Flexible Electronics

FLEXIBLE ELECTRONICS THE NEXT UBIQUITOUS PLATFORM Name:Dhiraj Bennadi Dept. of Electronics and Communication Gogte Institute of Technology Belagavi, India [email protected] Contact number:9008479560 PRN:106 Name:Amogh Balikai Dept. of Electronics and Communication Gogte Institute of Technology Belagavi, India [email protected] Contact number:9986186140 PRN:107 Abstract-Thin-film electronics in its myriad forms has underpinned much of the technological innovation in the fields of displays, sensors, and energy conversion over the past four decades. This technology also forms the basis of flexible electronics. Flexible electronics, also known as flex circuits, is a technology for assembling electronic circuits by mounting electronic devices on flexible plastic substrates, such as polyamide, PEEK or transparent conductive polyester film. The characteristic of Flexible Electronics is its reduced production cost and they have lightweight, thinner, non-breakable and new forms to create many new applications. Here we review the current status of flexible electronics along with its manufacturing and its applications mainly in healthcare and display. Keywords—PEEK; polyamide; Introduction Ever evolving advances in thin-film materials and devices have feuled many of the developments in the field of flexible electronics. These advances have been complemented with the development of new integration processes, enabling wafer-scale processes to be combined with flexible substrates. This has resulted in a wealth of demonstrators in recent years. Following substantial development and optimization over many decades, thin-film materials can now offer a host of advantages such as low cost and large area compatibility, and high scalability in addition to seamless heterogeneous integration. Just as the IC replaced discrete circuit board electronics, flexible electronics will almost inevitably by virtue of the ever-demanding end-user supersede solid-state ICs, in particular, applications where form factors are important. Reduced cost, large area, roll-to-roll, and flexible systems, such as low-cost flexible displays, require conformal, distributed, and integrated functionality, which is hitherto unavailable from more traditional brittle material and device platforms. This paper reviews the materials, design issues, and technologies for next-generation flexible electronics, and primarily considers future applications therein. This paper is foremost organized from the standpoint of applications, with some of the more pertinent questions posing the materials engineer introduced to stimulate discussion. We present examples of potential applications of flexible electronics in various societal sectors, including: healthcare; industry; human–machine interfaces; as well as market specific applications, such as: human–machine interactivity , while touching on the application of flexible electronics on ubiquitous displays throughout. manufacturing A. KEY ELEMENTS 1. Bond material The base material is the flexible polymer film which provides the foundation for the laminate. Under normal conditions, the flex circuit base material provides most primary physical and electrical properties of the flexible circuit. In the case of adhesive less circuit constructions, the base materials provide all of the characteristic properties. While a wide range of thickness is possible, most flexible films are provided in a narrow range of relatively thin dimension from 12 µm to 125 µm (1/2 mil to 5 mils) but thinner and thicker material are possible. There are a number of different materials used as base films including: polyester (PET), polyimide (PI), polyethylene napthalate (PEN), Poly-etheramide (PEI), along with various fluropolymers (FEP) and copolymers Polyimide films are most prevalent owing to their blend of advantageous properties electrical, mechanical, chemical and thermal. 2. Bonding adhesive Adhesives are used as the bonding medium for creating a laminate. Whenever it comes to temperature resistance, the adhesive is also typically the performance limiting element of a laminate especially when polyamide is the base material. As with the base films, adhesives come in different thickness. Thickness selection is usually a function of the application. For example, the different adhesive thickness is most commonly used in the creation of cover layers in order to meet the fill demands of different copper foil thickness which may be encountered. 3. Metal foil Metal foil is commonly used as the conductive element of flexible laminates. The metal foil is the material from which the circuit paths are normally etched. A wide variety of metal foils of varying thickness are available, however copper foils serve the vast majority of all flexible circuit applications. Copper’s excellent balance of cost and physical and electrical performance attributes make it an excellent choice. There are many different types of copper foil available. The IPC identifies eight different types of copper foils for printed circuits divided into two much broader categories, electro-deposited and wrought. As a result, there are a wide variety of copper foils available for flex circuit applications to serve the varied purposes of different end products. With most copper foils, a thin surface treatment is commonly applied to one side of the foil to improve its adhesion to the base film. B. FABRICATION PROCESS 1.BATCH PROCESS Electronic devices and circuits and display panels are made by a fabrication process known as batch processing. Flexible foil substrates which are cut to sheets, will be the drop-in replacement for the rigid glass plates or silicon wafers. Rigid substrates are best suited to free standing and loose mounting. Temporarily bonding the foil substrate to a rigid carrier for processing can improve the substrate’s dimensional stability. Bonding may be particularly advisable if inorganic device materials are deposited on compliant polymer substrates, because of the large strain this combination may generate. The adhesive must provide sufficient shear strength between the substrate and the carrier, resist the process chemicals, degas little and release few contaminants. At the end of processing it must be removed without damaging the electronics. Thermoplastic adhesives provide the necessary resistance against solvents and can be detached by heating. However, they impose a ceiling on the process temperature that necessarily is lower than the highest working temperature of the substrate. This requirement makes the process window narrow and therefore may degrade device performance. Because the mechanical force needed for de-bonding may cause damage to the devices and may reduce the yield, special equipment will be needed for de-bonding in a manufacturing setting. 