines, traction systems and HVDC drives that require high reliability, heat dissipation and partial discharge.
Active metal brazing, on the other hand, can bond very thick copper metal (up to 0.8mm thick) to relatively thin silicon nitride ceramics. Therefore, the current carrying capacity is high, and the heat transfer is also very good. Customers can customize the product layout, which is similar to a PCB circuit board.
2 AMB silicon nitride substrates have a low thermal expansion coefficient.
The thermal expansion coefficient (2.4 ppm/K) of silicon nitride ceramics is small, close to that of silicon chips (4 ppm/K), and has good thermal matching. Therefore, AMB silicon nitride substrates are very suitable for reliable packaging of bare chips, and the packaged components are not easy to fail during the product life cycle.
Basis for various ceramic substrate processes and material selection
Ceramics are chemically inert and resistant to corrosion, moisture and high temperatures, making them preferred over organic dielectrics that degrade in corrosive environments. When designing new substrates, electrical, thermal and mechanical properties are equally important. Dielectric strength is an important factor in meeting thermal insulation requirements and needs to be set according to the standards, codes and regulations of the target application. Too low thermal conductivity is not conducive to heat conduction between the chip and the surrounding environment. When the substrate is subjected to thermo-mechanical stress, its flexural strength and fracture toughness play an important role in prolonging the service life.
Major manufacturers of AMB ceramic substrates
The main international players are: Heraeus, Ferrotec, DOWA, NGK, Kyocera, Rogers…
Chinese manufacturers: there are currently more than 5 mainstream suppliers