Novel Materials and Joinings for Power Electronics Module Packaging
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Novel Materials and Joinings for Power Electronics Module Packaging Eckhard Wolfgang, Gerhard Mitic, Guy Lefranc, Herbert Schwarzbauer Siemens AG, Corporate Technology, Power Electronics D-81730 Munich, Germany ABSTRACT Power electronics modules consist of several layers of different materials according to their function. A base plate is necessary for mounting the module to the cooling unit, an insulating layer provides protection against high voltages, the power semiconductor chips with metal electrodes on both sides are used for switching currents, and finally passivation layers have to protect the chips against high electric fields and environmental impacts. The combination of semiconductors, metals and insulators and their different coefficients of thermal expansion leads to stresses and fatigue during temperature excursions. Some major trends require the use of new materials and joinings: higher voltages (up to 6.5 kV), higher currents (above 2000 A), and higher operating temperatures, e.g. in automotive applications (up to 200°C). To reduce the influence of the thermal mismatch between the base plate and the insulator (Alumina, AlN), metal matrix composites such as AlSiC have been used for several years in power modules. To minimize the partial discharge of the ceramics substrate, an amorphous coating (a-Si:H) was tested at high voltages up to 15 kV. A low-temperature joining technique based on silver powder processed under high pressure and at low temperatures works well at higher temperatures. Finally a concept for securing reliability is discussed. INTRODUCTION Power electronics devices and modules are designed to convert the current into a suitable form. A module contains a number of chips, for instance 24 IGBT chips and 12 diode chips. Where a sinusoidal current of variable frequency is generated by pulse width modulation for operating asynchronous motors, the power electronics module acts as the current switch. A link voltage of up to 3.3 kV (for 6.5 kV IGBT devices) is used to attain high switching power. Today, multi-chip power modules of this kind can switch currents of up to 2400 A. Because semiconductors have a finite electric resistance, a high power dissipation occurs which can assume values of up to 200 W/cm2. Power electronics module packages must therefore have the following properties: - low-inductance feed and output of the currents - effective insulation of the high voltage - dissipation of thermal losses - easy assembly onto bus bars and cooling devices - EMI protection. High reliability requirements are demanded for power modules. Thus 100 FIT is required in the case of traction applications: this means that only one failure may occur in 107 component hours (1141 years). The data sheet gives no specific information on reliability because the manufacturers of power modules cannot know how these will be stressed in the application. However, it does give the results of a series of reliability tests which the modules have passed in the type inspection [1] and thus contains several parameters w
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