Inorganic Substrates for Power Electronics Applications

The power electronic modules use ceramic insulation substrates mainly. This is caused by extreme requirements regarding power (voltage, current) and the very highrequired heat dissipation capability. This requirement is increasing to make use of higher junction temperature up to 175°C, 200°C and beyond possible. The right choice of ceramic material highly depends on the applications and certain insulation, reliability and thermal specifications. Of course, cost efficiency plays an import role beside the quality, therefore a perfect fit to each application is desirable. The most widely used traditional ceramic insulation material is aluminum oxide ceramics (Al2O3). For improved mechanical robustness, to achieve higher reliability, ZTA (Zirconia Toughened Alumina) is often used. ZTA is based of aluminum oxide material in which up to 30 % of ZrO2 are deposited in the ceramic matrix. It’s bending strength is higher than 500 MPa while aluminum oxide ceramics has about 400 MPa. For applications which are focused on thermal performance and which requires highest insulation capability thicker ceramic aluminum nitride (AlN) is mainly used. AlN offers superior thermal conductivity performance of about 180 W/mK (at r.t.), which is 7 to 8 times higher than that of aluminum oxide that has about 24 W/mK (at r.t.). However, aluminum nitride has relatively low bending strength of 300-400 MPa. Therefore, the reliability performance is limited and requires use of special base plate material, e.g. AlSiC to compensate this weakness. A further ceramic material is Silicon nitride (Si3N4). It provides the best mechanical robustness, e.g. the bending strength of silicon nitride is higher than 600 MPa. In addition, its thermal conductivity of approximately 90 W/mK (at r.t.) is by far superior to Al2O3 and ZTA ceramics. Due to favorable combination of mechanical and thermal properties, this type of ceramics is reasonable choice for applications that require high reliability and high power density. Several different bonding technologies for connecting of the metal and ceramic material to form the substrates will be discussed. The widely used technology is Direct Bonded Copper (DBC) – at high temperature a thin copperoxide on the copper foil is used to create a eutectic bond between the copper and ceramic. Another process is Active Metal Brazing (AMB) in which the copper and ceramic foils are joined by using a brazing process. For the sake of completeness, Direct Bonded Aluminum (DBA) has to be mentioned. It uses AlSi-brazing to bond the aluminum and the ceramic. Another method to form metal bonded ceramic substrates is Thick Printed Copper (TPC). Thick layers of copper paste are applied by screen printing process and are fired at 850 – 950 °C to sinter the metal powders and create a high adhesion bond between the metallic film and the ceramic substrate. This technology provides excellent temperature cycling reliability and opens possibility to combine power and logic components by parallel realization of thin and thick Cu pattern tracks. The increase of power density and junction temperatures drive the need for new packaging materials, e.g. sintering technology. These new materials put special requests on the functional surfaces of the substrates for best reliability the interconnect material and the interface layers must be perfectly matched together.