Scientific publications

A novel optical incremental and absolute encoder based on an optical application-specific integrated circuit (opto-ASIC) and an encoder disc carrying micro manufactured structures is presented. The physical basis of the encoder is the diffraction of light using a reflective phase grating. The opto-ASIC contains a ring of photodiodes that represents the encryption of the encoder. It also includes the analog signal conditioning, the signal acquisition, and the control of a light source, as well as the digital position processing. The development and fabrication of the opto-ASIC is also described in this work. A laser diode was assembled in the center on top of the opto-ASIC, together with a micro manufactured polymer lens. The latter was fabricated using ultra-precision machining. The encoder disc was fabricated using micro injection molding and contains micro structures forming a blazed grating. This way, a 10-bit optical encoder with a form factor of only 1 cm3 was realized and tested successfully.

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When electronic assemblies are experiencing high current loading conditions, thermo-mechanical mismatch induced from the local Joule heating of the component and electromigration (EM)-induced void formation can lead to the failure of the solder interconnect. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) are adopted to investigate the microstructural evolution of solder joints of shunt components subjected to current stressing. Voids and micro-cracks initiate at the corner of solder meniscus, where the maximal divergence of material flux density is located on the basis of finite element method (FEM) simulation. The EM-induced voids introduce higher accumulated plastic strain in the solder joints, which will result in reduction in the lifetime of the electronic assemblies.

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Radio frequency reflectometry is widely used in non-destructive characterization of various materials and can also be applied to automatic impedance matching systems in telecommunication devices. Here, a novel, low cost, software defined approach for measuring the reflection coefficient without using mixers or power detectors is proposed, enabled by using under-sampling down-conversion with an analog-digital-converter. Minimizing the analog part of the system removes possible error sources and a modular design allows application in different fields as well as an implementation as an integrated circuit. Computer simulations as well as measurement results on a printed circuit board setup for a system frequency of 400 MHz are presented, permitting extrapolation to other ultra-high frequencies.

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The fabrication of microstructured polymer optics enables a multitude of new options in the design of technical optics. However, challenges arise along the varying process chains of mold insert fabrication, integration into molding tools, replication by means of injection compression molding and metrology. In order to study the effects, diffractive optical elements (DOE) and microlens arrays (MLA) are fabricated using two different process chains. DOEs are fabricated using a laser direct writing (LDW) based mold insert fabrication. The MLA mold insert is produced using ultra-precision milling (UPmilling). Both optical parts are replicated using injection compression molding. The occurring effects are discussed and the results show, that with complete process control high quality microstructured polymer optical parts can be produced and characterized.

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In this work, a new process chain for the fabrication of curved micro-structured optical elements by injection compression molding is investigated. The fabrication is demonstrated for the example of a curved diffractive optical element (DOE). In a first process step a master substrate is fabricated using laser direct writing on a curved glass substrate. Blazed diffractive micro-structures with lateral feature sizes down to 5 μm and height of 1.6 μm are created. A subsequent electroplating process is applied to create a nickel stamper to be used as a tool insert for the molding process. During electroplating, a 3 mm nickel layer is formed, transferring the diffractive structures from the master substrate into a solid mold insert. The nickel stamper shows an accurate reproduction of the micro-structures. The mold insert is integrated into an injection compression molding tool to replicate the optical elements. Results show that the blazed diffractive structures are replicated with a high quality. Tests of the components within a chromatic-confocal measurement setup confirm that they can potentially replace expensive conventional elements.

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Polymer optics have gained increasing importance in recent years. With advancing requirements for the optical components, the fabrication process remains a challenge. In particular, the fabrication of the mold inserts for the replication process is crucial for obtaining high-quality optical components. This review focuses on fabrication technologies for optical mold inserts. Thereby, two main types of technologies can be distinguished: fabrication methods to create mold inserts with optical surface quality and methods to create optical microstructures. Since optical mold inserts usually require outstanding form accuracies and surface qualities, a focus is placed on these factors. This review aims to give an overview of available methods as well as support the selection process when a fabrication technology is needed for a defined application. Furthermore, references are given to detailed descriptions of each technology if a deeper understanding of the processes is required.

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Traceability of single parts is essential in automated manufacturing, especially considering recent developments in Industry 4.0 or Smart Manufacturing. In some cases, however, conventional traceability methods, e.g. barcode, DMC or RFID, are not sufficient to track individual components through their entire production chain. One solution is the exploitation of an object's inherent, individual surface patterns. This work demonstrates label-/tag-free traceability of approx. 1000 components in a molded interconnect device (MID) production chain. The parts' surface patterns are photographed several times along the MID production chain. Based on the captured images nearly 3,300 successful identification processes were performed without any failure. Using a hash algorithm to compress the relevant data of the surface pattern images reduces the required disk space and the runtime significantly. Since the results demonstrate the robustness against surface alterations, this work gives the proof of concept regarding to a method for fast label-/tag-free part tracking that is applicable in an industrial production line.

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This paper presents a simple approach for high accuracy assembly of multiple ultra-thin chips on flexible foil substrates. The chips are placed initially in cavities on a chip carrier substrate and transferred onto a thermally releasable adhesive tape. After application of epoxy-based adhesive onto the chip surface the chips are transferred onto the foil substrate. Finally, the epoxy adhesive is cured and the adhesive tape is removed. It was found that a wafer dicing tape with releasing temperature of 90 °C as thermally releasable adhesive tape and a two-component epoxy adhesive with a curing temperature of 65 °C is suitable for the process.  High placement accuracy of the chips of less than 30  µm in x- and y- orientation as well as rotational misalignment lower than 0.2° was obtained. The process is demonstrated on polyimide foil with a resulting chip flatness of ± 5 µm.

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Faster product lifecycles make long-term investments in machines for micro assembly riskier. Therefore, reconfigurable manufacturing systems gain more and more attention. But most companies are uncertain if a reconfigurable manufacturing system can fulfill their needs and justify the initial investment. New and improved techniques for product development have the potential to foster the utilization and decrease the investment risk for such systems. In this paper, four different methods for product development are reviewed. A set of criteria regarding micro assembly on reconfigurable manufacturing systems RMS is established. Based on those criteria and the assessment, a novel approach for a product development method is provided, which tries to combine the strengths of the beforehand presented approaches. It focuses on the conceptual design phase to overcome the customers’ uncertainty in the development process. For this, an abstract representation of a micro-assembly product idea as well as a decision tree for joining processes are established and validated by real product ideas using expert interviews. The validation shows that the conceptual design phase can be used as a useful tool in the product development process in the field of micro assembly.
 

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