Flexible electronics
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 polyester 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 are developed using photolithographic technology. An alternative approach to making flexible foil circuits or flexible flat cables is laminating very thin 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.
- Electrical connections where the assembly is required to flex during its normal use, such as folding cell phones.
- 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
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
s are generally used instead of a back-light for flexible displays, making a flexible organic light-emitting diode display.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, 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 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
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 electronic technologies into a broad range of low-cost consumer products of our 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.
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.
History
s 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 includes a brief discussion of creating circuits on what would have been flexible insulating materials. 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.