Flexible electronics is lightweight, rugged, bendable, roll-able, portable and potentially foldable. Ever-evolving advances in thin-film materials and devices have fuelled 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.

Fig. 1: Examples of basic and selected flexible circuit construction
Fig. 1: Examples of basic and selected flexible circuit construction

Diodes and transistors are two of the most common active thin-film devices used in digital and analogue circuits. While they have been successfully used in flexible platforms, their performance and applicability is limited by requirement of exotic device architectures and novel materials. In order to achieve the goal of full-system integration in next-generation flexible systems, a paradigm shift in design and fabrication is necessary.

There are many potential applications of flexible electronics in healthcare, automotive, human–machine interfaces, mobile communications and computing platforms, and embedded systems in both living and hostile environments. Besides, there are market-specific applications, such as human-machine interactivity, energy storage and generation, mobile communications and networking, touching the application of flexible electronics on ubiquitous computing platforms throughout.

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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. Further, in order to carry out daily tasks, flexibility is less likely to hinder over stiffness.

Materials
The fundamental properties of thin-film materials, as well as the quality of various device interfaces, give rise to inherent limitations in device performance. Each element of the flex circuit 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 circuit to assure ease of manufacture and reliability.

Fig. 2: Single-sided flex circuit
Fig. 2: Single-sided flex circuit

The materials and technologies behind flexible substrates are an important consideration for flexible electronics. Perhaps two of the main flexible substrate candidates are plastic and stainless steel. Although stainless steel is incompatible with standard deposition temperatures, it results in a substantially heavier system due to its higher mass density—a critical consideration for portable flexible technologies. Also, stainless steel is not particularly deformable, and is thus unsuitable, in particular, for wearable electronics. Plastic substrates are lighter and deformable alternatives.

Fig. 3: Sculptured flex circuit
Fig. 3: Sculptured flex circuit

Substrates need to be solvent-resistant, so that standard optical photolithography process can be used. Additional substrate requirements include low cost (allowing large area, mass production) and moisture resistance. Table I compares some of the more critical properties of some key plastic substrates.

One of the main challenges facing plastic as a next-generation substrate is the substantially reduced processing temperature window. The maximum fabrication temperature is related to the glass-transition temperature above which inelastic deformation takes place and the substrate no longer retains its original dimension, which is essential for photolithography. As an example, polyethylene naphthalate (PEN) satisfies all requirements and tolerates temperatures as high as 160oC.

More recently, a number of electronic devices and circuits have been demonstrated, utilising paper as a substrate and/or as a gate dielectric. Such approaches lead to electronic devices with the potential for en masse integration at low cost, which are also disposable and fully recyclable.

Base material
A polymer film provides the foundation as a base material for flexible electronics. Under normal circumstances, 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 material provides all of the characteristic properties.

Fig. 3: Sculptured flex circuit
Fig. 4: Double-sided flex circuits
Fig. 5: Multilayer flex circuits
Fig. 6: Standard stack-up of a 4-layer rigid-flex circuit
Fig. 6: Standard stack-up of a 4-layer rigid-flex circuit

While a wide range of thickness is possible, most flexible films are provided in a narrow range of relatively thin dimensions from 12µm to 125µm (1/2 mil to 5 mils). But thinner and thicker materials are possible. Thinner materials are, of course, more flexible. For most materials, stiffness increase is proportional to the cube of thickness. This 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 materials used as base films, including polyester (PET), polyimide (PI), polyethylene naphthalene (PEN) and polyetherimide (PEI). These, along with various fluropolymers (FEPs) and copolymers polyimide films, are most prevalent owing to their advantageous electrical, mechanical, chemical and thermal properties.

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Bonding adhesive

Fig. 7: Construction of polymer thick-film flex circuit
Fig. 7: Construction of polymer thick-film flex circuit

Adhesives are used as the bonding medium for creating a laminate. When it comes to temperature resistance, the adhesive is also typically a 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 thicknesses. Thickness selection is typically a function of the application.

Metal foil
A metal foil is most commonly used as the conductive element of a flexible laminate. A metal foil is the material from which the circuit paths are normally etched. 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. 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.

Types of flexible circuits
There are a few basic constructions of flexible circuits, but there is significant variation between the different types in terms of their construction. Following is a review of the most common types of flexible circuit constructions. Flexible circuits have a rich history and are extremely diverse in their nature. This diversity opens them to use in a wide range of applications, with new applications being developed on a regular basis.

