Structural Analysis and Material Characterization of Silver Conductive Ink for Stretchable Electronics

Stretchable electronic systems have become more popular in various applications such as medical, fabric, flexible sensors for personalized health care, etc. There are two major parts of flexible and stretchable circuit boards that are substrate (a plastic material) and conductive ink (formulated polymer with conductive metal). According to electrical measurements, conductive ink plays a very important role in stretchable electronic equipment. The main objective of this paper is to develop a silver (Ag) based conductive ink and characterize its mechanical and electrical properties. Conductive ink is prepared by mixing an epoxy resin, cross – linking agent, additives (adhesion promoter), organic solvent, catalyst and silver flakes all together. ASTM D412 Type C dog bone shaped cutter is used to make three samples of conductive ink. The stress-strain analysis of conductive ink is carried out using universal testing machine (UTM). The conductivity is measured using two-point probe digital multi-meter. Also, the microstructural analysis, morphology and characterization are done by scanning electron microscopy (SEM). The images are taken after curing and tensile testing. The formulated ink possesses high conductivity and stretchability up to 137% strain. The achieved conductivity of the ink is 4.167×104 S/m. The maximum stress before failure, yield stress, Young’s and tangent moduli are calculated as 1.195 MPa, 0.86 MPa, 5.72 MPa and 2.08 MPa, respectively. The SEM analysis indicates that the distribution of silver particles is uniform and in a good density throughout the sample

Screen Printing Silver Stretchable Conductive Paste to High Density Synthetic Fabric

This project investigated the viability of screen printing stretchable silver conductive paste directly onto fabric and how the resistance changed under cyclic mechanical loading. The paste tested was DuPont™ PE873 stretchable silver conductive paste, which forms a stretchable conductive path by suspending silver flakes in elastomer that can be elastically strained along with the underlying substrate. The silver pastes were printed directly onto two different high-density synthetic fabrics of different weaves. Other samples were prepared by first printing a base layer between the silver paste and the fabric. One base layer was a solvent-based dielectric (DuPont™ ME776) and a stretchable carbon conductor (DuPont™ PE671). This project determined that printing onto either base layer had a significantly smaller change in resistance under cyclic tensile tests than printing onto the fabric directly. Bend tests also revealed the possibility that the rate of change of strain had a higher impact on the change in resistance than the strain itself

The effect of different shape pattern of metal interconnects on the electrical and mechanical properties of stretchable conductive circuit

Electrically conductive adhesive (ECA) had been extensively studied to replace the Sn/Pb solder mainly found in printed circuit boards (PCBs) because of their harmful action towards human health and environment. In the production of stretchable PCBs, ECA mainly comprises of metallic filler and polymer matrix should perform good electrical and mechanical properties when straining being loaded. Therefore, determining the optimum shape pattern to be printed will contribute toward the desired traits of stretchable PCBs. In this study, commercial silver ink and thermoplastic polyurethane (TPU) as substrate was used. The ink was printed on the substrate by doctor-blade technique with different shape patterns with varies widths (1mm, 2mm and 3mm): (a) straight, (b) zigzag, (c) square and (d) sinusoidal. Then measurement of sheet resistance by four-point measurement was conducted on unloaded and loaded straining of shape pattern. This study exhibited that 3mm width zig zag shape pattern can elongate the highest straining (5% strained) compare than others patterns. In the meanwhile, straight and square shape pattern did not tolerate to any deformation which when straining at a minimum elongation of 0.1mm, the conductivity already lost. In conclusion, further study purpose, more analysis were suggested like analysis on the silver composition, curing temperature variation as well as the distribution of stress in printed shape pattern by 3D Finite Element Analysis (FEA) can be done for the more reliable study

