Transfer molding technology for smart power electronics modules - materials and processes

摘要

During the last years within power electronics packaging a trend towards compact power electronics modulesfor automotive and industrial applications could be observed, where a smart integrated control unit for motordrives is replacing bulky substrates with discrete control logic and power electronics. Most recent modulescombine control and power electronics yielding maximum miniaturization. Transfer molding is the method ofchoice for cost effective encapsulation of such modules due to robustness of the molded modules and moderatecost 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 thecomponent side and the substrate backside is exposed providing a heat path for optimized cooling. Thisasymmetric geometry is prone to yield warped substrates, preventing optimum thermal contact to the heatsinkand also putting thermomechanical stress on the encapsulated components, possibly reducing reliability.Such packages being truly heterogeneous, combining powerICs, wire bonds, SMDs, controlICs, substrate andleadframe 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°Cfor 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 leadframebased molded package. Taking into account the challenges mentioned above, a detailed description of materialselection for this module will be given, including material analysis as rheology, reactivity, change in εr andthermomechanical properties as f(t,T) and of media storage. Process development tools for module molding areused to ensure manufacturability and useability. Concluding rules for encapsulant material selection andpackage setup are provided.