Fraunhofer-Institut für Zuverlässigkeit und Mikrointegration IZM

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  • Publication
    Ferrites in Transfer-Molded Power SiPs: Challenges in Packaging
    ( 2020)
    Thomas, T.
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    Dijk, M. van
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    Dreissigacker, M.
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    Hoffmann, S.
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    Walter, H.
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    Becker, K.-F.
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    Schneider-Ramelow, M.
    Transfer-molding process is enjoying growing interest when aiming for novel high-power density system-in-packages (power SiPs), where not only transistors and diodes but also drivers, passives, coils, and transformers are supposed to be integrated in one package. Encapsulating modules in a transfer-molding process induces higher mechanical load onto module components compared with conventional silicone potting. Previous investigations have shown that integration of delicate components as ferrite cores into molded packages is not as trivial as integration of conventional surface-mount devices or power semiconductors; the brittle ferrites tend to fracture during the encapsulation process, resulting in higher ferrite core loss. The current study aims to identify main root causes for ferrite core cracking during manufacturing of molded power SiPs. The test vehicle is a symmetrical printed circuit board-based package with three pairs of E-shaped ferrite cores. The epoxy molding compound deployed here is characterized to enable filling simulations. Because technical datasheets of ferrites typically lack specifications of mechanical properties, ferrite materials are analyzed in more detail. Filling simulations and thermomechanical simulations are performed to gain insight into process-induced stress, which may induce cracks in the ferrites. In addition, different ferrite designs are evaluated regarding core losses and mechanical stability and, thus, their tendency to fracture.
  • Publication
    On the feasibility of fan-out wafer-level packaging of capacitive micromachined ultrasound transducers (CMUT) by using inkjet-printed redistribution layers
    ( 2020)
    Roshanghias, A.
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    Dreissigacker, M.
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    Scherf, C.
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    Bretthauer, C.
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    Rauter, L.
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    Zikulnig, J.
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    Braun, T.
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    Becker, K.-F.
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    Rzepka, S.
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    Schneider-Ramelow, M.
    Fan-out wafer-level packaging (FOWLP) is an interesting platform for Microelectromechanical systems (MEMS) sensor packaging. Employing FOWLP for MEMS sensor packaging has some unique challenges, while some originate merely from the fabrication of redistribution layers (RDL). For instance, it is crucial to protect the delicate structures and fragile membranes during RDL formation. Thus, additive manufacturing (AM) for RDL formation seems to be an auspicious approach, as those challenges are conquered by principle. In this study, by exploiting the benefits of AM, RDLs for fan-out packaging of capacitive micromachined ultrasound transducers (CMUT) were realized via drop-on-demand inkjet printing technology. The long-term reliability of the printed tracks was assessed via temperature cycling tests. The effects of multilayering and implementation of an insulating ramp on the reliability of the conductive tracks were identified. Packaging-induced stresses on CMUT dies were further investigated via laser-Doppler vibrometry (LDV) measurements and the corresponding resonance frequency shift. Conclusively, the bottlenecks of the inkjet-printed RDLs for FOWLP were discussed in detail.
  • Publication
    A numerical study on mitigation of flying dies in compression molding of microelectronic packages
    ( 2019)
    Dreissigacker, M.
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    Hoelck, O.
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    Bauer, J.
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    Braun, T.
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    Becker, K.-F.
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    Schneider-Ramelow, M.
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    Lang, K.-D.
    Compression molding with liquid encapsulants is a crucial process in microelectronic packaging. Material properties of highly filled systems of reactive epoxy molding compounds depend on process conditions in a complex manner, such as shear-thinning behavior, which is superimposed by a time- and temperature-dependent conversion rate, both strongly affecting viscosity. The focus is set on forces exerted on individual dice during encapsulation in fan-out wafer-level packaging (FOWLP). The presented framework consists of an analytical approach to calculate the melt front velocity and simulations carried out to capture the nonlinear kinematics, chemorheology, and to extract forces exerted on individual dice. It offers separate evaluation of pressure and shear contributions for two cases, 0° and 45° between the dice' frontal area and the melt front. Process parameters, such as compression speed, thus cycle time, and process temperature, are determined to keep the forces on the dice below the critical level, where drag forces exceed adhesive forces. As a result, process parameters are determined to minimize flying dice and thereby maximize yield. The approach is easily transferable to arbitrary geometries and is therefore well suited to face the challenges that come with the current efforts toward the transition from FOWLP to larger substrates.
  • Publication
    High viscosity paste dosing for microelectronic applications
    ( 2016)
    Thomas, Tina
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    Voges, S.
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    Braun, T.
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    Raatz, S.
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    Kahle, R.
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    Becker, K.-F.
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    Koch, M.
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    Fliess, M.
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    Bauer, J.
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    Schneider-Ramelow, M.
    ;
    Lang, K.-D.
