Universität Stuttgart
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Item Open Access Embedding of ultrathin chips in highly flexible, photosensitive solder mask resist(2021) Janek, Florian; Eichhorn, Nadine; Weser, Sascha; Gläser, Kerstin; Eberhardt, Wolfgang; Zimmermann, AndréThis work presents an embedding process for ultrathin silicon chips in mechanically flexible solder mask resist and their electrical contacting by inkjet printing. Photosensitive solder mask resist is applied by conformal spray coating onto epoxy bonded ultrathin chips with daisy chain layout. The contact pads are opened by photolithography using UV direct light exposure. Circular and rectangular openings of 90 µm and 130 µm diameter respectively edge length are realized. Commercial inks containing nanoparticular silver and gold are inkjet printed to form conductive tracks between daisy chain structures. Different numbers of ink layers are applied. The track resistances are characterized by needle probing. Silver ink shows low resistances only for multiple layers and 90 µm openings, while gold ink exhibit low resistances in the single-digit Ω-range for minimum two printed layers.Item Open Access Integration of mechatronic functions on additively manufactured components via laser‐assisted selective metal deposition(2024) Vieten, Tobias; Weser, Sascha; Schilling, Alexander; Gläser, Kerstin; Zimmermann, AndréThe current industrial revolution derives much of its momentum from value creation based on interconnected products and related data based services. Such products must fulfill both mechanical and electrical requirements, making them mechatronic systems. The production of such systems via additive manufacturing (AM) processes offers advantages in achievable complexity, reduction of the amount of individual components, and cost‐effective as well as sustaina ble production of small quantities. In this work, a process chain is presented that allows for refining additively manufactured 3D structures made from industry‐standard materials into mechatronic components by creating electrically conductive structures directly on their surfaces. The process chain is based on masking the component's surface and selectively removing the masking according to the circuit geometry using laser radiation. In a wet-chemical bath process, the surface is then exposed to palladium nuclei, the masking is fully removed and metal layers (copper/nickel/gold) are deposited by electroless plating. The procedure is developed using stereolithography as a model process for AM and transferred to four additional AM methods. In all cases, despite markedly different surface properties, good selectivity of metal deposition is observed as well as adhesion strength and conductivity comparable to industrially common injection‐molded laser direct structured mechatronic interconnect devices.Item Open Access Inkjet-printed temperature sensors characterized according to standards(2022) Jäger, Jonas; Schwenck, Adrian; Walter, Daniela; Bülau, André; Gläser, Kerstin; Zimmermann, AndréThis paper describes the characterization of inkjet-printed resistive temperature sensors according to the international standard IEC 61928-2. The goal is to evaluate such sensors comprehensively, to identify important manufacturing processes, and to generate data for inkjet-printed temperature sensors according to the mentioned standard for the first time, which will enable future comparisons across different publications. Temperature sensors were printed with a silver nanoparticle ink on injection-molded parts. After printing, the sensors were sintered with different parameters to investigate their influences on the performance. Temperature sensors were characterized in a temperature range from 10 °C to 85 °C at 60% RH. It turned out that the highest tested sintering temperature of 200 °C, the longest dwell time of 24 h, and a coating with fluoropolymer resulted in the best sensor properties, which are a high temperature coefficient of resistance, low hysteresis, low non-repeatability, and low maximum error. The determined hysteresis, non-repeatability, and maximum error are below 1.4% of the full-scale output (FSO), and the temperature coefficient of resistance is 1.23-1.31 × 10-3 K-1. These results show that inkjet printing is a capable technology for the manufacturing of temperature sensors for applications up to 85 °C, such as lab-on-a-chip devices.Item Open Access Inkjet-printing of nanoparticle gold and silver ink on cyclic olefin copolymer for DNA-sensing applications(2020) Trotter, Martin; Juric, Daniel; Bagherian, Zahra; Borst, Nadine; Gläser, Kerstin; Meissner, Thomas; Stetten, Felix von; Zimmermann, AndréInkjet technology as a maskless, direct-writing technology offers the potential for structured deposition of functional materials for the realization of electrodes for, e.g., sensing applications. In this work, electrodes were realized by inkjet-printing of commercial nanoparticle gold ink on planar substrates and, for the first time, onto the 2.5D surfaces of a 0.5 mm-deep microfluidic chamber produced in cyclic olefin copolymer (COC). The challenges of a poor wetting behavior and a low process temperature of the COC used were solved by a pretreatment with oxygen plasma and the combination of thermal (130 °C for 1 h) and photonic (955 mJ/cm²) steps for sintering. By performing the photonic curing, the resistance could be reduced