Fig. 8: Fabrication of short-channel transistors using spin-coating-induced edge effect (I, side view) and the specific process used for TFT fabrication (II, top view)
Fig. 8: Fabrication of short-channel transistors using spin-coating-induced edge effect (I, side view) and the specific process used for TFT fabrication (II, top view)
Fig. 9: TFT fabrication process by self-aligned, one-step, multilayered patterning
Fig. 9: TFT fabrication process by self-aligned, one-step, multilayered patterning

It is hard to predict where the technology will go next. However, roll-to-roll processing is likely to play an important part. Flexible electronics opens the door to foldaway smartphone displays, solar cells on a roll of plastic and advanced medical devices—if we can figure out how to make them.

Single-sided flex circuits
A single-layer flex circuit is the most basic and consists of a flexible polyimide film laminated to a thin sheet of copper. The copper layer is then chemically etched to produce a circuit pattern specific to your design requirements. A polyimide cover lay is then added for insulation and environmental protection of the circuit.

A single-sided flex is used for:
1. Dynamic flexing applications
2. Unusual folding and forming applications
3. Installation/service applications/repair
4. When there are limitations on space/thickness
Some important features of single-sided flex are:
1. Very thin construction, under 0.1mm-0.2mm (0.004-0.008 inch)
2. One conductor layer
3. Reverse-bared or back-bared pads
4. Supported and unsupported finger areas

Back-bared flex circuits
Double access flex, also known as back-bared flex, are flexible circuits having a single conductor layer but they allow access to selected features of the conductor pattern from both sides. While this type of circuit has certain benefits, the specialised processing requirements for accessing the features limit its use.

A dual-access flex circuit is simply a single-sided flex circuit that is manufactured such that its conductive material can be accessed from both sides of the circuit. Flexible Circuit is a front runner in this technology, which utilises specialised lasers and processing to skive open the polyimide layer of the base material to allow dual access to the single copper layer. While this type of circuit has certain benefits, the specialised processing requirements for accessing the features limit its use.

These flex circuits find application in such fields as:
1. Medical
2. Environmental
3. Test and Measurement
4. Automotive and other general electronic devices

Sculptured flex circuits
Sculptured flex circuits are a novel subset of normal flexible circuit structures. Their manufacturing process involves a special flex circuit multi-step etching method to produce a flexible circuit having finished copper conductors whose thickness differs at various places along their length. The conductors are generally thin in flexible areas and thick at interconnection points.

Sculptured circuit technology enables the thickness of the copper conductors to vary at any point on the circuit. In addition, selective application of the supporting dielectric enables integral exposed fingers to be produced. All holes are etched, providing the flexibility to create precisely-located apertures of any shape or size at any position on the circuit.

Sculptured circuits are in use across a wide spectrum of applications from motor sports to missiles. Sculptured circuits, when combined with flexible or rigid circuits, provide an extremely cost-effective solution to complex interconnect problems by reducing cost, simplifying assembly and increasing reliability.

Sculptured flex circuits offer many benefits, including robust contact areas, nil or low tooling cost, reliability and cost-effectiveness. Their applications include power circuits and custom-built circuits.

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Double-sided flex circuits
Double-sided flex circuits are flex circuits having two conductor layers. These 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, the connection features are accessed from one side only, and the circuit is called Type V (5), according to military specifications. It is not a common practice but it is an option.

Because of the plated-through hole, terminations for electronic components are provided for 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 cover layers on one, both or neither side of the completed circuit. Generally, these are produced with the protective layer on both sides.

One major advantage of this type of substrate is that, it allows crossover connections to be made very easily. Many single-sided circuits are built on a double-sided substrate, just because they have one or two crossover connections. An example of this use is the circuit connecting a mouse pad to the motherboard of a laptop. All connections on that circuit are located on only one side of the substrate, except a very small crossover connection which uses the second side of the substrate.

Some situations where double-sided flex is used are:
1. When circuit density and layout cannot be routed on a single layer
2. Ground and power plane applications
3. For shielding applications
4. For dense surface-mount assembly

Salient features of the double-sided flex are:
1. Component assembly available on both sides
2. Two conductive layers
3. Fingers are an integral part of the conductor patterns

Multilayer flex circuits
Flex circuits having 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 a requirement by definition; it is possible to provide openings to access lower circuit level features.

The layers of a multilayer flex circuit may or may not be continuously laminated together throughout the construction, with the obvious exception of the areas occupied by plated-through holes. The practice of discontinuous lamination is common in cases where maximum flexibility is required. This is accomplished by leaving unbounded the areas where flexing or bending is to occur.

Multilayer flex circuits are used when circuit density and layout cannot be routed on a single layer, and for:
1. Ground and power plane applications
2. Shielding applications
3. Dense surface-mount assembly
4. Increased circuit density
5. EMI/RFI shielding
6. Controlled impedance with shielding

Multilayer flex circuits are used in high-density SMT electronic applications, and such sectors as automotive, aerospace, medical, and test and measurement.

