Flexibility-Aware System-on-Polymer (SoP): Concept to Prototype

This articles in this special issue on flexible hybrid electronics provides insight into the technical problems that must be overcome as well as the new capabilities they will provide. The term, 'flexible hybrid electronics' means different things based on the context in which it is being discussed. It could mean adaptable for multiple applications, but, here the term refers to mechanical properties, although not necessarily precisely defined. Mechanically flexible might mean anything from conformable (nonlinear shape) to bendable or rollable (around a defined radius of curvature), or even foldable and stretchable. Although not well defined, the intent is to differentiate from the rectangular, rigid form factor of conventional semiconductor electronics. Hybrid means heterogeneous integration of conventional electronics with other electronic components (passive devices, sensors, etc.) that could be fabricated using conventional thin-film transistor (TFT) process technology by additive printing (ink jet, gravure, etc.). The purpose is to provide an electronics solution that has adequate performance, but in a low cost, novel form factor because printing offers the ability to inexpensively fabricate structures over large areas on mechanically flexible substrates (plastics, metal sheets, bendable glass). However, integration with conventional electronics such as silicon CMOS is necessary to achieve required performance because TFT and printed devices are ignificantly inferior to conventionally fabricated devices.

How is flexible electronics advancing neuroscience research?

Flexible devices, which are seen as the future of the electronics industry, require encapsulation for protection while meeting the flexibility requirements of end applications. Flexible electronics have lower production costs and are thinner, lighter, and nonbreakable, resulting in a new form of application for electronic devices. One such use is the employment of electronic gadgets in the daily surroundings to monitor one's vitals. These devices are frequently exposed to dust, perspiration, and moisture. They are frequently subjected to bending and folding action, which causes stresses to accumulate in those devices. These stressors and the hostile environment are frequently minimized by using potting encapsulants to increase durability. In our investigation, we picked six distinct encapsulant formulations and exposed them to varied cure profiles to measure the adhesive bond strength of the encapsulants. The benchmark peel strength was constructed using a Finite element model of the AU-biometric band. The encapsulants peel strength was used to determine which material performed best under experimental conditions. This study presents a sample geometry comprising six different encapsulants and two distinct substrates, polyimide and PET, which were evaluated at four different cure schedules and cleaned using two different cleaning procedures. The encapsulants are ranked against one another to determine their potential future usage in flexible hybrid electronics (FHE) devices.

Wireless charging delivers power wirelessly across an air gap to recharge electronic devices without requiring direct physical connections. The rapid advancements in wireless charging technologies and the emergence of commercial products have introduced an alternative to overcome the energy limitations of traditional portable, battery‐operated devices. This paper offers an in‐depth review of radio frequency (RF)‐based wireless charging, emphasizing its applications in flexible electronics. We begin with a comprehensive overview of wireless charging methods, followed by a detailed introduction to RF‐based charging principles. We then review the applications of RF‐based charging in wearable devices, biomedical systems, integrated flexible and stretchable electronics, and on‐body wireless data transmission. Wireless charging, RF, flexible electronic, wearable devices.

Flexible hybrid electronics (FHE) is emerging as a promising solution to combine the benefits of printed electronics and silicon technology. FHE has many high-impact potential areas, such as wearable applications, health monitoring, and soft robotics, due to its physical advantages, which include light weight, low cost and the ability conform to different shapes. However, physical deformations in the field can lead to significant testing and validation challenges. For example, designers must ensure that FHE devices continue to meet their specs even when the components experience stress due to bending. Hence, physical deformation, which is hard to emulate, has to be part of the test procedures for FHE devices. This paper is the first to analyze stress experience at different parts of FHE devices under different bending conditions. We develop a novel methodology to maximize the test coverage with minimum number of text vectors with the help of a mixed integer linear programming formulation. We validate the proposed approach using an FHE prototype and COMSOL Multiphysics simulations.

With the emergence of the Internet-of-Things (IoT) and wearable devices in the recent years, Flexible Hybrid Electronics (FHEs) has attracted significant attention. Chip-foil packages (also known as flexible interposers) fabricated by integrating ultra-thin silicon ICs onto or embedded into polymer foils comprising of metal interconnects (wiring lines, through hole via interconnects etc.) for powering up the system and transmission of data signals / IO commands are ideal candidates for FHE integration. The principal advantages of chip-foil packages over standard Surface Mount Device (SMD) components are their bend ability and conformability. However, the behavior of chip-foil packages under repeated bending must be analyzed in detail through extensive investigations to enable the transfer of the technology from research labs to industrial manufacturing platforms. Hence, we conducted two different types of bending tests to examine the repeated or dynamic bending reliability of the wiring lines and the via interconnects of the chip-foil packages. The experimental results highlight the need for design optimized dimensioning of the wiring lines and the via interconnects for manufacturing FHEs with high performance and good dynamic bending reliability. Such results complement the state-of-the-art information available regarding the electrical as well as mechanical reliability of chip-foil packages and are required as essential information for establishing guidelines for handling chip-foil packages during integration as well as for producing FHEs that are more reliable.

