Flexible electronics

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Image of Miraco flexible printed circuits prior to de-panelization. FPC PANEL.pdf
Image of Miraco flexible printed circuits prior to de-panelization.
An Olympus Stylus camera without the case, showing the flex circuit assembly. Olympus Stylus.jpg
An Olympus Stylus camera without the case, showing the flex circuit assembly.

Flexible electronics, also known as flex circuits, is a technology for assembling electronic circuits by mounting electronic devices on flexible plastic substrates, such as polyimide, PEEK or transparent conductive polyester [1] film. Additionally, flex circuits can be 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.

Contents

Manufacturing

Flexible printed circuits (FPC) are made with a photolithographic technology. An alternative way of 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 which is thermosetting, and will be activated during the lamination process. FPCs and FFCs have several advantages in many applications:

Advantage of FPCs

Disadvantages of FPCs

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 many 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 like 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 normally 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, and 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, [2] 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. [3] 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. [4] [5]

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. [6]

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. Thomas Edison’s lab books from the period indicate that he was thinking to coat 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.[ citation needed ][ citation needed ]

The 1947 publication "Printed Circuit Techniques" by Brunetti and Curtis [7] 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 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.[ citation needed ]

Flexible circuits are variously known around the world variously 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.[ citation needed ]

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

Flex circuits with three or more layers of conductors are known as multilayer flex circuits. Commonly the layers are interconnected by means of plated through holes, though this is not required because it is possible to provide openings to access lower circuit level features. The layers may or may not be continuously laminated together throughout the construction with the exception of the areas occupied by through-holes. Discontinuous lamination is common in cases requiring maximum flexibility. This is accomplished by leaving unbonded the areas where flexing or bending is to occur.

Rigid-flex circuits

Rigid-flex circuits are a hybrid combining rigid and flexible substrates laminated into a single structure. Rigid-flex circuits are not rigidized flex constructions, which are flex circuits to which a stiffener is attached to support the weight of the components. A rigidized or stiffened flex circuit can have one or more conductor layers. The terms represent quite different products.

The layers are normally interconnected by plated through holes. Rigid-flex circuits are often chosen by military product designers and increasingly in commercial products. Compaq Computer chose the approach for laptop computer boards in the 1990s. While the computer's main rigid-flex PCBA did not flex during use, subsequent Compaq designs utilized rigid-flex circuits for the hinged display cable, passing tens of thousands of flexures during testing. By 2013, the use of rigid-flex circuits in consumer laptop computers had become common.

Rigid-flex boards are normally multilayer structures; however, two metal layer constructions are sometimes used. [8]

Polymer thick film flex circuits

Polymer thick film (PTF) flex circuits print circuit elements onto a polymer film. They are typically single conductor layer structures, however two or more metal layers can be printed sequentially separated by printed insulating layers. While lower in conductivity and thus limited to certain applications, PTF circuits have found a home in low-power applications at slightly higher voltages. Keyboards are a common application.

Flexible circuit materials

Each element of the flex circuit construction must be able to consistently meet the demands placed upon it for the life of the product. In addition, the material must work reliably in concert with the other elements of the flexible circuit construction to assure ease of manufacture and reliability. Following are brief descriptions of the basic elements of flex circuit construction and their functions.

Base material

The base material is the flexible polymer film which provides the foundation for the laminate. Under normal circumstances, the flex circuit base material provides most primary physical and electrical properties of the flexible circuit. In the case of adhesiveless circuit constructions, the base material provides 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. Thinner materials are of course more flexible and for most material, stiffness increase is proportional to the cube of thickness. Thus for example, means that if the thickness is doubled, the material becomes eight times stiffer and will only deflect 1/8 as much under the same load. There are a number of different materials used as base films including: polyester (PET), polyimide (PI), polyethylene naphthalate (PEN), polyetherimide (PEI), along with various fluropolymers (FEP) and copolymers. Polyimide films are most prevalent owing to their blend of advantageous electrical, mechanical, chemical and thermal properties.

Bonding adhesive

Adhesives are used as the bonding medium for creating a laminate. When it comes to temperature resistance, the adhesive is typically the performance limiting element of a laminate especially when polyimide is the base material. Because of the earlier difficulties associated with polyimide adhesives, many polyimide flex circuits presently employ adhesive systems of different polymer families. However some newer thermoplastic polyimide adhesives are making important in-roads. As with the base films, adhesives come in different thickness. Thickness selection is typically a function of the application. For example, different adhesive thickness is commonly used in the creation of cover layers in order to meet the fill demands of different copper foil thickness which may be encountered.

