Direct Printing of Heat Sinks, Cases and Power Connectors on Insulated Substrate Using Selective Laser Melting Techniques

Abstract

The market introduction of high temperature wide bandgap electronics power semiconductor devices with junction temperature exceeding 200°C significantly accelerates the trend towards high power density and severe ambient temperature electronics applications. Such evolution may have a great impact in aeronautics applications, especially with the development of More Electric Aircraft (MEA), since it can allow to reduce the mass and volume of the power electronics systems. As a consequence, the aircraft operating cost can decrease. However, for electronics used under such harsh conditions, heat evacuation is a very critical issue for the operation and long-term reliability of power modules. Among materials used in the power assembly, Thermal Interface Material (TIM) plays a significant role in improving the thermal contact resistance between the power module package and the heat sink. However, TIM suffers from its high thermal resistance in case of soft material use (thermal grease, phase changing films, elastomers) and from high thermomechanical stresses in case of solder use. This paper focuses on a new approach allowing direct printing of high performance heat sink on the back side of the insulated ceramic substrate leading to the removal of the TIM. In addition, the module case and the electrical connectors can also be built-up on the upper face of the insulated ceramic substrate. Selective Laser Melting (SLM) process is used to achieve complex three dimensional structures with AlSi7Mg0.6 powder alloy on the both sides of direct bonded aluminum substrate. Using this bi-material substrate involves the development of a specific tray to allow ceramic stress relaxation during process. Various heat sink designs including lattice structure, array structures (pin fins, rectangular fins, elliptical fins and water drop shaped fins) as well as channel and cold plate structure were printed. It has been shown that the design has a strong impact on the residual stresses induced during the process, and the latter can induce in some cases, significant substrate warpage and even cracks in the ceramic. Based on the experimental results, design recommendations allowing the reduction of the residual stresses in the structure are briefly introduced. Shear strength measurements have been performed to evaluate the adhesion between the built material and the substrate metallization and shear strength values higher than 20 MPa have been obtained illustrating a good interfacial joint. Finally, the thermal performance of the air cooled direct printed heat sinks was evaluated using thermo-fluidic models and results have been compared to conventional assembly with TIM.