Flexible electronics, also known as flex circuits, encompass various technologies that are used for assembling electronic circuits by mounting electronic components on flexible plastic substrates, such as polyimide, PEEK, or transparent conductive polyesterD. Shavit: The developments of LEDs and SMD Electronics on transparent conductive Polyester film, Vacuum International, 1/2007, S. 35 ff film. Additionally, flex circuits can have the form of screen-printed silver circuits on polyester. Flexible electronic assemblies may be manufactured using identical components used for rigid printed circuit boards, allowing the board to conform to a desired shape or to flex during its use.
Manufacturing Flexible printed circuits (FPCs) are developed using photolithographic technology. An alternative approach to making flexible foil circuits or flexible flat cables (FFCs) is laminating very thin (0.07 mm) copper strips in between two layers of PET. These PET layers, typically 0.05 mm thick, are coated with an adhesive that is thermosetting, and will be activated during the lamination process. FPCs and FFCs have several advantages in many applications: Tightly assembled electronic packages, where electrical connections are required in 3 axes, such as cameras (static application). Electrical connections where the assembly is required to flex during its normal use, such as folding cell phones (dynamic application). Electrical connections between sub-assemblies to replace wire harnesses, which are heavier and bulkier, such as in cars, rockets, and satellites. Electrical connections where board thickness or space constraints are driving factors.
Advantages of FPCs Potential to replace multiple rigid boards or connectors Single-sided circuits are ideal for dynamic or high-flex applications Stacked FPCs in various configurations
Disadvantages of FPCs Cost increase over rigid PCBs Increased risk of damage during handling or use More difficult assembly process Repair and rework can be difficult or impossible Generally worse panel utilization resulting in increased cost
Applications Flex circuits are often used as connectors in various applications where flexibility, space savings, or production constraints limit the serviceability of rigid circuit boards or hand wiring.
Most flexible circuits are passive wiring structures that are used to interconnect electronic components such as integrated circuits, resistors, capacitors, and the like; however, some are used only for making interconnections between other electronic assemblies either directly or by means of connectors. Consumer electronics devices make use of flexible circuits in cameras, personal entertainment devices, calculators, or exercise monitors. Flexible circuits are found in industrial and medical devices where numerous interconnections are required in a compact package. Cellular telephones are another widespread example of flexible circuits.
Input Devices A common application of flex circuits is in input devices such as computer keyboards; most keyboards use flex circuits for the switch matrix.
Displays
LCD displays In LCD fabrication, glass is used as a substrate. If thin flexible plastic or metal foil is used as the substrate instead, the entire system can be flexible, as the film deposited on top of the substrate is usually very thin, on the order of a few micrometres.
OLED displays Organic light-emitting diodes (OLEDs) are generally used instead of a back-light for flexible displays, making a flexible organic light-emitting diode display.
Flexible batteries Flexible batteries are batteries, both primary and secondary, that are designed to be conformal and flexible, unlike traditional rigid ones.
Automotive circuits In the automotive field, flexible circuits are used in instrument panels, under-hood controls, circuits to be concealed within the headliner of the cabin, as well as in ABS systems.
Printers In computer peripherals, flexible circuits are used on the moving print head of printers and to connect signals to the moving arm carrying the read/write heads of disk drives.
Solar cells
Flexible, thin-film solar cells have been developed for powering satellites. These cells are lightweight, can be rolled up for launch, and are easily deployable, making them a good match for the application. They can also be sewn into backpacks or outerwear,See for instance the Scottevest solar jacket and the Voltaic and similar solar backpacks. among many other types of consumer-oriented applications.
The growing markets related with flexible and/or portable electronics, such as for self-powered IoT systems, have driven the development of bendable thin-film photovoltaics (PV) in view of enhancing the energetic autonomy of such off-grid devices. It has been shown that this class of PV technologies is already capable of reaching high solar-to-electricity efficiencies, at the level of rigid wafer-based solar cells, particularly when integrated with effective light-trapping structures. Such photonic schemes allow high broadband absorption in the thin PV absorber materials, despite their reduced thickness required for mechanical bendability.
Skin-like circuits In December 2021, engineers from Keio University in Tokyo and Stanford University announced the creation of stretchable and skin-like semiconductor circuits. In the future, these wearable electronics may be used to send health data to doctors wirelessly.
Printed Electronics Printed electronics are in use or under consideration, including wireless sensors in packaging, skin patches that communicate with the internet, and buildings that detect leaks to enable preventative maintenance. Most of these applications are still in the prototyping and development stages. There is a particularly growing interest in flexible smart electronic systems, including photovoltaic, sensing, and processing devices, driven by the desire to extend and integrate the latest advances in (opto-)electronic technologies into a broad range of low-cost (even disposable) consumer products of everyday life, and as tools to bring together the digital and physical worlds.
