Stretchable Circuits Research Articles

SCB and SMI: two stretchable circuit technologies, based on standard printed circuit board processes

PurposeIn the past 15 years stretchable electronic circuits have emerged as a new technology in the domain of assembly, interconnections and sensor circuits and assembly technologies. In the meantime a wide variety of processes with the use of many different materials have been explored in this new field. The purpose of the current contribution is for the authors to present an approach for stretchable circuits which is inspired by conventional rigid and flexible printed circuit board (PCB) technology. Two variants of this technology are presented: stretchable circuit board (SCB) and stretchable mould interconnect (SMI).Design/methodology/approachSimilarly as in PCB 17 or 35 μm thick sheets of electrodeposited or rolled‐annealed Cu are structured to form the conductive tracks, and off‐the‐shelf, standard packaged, rigid components are assembled on the Cu contact pads using lead‐free solder materials and reflow processes. Stretchability is obtained by shaping the Cu tracks not as straight lines, like in normal PCB design, but as horseshoe shaped meanders. Instead of rigid or flexible board materials, elastic materials, predominantly PDMS (polydimethylsiloxane), are used to embed the conductors and the components, thus serving as circuit carrier. The authors include some mechanical modeling and design considerations, aimed at the optimization of the build‐up and combination of elastic, flexible and rigid materials towards minimal stress and maximum mechanical reliability in the structures. Furthermore, details on the two production processes are given, reliability findings are summarised, and a number of functional demonstrators, realized with the technologies, are described.FindingsKey conclusions of the work are that: supporting the metal meanders with a flexible carrier prior to embedding in an elastic substrate substantially increases the reliability under mechanical stress (cyclic uniaxial stretching) of the stretchable interconnect and the transition areas between rigid components and stretchable interconnects are the zones which are most sensitive to failure under mechanical stress. Careful design and technology implementation is necessary, providing a gradual transition from rigid to flexible to stretchable parts of the circuit.Originality/valueTechnologies for stretchable circuits, with the same level of similarity to standard PCB manufacturing and assembly, and thus with the same high potential for transfer to an industrial environment and for mass production, have not been shown before.

Integration of stretchable and washable electronic modules for smart textile applications

Today electronics in “wearable systems” or “smart textiles” are mainly realised on traditional interconnection substrates, like rigid printed circuit boards or mechanically flexible substrates. The electronic modules are detachable to allow cleaning and washing of the textile. In order to achieve a higher degree of integration and user comfort, a technology for flexible and stretchable electronic circuits was developed. The electronic system is completely embedded in an elastomer material like polydimethylsiloxane (PDMS, silicone), resulting in soft and stretchable electronic modules. The technology uses standard packaged components (ICs) and meander-shaped copper tracks, so that stretchable systems with complex functionality can be achieved. This paper describes how these stretchable modules can be integrated in textiles. Also the influence of the PDMS on the stretchability of the textile has been examined. A basic electronic module with blinking light emitting diodes (LEDs) was designed and produced to illustrate the textile integration and to perform initial washing tests.

Materials for stretchable electronics

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Printed circuit board technology inspired stretchable circuits

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Stretchable organic integrated circuits for large-area electronic skin surfaces

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Wavy Ribbons of Carbon Nanotubes for Stretchable Conductors

AbstractWavy ribbons of carbon nanotubes (CNTs) are embedded in elastomeric substrates to fabricate stretchable conductors that exhibit excellent performance in terms of high stretchability and small resistance change. A CNT ribbon with a thin layer of sputtered Au/Pd film is transferred onto a prestrained poly(dimethylsiloxane) (PDMS) substrate and buckled out‐of‐plane upon release of the prestrain. Embedded in PDMS, the wavy CNT ribbon is able to accommodate large stretching (up to the prestrain) with little change in resistance. For a prestrain of 100%, the resistance increases only about 4.1% when the wavy CNT ribbon is stretched to the prestrain. A simple stretchable circuit consisting of a light‐emitting diode and two wavy ribbons is demonstrated and shows constant response on significant twisting, folding, or stretching. Fabricated with a simple buckling approach, the wavy CNT‐ribbon‐based stretchable conductors (e.g., interconnects and electrodes) could play an important role in stretchable electronics, sensors, photovoltaics, and energy storage.