Prediction of the interpass temperature of a wire arc additive manufactured wall: FEM simulations and artificial neural network

Flexible hybrid electronics (FHE) integrate both traditional printed circuits, solder assembled standard and thinned silicon chips along with printable electronic materials and sensors. The combination results in high performance from thin, light weight, flexible devices that potentially could be manufactured at low cost. In this paper, flexible hybrid electronics technology is being used to develop a wearable ECG and skin temperature monitor. All components and materials were commercially available, and all fabrication processes were executed in manufacturing environments. The monitor is composed of a flexible polyimide substrate with printed ECG electrodes, a printed thermistor, and connecting traces printed on one surface, and the electronic components mounted on other. Both sides have copper metal circuits connected by copper plated through hole vias (THV). ECG signals are amplified, preconditioned and wirelessly transmitted via Bluetooth to a nearby handheld mobile phone or computer. The wearable monitor is 2x2 inches in size and has been demonstrated to produce high fidelity ECG signals at the host from both certified archived human ECG signals and ECG signals from human volunteers. The monitor reproduced the archived signals at the host from which a set of clinical parameters were calculated that closely matched those of the archived signals. Manufacturing challenges and device reliability will be discussed. Current work includes building upon this platform and integration of other monitoring and sensor devices included those that monitor for biomarkers in sweat. This work was sponsored by the NanoBio Manufacturing Consortium administered by the Flextech Alliance and funded by the US Air Force Research Laboratory.

Selective metallization is not be a new concept, but when applied to three-dimensional (3D) geometries, numerous challenges are apparent. Various approaches are explored for utilizing additive copper plating techniques to achieve selective metallization on complex two-dimensional (2D) and 3D structures. With a focus on enhancing the manufacturing of conformal antennas, frequency selective surfaces, and flexible hybrid electronics, this study evaluates three additive copper plating methodologies, coupled with subsequent selective plating and etching techniques, to achieve precise metallization control. Each process implements a unique way to additively build up copper, including electroless copper plating, deposition of conductive inks, and sputtered conductive coatings. Through an investigation assessing each strategy on various materials, key variables were identified to enable accurate and repeatable selective metallization. Building on this success, the approach will be evolved for 3D substrates, establishing a foundation for the creation of intricate conductive patterns on complex shapes. Furthermore, this process will be developed for maturation and scalability in manufacturing, with a focus on applications in the defense industry, enabling the production of complex conformal electronics with enhanced performance, reliability, and efficiency. The results of this study have significant implications for additive electronics, providing new opportunities for advancements in manufacturing of conformal antennas, frequency selective surfaces, and flexible hybrid electronics, and enabling the creation of intricate conductive patterns on complex shapes.

24/7 real-time body signal monitoring for decease check and life quality enhancement is becoming a megatrend these years. However, the precious yet limited clinical facility and medical professionals cannot fulfill the emerging needs out-of- hospital. A wearable device such like smart watch utilizing biosensor-integrated System-in-Package (SiP) module can provide a real time and personal-based bio data collection as well as better health management through AI assistance. For performing a more comfortable experience, flexible hybrid electronics with ECG SiP embedded was proposed with lower profile and higher structure flexibility. The patch durability by 100,000 times rolling test as simulating daily use condition was also examined.

We have reported a new flexible hybrid electronics (FHE) whose structure is consisting of heterogeneous small/thin/rigid chiplets embedded in 2D molded polymeric substrates such as PDMS and hydrogel on which fine-pitch fanout RDLs are formed by die-first or RDL-first FOWLP. In this study, we apply this FHE methodology to flexible 3D In-Mold Electronics (IME) for highly integrated 3D electronics. 1-mm-square Si chiplets are embedded in PDMS and biocompatible Au RDLs are formed on the elastomer in die-first FOWLP processing, and finally, the resulting 2D FHE (2D-PDMS) is re-molded again to give contact lens-shaped flexible 3D IME (3D-PDMS). In addition, we design the flexible 3D interconnections with self-assembled corrugation of out-of-plane tortuousness by controlling a CTE mismatch between the Au layer and the underneath insulator of flexible epoxy formed on the PDMS.

