Electromechanical Reliability Testing of Flexible Hybrid Electronics Incorporating FleX Silicon-On-Polymer Ics

Research on flexible electronics has grown exponentially over the last decade. Researchers around the globe are developing a wide range of flexible systems, including displays [1, 2], sensors [3–5], RFID tags [6, 7] and other similar devices [8]. Innovations in materials have been key to the increased research success in this field of research in recent years [9]. Transistors, interconnects, memory cells, passive components and other assorted devices all have challenging material demands for flexible electronics to become a reality. Nanomaterials of various kinds have been found to represent a tremendously powerful tool, with nanoparticles [10], nanotubes, nanowires [3, 11] and engineered organic molecules [12, 13] contributing to the realization of high-performance semiconductors, dielectrics and conductors for flexible electronics applications.Nanomaterials offer tunability in terms of performance, solution processability and processing temperature requirements, which makes them very attractive as building blocks for flexible electronic systems. Indeed, such systems represent some of the largest families of commercially produced nanomaterials today, and numerous commercial products based on nanoparticle formulations are widely available.This special issue focuses on the rapidly blossoming field of flexible electronics, with a particular focus on the use of nanotechnology to facilitate flexible electronic materials, processes, devices and systems. Contributions to the issue describe the development of nanomaterials—including nanoparticles, nanotubes, nanowires and carbon-based thin films—for use in conductors, transparent electrodes, semiconductors and dielectrics. The articles feature innovations in nanomanufacturing and novel materials, as well as the application of these technologies to advanced flexible devices and systems.As flexible electronics systems move rapidly towards successful commercial deployment, it is extremely likely that they will exploit nanomaterials as building blocks. Developments in the field will help to leverage the power of these materials to realize novel functionalities in flexible form factors. This special issue provides a view of the state of the art in these technologies, and gives a vision of the coming innovations that will make flexible electronics a reality.

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.

Predicting the reliability is an important task of product life cycle analysis, especially during the product development stages. The uncertainty in the operating conditions and the presence of multistress factors makes this reliability prediction more difficult. In this work, a probabilistic reliability prediction framework is proposed using the linear damage accumulation and degradation modeling for multistress conditions. The multistress-life (S-L) curve for corresponding multistress has been developed using the equivalent stress. The multi S-L model allows extrapolating the expected life product under given operating or test conditions as well as provides input to estimate the reliability as a function of time for both a single multistress factor level and a sequence of multistress-factor levels. The proposed methodology has been demonstrated with newly developed flexible hybrid electronics products.

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.

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.

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.

Flexible hybrid electronics (FHE) is an emerging technology enabled through the integration of semiconductor devices and additive manufacturing technology. It unlocks tremendous market potential by realizing low-cost flexible circuits and systems that can be conformally integrated into various applications. However, the operating frequencies of most reported FHE systems are relatively low. It is also worth noting that reported FHE systems have been limited to simple design concepts since complex systems will impose challenges in aspects such as multilayer interconnections, printing materials, and notably, bonding layers. In this article, an additively manufactured flexible millimeter-wave Doppler radar is reported. Rogers RO4350B and flexible high-temperature filament TPC are chosen as the substrate. High-heat insulation epoxy is used as the bonding layer to increase the mechanical strength of the printing radar. Additionally, the RF properties of TPC and epoxy materials have been characterized from 20 to 50 GHz, paving the way toward fully additively manufactured radar systems. Mechanically drilled through vias and blind vias are employed to realize interconnections between each conductive layer. The realized gain of the unbent <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\backslash $</tex-math> </inline-formula> bent printed millimeter-wave circuits was characterized, with a value of 12.80 and 9.355 dBi, matching well with expectations. The sensing performance and flexibility of the printed radar are also characterized and validated by general laboratory tests, field tests, and on-object tests. Our results demonstrate the feasibility of developing additively manufactured high-frequency multilayer FHE, which can be conformally and seamlessly integrated into irregular surfaces for applications such as vehicle radars.

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.

