Numerical simulation of electrohydrodynamic jet and printing micro-structures on flexible substrate

By today, die bonding on flexible circuit boards is only possible with adhesive connections or can be realized in combination by rigid-flex technology via soldering. The flex technology has a big potential with regard to new application fields such as automotive interiors (e.g. new lighting concepts, integration of electronics as functional components of the equipment), autonomous driving (new camera and sensor concepts, radar, car-to-x communication), building integration (e.g. smart home integration of functional electronic assemblies in building walls) but also medical technology and textile electronics. The application areas and the reliability of assemblies with the use of adhesive technology minimizes the application areas of flex technology. Great opportunities to new reliable polymer-based electronics could, however, be made up by transferring a low-temperature silver sintering process from power electronics to the microelectronic field. But the transfer of this bonding process also contains the risk of electrochemical failure processes (e.g. due to residual moisture in flexible polymer substrates, Ag migration, acceleration of failure modes due to mobile ions). In our studies we analyzed electrochemical corrosion and migration processes at silver sintered flexible substrates. Therefore, in a first step, we investigated technical processes to produce reliable low temperature silver sintered systems at polyimide based flexible substrates. Afterwards the silver sintered systems were tested by biased humidity testing and salt spray testing. The materials, surfaces and reaction products were investigated by high resolution microstructural and physical methods like $\mu$ XRF and SEM-EDS. The results achieved by this work give a good overview for the possibilities and limits of pressureless silver sintered systems on flexible substrates.

Recently, metal nanoparticle (NP) based ink solutions are widely used for fabricating flexible electronic devices due to many advantages such as low temperature process, simple and low-cost manufacturing, and compatibility with various flexible substrates. The electromechanical characteristics of metal NP thin films on flexible substrates are very important because they often operate under various mechanical loading conditions. Here, we report an improvement of electromechanical properties of silver (Ag) NP thin films by composite formation with short Ag nanowires (Ag NWs). We fabricated Ag NP-Ag NW hybrid thin films on the flexible polymer substrates, which presented more reliable electromechanical characteristics than the Ag NP thin film. From in situ scanning electron microscope images, we observed a bridging behavior of Ag NWs inhibiting the growth of cracks on the surface as well as in the inside of hybrid thin films. The improvement of electromechanical characteristics by composite formation with short Ag NWs could be also observed in the microscale line patterns, both in the static and cyclic tensile loading conditions.

Numerical simulation of stable electrohydrodynamic cone-jet formation and printing on flexible substrate

Multifunctional self-assembled BaTiO3-Au nanocomposite thin films on flexible mica substrates with tunable optical properties

Probing the wear characteristics of graphene on flexible polymer substrates using a heated atomic force microscopy tip

Flexible electronics have been of great interest in the past few decades for their wide-ranging applications in health monitoring, human-machine interaction, artificial intelligence, and biomedical engineering. Currently, transfer printing is a popular technology for flexible electronics manufacturing. However, typical sacrificial intermediate layer-based transfer printing through chemical reactions results in a series of challenges, such as time consumption and interface incompatibility. In this paper, we have developed a time-saving, wafer-recyclable, eco-friendly, and multiscale transfer printing method by using a stable transferable photoresist. Demonstration of photoresist with various, high-resolution, and multiscale patterns from the donor substrate of silicon wafer to different flexible polymer substrates without any damage is conducted using the as-developed dry transfer printing process. Notably, by utilizing the photoresist patterns as conformal masks and combining them with physical vapor deposition and dry lift-off processes, we have achieved in situ fabrication of metal patterns on flexible substrates. Furthermore, a mechanical experiment has been conducted to demonstrate the mechanism of photoresist transfer printing and dry lift-off processes. Finally, we demonstrated the application of in situ fabricated electrode devices for collecting electromyography and electrocardiogram signals. Compared to commercially available hydrogel electrodes, our electrodes exhibited higher sensitivity, greater stability, and the ability to achieve long-term health monitoring.