Thin-film stretchable electronics technology based on meandering interconnections: fabrication and mechanical performance

A new fabrication technology for stretchable electrical interconnections is presented. This technology can be used to connect various non-stretchable polyimide islands hosting conventional electronic components. The interconnections are realized by patterning a 200 nm thick sputter-deposited gold film into meandering horseshoe shapes, functioning as ‘two-dimensional springs’ when embedded in a silicone elastomer. Polyimide support is introduced around the meandering conductors as a means to improve the mechanical performance. Processing is done on a temporary carrier; the islands and interconnections are embedded in polydimethylsiloxane in a final stage. To this end, a release technique compatible with high temperatures up to 350 based on the evaporation of a 400 nm thick layer of potassium chloride is developed. Test structures consisting of stretchable interconnections with a varying polyimide support width were fabricated. These were strained up to twice their original length without compromising their functionality. Also cyclic mechanical loading at various strains was performed, indicating the influence of the polyimide support width on the lifetime. At strains of 10%, a minimum lifetime of 500 000 cycles is demonstrated. The presented technology thus provides a promising route towards the fabrication of stretchable electronic circuits with enhanced reliability.

Elastically Stretchable Insulation and Bilevel Metallization and Its Application in a Stretchable RLC Circuit

Stretchable electronics need stretchable wiring membranes that are equivalent to printed wiring boards but with elastically stretchable insulators and multilevel metallization. We have developed technology for elastically stretchable two-level metallization on an elastomeric membrane. Two levels of conductors were separated by a photopatternable elastomeric dielectric and connected through via holes. They were evaluated at uniaxial tensile strains of up to 30% and then used to create an elastomeric resistor–inductor–capacitor (RLC) circuit, whose alternating-current (AC) response was measured at biaxial tensile strains of up to 6%. We describe the fabrication process, morphology, and electrical performance of the bilevel metallization and the RLC circuit.

Soft generators using dielectric elastomers

The potential to produce light-weight, low-cost, wearable dielectric elastomer generators has been limited by the requirement for bulky rigid, and expensive external circuitry. In this letter, we present a soft dielectric elastomer generator whose stretchable circuit elements are integrated within the membrane. The soft generator achieved an energy density of 10 mJ/g at an efficiency of 12% and simply consisted of low-cost acrylic membranes and carbon grease mounted in a frame.

Design and implementation of flexible and stretchable systems

This paper presents a summary of the modeling and technology developments for flexible and stretchable electronics. These technologies can achieve mechanically bendable and stretchable subsystems by incorporating the electronic circuit into a matrix made of a soft polymer. The base substrate used for the fabrication of flexible circuits is a uniform polyimide layer, while silicones or polyurethanes materials are preferred for the stretchable circuits. The method developed for chip embedding and interconnections is named Ultra Thin Chip Package (UTCP). Extensions of this technology can be achieved by stacking and embedding thin dies in polyimide, providing large benefits in electrical performance and still allowing some mechanical flexibility. These flexible circuits can be converted into stretchable circuits by replacing the polyimide by a soft and elastic silicone material. The integration of ultra thin dies at package level, with thickness in the range of 10–30 μm, into flexible and/or stretchable materials are demonstrated. Furthermore, the design and reliability test of stretchable metal interconnections at board level are analyzed by both experiments and finite element modeling. We have shown through finite element modeling and experimental validation that an appropriate thermo-mechanical design is necessary to achieve mechanically reliable circuits and thermally optimized packages.

Isotropically stretchable gold conductors on elastomeric substrates

Designers of stretchable circuits need to orient stretchable interconnects in any direction. Conductors of gold film deposited on radially pre-stretched elastomeric substrates meet this condition because their wrinkles are oriented randomly. The continuous nature of those conductors makes them attractive as large area electrodes in thin film electronics. The electrical resistance of such conductors changes less with applied uniaxial strain than that of microcracked gold films and of an ideal strain gauge.

Stretching-induced interconnect delamination in stretchable electronic circuits

Stretchable electronics offer increased design freedom of electronic products. Typically, small rigid semiconductor islands are interconnected with thin metal conductor lines on top of, or encapsulated in, a highly compliant substrate, such as a rubber material. A key requirement is large stretchability, i.e. the ability to withstand large deformations during usage without any loss of functionality. Stretching-induced delamination is one of the major failure modes that determines the amount of stretchability that can be achieved for a given interconnect design. During peel testing, performed to characterize the interface behaviour, the rubber is severely lifted at the delamination front while at the same time fibrillation of the rubber at the peel front is observed by ESEM analyses. The interface properties are established by combining the results of numerical simulations and peeling experiments at two distinct scales: the global force–displacement curves and local rubber lift geometries. The thus quantified parameters are used to predict the delamination behaviour of zigzag- and horseshoe-patterned interconnect structures. The accuracy of these finite element simulations is assessed by a comparison of the calculated evolution of the shape of the interconnect structures and the fibrillation areas during stretching with experimental results obtained by detailed in situ analyses.

