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The electrical performance of screen‐printed dry electrodes on thermoplastic polyurethane substrates are evaluated according to their conformance to international standards, skin‐electrode impedance, and ability to gather high quality electrocardiograms. The electrical behavior of seven screen‐printed electrodes (Ag, Ag/PEDOT:PSS, Ag/AgCl, Ag/AgCl/PEDOT:PSS, C, C/AgCl, and PEDOT:PSS) is compared to two commercially available gel electrodes (3M RedDot 50 and Philips NeoLead) which represent the state‐of‐the‐art. After basic standard electrical characterization (electrode‐electrode impedance, DC offset voltage, defibrillation recovery, bias stress, and noise), it is shown that layers like AgCl or PEDOT:PSS help to reduce the contact impedance of dry electrodes. However, the quality of the electrical signal is primarily governed by the skin impedance, which, without physical disruption or preparation, maintains a high impedance. For this reason, the impedance of all screen‐printed dry electrodes allows the collection of electrocardiograms with a quality equal to that of gel electrodes. Dry electrodes, while capable of obtaining similar electrical signal quality also bring the advantages of reusability, comfort, and the capability for long‐term measurements.

One-dimensional metal nanowires offer great potential in printing transparent electrodes for next-generation optoelectronic devices such as flexible displays and flexible solar cells. Printing fine patterns of metal nanowires with widths 90%. In this method, the AgNW dispersion, which is swept by a glass rod, is spontaneously deposited to the hydrophilic areas patterned on a hydrophobic-coated substrate. The alignment and accumulation of AgNWs at the pattern periphery are enhanced by employing a high sweeping rate of >3.2 mm s-1, improving electrical conductivity and pattern definition. The more aligned and more accumulated AgNWs lower the sheet resistance by a factor of up to 6.8. In addition, a high pattern accuracy ?3.6 ?m, which is the deviation from the pattern designs, is achieved. Quantitative analyses are implemented on the nanowire alignment to understand the nanowire geometry. This fine-printing method of the AgNW electrodes will provide great opportunities for realizing flexible and high-performance optoelectronic devices.

Surfaces which can accurately distinguish spatial and temporal changes in temperature are critical for not only flow sensors, microbolometers, process control, but also future applications like electronic skins and soft robotics. Realizing such surfaces requires the deposition of thousands of thermal sensors over large areas, a task ideally suited for printing technologies. Negative temperature coefficient (NTC) ceramics represent the industry standard in temperature sensing due to their high thermal coefficient and excellent stability. A drawback is their complex and high temperature fabrication process and high stiffness, prohibiting their monolithic integration in large area or flexible applications. As a remedy, a printable NTC composite that combines a rapid and scalable all-printed fabrication process with performances that are on par with conventional NTC ceramics is demonstrated. The composite consists of micrometer-sized manganese spinel oxide particles dispersed in a benzocyclobutene matrix. The sensor has a B coefficient of 3500 K, with a 4.0% change in resistance at 25 °C, comparable to bulk ceramics. The selected polymer binder yields a composite exhibiting less than a 1 °C change in resistance to changes in humidity. The sensor's scalability is validated by demonstration of a Q4 A4-sized temperature sensing sheet consisting of over 400 sensors.

Silver nanowires (AgNWs) are excellent candidate electrode materials in next-generation wearable devices due to their high flexibility and high conductivity. In particular, patterning techniques for AgNWs electrode manufacture are very important in the roll-to-roll printing process to achieve high throughput and special performance production. It is also essential to realize a non-contact mode patterning for devices in order to keep the pre-patterned components away from mechanical damages. Here, we report a successful non-contact patterning of AgNWs-based stretchable and transparent electrodes by laser-induced forward transfer (LIFT) technique. The technique was used to fabricate a 100% stretchable electrode with a width of 200 μm and electrical resistivity 10−4 Ωcm. Experiments conducted integrating the stretchable electrode on rubber substrate in which LED was pre-fabricated showed design flexibility resulting from non-contact printing. Further, a patterned transparent electrode showed over 80% in optical transmittance and less than 100 Ω sq−1 in sheet resistance by the optimized LIFT technique.