Stretchable wearable electronics often require integrating mechanically different materials to fabricate functional devices that are stretchable, conformal, and sufficiently reliable. Common methods to integrate highly stretchable conductors with other Flexible Hybrid Electronics (FHE) components include solder and isotropic/anisotropic conductive adhesives or films. Due to the nature of the stretchable conductors/substrate, each of those methods has its limitations in wearable and stretchable devices. A magnetically aligned Anisotropic Conductive Epoxy (ACE) has promising potential for such integration due to its excellent adhesion to most substrates of interest, low curing temperature, and no pressure processing requirements. This study investigates the effect of the mechanical and environmental stresses on a magnetically aligned ACE as a bonding material between polymerized liquid metal networks (Poly-LMNs) and copper, e-textile, and Surface Mount Device (SMD) resistor. The test coupons were subjected to various mechanical and environmental stresses through a tensile test, fatigue cycling, and exposure to temperature and humidity conditions. The results showed that the ACE exhibited robustness and low contact resistance during electromechanical testing. In addition, the Poly-LMNs to e-textile or Cu-Flex assembly through magnetically aligned ACE showed no noticeable increase in the electrical resistance from cycle to cycle during fatigue cycling. The environmental conditions showed no significant impact on direct contact joints; however, there was a permanent failure to via connections due to moisture absorption, which led to cracks/delamination.

Flexible hybrid electronics allow for closer integration of the electronics circuits with the underlying structures concurrent with the added benefit of weight reduction compared to rigid glass-epoxy-impregnated printed circuit boards. Many additive fabrication methods have emerged for manufacturing single-layer and multilayer circuits, including direct-write and inkjet. The processes and parameters used for the fabrication and assembly differ significantly from those used in manufacturing rigid substrates that use plateand-etch techniques. The effect of process parameters on the realized performance and reliability of the additively fabricated circuits is poorly understood. Furthermore, several flexible substrates, including PET and PEN, require lower temperature processing than traditionally used for rigid printed circuit boards assembled using SnAgCu alloys. This paper has taken a systematic approach to study the process parameters used in the direct-write and inkjet fabrication methods. The effect of the process parameters on the realized mechanical performance, electrical performance, and reliability of the circuits has been studied under flex-to-install applications. The realized circuits have been studied using both copper and silver metallization.

Flexible electronics have inspired many novel and very important applications in recent years and various flexible electronic devices such as diodes, transistors, circuits, sensors, and radiofrequency (RF) passive devices including antennas and inductors have been reported. However, the lack of a high-performance RF resonator is one of the key bottlenecks to implement flexible wireless electronics. In this study, for the first time, a novel ultra-flexible structured film bulk acoustic resonator (FBAR) is proposed. The flexible FBAR is fabricated on a flexible polyimide substrate using piezoelectric thin film aluminum nitride (AlN) for acoustic wave excitation. Both the shear wave and longitudinal wave can be excited under the surface interdigital electrodes configuration we proposed. In the case of the thickness extension mode, a flexible resonator with a working frequency as high as of 5.2325 GHz has been realized. The resonators stay fully functional under bending status and after repeated bending and re-flattening operations. This flexible high-frequency resonator will serve as a key building block for the future flexible wireless electronics, greatly expanding the application scope of flexible electronics.

The fabrication of flexible and stretchable electronics is a critical requirement for the successful application of wearable healthcare devices. Although such flexible electronics have been commonly fabricated by microelectromechanical system (MEMS) technologies, they require a specialised equipment for vacuum deposition, photolithography, and wet and dry etching. A photolithography-free simple patterning method using a desktop plotter cutter has been proposed; however, the metal formation and electrode opening still rely on the MEMS technology. To address this issue, we demonstrate a simple, rapid, cost-effective, and a complete microfabrication process for flexible and stretchable sensor platforms encompassing conductor formation and patterning to encapsulate and open sensing windows, which only require an economic plotter cutter and readily available supplies. Despite its simplicity, the proposed process could stably create microscale features of 200 μm wide conductor lines and 1 mm window openings, which are in the useful range for various wearable applications. The feasibility of the simple fabrication of multi-functional sensors for various physiological monitoring applications was successfully demonstrated in electrochemical (glucose), electrical (electrocardiogram), mechanical (strain), and thermal (body temperature) modalities.