On the Feasibility of Fan-Out Wafer-Level Packaging of Capacitive Micromachined Ultrasound Transducers (CMUT) by Using Inkjet-Printed Redistribution Layers

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.EC/H2020/737487/EU/(Ultra)Sound Interfaces and Low Energy iNtegrated SEnsors/SILENS

Evaluation of the Sheet Resistance of Inkjet-Printed Ag-Layers on Flexible, Uncoated Paper Substrates Using Van-der-Pauw’s Method

With the growing significance of printed sensors on the electronics market, new demands on quality and reproducibility have arisen. While most printing processes on standard substrates (e.g., Polyethylene terephthalate (PET)) are well-defined, the printing on substrates with rather porous, fibrous and rough surfaces (e.g., uncoated paper) contains new challenges. Especially in the case of inkjet-printing and other deposition techniques that require low-viscous nanoparticle inks the solvents and deposition materials might be absorbed, inhibiting the formation of homogeneous conductive layers. As part of this work, the sheet resistance of sintered inkjet-printed conductive silver (Ag-) nanoparticle cross structures on two different, commercially available, uncoated paper substrates using Van-der-Pauw’s method is evaluated. The results are compared to the conductivity of well-studied, white heat stabilised and treated PET foil. While the sheet resistance on PET substrate is highly reproducible and the variations are solely process-dependent, the sheet resistance on uncoated paper depends more on the substrate properties themselves. The results indicate that the achievable conductivity as well as the reproducibility decrease with increasing substrate porosity and fibrousness

Photoluminescence Imaging for the In-Line Quality Control of Thin-Film Solar Cells

Renewable energy sources such as photovoltaic (PV) technologies are considered to be key drivers towards climate neutrality. Thin-film PVs, and particularly copper indium gallium selenide (CIGS) technologies, will play a crucial role in the turnaround in energy policy due to their high efficiencies, high product flexibility, light weight, easy installation, lower labour-intensiveness, and lower carbon footprint when compared to silicon solar cells. Nonetheless, challenges regarding the CIGS fabrication process such as moderate reproducibility and process tolerance are still hindering a broad market penetration. Therefore, cost-efficient and easily implementable in-line process control methods are demanded that allow for identification and elimination of non-conformal cells at an early production step. As part of this work, a practical approach towards industrial in-line photoluminescence (PL) imaging as a contact-free quality inspection tool is presented. Performance parameters of 10 CIGS samples with 32 individually contacted cells each were correlated with results from PL imaging using green and red excitation light sources. The data analysis was fully automated using Python-based image processing, object detection, and non-linear regression modelling. Using the red excitation light source, the presented PL imaging and data processing approach allows for a quantitative assessment of the cell performance

Low-Cost Inkjet-Printed Temperature Sensors on Paper Substrate for the Integration into Natural Fiber-Reinforced Lightweight Components

In a unique approach to develop a “green” solution for in-situ monitoring, low-cost inkjet-printed temperature sensors on paper substrate were fully integrated into natural fiber-reinforced lightweight components for which structural health monitoring is becoming increasingly important. The results showed that the sensors remained functional after the vacuum infusion process; furthermore, the integration of the sensors improved the mechanical integrity and stability of the lightweight parts, as demonstrated by tensile testing. To verify the qualification of the printed sensors for the target application, the samples were exposed to varying temperature and humidity conditions inside of a climate chamber. The sensors showed linear temperature dependence in the temperature range of interest (−20 to 60 °C) with a TCR ranging from 1.576 × 10−3 K−1 to 1.713 × 10−3 K−1. Furthermore, the results from the tests in humid environments indicated that the used paper-based sensors could be made almost insensitive to changes in ambient humidity by embedding them into fiber-reinforced lightweight materials. This study demonstrates the feasibility of fully integrating paper-based printed sensors into lightweight components, which paves the way towards integration of other highly relevant sensing devices, such as strain and humidity sensors, for structural health monitoring of smart, sustainable, and environmentally compatible lightweight composite materials

Evaluation of Standard Electrical Bonding Strategies for the Hybrid Integration of Inkjet-Printed Electronics