2. ROLL-TO- ROLL PROCESS Flexible electronics are usually associated with roll-to-roll processing .This fabrication of large-area electronics, including solar cells, is most desirable for cost reduction. Indeed, amorphous silicon solar cells are manufactured on flexible steel and PI foils by roll-to-roll process. The solar cells on PI even take advantage of the easy through-substrate connection that can be had by punching holes into the plastic foil substrate. In contrast to solar cell manufacture, making displays and other active electronic circuitry requires a large number of patterning steps. The device layers can be patterned by the additive processes of directly printing the active materials, shadow masking, or subtractive patterning by photolithography. All of these techniques can be adapted to web processing. The big challenge is that backplane circuits need high precision, accuracy, and yield. The roll-to-roll photolithography and etching tools available today are not capable of 2-μm resolution and overlay registration, particularly when combined with the tensioning applied for winding and with process cycles at elevated temperature, both of which cause substrate deformation. The goal of roll-to-roll fabrication of flexible electronics is stimulating innovations in equipment and process design, process recipes, and system integration. Tools for roll-to-roll processing that are available today include web cleaner, PECVD, sputtering, plasma etcher, develop etch strip line (for printed circuit boards with high-density interconnects), die punch (which can be modified for control of alignment and winding/unwinding), evaporator (with linear source to improve film uniformity in roll-to-roll applications), laser writer, inkjet printer, screen printer, and inspection devices. 3. ADDITIVE PRINTING If flexible electronics could be fabricated by additive printing, their cost could become as low as a few dollars per square meter . Additive printing is roll-to-roll process compatible, is a high-throughput process, uses device materials efficiently, may not require vacuum, and may provide a solution to overlay registration problem through digital compensation. Noble-metal conductors, organic conductors, semiconductors, and insulators can be printed. The print ability of organic materials has stimulated experiments on the printing of TFTs. Masks for etching or lift-off patterns, as well as certain inorganic materials , can be printed . Printing metallic conductors from nano-particles may reduce the required sintering temperature to values acceptable for plastic substrates. The printing of high quality gate dielectrics and the operational stability of printed devices are issues that remain to be resolved. APPLICATIONS Flexible circuits are often used as connectors in various applications where flexibility, space savings, or production constraints limit the service ability of rigid circuit boards or hand wiring. Flexible circuits in display have gained a lot of momentum in the current scenario of technology. Many electronic companies have come forward with their bendable displays. The main component of such a display is an OLED called FOLED which stands for Flexible Organic Light Emitting Diodes. A FOLED is a type of organic light-emitting diode (OLED) incorporating a flexible plastic substrate on which the electroluminescent organic semiconductor is deposited by various processes. This enables the device to be bent or rolled to an extent while still operating. An OLED emits light due to the electroluminescence of thin films of organic semiconductors approximately 100 nm thick. Regular OLEDs are usually fabricated on a glass substrate, but by replacing glass with a flexible plastic such as polyethylene terephthalate (PET) among others, OLEDs can be made both bendable and light weight. Flexible OLEDs may be used in the production of roll able displays, electronic paper, or bendable displays which can be integrated into clothing, wallpaper or other curved surfaces. Prototype displays have been exhibited by companies such as Sony, Xerox, Samsung which are capable of being rolled around the width of a pen. Demonstration of an flexible oled. A.HEALTHCARE Flexibility in electronic materials is very attractive for medical and bioengineering. Living organisms are intrinsically flexible and malleable. Thus, flexibility is a necessity for successful integration of electronics in biological systems. Furthermore, in order to carry out daily tasks, flexibility is less likely to hinder over stiffness . Recently, some electronics have been integrated into human bodies. 1. BIONIC EYE: A visual prosthesis, usually referred to as a bionic eye, is an experimental visual device intended to restore functional vision in those who are suffering from partial or total blindness. Here a vision-compromised patient requires an electrically active addressable matrix array, with each unit or pixel recording an image and transmitting this to the patient via the optic nerve. Such technology is not limited to vision, and is also applicable to many other types of sensation. 2. BIONIC EAR: The bionic ear offers an ideal platform for flexible thin-film electronics. In auditory systems, in particular inside the cochlear, the basal membrane of the organ oscillation is key for listening and fine tuning. With a unique stiffness and geometry, a thin film coupled together with pressure sensing arrays acts as a bio-mimicking auditory system. At a specific frequency and sound pressure, the basal membrane vibrates at a specific location with predefined amplitude. A microarray pressure sensor can be activated for each specific location, emitting a signal of known pitch and loudness, mimicking the incident sound. Bionic Ear ADVANTAGES Ease for manufacturing or assembly. Single-Sided circuits are ideal for dynamic or high-flex applications. Stacked FPCs in various configurations. Light weight. Smaller dimensions required. Foldable and bendable . Flexible circuitry reduces the size and weight of the product. It allows increased circuit density and eliminates bulky connections and wiring. DISADVANTAGES Initial investment may be expensive. Precision machines required . Integration of components is difficult. Lifetime is reduced compared to normal circuit boards. CONCLUSION In this paper, we have considered some of the unique properties and applications of thin-film flexible electronics. Based on the current socioeconomic trends, we outlined some of the more likely technological future needs and discussed the potential exploits of thin-film flexible electronics in various market sectors. We have shown that the novel properties of thin-film, flexible electronics such as low weight, mechanical flexibility and durability, simple device integration, along with low-cost and large-area process ability allow them to be utilized in a wide range of applications Future developments in flexible thin-film technology are likely to enhance the performance of the devices discussed here, leading to more widespread applications. REFERENCES [1] Wikipedia-An article on Flexible Electronics http://en.wikipedia.org/wiki/Flexible_electronics [2] Development of Electronic Displays by Stanley W Stephenson [3] "Printed Circuit Techniques" by Cledo Brunetti and Roger w.Curtis [4] IEEE LIBRARY [5] http://www.technologyreview.com/tagged/flexible-electronics/