In this contribution, a conformable 64×45 RGB light-emitting diode (LED) matrix is presented. A design concept of stretchable electronics is used: non-stretchable polymer islands host rigid LEDs and are interconnected by horseshoe shaped metallic conductors, which are able to deform together with an elastomeric substrate. Thin-film metallization is used to enable a pixel pitch of 1 mm, while polyimide is used to realize the non-stretchable islands, but also to support the meandering conductors. The design of the stretchable interconnections has been optimized with respect to their mechanical and electrical performance. Thermoplastic polyurethane is introduced at a final stage of the fabrication process to ensure its compatibility with conventional thin-film processing. A passive matrix addressing scheme is used to drive the LED matrix. At a supply voltage of 10 V, the brightness of the display reaches up to 700 cd/m2.

The reliability of a commercially available isotropic conductive adhesive (ICA) deposited via laser induced forward transfer (LIFT) printing is reported. ICAs are particularly important for surfacemount device (SMD) integration onto low-cost, large-area system-in-foil (SiF) applications such as radio frequency identification (RFID) transponder tags. For such tags, and for SiF in general, the reliability of the printed interconnects under harsh circumstances is critical. In this study, the reliability of surface mounted resistors bonded onto screen-printed conductive circuitry on polymer foil was assessed. The prepared samples were subjected to thermal shock testing (TST), accelerated humidity testing (AHT) and flexural testing, while electrical measurements were conducted at regular intervals. Die shear testingwas performed to evaluate the bond strength. The reliability characteristics of the LIFT-printed sampleswere benchmarked against current industry standard stencil printing process. Finally, the applicability of the LIFT–ICA process for practical applications is demonstrated using RFID transponder integration and testing.

We present a stretchable and foldable passive matrix driven display using 45 by 80 RGB LED’s mounted on a meandering printed circuitry embedded in a polyurethane film. The meander interconnections have been optimized with respect to their electrical and mechanical properties to provide a display with a 3 mm pitch between the pixels and a stretchability of up to 10%. At an operating supply voltage of 5 V, the brightness of the display exceeds 30 cd/m2.

Ultrathin bare die chips and small-size surface mount device components were successfully soldered using a novel roll-to-roll compatible soldering technology. A high-power xenon light flash was used to successfully solder the components to copper tracks on polyimide (PI) and polyethylene terephthalate (PET) flex foils by using a lead-free solder paste. Results are compared with oven-reflowed solder joints on PI substrates. The delicate PET foil substrates were not damaged owing to the selectivity of light absorption, leading to a limited temperature increase in the PET foil while the chip and copper tracks were heated to a temperature high enough to initiate soldering. The microstructure of the soldered joints was investigated and found to be dependent on the photonic flash intensity. Reliability of the photonically soldered joints during damp heat testing and dynamic flexing testing was comparable with the reflowed benchmark and showed increased reliability compared with anisotropic conductive adhesives bonded on PET foils.

This paper describes and compares two integration methods to structurally laminate and interconnect foil-based components with flexible polymeric substrates. The first method uses isotropic conductive adhesives (ICA) confined in laser-ablated through foil vias (TFV), while the second one uses an anisotropic conductive adhesive (ACA). Both procedures were successfully demonstrated by interconnecting silverbased inkjet printed and gold-sputtered interdigitated capacitive humidity sensors onto flexible PEN carrier substrates, showing functionality, high process yield and low-complexity. The robustness of the assemblies was tested and compared for adhesion, bending, high humidity and temperature cycling. Confined ICA vias show higher mechanical robustness to bending, while both methods remained functional after more than 900 h of environmental ageing. Finally, the interconnections were fully validated at different levels of relative humidity (RH) by comparing the sensor response to that of a commercial sensor. (C) 2013 Elsevier B.V. All rights reserved.

Soldering of packaged electronic components using industry standard Sn-Ag-Cu (SAC) lead-free solders on low-cost foils, which are often the substrate of choice for flexible electronics, is challenging. This is mainly originating from the fact that the reflow temperatures of these solder alloys are normally higher than the maximum processing temperature of the low-cost flex foils. To enable component integration on the low-cost foils a novel method for soldering has been introduced by Holst Centre as an alternative to oven reflow, termed 'photonic soldering'. In this method high intensity photonic flashes are used to deliver the thermal energy required for soldering. By taking advantage of the selectivity of light absorption, the required energy for soldering is delivered to the components and circuit tracks while excessive heating of the foils is avoided. This paper presents successful photonic flash soldering of packaged LED components on low-cost polyethylene terephthalate (PET) foils using conventional SAC solders as a demonstration of the capabilities of this novel soldering technology.