Rigid-flex circuits
Rigid-flex circuits are a hybrid construction with rigid and flexible substrates which are laminated together into a single structure. Rigid-flex circuits should not be confused with rigidised flex constructions that are simply flex circuits to which a stiffener is attached to support the weight of the electronic components locally. A rigidised or stiffened flex circuit can have one or more conductor layers. Thus, while the two terms may sound similar, they represent products that are quite different.

The layers of a rigid flex are also normally electrically interconnected by means of plated-through holes. Over the years, rigid-flex circuits have enjoyed tremendous popularity among military product designers. However, the technology has found increased use in commercial products. While often considered a specialty product for low-volume applications because of the challenges, rigid-flex boards are normally multilayer structures, but two-metal-layer constructions are also sometimes used.

Benefits of rigid-flex circuits include:
1. Use of third dimension creates an optimal solution for applications with extreme space limitations
2. Replace bulky wire harnesses with a compact, yet robust design
3. Flexible assemblies reduce connectors as well as labour, yield, transmission and reliability issues
4. These hybrid circuits can have up to sixteen layers

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Polymer thick-film flex circuits
Polymer thick-film (PTF) flex circuits are true printed circuits wherein the conductors are printed onto a polymer base film. They are typically single-conductor-layer structures. However, two or more metal layers can be printed sequentially, with insulating layers printed between printed conductor layers.

While lower in conductivity, PTF circuits have successfully served in a wide range of low-power applications at slightly higher voltages. Keyboards are a common application, but there is a wide range of potential uses for this cost-effective approach to flex circuit manufacture.

PTF technology consists of a simple set of basic building block materials, such as substrate, conductive inks, dielectrics, conductive adhesives and non-conductive adhesives/encapsulants. Underfills, required for flip chips, are commonly classified as packaging materials, but they are polymer-based systems that are a close kind of PTF.

Key capabilities of polymer thick-film flex circuits are:
1. Single-layer or double-layer screen through-hole circuitry
2. Low-cost, high-volume, roll-to-roll fabrication technology
3. Low-cost, screen-printed ink surface dielectrics
4. Economical choice for cost-sensitive applications

Integration process
Fabrication methods have an important effect on the characteristics, cost and stability. For example, instead of using a high-mobility material to achieve high device transconductance, it is possible to adjust the architecture. Employing short channel lengths is one way to achieve this. In conventional planar TFTs, the channel length is ultimately limited by the diffraction limit in the photolithography process. Vertical transistors, where the channel length is set by the thickness of the semiconductor, have been demonstrated to achieve submicrometer channel lengths, paving the way for high-transconductance devices using conventional materials such as a-Si.

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Alternative approaches have also been developed to fabricate organic submicrometer TFTs uniformly over large areas. One such approach is based on a novel edge effect that is induced by spin-coating a polymer onto a prepatterned structure, as shown in Fig. 8. Polymer TFTs, with channel widths as narrow as 400nm, can be fabricated by this method.

One key advantage of this method is that, it facilitates the use of low-resolution patterning techniques, such as shadow masking, to create highly reproducible submicrometer features, thereby obviating more conventional, time-consuming lithographic processes. This, combined with inkjet printing, provides an exciting opportunity to apply on-demand material deposition and desktop-programmable wiring of ad hoc patterns. The latter has already been demonstrated for CNT and graphene-based inks.Though rather exotic, such innovative fabrication techniques will facilitate the technologies’ widespread usage for fabrication of TFT with high transconductance.

Any manufacturable device has following essential characteristics:
1. Superior and prespecified performance, with reproducibility, uniformity and reliability
2. High yield to acceptable tolerance
3. Simulations exist for both reverse engineering during development and right-first-time design

Some novel processes have been developed to fabricate solution-processable TFTs with one-step self-aligned dimensions in all functional layers. The TFT-channel, semiconducting materials and effective gate dimension were controlled by a one-step imprint process and subsequent pattern transfer, without the need for multiple patterning and mask alignment, as shown in Fig. 9. Both p- and n-type organic TFTs have been demonstrated using this method.

In the case of n-type TFTs, Li et al reported that 20 transistors were fabricated without process optimisation, with a yield of 100% and a variation in mobility and on/off current ratio of a factor of 3 and 5, respectively. All the used techniques (imprinting, wet/dry etching and inkjet printing) are already available in roll-to-roll processes. The demonstrated high-resolution features, mask-alignment-free process and compatibility to roll-to-roll fabrication show that these and similar techniques are commercially attractive, inexpensive and ready to use. It is expected that these methods can be extended to the level of integrated complementary metal–oxide–semiconductor (CMOS) circuit fabrication.

Part 2 of the article, in next issue, will cover some very interesting aspects of the application of flexible electronics in various sectors.
To be concluded next month


The author is a final year student of KMIT afflicted to JNTUH. This article is based on a paper he prepared for a technical seminar on flexible electronics

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