Metal foil

A metal foil is most commonly used as the conductive element of a flexible laminate. 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 from which to choose and create a flex circuit, 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 actually many different types of copper foil. The IPC identifies eight different types of copper foil for printed circuits divided into two much broader categories, electrodeposited and wrought, each having four sub-types.) As a result, there are a number of different types of copper foil available for flex circuit applications to serve the varied purposes of different end products. With most copper foil, a thin surface treatment is commonly applied to one side of the foil to improve its adhesion to the base film. Copper foils are of two basic types: wrought (rolled) and electrodeposited and their properties are quite different. Rolled and annealed foils are the most common choice, however thinner films which are electroplated are becoming increasingly popular.

In certain non standard cases, the circuit manufacturer may be called upon to create a specialty laminate by using a specified alternative metal foil, such as a special copper alloy or other metal foil in the construction. This is accomplished by laminating the foil to a base film with or without an adhesive depending on the nature and properties of the base film.[ citation needed ]

Flexible circuit industry standards and specifications

Specifications are developed to provide a common ground of understanding of what a product should look like and how it should perform. Standards are developed directly by manufacturer's associations such as the Association Connecting Electronics Industries (IPC) and by users of flexible circuits.

Scientific Publications

IEEE Journal on Flexible Electronics (J-FLEX) [9]

Scientific Conferences

IEEE International Flexible Electronics Technology Conference (IFETC) [10]

See also

Related Research Articles

<span class="mw-page-title-main">Organic electronics</span> Field of materials science

Organic electronics is a field of materials science concerning the design, synthesis, characterization, and application of organic molecules or polymers that show desirable electronic properties such as conductivity. Unlike conventional inorganic conductors and semiconductors, organic electronic materials are constructed from organic (carbon-based) molecules or polymers using synthetic strategies developed in the context of organic chemistry and polymer chemistry.

<span class="mw-page-title-main">Printed circuit board</span> Board to support and connect electronic components

A printed circuit board (PCB), also called printed wiring board (PWB), is a medium used to connect or "wire" components to one another in a circuit. It takes the form of a laminated sandwich structure of conductive and insulating layers: each of the conductive layers is designed with a pattern of traces, planes and other features etched from one or more sheet layers of copper laminated onto and/or between sheet layers of a non-conductive substrate. Electrical components may be fixed to conductive pads on the outer layers in the shape designed to accept the component's terminals, generally by means of soldering, to both electrically connect and mechanically fasten them to it. Another manufacturing process adds vias, plated-through holes that allow interconnections between layers.

<span class="mw-page-title-main">BoPET</span> Polyester film

BoPET is a polyester film made from stretched polyethylene terephthalate (PET) and is used for its high tensile strength, chemical and dimensional stability, transparency, reflectivity, gas and aroma barrier properties, and electrical insulation. A variety of companies manufacture boPET and other polyester films under different brand names. In the UK and US, the best-known trade names are Mylar, Melinex, Lumirror and Hostaphan.

In the field of electronic devices, roll-to-roll processing, also known as web processing, reel-to-reel processing or R2R, is the process of creating electronic devices on a roll of flexible plastic, metal foil, or flexible glass. In other fields predating this use, it can refer to any process of applying coating, printing, or performing other processes starting with a roll of a flexible material and re-reeling after the process to create an output roll. These processes, and others such as sheeting, can be grouped together under the general term converting. When the rolls of material have been coated, laminated or printed they can be subsequently slit to their finished size on a slitter rewinder.

<span class="mw-page-title-main">Lamination</span> Technique of fusing layers of material

Lamination is the technique/process of manufacturing a material in multiple layers, so that the composite material achieves improved strength, stability, sound insulation, appearance, or other properties from the use of the differing materials, such as plastic. A laminate is a permanently assembled object created using heat, pressure, welding, or adhesives. Various coating machines, machine presses and calendering equipment are used.

FR-4 is a NEMA grade designation for glass-reinforced epoxy laminate material. FR-4 is a composite material composed of woven fiberglass cloth with an epoxy resin binder that is flame resistant (self-extinguishing).