Cyclic endurance reliability of stretchable electronic substrates

Stretchable electronic circuit boards have been developed based on three different technologies. Such substrates serve to connect rigid interposers or electronic components. The conducting traces have a meandering shape and consist of Cu-foil or screen-printed Ag-paste. These conducting traces are attached to or embedded in polyurethane, polydimethylsiloxane, or breathable non-woven stretchable substrate material. The long-term endurance behavior of this novel type of boards is studied by cyclic elongation at strain ranges of up to 20% and monitoring the electrical connectivity. The main failure mode in the Cu-foil based technologies is fatigue of the conducting traces and can be described in terms of the Manson–Coffin relation. Indications for high-cycle fatigue were found. The screen-printed conductors on non-woven substrates fail by breaking of the connection between the metallic grains. The application areas are electronic monitoring systems that need to be placed directly on the skin, or conformable systems for curved surfaces.

Stretchable Microfluidic Radiofrequency Antennas

www.MaterialsViews.com C O M Stretchable Microfl uidic Radiofrequency Antennas M U N I By Masahiro Kubo , Xiaofeng Li , Choongik Kim , Michinao Hashimoto , Benjamin J. Wiley , Donhee Ham , and George M. Whitesides * C A IO N This paper describes a new method for fabricating stretchable radiofrequency antennas. The antennas consist of liquid metal (eutectic gallium indium alloy, EGaIn [ 1 , 2 ] ) enclosed in elastomeric microfl uidic channels. In particular, a microfl uidic structure made of two types of elastomers (polydimethylsiloxane (PDMS) and Ecofl ex (type 0030, Reynolds Advanced Materials)) with different stiffness has been developed to improve the stretchability and mechanical stability of the antennas. These antennas can be stretched up to a strain [defi ned as the percentage change in length or ( l – l 0 )/ l 0 ] of 120 %. This high stretchability allows the resonance frequencies of the antennas to be mechanically tuned over a wide range of frequencies. The antennas can also be repeatedly stretched, while retaining a high effi ciency (> 95 %) in radiation. “Stretchability” in electronics has the potential to open new opportunities, particularly for large-area devices and systems, and in systems that require the device to conform to a nonplanar surface, or to bend and stretch while in use. [ 3–5 ] Compared to “fl exible” electronics built on nonstretchable polymer or paper substrates, [ 6 , 7 ] stretchable electronics can cover almost arbitrarily curved surfaces and movable parts. Mechanical compliance may increase the comfort of the user for wearable electronics or implantable medical devices, and simplify the integration for a range of applications. [ 3–5 , 8 ] New approaches to stretchable electronics are now being developed. In a recent advance, Rogers et al. [ 4 , 5 ] described stretchable integrated circuits with elongation of up to 100 % using wavy, thin silicon ribbons on pre-stretched elastic substrates. Antennas offer new, attractive applications for stretchable electronics; these applications might include reconfi gurable antennas, [ 9 ] antennas for limited and nonplanar spaces, [ 10 ] and wearable sensors. Two methods are commonly used to build antennas for commercial applications. The most common method uses sheet-metal processing; in this method, a metal sheet is punched, bent, and welded into the desired structure. A second method uses chemical etching and plating to make small patterns of metal. This method can make fl exible antennas by patterning metal on a fl exible substrate. Neither

The effect of pitch on deformation behavior and the stretching-induced failure of a polymer-encapsulated stretchable circuit

The deformation behavior and failure mechanisms of parallel-aligned, horseshoe-patterned, stretchable conductors encapsulated in a polymer substrate were investigated by numerical and experimental analyses. A design guideline for the optimal pitch between the conductors was proposed through numerical analysis, and two extreme cases—fine and coarse pitches—were investigated by in situ experimental observations. The experimental results demonstrate that the stretchable conductors enable elongation up to 123 and 135% without metal rupture for the fine and coarse pitches, respectively. The difference between these numbers is much smaller (12%) than expected from the simulations. It is found and confirmed by a modified simulation model that the reason for this is interfacial delamination, the onset of which depends on the pitch of the conductors and occurs before metal rupture of the conductors. The definition of ‘stretchability of the electronic interconnects’ is discussed based on the facts that two different failure mechanisms occur: interfacial delamination and metal rupture.

