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Smart Power Module Molding Advances: Evaluating high temperature suitability of Molding Compounds
Keywords: Smart Power Module, Transfer Molding, High Temperature Packaging
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 long time, e.g. operate at 200 °C for 1000 h and more, so high thermal stability is of ample importance. Within this paper the results of a project funded by ECPE are provided. Project focus is to identify transfer molding compounds suitable for the encapsulation of smart power modules, ready to be used at 200 °C and determine the actual max. temperature of use of such high performance molding compounds currently available in the market. This is done on bulk material level determine a large number of material properties in initial state and after material ageing at 200 °C, 220 °C and 250 °C. Material properties evaluated are degradation temperature, CTE, Young’s modulus, glass transition temperature Tg, adhesion strength on leadframe and ceramics and the dielectric properties using interdigital capacitors integrated into mold packages. On package level, material suitability is determined using a functional test package integrating MOSFET power ICs and SMD devices assembled on HT-PCB in a single sided leadframe-based mold package. Package integrity is evaluated in initial state using non-destructive analysis as x-ray and CSAM, and also by measuring warpage of this asymmetric package, as only low warpage ensures good thermal interconnection to the heat sink necessary. Also thermal cycling tests and high temperature storage will be applied to the modules to determine long term reliability of such molded Smart Power Modules. Summarized a detailed description of the high temperature suitability of high performance molding compounds is provided – additionally an extended test methodology is described to facilitate future material evaluation for HT or harsh environment use of polymeric materials as encapsulants or base materials.
Karl-F. Becker, Group Manager Assembly & Encapsulation
Fraunhofer IZM
Berlin, Berlin

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