<span class="mw-page-title-main">Polyimide</span> Class of polymers

Polyimide is a polymer containing imide groups belonging to the class of high-performance plastics. With their high heat-resistance, polyimides enjoy diverse applications in roles demanding rugged organic materials, such as high temperature fuel cells, displays, and various military roles. A classic polyimide is Kapton, which is produced by condensation of pyromellitic dianhydride and 4,4'-oxydianiline.

A thin film is a layer of material ranging from fractions of a nanometer (monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films is a fundamental step in many applications. A familiar example is the household mirror, which typically has a thin metal coating on the back of a sheet of glass to form a reflective interface. The process of silvering was once commonly used to produce mirrors, while more recently the metal layer is deposited using techniques such as sputtering. Advances in thin film deposition techniques during the 20th century have enabled a wide range of technological breakthroughs in areas such as magnetic recording media, electronic semiconductor devices, integrated passive devices, LEDs, optical coatings, hard coatings on cutting tools, and for both energy generation and storage. It is also being applied to pharmaceuticals, via thin-film drug delivery. A stack of thin films is called a multilayer.

<span class="mw-page-title-main">Membrane switch</span>

A membrane switch is a custom switch assembly that can open or close the conducting path in an electrical circuit and requires at least one contact made of or attached to a flexible substrate. Its assembly differs from traditional mechanical switches: a membrane switch's construction consists of various thin layers sandwiched together using pressure-sensitive adhesives. Each layer in a membrane switch assembly serves a different purpose, and custom features require the addition of specialty layers. Typical implementations arrange multiple membrane switches across its layered structure to form a keypad interface that allows human interaction to control electronic systems.

A diffusion barrier is a thin layer of metal usually placed between two other metals. It is done to act as a barrier to protect either one of the metals from corrupting the other.

The role of the substrate in power electronics is to provide the interconnections to form an electric circuit, and to cool the components. Compared to materials and techniques used in lower power microelectronics, these substrates must carry higher currents and provide a higher voltage isolation. They also must operate over a wide temperature range.

On 8 September 2011 Nuon announced the pilot plant would be closed down since no investor for production expansion could be found. However, on 7 May 2012 Nuon announced that Helianthos has been sold to HyET Solar.

<span class="mw-page-title-main">Printed electronics</span> Electronic devices created by various printing methods

Printed electronics is a set of printing methods used to create electrical devices on various substrates. Printing typically uses common printing equipment suitable for defining patterns on material, such as screen printing, flexography, gravure, offset lithography, and inkjet. By electronic-industry standards, these are low-cost processes. Electrically functional electronic or optical inks are deposited on the substrate, creating active or passive devices, such as thin film transistors; capacitors; coils; resistors. Some researchers expect printed electronics to facilitate widespread, very low-cost, low-performance electronics for applications such as flexible displays, smart labels, decorative and animated posters, and active clothing that do not require high performance.

<span class="mw-page-title-main">IPC (electronics)</span> Trade association for electronics

IPC is a trade association whose aim is to standardize the assembly and production requirements of electronic equipment and assemblies. IPC is headquartered in Bannockburn, Illinois, United States with additional offices in Washington, D.C. Atlanta, Ga., and Miami, Fla. in the United States, and overseas offices in China, Japan, Thailand, India, Germany, and Belgium.

<span class="mw-page-title-main">Stretchable electronics</span>

Stretchable electronics, also known as elastic electronics or elastic circuits, is a group of technologies for building electronic circuits by depositing or embedding electronic devices and circuits onto stretchable substrates such as silicones or polyurethanes, to make a completed circuit that can experience large strains without failure. In the simplest case, stretchable electronics can be made by using the same components used for rigid printed circuit boards, with the rigid substrate cut to enable in-plane stretchability. However, many researchers have also sought intrinsically stretchable conductors, such as liquid metals.

<span class="mw-page-title-main">Thermal copper pillar bump</span>

The thermal copper pillar bump, also known as the "thermal bump", is a thermoelectric device made from thin-film thermoelectric material embedded in flip chip interconnects for use in electronics and optoelectronic packaging, including: flip chip packaging of CPU and GPU integrated circuits (chips), laser diodes, and semiconductor optical amplifiers (SOA). Unlike conventional solder bumps that provide an electrical path and a mechanical connection to the package, thermal bumps act as solid-state heat pumps and add thermal management functionality locally on the surface of a chip or to another electrical component. The diameter of a thermal bump is 238 μm and 60 μm high.