Flexible electronics have received considerable attention in academies and industries for their promising applications in enormous fields, such as flexible displays, wearable sensors, artificial skins, and flexible energy devices. Challenges remain in developing a flexible and scalable manufacturing method to facilitate the fabrication of multi-functional structures in a flexible electronic system. Nanoimprint lithography is a high resolution and low-cost approach to fabricate nanostructures over a large area. This paper reviews recent progress of nanoimprint lithography and its applications in flexible electronics. The basic principles, classification, research focus, and critical issues of nanoimprint lithography are elaborated. Then, the advantages of nanoimprint lithography are demonstrated in several typical applications related to flexible electronics, including conductive films, optoelectronic devices, flexible sensors, energy harvesting and storage devices, and bioinspired electronic devices. Finally, the challenges and perspectives of nanoimprint lithography in flexible electronic systems are discussed.

Mechanically flexible, printed, and stretchable electronics are gaining momentum. While rapid advancement is well underway at the device and circuit levels, researchers have yet to envision the system design in a flexible form. This paper introduces the concept of <i>System-on-Polymer (SoP)</i> based on flexible hybrid electronics (FHE) to combine the advantages of flexible electronics and traditional silicon technology. First, we formally define flexibility as a new design metric in addition to existing power, performance, and area metrics. Then, we present a novel optimization approach to place rigid components onto a flexible substrate while minimizing the loss in flexibility. We show that the optimal placement leads to as much as 5.7 <inline-formula><tex-math notation="LaTeX"> $\times$</tex-math></inline-formula> enhancement in flexibility compared to a naïve placement. We confirm the accuracy of our models and optimization framework using a finite element method (FEM) simulator. Finally, we demonstrate the SoP concept using a concrete hardware prototype, and discuss the major challenges in the architecture and design of SoPs.

Flexible Hybrid Electronics (FHE) is in the limelight for next generation electronics, it has good potential for further development as the priority areas for focused study, not only can it create new products, but also meet the needs of flexible, bendable, and stretchable characteristics. Furthermore, FHE brings new business opportunities to the semiconductor industry, and it has niche growth potential in the fields of healthcare, aviation, soft robotics and Internet of Things (IoT). Fan-out panel-level packaging (FOPLP) technology have the advantages of thinner packaging, good heat dissipation and high electrical performance. In this paper, the RDL-first FOPLP techniques is adopted to develop the FHE systems with ITRI's core technologies, i.e., FlexUPTM and mechanical de-bonding. With ITRI's FlexUPTM and flexible display technology, the related problems can be overcame such as broken circuit and poor electrical performance, which were caused by bending and stretching the circuit of the RDL substrate. The results showed that ITRI's core technology could meet the demand of FHE systems. It also provide a solution to solve the contradiction between the characteristics of the rigid structure and flexible requirements.

The U.S. National Science Foundation (NSF)’s ‘new’ Advanced Manufacturing (AM) program is an amalgam of previous programs that explored nano-scale, additive or subtractive manufacturing, manufacturing machines or materials engineering or cybermanufacturing. The AM program seeks new ideas in, across and outside these domain areas, especially manufacturing at different scales and their integration to generate complex systems. A key component of the AM program is nanomanufacturing and an important application area for nanomanufacturing is flexible hybrid electronics (FHE). This paper describes the AM program and achievements in nanomanufacturing research. Nanomanufacturing research efforts through the core program, solicitations and center-level are highlighted. Projects in FHE that address challenges in processes, such as printing, imprinting and self-assembly, in materials, such as inks and substrates, and in scale-up, e.g., wafer-scale or continuous roll-to-roll are described. The paper is an overview of activities in these areas and is meant to serve as a resource for information and collaboration.

Au redistribution layers 10 to 100 μm wide were fabricated on heterogeneously integrated advanced flexible hybrid electronics (FHE) substrates formed by a die-first approach based on fan-out wafer-level packaging. The formed Au metal wiring was meticulously studied for locally induced mechanical stress upon bending (bending radius, BR 20 mm) using Laue microdiffraction (LμD) with synchrotron radiation. It was inferred from the LμD data that upon bending the FHE substrate up to the BR of 20 mm, the Au metal wiring (10 mm long, 100 μm wide, and 500 nm thick) experienced mechanical bending stress amounting to 250 ∼ 300 MPa. The stress values obtained from the LμD studies were close to the stress value of 350 MPa obtained by simulation.