Ultrathin bare die chips were soldered using a novel soldering technology. Using homogeneous flash light generated by high-power xenon flash lamp the dummy components and the bare die NFC chips were successfully soldered to copper tracks on polyimide (PI) and polyethylene terephthalate (PET) flex foils by using industry standard Sn-Ag-Cu lead free alloys. Due to the selectivity of light absorption, a limited temperature increase was observed in the PET substrates while the chip and copper tracks were rapidly heated to a temperatures above the solder melting temperature. This allowed to successfully soldered components onto the delicate polyethylene foil substrates using lead-free alloys with liquidus temperatures above 200 °C. It was shown that by preheating components above the decomposition temperature of solder paste flux with a set of short low intensity pulses the processing window could be significantly extended compared to the process with direct illumination of chips with high intensity flash pulse. Furthermore, it was demonstrated that with localized tuning of pulse intensity components having different heat capacity could be simultaneously soldered using a single flash pulse.

Our increasing life expectancy also implies that many of us will be suffering from one or more chronic illnesses during a larger part of our lives. Medical-grade wearables have the grand promise and the largely untapped potential to become a cornerstone technology in the care cycle. For chronic patients, tools are needed that improve the risk stratification, follow-up and management and are able to monitor disease progression and prevent relapse. Aside from the already existing cardiac rhythm monitoring patches (1), studies with emerging wearable sensors are aim to track congestive heart failure (2), sleep apnea and hypertension. In these care-focused devices, quality of sensing comes first, followed by miniaturization and power autonomy. A massive opportunity for wearable sensing concerns behavior change. Frictionless technology, personalized algorithms/feedback and power autonomy are key requirements for widespread user adoption in this space. cop. 2015 IEEE.

Flexible electronics are starting to emerge with all-printed but also hybrid cost effective, smart electronic products that will find a wide range of applications in large quantities in our society. Such products have to be built on low cost substrate materials like PEN or PET foils. Because of the low thermal stability and limited chemical resistance of these foils, established interconnection technologies are not suitable. The current paper describes a novel technology for making electronic circuitry which does not need incompatible thermal and chemical processes during fabrication. The technology called 'embedded circuitry' uses laser ablation to write the circuitry patterns in the flexible foils. These patterns are subsequently filled with conductive pastes using mask-less squeegee filling techniques. Conducting lines in flexible PEN foil with widths down to [email protected], line gaps down to [email protected] and resistances down to [email protected]/mm are demonstrated. Finite element modeling and bending tests revealed good flexibility of this low cost circuitry. Also the circuitry can be directly used for chip attachment through flipchip bonding.

Measuring the quality of human health and well-being is one of the key growth areas in our society. Preferably, these measurements are done as unobtrusive as possible. These sensoric devices are then to be integrated directly on the human body as a patch or integrated into garments. This requires the devices to be very thin, flexible and sometimes even stretchable. An overview of recent technology developments in this domain and concrete application examples will be discussed.

The technology development for a low-cost, roll-to-roll compatible chip embedding process is described in this paper. Target applications are intelligent labels and disposable sensor patches. Two generations of the technology are depicted. In the first version of the embedding technology, the chips are embedded in an adhesive layer between a copper foil and a PET film. While this results in a very thin (< 200 µm) and flexible system, the single-layer routing and the incompatibility with passive components restricts the application of this first generation. The double-sided circuitry embedding technology is an extension of the single-sided, foil-based chip embedding, where the PET film is replaced by a second metal foil. To obtain sufficient mechanical strength and to further reduce cost, the adhesive film is replaced by a substrate material which is compatible with the chip embedding concept. Both versions of the foil-based embedding technology are very versatile, as they are compatible with a broad range of polymer materials, for which the specifications can be tuned to the final application.

Conformable electronics, i.e., electronics that can be applied on curved surfaces, is demanded nowadays in place of conventional rigid printed circuit board (PCB) based electronics for a number of applications. In the field of stretchable electronics there has been a swift progress in recent years. In this paper we are presenting our contribution to this ever growing topic, including thin-film based polyimide (PI), supported Au stretchable meanders as well as PCB based Cu meanders. These meanders are supported by PI or poly(ethylene naphthalate)/poly(ethylene terephthalate) (PEN/PET) films. Thin-film based stretchable interconnects is targeting mainly the biocompatible environments with demands for strong miniaturization while the PCB based technology is used more for large area applications. Both approaches are reviewed in this paper in terms of fabrication processes, materials and cyclic fatigue reliability. For each technology fabricated demonstrators are presented as well.