    Today's microelectronics packaging especially for SiPs relies on the processing of a wide variety of materials. Materials for components, for substrates, for contact materials (solder & adhesives) and encapsulants. Most materials are processed as bulk material but precision dosing of pastes is key to many assembly processes. Examples are dosing of solder paste, typically done by stencil printing, Underfilling for Flip Chip encapsulation, typically done by dispensing or jetting, or Glob Top encapsulation of Chip on Board assemblies, where also dispensing is the typical process. When working with those paste materials, viscosity is one of the key parameters for processing, and viscosities too high do not allow dosing of the materials, not even to transport the material from a reservoir to the dosing head, which may be a simple needle or a jet valve. [i, ii] To overcome this obstacle, i.e. to dose materials of high viscosity precisely and homogeneously from a syringe to the dosing head, a research program has been set up, where Vermes microdispensing as a valve manufacturer and TU Berlin/IZM as a research institute are cooperating. TU Berlin is working on material rheology effects and flow models; Vermes is researching valves modifications and material flow path optimization. Core of the research is to find methods that allow a reduction of paste viscosity without leading to irreversible changes in the material, as would be the case when simply applying heat to the paste. As reference process for material dosing, FO-WLP has been chosen, materials selected for the investigations are GlobTop dam and fill material and liquid molding compound - using both rheological experiments as well as actual material dosing and processing. Apart from temperature, mechanical and ultrasonic stimulation of the material have been evaluated to achieve optimized dosing of high viscous pastes, As a result, a first description of paste behavior during processing is given, being the basis for future work towards homogeneous precision dosing of high viscous pastes for microelectronic applications.
  • Publication
    In-situ measuring module for transfer molding process monitoring
    ( 2016)
    Kahle, Ruben
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    Braun, T.
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    Bauer, J.
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    Becker, K.-F.
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    Schneider-Ramelow, M.
    ;
    Lang, K.-D.
    While Moore's Law is slowing down heterogeneous integration and System-in-Package (SiP) are taking up the challenge towards further miniaturization. To ensure reliability of these packages often encapsulation by transfer molding is used - providing a highly productive and cost effective device housing. Though transfer molding is the dominant process for microelectronics encapsulation, the process details are typically not accessible directly but only via machine settings. To understand more of the mold process further research needs to be conducted to get inside information from the process. A sensor based system was developed to in-situ measure transient material data and process parameters. The temperature of the tool and melt front, the cavity pressure and cure related dielectric material data was measured with a first prototype. Summarized this paper presents the development of a sensor based system to in-situ measure characteristic material properties and process parameters in a transfer mold machine. The in-situ measurement tool allows a live documentation, optimization and knowledge extension of transfer mold processes parallel to FEM simulation and external measuring like DSC analysis. The sensor approach can lead to material driven self-adjusting transfer mold machines with an economized process.
  • Publication
    Transfer molding technology for smart power electronics modules: Materials and processes
    ( 2012)
    Becker, K.-F.
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    Joklitschke, D.
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    Braun, T.
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    Koch, M.
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    Thomas, T.
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    Schreier-Alt, T.
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    Bader, V.
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    Bauer, J.
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    Nowak, T.
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    Bochow-Ness, O.
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    Aschenbrenner, R.
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    Schneider-Ramelow, M.
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    Lang, K.-D.
    In recent years, within power electronics packaging, there has been a trend toward compact power electronics modules for automotive and industrial applications, 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, these packages are asymmetric, and thus 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 yielding warped substrates, preventing optimum thermal contact to the heat sink and also putting thermomechanical stress on the encap sulated components, possibly reducing reliability. Such packages are truly heterogeneous, combining power ICs, wire bonds, SMDs, control ICs, substrate, and lead frame surfaces. As a result, the encapsulant used needs to adhere sufficiently to all surfaces present. Additionally, those packages need to operate at elevated temperatures for extended time periods, for example, at 150°C for 2000 h and more, so high thermal stability is of prime importance. Within this paper, a reference application is described integrating power and control logic inside a lead frame 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, such as rheology, reactivity, and change in εr; and thermomechanical properties, in initial stage as f(t,T) and after media storage. Process development tools for module molding are used to ensure manufacturability and usability.
  • Publication
    Thermo-mechanical reliability during technology development of power chip-on-board assemblies with encapsulation
    ( 2009)
    Wunderle, B.
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    Becker, K.-F.
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    Sinning, R.
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    Wittler, O.
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    Schacht, R.
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    Walter, H.
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    Schneider-Ramelow, M.
    ;
    Halser, K.
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    Simper, N.
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    Michel, B.
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    Reichl, H.
    In this paper we examine the thermo-mechanical reliability of polymer-encapsulated chip-on-board (COB) assemblies for power applications by simulation and experiment. Thereby the focus is set on the low cycle fatigue failure behaviour of the die-attach material under thermal cycling conditions. As die-attach material different solder materials and Ag-filled thermal adhesives have been used. The encapsulation was performed with a soft silicone-based and hard silica-reinforced epoxy-based material, respectively. An other process variable takes into account die-tilt. The study was carried out as a feasibility analysis in the course of a COB technology development. To this end lifetime models have been employed to correlate crack growth in the, i.e. attach to a computational accumulative failure criterion which allows to consistently describe ad predict quantitatively the lifetime of the assemblies. Thereby a considerable influence of the encapsulation was found. In particular it could be shown that a hard encapsulation largely increases reliability for solder die-attach.