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Combining advantages of rheometry and inline viscometry for improved viscosity modeling
Keywords: In-line viscometry, material modelling, microelectronic encapsulation
In microelectronic packaging, encapsulation by compression and transfer molding is a crucial process block to ensure device reliability. Material properties of encapsulants, highly filled systems of reactive epoxy molding compounds (EMC), strongly depend on process conditions in a complex manner and vary over time. Shear-thinning behavior, as well as time- and temperature-dependent conversion strongly impact the viscosity of the polymer melt. In all fields of application, such as automotive or IoT, demands towards miniaturization, lifetime and environmental conditions increase. Thus, detailed understanding of the complex material behavior is of vital importance. Typically, shear-thinning behavior of polymer melts is characterized using a conventional rheometer in oscillation mode under varying shear-rates and temperatures. Limitations of this approach are, that measurements at process temperature typically cannot be performed due to the high reactivity of the encapsulant at these temperatures (e.g. 175 C for transfer molding). Therefore extrapolation to the correct temperature range is required. Furthermore, measurements in oscillation mode cannot necessarily be transferred to real process conditions, where a continuous flow is present. To overcome these limitations the inline viscometer can be used, a specially designed measurement tool for a transfer molding machine developed by Fico/Besi (Kersjes 2016). The polymer melt is pressed through a narrow slit under known volumetric flow at process temperature. By measuring the pressure difference before and after the slit, the viscosity can be calculated. In order to better understand and also predict material behavior, inline viscosimetry is combined with rheometer measurements. This allows to maintain the advantages of conventional rheometry regarding material consumption and large shear-rate measuring range. At the same time, the inline approach provides relevant data under process conditions. The synthesis of both approaches yields a correction of the rheometer measurements, ultimately improving viscosity modeling and being a validated basis for process simulation.
Strobelt, Jonas, Student researcher
Fraunhofer IZM
Berlin, Berlin

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