Reliability testing on stretchable electronics:printed conductors under strain

Abstract. The aim of stretchable electronics research is to develop stretchable circuit boards and devices that can replace today’s hard and rigid circuit boards and devices. Stretching would allow for more comfortable, flexible and stretchable devices. The aim of this thesis is to investigate the properties of printed conductors on a stretched substrate, the requirements for testing, and to produce a device for stretch testing which meets desired requirements. The properties of the printed conductor are influenced by the used fabrication technique and the drying times, as well as materials such as printing ink and substrate. The properties affect the reliability and durability of the conductor, for example, failure mechanisms such as cracking and delamination of the conductor. Requirements for the test device is 1 % accuracy as a standard for the measuring instrument, which was achieved for the measuring instruments in this thesis. With the test apparatus implemented in the thesis, the stretchability limit test was performed on samples made using silk screen printing. The samples used five different printing inks, six drying times, three substrate thicknesses, four wire patterns and horizontal and vertical printing directions. The best wire pattern for the stretchability limit test was the U-shaped conductor. The horizontal printing direction was better than the vertical and the thickest 150 µm substrate was better than thinner ones. The second cyclic stretchability test performed by the test device was performed on samples with the evenly performed ink in the stretchability limit test. The measurement was performed on ten identical samples giving very similar results, which were made using the roll-to-roll technique. Based on the first test, the samples were horizontally printed on a 150 µm substrate, and the wire was U-shaped. All samples failed between 111–168 cycles when limit of failure was 100 Ω average resistance per cycle. Based on the Weibull distribution of the results, 63.2% of the tested samples failed at cycle 149.Venyvän elektroniikan luotettavuustestausta : painetut johtimet venytyksessä. Tiivistelmä. Venyvän elektroniikan tutkimuksen tavoitteena on kehittää venyviä alustoja ja venyviä laitteita, joilla voidaan korvata kovia ja kiinteitä nykypäivän piirilevyjä ja laitteita. Venyvyys mahdollistaisi mukavampien, taipuisien ja venyvien laitteiden valmistamisen. Tämän tutkielman tavoitteena on perehtyä venyvälle substraatille painetun johtimen ominaisuuksiin, testauksen vaatimuksiin ja toteuttaa vaatimusten mukainen laite venytystestaukseen. Painetun johtimen ominaisuuksiin vaikuttavat käytetyt valmistusmenetelmät ja kuivausajat sekä materiaalit kuten painomuste ja substraatti. Ominaisuudet vaikuttavat johtimen luotettavuuteen ja kestävyyteen, esimerkiksi vikaantumismekanismeihin, kuten johtimen halkeiluun ja delaminoitumiseen. Testilaitteen vaatimuksena käytetään standardeissa mittalaitteille vaadittua 1 % tarkkuutta, minkä tutkielmassa toteutettu mittalaite täyttää kaikilta osin. Tutkielmassa toteutetulla testilaitteella suoritettiin maksimivenymän testi tasosilkillä valmistetulle näytteille. Näytteissä käytettiin viittä eri painomustetta, kuutta kuivausaikaa, kolmea substraatin paksuutta, neljää johdinkuviota, sekä horisontaalista että vertikaalista painosuuntaa. Painokuvioista parhaiten maksimivenymän testissä kesti U-muotoinen johdin, horisontaalinen painosuunta oli vertikaalista parempi ja paksuin 150 µm substraatti ohuempia parempi. Testilaitteella toteutettu toinen syklisen venytyksen kesto mittaus suoritettiin maksimivenymän testissä tasaisimmin suoriutuneelle musteelle. Mittaus suoritettiin kymmenelle identtiselle roll-to-roll menetelmällä valmistetulle näytteelle, joista saatiin hyvin tasaiset tulokset. Näytteet olivat ensimmäisen testin perusteella horisontaalisesti 150 µm substraatille painettuja ja johdin oli U-muotoinen. Kaikki näytteet vikaantuivat 111–168 syklien välissä, kun vikaantumisen määritelmä oli syklin keskimääräisen resistanssin ylittäminen 100 Ω raja-arvon. Tuloksista muodostetun Weibullin jakauman perusteella testatun kaltaisista näytteistä 63,2 prosenttia on vikaantunut 149 syklin kohdalla