Stretchable and Washable Electronics for Embedding in Textiles

AbstractElectronics in “wearable systems” or “smart textiles” are nowadays mainly realized on traditional interconnection substrates, like rigid Printed Circuit Boards (PCB) or mechanically flexible substrates. The electronic modules are detachable to allow cleaning and washing of the textile. In order to achieve a higher degree of integration and user comfort, IMEC-UGent/CMST developed a technology for flexible and stretchable electronic circuits. The electronic system is completely embedded in an elastomer material like PDMS (silicone), resulting in soft and stretchable electronic modules. The technology uses standard packaged components (IC's) and meander shaped copper tracks, so that stretchable systems with complex functionality can be achieved. Testing methods for washability were selected and developed. First tests are showing promising results, leveling the path to washable electronics in textiles. In order to show the possibilities of the technology in the field of textile applications a 7x8 single color stretchable LED-matrix was designed and integrated in textile. This LED-matrix can be applied for example in wearable signage applications.

Interlaced Circuits for Multidirectional Stretchable Electronics

AbstractStretchable electronic circuits have the potential to the fields where electronics have to be conformable, deformable and stretchable into three dimensional surfaces. In this work, an “interlaced” structure is developed for multidirectional stretchable circuit. The shape of the conductor is loop-like configuration. A knitted structure is employed for the elastic substrate due to its flexibility, high stretchability, low cost and simple fabrication. The electro-mechanical behavior of the interlaced circuit is investigated in three different directions, i.e., 0-degree, 45-degree, and 90-degree, respectively. A significant improvement in stretchability is achieved in 0-degree direction. Then, a preliminary theoretical analysis is made in the electro-mechanical mechanism of the interlaced circuit. From the experimental investigation and theoretical analysis, it is found that the interlaced structure gives the conductor more freedom to move in the substrate, decreasing the stress concentration in the crest and trough parts of the loop when it is stretched.

Ultrathin Silicon Circuits With Strain‐Isolation Layers and Mesh Layouts for High‐Performance Electronics on Fabric, Vinyl, Leather, and Paper

We present various stretchable high-performance CMOS circuit demonstrations on unconventional substrates, such as fabric, vinyl, leather, and paper. Electronics on such substrates, especially paper, open up new and important application possibilities for electronics. Theoretical analysis reveals the underlying mechanics of these systems; electrical tests under mechanical cycling demonstrate the robustness of the designs.

Design and performance of metal conductors for stretchable electronic circuits

PurposeThe purpose of this paper is to present an update on the progress of the design and reliability of stretchable interconnections for electronic circuits.Design/methodology/approachFinite element modelling (FEM) is used to analyse the physical behaviour of stretchable interconnects under different loading conditions. The fatigue life of a copper interconnect embedded into a silicone matrix has been evaluated using the Coffin‐Manson relation and FEM.FindingsThe mechanical properties of the substrate and the design of the metal interconnection play an important role on the fatigue lifetime of circuit. In the case of copper embedded into a PDMS Sylgard 186, more than 2,500 tensile cycles have been observed for a periodic deformation of 10 per cent.Research limitations/implicationsReliability results are limited and need further work to create a more accurate empirical model to estimate the lifetime of stretchable interconnections.Originality/valueThe combined use of FEM and experimental analysis enable a more reliable design of the stretchable metal interconnections. The proposed horseshoe design offers the benefit of reduced permanent damage during elongation.

Complementary metal oxide silicon integrated circuits incorporating monolithically integrated stretchable wavy interconnects

Stretchable complementary metal oxide silicon circuits consisting of ultrathin active devices mechanically and electrically connected by narrow metal lines and polymer bridging structures are presented. This layout—together with designs that locate the neutral mechanical plane near the critical circuit layers—yields strain independent electrical performance and realistic paths to circuit integration. A typical implementation reduces the strain in the silicon to less than ∼0.04% for applied strains of ∼10%. Mechanical and electrical modeling and experimental characterization reveal the underlying physics of these systems.