Thin film flexible electronics refer to a class of electronic devices built on flexible substrates. Examples includes printed Li batteries, Thin film flexible electronics refer to a class of electronic devices manufactured by multiple layering and scribing on flexible polymer substrates. Examples of such devices includes printed Li batteries, flexible photovoltaic cells and light emitting diodes. These devices are often mass manufactured by Roll-to-Roll processing (R2R). Whilst the basic technology is well established, the increasing demands on precision environmental protection and multi layering of devices means that in-process measurement of printed surface features is a critical bottle neck in terms of developing R2R as a process route. The purpose of the present paper is to review the current critical dimensional metrology needs in R2R manufacture and in particular to highlight the development of a new inprocess surface metrology system based on Multi-wavelength Polarizing Interferometry (MPI). The system is capable of measurement in real time, is environmentally robust and has nanometre resolution. The paper concludes by highlighting an example of the first trial implementation of the MPI on a production level R2R machine and discussed issues with quantification of film dimensions and associated signal processing

Cold spray-based rapid and scalable production of printed flexible electronics

Mitigating early fracture of amorphous metallic thin films on flexible substrates by tuning substrate roughness and buffer layer properties

Organic light emitting diode (OLED) based displays are well suited for fabrication on flexible polymer substrates. In order to achieve the vision of a highly flexible, rollable display, significant progress must be made in understanding and overcoming the mechanical limitations of the component materials. Methodologies and metrics for evaluating thin film components as well as full devices on flexible substrates need to be developed and standardized. A new technique for bend testing is described here and compared to a more traditional technique. One thin film component of flexible OLEDs that must be very mechanically robust is the permeation barrier. It is desirable to have a rapid method to characterize the onset of cracking upon bending of a barrier-coated substrate. We have developed a convenient method that allows characterization of mechanical cracking of flexible permeation barriers. In addition, transparent conductors with mechanical properties superior to those of ITO are desired for improved flexibility of displays. We report dramatically improved mechanical properties of multilayer ITO-Ag-ITO films that also exhibit excellent electrical and optical properties. Finally, we show preliminary results from bend tests of complete OLED devices on polymer substrates. The results demonstrate that in addition to evaluating component materials, other factors must be considered for integrated devices, and further investigation on this subject is needed.

Formation and characterization of silicon films on flexible polymer substrates

The role of the applied potential to the electrodeposition process of ZnO onto flexible polymeric substrates was investigated. ZnO nanostructures were prepared on indium-tin oxide conducting flexible substrates, using the electrodeposition method at 70°C in a zinc nitrate electroplating bath for different deposition potential, Edep, which has been divided in three distinct regions named as A, B and C. Chronoamperometric analysis reveals that the nucleation growth mechanism is strongly affected by the deposition conditions. The analysis of the current–time transients indicates that the ZnO electrodeposition occurs by progressive three-dimensional nucleation with diffusion-controlled growth in region B, −1000 < Edep< −1200 mV. For −700 < Edep < −1000 mV (region A), it is assumed that the electrodeposition process is mainly controlled by the slowest kinetic electroreduction of dissolved oxygen and nitrate ions resulting a low nucleation rate. Furthermore, in region C, related to more negative applied potentials, the electrodeposition process is conditioned by the substrate stability. The kinetic parameters values obtained from the diffusion controlled model are in good accordance to the dimensionless analysis of the transients. X-ray diffraction analysis confirmed the formation of (002) textured ZnO structures prepared in regions B and C.

Fabricating metallic wire grating inside a polymeric substrate by insertion nanoimprint

Characteristics of micro DMFCs array fabricated on flexible polymeric substrate

Electrohydrodynamic jet (E-jet) printing has broad application prospects in the preparation of flexible electronics and optical devices. Ejection cycle time and droplet size are two key factors affecting E-jet-printing quality, but due to the complex process of E-jet printing, it remains a challenge to establish accurate relationships among ejection cycle time and droplet diameter and printing parameters. This paper develops a model based on random forest regression (RFR) for E-jet-printing prediction. Trained with 72 groups of experimental data obtained under four printing parameters (voltage, nozzle-to-substrate distance, liquid viscosity, and liquid conductivity), the RFR model achieved a MAPE (mean absolute percent error) of 4.35% and an RMSE (root mean square error) of 0.04 ms for eject cycle prediction, as well as a MAPE of 2.89% and an RMSE of 0.96 μm for droplet diameter prediction. With limited training data, the RFR model achieved the best prediction accuracy among several machine-learning models (RFR, CART, SVR, and ANN). The proposed prediction model provides an efficient and effective way to simultaneously predict the ejection cycle time and droplet diameter, advancing E-jet printing toward the goal of accurate, drop-on-demand printing.