by about 50% to 22.7 µΩ cm. The printed gold structures were mechanically stable (optimal cross-cut value) and porous (roughness factors between 8.6 and 24.4 for 3 and 9 inkjet-printed layers, respectively). Thiolated DNA probes were immobilized throughout the porous structure without the necessity of a surface activation step. Hybridization of labeled DNA probes resulted in specific signals comparable to signals on commercial screen-printed electrodes and could be reproduced after regeneration. The process described may facilitate the integration of electrodes in 2.5D lab-on-a-chip systems.Item Open Access Image analysis based evaluation of print quality for inkjet printed structures(2023) Horter, Tim; Ruehl, Holger; Yang, Wenqi; Chiang, Yu-Sheng; Gläser, Kerstin; Zimmermann, AndréInkjet printing for printed electronics is a growing market due to its advantages, including scalability, various usable materials and its digital, pixel based layout design. An important quality factor is the wetting of the ink on the substrate. This article proposes a workflow to evaluate the print quality of specific layouts by means of image analysis. A self-developed image analysis software, which compares a mask with the actual layout, enables a pixel-based analysis of the wetting behavior by the implementation of two parameters called over- and underwetting rate. A comparison of actual and targeted track widths can be performed for the evaluation of different parameters, such as the tested plasma treatment, drop spacing (DS) and substrate temperature. To prove the functionality of the image analyses tool, the print quality of Au structures inkjet printed on cyclic olefin copolymer (COC) substrates was studied experimentally by varying the three previously mentioned parameters. The experimental results showed that the wetting behavior of Au ink deposited on COC substrates influences various line widths differently, leading to higher spreading for smaller line widths. The proposed workflow is suitable for identifying and evaluating multiple tested parameter variations and might be easily adopted for printers for in-process print quality control in industrial manufacturing.Item Open Access Aerosol jet printing and interconnection technologies on additive manufactured substrates(2022) Werum, Kai; Mueller, Ernst; Keck, Juergen; Jäger, Jonas; Horter, Tim; Gläser, Kerstin; Buschkamp, Sascha; Barth, Maximilian; Eberhardt, Wolfgang; Zimmermann, AndréNowadays, digital printing technologies such as inkjet and aerosol jet printing are gaining more importance since they have proven to be suitable for the assembly of complex microsystems. This also applies to medical technology applications like hearing aids where patient-specific solutions are required. However, assembly is more challenging than with conventional printed circuit boards in terms of material compatibility between substrate, interconnect material and printed ink. This paper describes how aerosol jet printing of nano metal inks and subsequent assembly processes are utilized to connect electrical components on 3D substrates fabricated by Digital Light Processing (DLP). Conventional assembly technologies such as soldering and conductive adhesive bonding were investigated and characterized. For this purpose, curing methods and substrate pretreatments for different inks were optimized. Furthermore, the usage of electroless plating on printed metal tracks for improved solderability was investigated. Finally, a 3D ear mold substrate was used to build up a technology demonstrator by means of conductive adhesives.Item Open Access Inkjet-printed low temperature co-fired ceramics: process development for customized LTCC(2024) Jäger, Jonas; Ihle, Martin; Gläser, Kerstin; Zimmermann, AndréThis paper investigates the utilization of digital printing technologies for the fabrication of low temperature co-fired ceramics (LTCC). LTCC offer great opportunities for applications such as antennas, sensors or actuators due to their outstanding properties like low dielectric loss, low permittivity, low coefficient of thermal expansion and at the same time high reliability in harsh environments (heat, humidity, and radiation). LTCC are multilayer circuits that are typically functionalized by screen-printing. This publication investigates the replacement of screen-printing by digital printing processes, such as inkjet and Aerosol Jet printing, to facilitate a more resource-friendly and customizable manufacturing of LTCC. The use of digital printing technologies not only streamlines small-scale productions and development processes but also offers the advantage of achieving miniaturization down to single-digit microns. In this publication, digital printing processes, filling of vias, lamination processes, co-firing at 850 °C and printing on fired LTCC were investigated. Three layers of nanoparticle silver ink were printed on green LTCC tape and 100% of the embedded printed structures were conductive after co-firing. Filling of vias with inkjet printing was investigated and the most important process parameters were found to be the clustering of vias, the amount of active nozzles and the substrate temperature. Printing on fired LTCC demonstrated high precision, and sintering at 600 °C achieved strong adhesion of printed structures to LTCC. These successful findings culminate in presenting a process chain for fully maskless structured, multilayer LTCC.