<span class="mw-page-title-main">Integrated passive devices</span>

Integrated passive devices (IPDs), also known as integrated passive components (IPCs) or embedded passive components (EPC), are electronic components where resistors (R), capacitors (C), inductors (L)/coils/chokes, microstriplines, impedance matching elements, baluns or any combinations of them are integrated in the same package or on the same substrate. Sometimes integrated passives can also be called as embedded passives, and still the difference between integrated and embedded passives is technically unclear. In both cases passives are realized in between dielectric layers or on the same substrate.

<span class="mw-page-title-main">Copper indium gallium selenide solar cell</span>

A copper indium gallium selenide solar cell is a thin-film solar cell used to convert sunlight into electric power. It is manufactured by depositing a thin layer of copper indium gallium selenide solid solution on glass or plastic backing, along with electrodes on the front and back to collect current. Because the material has a high absorption coefficient and strongly absorbs sunlight, a much thinner film is required than of other semiconductor materials.

<span class="mw-page-title-main">Transparent conducting film</span> Optically transparent and electrically conductive material

Transparent conducting films (TCFs) are thin films of optically transparent and electrically conductive material. They are an important component in a number of electronic devices including liquid-crystal displays, OLEDs, touchscreens and photovoltaics. While indium tin oxide (ITO) is the most widely used, alternatives include wider-spectrum transparent conductive oxides (TCOs), conductive polymers, metal grids and random metallic networks, carbon nanotubes (CNT), graphene, nanowire meshes and ultra thin metal films.

<span class="mw-page-title-main">Copper foil</span> Thin sheet of metal

Copper foil is a thin sheet of copper metal that is widely used in various applications due to its excellent electrical conductivity, malleability, and corrosion resistance. It is an essential material in the electronics industry, especially for manufacturing printed circuit boards (PCBs) and other electronic components.

References

  1. D. Shavit: The developments of LEDs and SMD Electronics on transparent conductive Polyester film, Vacuum International, 1/2007, S. 35 ff
  2. See for instance the Scottevest solar jacket and the Voltaic Archived 2014-01-15 at the Wayback Machine and similar solar backpacks.
  3. Vicente, António T.; Araújo, Andreia; Mendes, Manuel J.; Nunes, Daniela; Oliveira, Maria J.; Sanchez-Sobrado, Olalla; Ferreira, Marta P.; Águas, Hugo; Fortunato, Elvira; Martins, Rodrigo (2018-03-29). "Multifunctional cellulose-paper for light harvesting and smart sensing applications" . Journal of Materials Chemistry C. 6 (13): 3143–3181. doi:10.1039/C7TC05271E. ISSN   2050-7534.
  4. Haque, Sirazul (2022). "Photonic-Structured Perovskite Solar Cells: Detailed Optoelectronic Analysis" . ACS Photonics. 9 (7): 2408–2421. doi:10.1021/acsphotonics.2c00446. S2CID   250013073 via ACS.
  5. Boane, Jenny L. N.; Centeno, Pedro; Mouquinho, Ana; Alexandre, Miguel; Calmeiro, Tomás; Fortunato, Elvira; Martins, Rodrigo; Mendes, Manuel J.; Águas, Hugo (2021). "Soft-Microstructured Transparent Electrodes for Photonic-Enhanced Flexible Solar Cells". Micro. 1 (2): 215–227. doi: 10.3390/micro1020016 . hdl: 10362/135394 . ISSN   2673-8023.
  6. Fox, Dan (2021-12-14). "Stretchy electronics go wireless for flexible wearables". Nature . Retrieved 2021-12-18.
  7. "Printed Circuit Techniques" by Cledo Brunetti and Roger w. Curtis (National Bureau of Standards Circular 468 first issued 15 November 1947)
  8. "Rigid Flex Circuits, Flexible Circuits, Flex Circuit Boards, PCB Flex | GC Aero Flexible Circuitry, Inc". gcaflex.com. Retrieved 2018-02-27.
  9. "IEEE Journal on Flexible Electronics - IEEE Journal on Flexible Electronics (J-FLEX) publishes on sensors, transistors, related devices, circuits, systems on flexible, disposable, stretchable and degradable substrates". ieee-jflex.org.
  10. 2021 IEEE International Flexible Electronics Technology Conference (IFETC)

Further reading