Effect of Tensile Load on Electrical Resistivity of Stretchable Conductive Ink (SCI)

To date, research has tended to focus on emerging Electrical Conductive Adhesive (ECA) with stretchable and flexible substrate or known as Stretchable Conductive Ink (SCI). SCI is more flexible, stretchable and multi-purpose compare with the traditional printed circuit. Limitation on the chatacreization of SCI performance especially on it electrical performane under tensile stress has motivate this study. The aim of this research is to investigate the conductivity of the conductive ink under tensile stress at different elongation. The conductive ink carbon black was used to print on the thermoplastic polyurethane (TPU) and cure in the oven at 120°C for 30 minutes. The conductive ink was clamp using in-house stretching equipment with different elongation. The resistivity was measured by four-point probe while surface structure was observed by using Axioscope 2 MAT microscope. The result shows that the resistance increased when the elongation increased. For 40mm length of conductive ink, the initial resistance is 0.562 kΩ and its become 1.217 kΩ when stretch until 18% of its initial length. The sheet resistance of the conductive ink also increased due to the defection (porosity) on the surface of conductive ink after stretching. The strain level for 40mm and 60mm also increase form 0.14 to 0.16 that cause incerase in resistance. However, since there are no crack/defection observes at 80mm after maximum elongaton, the resistance start to decrease that cause increase in SCI conductivity

Printed electronics as prepared by inkjet printing

Inkjet printing has been used to produce a range of printed electronic devices, such as solar panels, sensors, and transistors. This article discusses inkjet printing and its employment in the field of printed electronics. First, printing as a field is introduced before focusing on inkjet printing. The materials that can be employed as inks are then introduced, leading to an overview of wetting, which explains the influences that determine print morphology. The article considers how the printing parameters can affect device performance and how one can account for these influences. The article concludes with a discussion on adhesion. The aim is to illustrate that the factors chosen in the fabrication process, such as dot spacing and sintering conditions, will influence the performance of the device

Venyvän elektroniikan vaihtoehtoiset valmistus- ja tutkimusmenetelmät

Stretchable electronics are used in wearable applications to implement intelligent features. The main characteristic of stretchable electronics is stretchability enabling deformation required in wearable objects such as bandages and clothes. In this thesis, the stretchable electronics consist of elastic substrates, printed stretchable interconnections, adhesives and rigid modules with traditional electronic components. The modules on the elastic substrate form rigid islands that allow the substrate to stretch. Stretchable electronics can endure only a specific amount of elongation before their electrical interconnections fail. Adhesion and deformation mechanisms in the joint and in the joint area of the module and the substrate affect elongation. The durability of stretchable electronics can be improved by improving adhesion and controlling the deformations via optimizing the structure of the joint and the joint area. In this thesis, the stretchable electronics were studied on several levels. A thermoplastic polyurethane (TPU) film was used as the elastic substrate. Wettability and effectiveness of pre-treatments on wettability were examined. The substrate was investigated by measuring contact angles of droplets with a drop shape analyzer. Adhesion and peel behavior of non-conductive adhesives between the TPU-film and the rigid substrates were studied with a floating roller peel test setup. Finally, tensile testing was used to investigate deformations and elongation of the fabricated stretchable electronics samples. In the tensile test samples, width of the interconnection, the amount of the conductive adhesive and the use of a supportive frame structure were varied. The tests presented new results that can be adopted alone or as whole. The wettability of the TPU-film improved most with a plasma pre-treatment that decreased the contact angles up to 63 percent. The peel tests showed that the sample with one cyanoacrylate adhesive with a primer had the highest momentary bond strength (0,5 N/mm). The high bond strength made the TPU-film elongate during the peeling test. Unlike the tested structural adhesives, an elastic transfer tape adhesive had the most even peeling force during the tests (between 0,2 – 0,3 N/mm) and was the easiest adhesive to process. According to the stress peaking concept, in the tensile testing, when the samples elongated, stress concentrated close to the attached module and broke the samples. The strongest interconnection elongated 91,7 % before failure. The referred sample type had the supportive frame and conductive adhesive only under the contacts. Similarly, according to the concept, the stress exerted on this sample was more uniform compared to the other tensile test samples, which explains the good results

Micro-patterned graphene-based sensing skins for human physiological monitoring.