Sensors integrated into food packages could benefit consumers by ensuring freshness and quality while allowing retail industry to more efficiently manage food stocks and product authenticity. Here we present smart radio-frequency labels with sensors able to measure temperature, humidity and the presence of volatile amine compounds. The labels are made via high quality screen printing and lamination technologies on low cost foils in combination with pick and place technology. As a case study the smart labels are used to quantify the freshness of fish.

This paper reports a study on the dynamics of foil-based functional component self-alignment onto patterned test substrates and its demonstration when integrating a flexible sensor onto a printed circuitry. We investigate the dependence of alignment time and final precision of stacking of mm- and cm-sized foil dies on a number of system parameters, such as amount of assembly medium dispensed on target positions, size and weight of assembling dies and their initial misalignment. Using water as a medium for direct self-alignment, mm- and cm-sized square-shaped pre-marked foil dies were aligned with accuracy down to 30 μm and smaller, which reflects a high precision relatively to their lateral dimensions on patterned marked carriers. High-speed camera stage and image recognition tools were used for analyzing rapid capillary-driven self-alignment processes of marked foil dies. It is shown that there is a definite range of initial misalignment values allowing dies to align with high accuracy and yield within the same time window, whereas both under smaller and larger initial offsets, i.e. with dies correspondingly too close or too far from their target positions, yield and alignment precision is significantly lower. The high-alignment accuracy of mm- to cm-sized functional foils was demonstrated by means of the integration of an interdigitated electrodes (IDE) capacitive sensor to a flexible polymeric substrate. Additionally, high-yield electrical interconnection was performed using anisotropic conductive adhesives (ACA). The latter opens the perspective of efficiently assembling interesting new systems such as separately manufactured sensors, paper batteries and RFIDs components through the direct capillary-driven self-alignment approach and ACA electrical interconnection.

All-printed, cost effective, smart electronic products are expected to be used in a wide range of applications and in large quantities in our society. The substrate material for these applications will be low cost materials like PEN or PET foils. For the functionality of the printed electronics product it often will still be required to integrate a Si chip. To keep the flexibility of the package and not to add too much to the thickness, the chip needs to be integrated into the product as a bare, thinned die. Because of the low temperature stability of the PEN and PET and the use of printed conductors it is necessary to interconnect the chip using an adhesive. The current paper specifically addresses the challenges associated with this. Research efforts will be discussed on the flip chip bonding of ultrathin (i.e. thickness 20 µm) bare chips on printed circuitry on both PEN and PET foils using a typical anisotropic conductive adhesive (ACA). Based on the results it can be concluded that a reproducible, low contact resistance and a good lifetime and flexural durability can be achieved over a wide range of bonding forces and temperatures.

A study on via drilling and channel scribing on PEN and PET substrates for flexible electronic application is discussed in this paper. For the experiments, both KIF excimer laser (248 nm) and frequency tripled Nd:YAG (355 nm) laser are used. Different measurement techniques like optical microscopy, Dektak profilometer, Confocal microscopy and scanning electron microscopy (SEM) were employed to characterize the quality of the channels and vias. The patterned structures were filled by three different methods using a conductive paste or ink that is cured in an oven at an elevated temperature. The cross-sectional measurements of channels and vias were carried out using SEM to study the uniformity of filling.

A novel, cost effective technology to manufacture high density embedded electronic circuitry is demonstrated. The process consists of laser photoablation of the circuitry into a substrate through a mask and subsequent filling using a polymer thick film paste. Because the volume of the substrate is used it is possible to make thick and thereby highly conductive lines using low cost materials and processes. The process is demonstrated for a fan out circuitry in 100 µm thick polyethylene naphthalate (PEN). The fan out circuitry has linewidths of 50 µm and line spacings of 100 µm. The usability of the circuitry is demonstrated by the successful flipchip bonding of a thinned Si daisy chain dummy chip with 176 IO's.

Measuring the quality of human health and well-being is one of the key growth areas in our society. Preferably, these measurements are done as unobtrusive as possible. These sensoric devices are then to be integrated directly on the human body as a patch or integrated into garments. This requires the devices to be very thin, flexible and sometimes even stretchable. An overview will be given of recent technology developments in this domain and concrete application examples will be shown.