Norwegian company ThinFilm demonstrated roll-to-roll printed organic memory in 2009.Thinfilm and InkTec awarded IDTechEx' Technical Development Manufacturing Award IDTechEx, April 15th 2009PolyIC, ThinFilm announce pilot of volume printed plastic memories EETimes, September 22nd 2009All set for high-volume production of printed memories Printed Electronics World, April 12th 2010Thin Film Electronics Plans to Provide ‘Memory Everywhere’ Printed Electronics Now, May 2010
Another company, Rotimpres, based in Spain, has successfully introduced applications on different markets as for instance; heaters for smart furniture or to prevent mist and capacitive switches for keyboards on white goods and industrial machines.Revolutionize Your Industrial Heating with Rotimpres Heater, October 14th 2024Capacitive keyboards - Rotimpres Capacitive switch, October 14th 2024
History Patents issued at the turn of the 20th century show interest in flat electrical conductors sandwiched between layers of insulating material. The resulting electrical circuits were to serve in early telephony switching applications. One of the earliest descriptions of what could be called a flex circuit was unearthed by Dr Ken Gilleo and disclosed in a 1903 English patent by Albert Hansen that described a construction consisting of flat metal conductors on paraffin-coated paper. Moreover, Thomas Edison’s lab books from the period indicate that he was thinking of coating cellulose gum applied to linen paper with graphite powder to create what would have clearly been flexible circuits, though no evidence indicates that it was reduced to practice.
The 1947 publication "Printed Circuit Techniques" by Brunetti and Curtis"Printed Circuit Techniques" by Cledo Brunetti and Roger w. Curtis (National Bureau of Standards Circular 468 first issued 15 November 1947) includes a brief discussion of creating circuits on what would have been flexible insulating materials (e.g., paper). In the 1950s, Dahlgren and Sanders made significant strides in developing and patenting processes for printing and etching flat conductors on flexible base materials to replace wire harnesses. An advertisement from the 1950s, placed by Photocircuits Corporation, demonstrated their interest in flexible circuits.
Flexible circuits are known by various names around the world, such as flexible printed wiring, flex print, flexi circuits, are used in many products. Credit is due to the efforts of Japanese electronics packaging engineers who have found ways to employ flexible circuit technology. Flexible circuits are one of the fastest-growing interconnection product market segments. One variation on flexible circuit technology is called "flexible electronics". It involves the integration of both active and passive functions in the device.
Flexible circuit structures Flexible circuits display significant variation in their construction.
Single-sided flex circuits Single-sided flexible circuits have a conductor layer made of either a metal or conductive (metal filled) polymer on a flexible dielectric film. Component termination features are accessible only from one side. Holes may be formed in the base film to allow component leads to pass through for interconnection, normally by soldering. Single sided flex circuits can be fabricated with or without such protective coatings as cover layers or cover coats, however the use of a protective coating over circuits is the most common practice. The development of surface mounted devices on sputtered conductive films has enabled the production of transparent LED Films, which is used in LED Glass but also in flexible automotive lighting composites.
Double access or back-bared flex circuits Double access flex, also known as back-bared flex, are flexible circuits that have a single conductor layer, but allow access to selected features of the conductor pattern from both sides. While this type of circuit has benefits, the specialized processing requirements for accessing the features limits its use.
Sculptured flex circuits Manufacturing sculptured flex circuits involves a special flex circuit multi-step etching method that yields a flexible circuit having finished copper conductors wherein the conductor thickness differs at various places along their length. (i.e., the conductors are thin in flexible areas and thick at interconnection points).
Double-sided flex circuits Double-sided flex circuits have two conductor layers. They can be fabricated with or without plated through holes, though the plated through hole variation is much more common. When constructed without plated through holes, connection features are accessed from one side only, and the circuit is defined as a "Type V (5)" according to military specifications. Because of the plated through hole, terminations are provided on both sides of the circuit, thus allowing components to be placed on either side. Depending on design requirements, double-sided flex circuits can be fabricated with protective coverlayers on one, both or neither side of the completed circuit, but are most commonly produced with the protective layer on both sides. One major advantage is that it allows easy crossover connections. Many single sided circuits are built on a double sided substrate because of the crossover connections. An example of this use is the circuit that connects a mousepad to the motherboard. All connections on that circuit are located on only one side of the substrate, except a small crossover connection that uses the other side.
Multilayer flex circuits