Ultrathin, flexible, conformal, and skin-like electronic transducers are emerging as promising candidates for noninvasive and nonintrusive human health monitoring. In this work, a wearable sensing membrane is developed by patterning a graphene-based solution onto ultrathin medical tape, which can then be attached to the skin for monitoring human physiological parameters and physical activity. Here, the sensor is validated for monitoring finger bending/movements and for recognizing hand motion patterns, thereby demonstrating its future potential for evaluating athletic performance, physical therapy, and designing next-generation human-machine interfaces. Furthermore, this study also quantifies the sensor's ability to monitor eye blinking and radial pulse in real-time, which can find broader applications for the healthcare sector. Overall, the printed graphene-based sensing skin is highly conformable, flexible, lightweight, nonintrusive, mechanically robust, and is characterized by high strain sensitivity

Integration of conductive materials with textile structures : an overview

In the last three decades, the development of new kinds of textiles, so-called smart and interactive textiles, has continued unabated. Smart textile materials and their applications are set to drastically boom as the demand for these textiles has been increasing by the emergence of new fibers, new fabrics, and innovative processing technologies. Moreover, people are eagerly demanding washable, flexible, lightweight, and robust e-textiles. These features depend on the properties of the starting material, the post-treatment, and the integration techniques. In this work, a comprehensive review has been conducted on the integration techniques of conductive materials in and onto a textile structure. The review showed that an e-textile can be developed by applying a conductive component on the surface of a textile substrate via plating, printing, coating, and other surface techniques, or by producing a textile substrate from metals and inherently conductive polymers via the creation of fibers and construction of yarns and fabrics with these. In addition, conductive filament fibers or yarns can be also integrated into conventional textile substrates during the fabrication like braiding, weaving, and knitting or as a post-fabrication of the textile fabric via embroidering. Additionally, layer-by-layer 3D printing of the entire smart textile components is possible, and the concept of 4D could play a significant role in advancing the status of smart textiles to a new level

Development of a new Stretchable and Screen Printable Conductive Ink

Stretchable conductive ink is a key enabler for stretchable electronics. This thesis research focuses on the development of a new stretchable and screen printable conductive ink. After print and cure, this ink would be capable of being stretched by at least 500 cycles at 20% strain without increasing its resistance by more than 30 times the original resistance, while maintaining electrical and mechanical integrity. For a stretchable and screen-printable conductive ink, the correct morphology of the metal powder selected and the ability of the binder to be stretched after the sintering process, are both indispensable. This research has shown that a bi-modal mixture of fine and large-diameter silver flakes will improve stretchability. While the smaller flakes increase the conductivity and lower the sintering temperature, the larger flake particles promote ohmic connectivity during stretching. The bi-modal flake distribution increases connection points while enhancing packing density and lowering the thermal activation barrier. The polymer binder phase plays a crucial role in offering stretchability to the stretchable conductive inks. The silver flakes by themselves are not stretchable but they are contained within a stretchable binder system. The research demonstrates that commonly used printable ink binder when combined with large-chain polymers through a process known as ‘elastomeric chain polymerization’ will enable the conductive ink to become more stretchable. This research has shown that the new stretchable and screen printable silver conductive ink developed based upon the two insights mentioned above; (1) bi modal flakes to improve ohmic connectivity during stretching and (2) elastomeric chain polymerized binder system which could stretch even after the ink is sintered to the substrate, can exhibit an ink stretchability of at least 500 cycles at 20% strain while increasing the resistance by less than 30 times the original resistance. Wavy print patterns can enhance the stretchability of stretchable conductors. The research also demonstrates that FEA modeling, simulating the total principal strain on the printed patterns, can be used to estimate the comparative resistance changes caused by stretching and these changes can be explained by some basic equations from Percolation Theory