Different conductive bonding strategies for the hybrid integration of flexible, inkjet-printed electronics are investigated. The focus of the present work lies on providing a practical guide comprising standard techniques that are inexpensive, easily implementable and frequently used. A sample set consisting of identical conductive test structures on different paper and plastic substrates was prepared using silver (Ag) nanoparticle ink. The sintered specimens were electrically contacted using soldering, adhesive bonding and crimping. Electrical and mechanical characterization before and after exposing the samples to harsh environmental conditions was performed to evaluate the reliability of the bonding methods. Resistance measurements were done before and after connecting the specimens. Afterwards, 85 °C/85% damp-heat tests and tensile tests were applied. Adhesive bonding appears to be the most suitable and versatile method, as it shows adequate stability on all specimen substrates, especially after exposure to a 85 °C/85% damp-heat test. During exposure to mechanical tensile testing, adhesive bonding proved to be the most stable, and forces up to 12 N could be exerted until breakage of the connection. As a drawback, adhesive bonding showed the highest increase in electrical resistance among the different bonding strategies

Printed 2D Proton Sensor for In-Situ Measurement in Glue Lines

A screen printed pH sensor was developed using a PANI layer as a proton sensitive material for the in-situ measurements of matrix cross-linking. The sensor showed a linear response in a broad pH range (3–10) and had an evident cross-talk to Cl− ions. Preliminary in-situ measurements showed a substantial signal change during the cross-linking process

Printed Electronics Technologies for Additive Manufacturing of Hybrid Electronic Sensor Systems

Abstract Requirements for the miniaturization of electronics are constantly increasing as more and more functions are aimed to be integrated into a single device. At the same time, there are strong demands for low‐cost manufacturing, environmental compatibility, rapid prototyping, and small‐scale productions due to fierce competition, policies, rapid technical progress, and short innovation times. Altogether, those challenges cannot be sufficiently addressed by simply using either printed or silicon electronics. Instead, the synergies from combining those two technologies into so‐called hybrid electronics create novel opportunities for advanced capabilities and new areas of applications. In the first part of this review, printing and patterning technologies are presented with potential compatibility with conventional electronics manufacturing techniques. They can be utilized for the fabrication of highly complex structures. Nonetheless, up‐scalability, integration, and adaptation for industrial fabrication remain challenging due to technically limiting factors. Consequently, a special focus is placed on the up‐scalability, availability of commercial printing, and manufacturing machines, as well as processing challenges for high‐volume industrial applications. The second part of this review further provides an overview of exciting and innovative application possibilities of printed electronics, emphasizing sensor applications, as well as additively manufactured integrated circuits

Sustainable Multifunctional Biface Sensor Tag

Abstract In this article, a sustainable, multifunctional, low‐cost, wireless sensor tag is presented. The sensor tag combines three different environmental sensors in one single platform for the dedicated purpose of wireless structural health monitoring of a variety of applications. However, the adaptive design allows the integration of different sensors depending on the specific sensing task. The material consumption is minimized by double‐sided printing, resulting in compact, resource‐efficient sensor solutions. On one side, the tag is equipped with a printed antenna, a fully passive silicon‐based near‐field communication chip and a carbon‐based strain sensor, while the environmental sensors for humidity and temperature are printed on the other side. Due to its low cost, the usage of environmentally friendly materials and the absence of a battery, the biface sensor tag is a milestone in the field of wireless, sustainable electronics for ubiquitous sensing applications. The fabrication itself comprises a series of processes with a focus on efficient additive manufacturing. The characterization of the three sensors shows sensitivity values and characteristics comparable to those found in literature and industrially manufactured sensors. The utilization of a smartphone for reading out the sensor signals further emphasizes the sustainable approach of this sensor system

Future Thread: Printing Electronics on Fibers

This article introduces a methodology to increase the integration density of functional electronic features on fibers/threads/wires through additive deposition of functional materials via printed electronics. It opens the possibility to create a multifunctional intelligent system on a single fiber/thread/wire while combining the advantages of existing approaches, i.e., the scalability of coating techniques and the microfeatures of semiconductor-based fabrication. By directly printing on threads (of diameters ranging from 90 to 1000 μm), micropatterned electronic devices and multifunctional electronic systems could be formed. Contact and noncontact printing methods were utilized to create various shapes from serpentines and meanders to planar coils and interdigitated electrodes, as well as complex multilayer structures for thermal and light actuators, humidity, and temperature sensors. We demonstrate the practicality of the method by integrating a multifunctional thread into a FFP mask for breath monitoring. Printing technologies provide virtually unrestricted choices for the types of threads, materials, and devices used. They are scalable via roll-to-roll processes and offer a resource-efficient way to democratize electronics across textile products

Early postnatal cardiac follow-up of survivors of twin–twin transfusion syndrome treated with fetoscopic laser coagulation

Research into fetal development and medicin