AlSiC Baseplates for Power IGBT Modules: Design, Performance and Reliability

Improved baseplate materials are required to provide superior reliability and heat transfer as IGBT power density increases. The key is for the baseplate thermal coefficient of expansion (TCE) to be matched to the module design and to have sufficient thermal conductivity (κ). Aluminum Silicon Carbide (AlSiC), a metal matrix composite material, provides a TCE that is compatible with the attachment of dielectric substrates and IGBT silicon devices. Matching the AlSiC baseplate TCE to other materials within the IGBT module can provide more than two times longer module life by minimizing thermal stresses that cause high cycle fatigue failure. A matched TCE also eliminates the need for stress compensating compliant layers and expansion graded thick solders that increase thermal resistance and complexity of assembly. The TCE of AlSiC can be adjusted for the IGBT module design by control of the SiC volume fraction in the AlSiC composite. The AlSiC average TCE can be controlled between 7.5 and 12 ppm/°C (30 – 150°C). An AlSiC composition was chosen for the IGBT module with a TCE value of 8.39 ppm/°C (30° 150°C) and a κ value of 180 W/mK. IGBT modules with AlSiC baseplates have equivalent power dissipation and more than two-fold increased reliability over the same module with a Cu baseplate. The IGBT module reliability improves with the AlSiC baseplate because the TCE is matched to the IGBT module design. Cu baseplates have a κ value of 398 W/mK. However, the Cu baseplate TCE of 17 ppm/°C requires thermal stress compensation layers between the Cu baseplate and the dielectric ceramic. The benefit of the high Cu κ value is not fully realized because of the thermal resistance penalty associated with the stress compensating layers. The Ceramics Process Systems AlSiC module baseplates are cost-effectively fabricated to net-shape, attaining close dimensional tolerances with minimal machining. Additionally, this process allows for the fabrication of engineered bow profiles in the cast module base. The AlSiC process flow will also be outlined to illustrate how the process is used to fabricate IGBT baseplates with integrated advanced high thermal conductivity (>1000 W/mK) heat-spreading materials and recirculation cooling paths for higher power applications. Design rules, process capability, fabrication and assembly of AlSiC module baseplates will be discussed in terms of current production designs.