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Transfer Molding Technology for Smart Power Electronics Modules - Materials and Processes
Keywords: Transfer Molding , Power Electronics, Automotive Packages
During the last years within power electronics packaging a trend towards compact power electronics modules for automotive and industrial applications could be observed, where a smart integrated control unit for motor drives is replacing bulky substrates with discrete control logic and power electronics. Most recent modules combine control and power electronics yielding maximum miniaturization. Transfer molding is the method of choice for cost effective encapsulation of such modules due to robustness of the molded modules and moderate cost of packaging. But there are challenges with this type of package: Typically those packages are asymmetric, a substrate with single sided assembly is overmolded on the component side and the substrate backside is exposed providing a heat path for optimized cooling. This asymmetric geometry is prone to yield warped substrates, preventing optimum thermal contact to the heatsink and also putting thermomechanical stress on the encapsulated components, possibly reducing reliability. Such packages being truly heterogeneous, combining powerICs, wire bonds, SMDs, controlICs, substrate and leadframe surfaces, the encapsulant used needs to adhere sufficiently to all surfaces present. Additionally those packages need to operate at elevated temperatures for increased times, e.g. operate at 150 °C for 2000 h and more, so high thermal stability is of ample importance. Within this paper a reference application is described, integrating power and control logic inside a leadframe based molded package. Taking into account the challenges mentioned above, a detailed description of material selection for this module will be given, including material analysis as rheology, reactivity, change in r and thermomechanical properties as f(t,T). Process development for module molding is described accompagnied by nondestructive analysis as ultrasonic / x-ray microscopy and package warpage measurement, evaluating the optimum combination of molding process parameters and selected materials. Concluding rules for encapsulant material selection and package setup are provided.
Karl-Friedrich Becker, Group Manager Assembly & Encapsulation
Fraunhofer IZM
Berlin 13355,
Germany


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