Wearable Flexible Silicon Circuits

ABSTRACT: Flex circuits are flexible electronic circuits made of thin sheets of silicon mounted on rubber or plastic medium that are foldable, stretchable and bendable into any desired shape that can be weared or implanted. Present circuits are mounted on rigid base and this rigidity pose shape limitations for electronic equipments. The flex circuits reduce the size& weight of the finished product. It allows increased circuit density and eliminates bulky connections. These flex circuits can be designed in configurations from simple single sided conductive paths to complex three dimensional assemblies utilizing variety of fabrication materials. These circuits can be made to fold or stretch and still match the performance of rigid circuits. There are many reasons for opting flex circuits like size& weight reduction, increased reliability, reduced assembly time& cost, improved heat dissipation and three dimensional packaging capabilities. They are mainly used in tightly assembled packages, dynamic flexing applications, human implants and wearable products such as an external wearable thermometer to an extremely high-tech artificial eye that could provide visibility to the humans who might have lost their eyesight badly. These flex circuits would likely bring more hopes of new innovations in various engineering sections owing to its unmatched advantageous capabilities. The way to lighter, highly efficient and nonbulky electronics is made possible through these flexible circuits.

INTRODUCTION: Flexible electronic circuits made of thin sheets of silicon on plastic or rubber can be stretched and bent without breaking and can be used as wearable computers, implanted inside the human body, for implantable health monitoring systems, and many other areas where the conventional circuits cannot be used. The stretchable silicon integrated circuit can be wrapped around complex shapes such as spheres, body parts and aircraft wings, and can operate during stretching, compressing, folding and other types of extreme mechanical deformations, without a reduction in electrical performance. Flexible circuitry reduces the size and weight of a finished product. It allows increased circuit density and eliminates bulky connections and wiring. And the added ability to fold the circuit expands the boundaries of design and packaging.

FLEX CIRCUITS: Flexible electronics also known as flex circuits or flex circuit boards, is a technology for assembling electronic circuits by mounting electronic devices on flexible high-performance plastic substrates, such as polyimide. Circuits can be designed in configurations from simple, single-sided conductive paths to complex high density three-dimensional assemblies utilizing a variety of fabrication materials. 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(Fig.1) during its use.

Researchers have shown that electrical circuits can be made to fold and stretch and still match the performance of circuits built on rigid wafers. The new work exploits that ultra thin geometry to make two types of circuits. One type is merely foldable: silicon-based circuits were placed on unstrained plastic sheets, resulting in circuitry that can fold up like a piece of paper. To ensure that the circuit would work well no matter what direction it is twisted or bent, the researchers place the silicon, or whatever part of the circuit is most fragile, at a distance between the top and bottom on the circuit sheet that experiences the least amount of strain. Placing the fragile components of the circuit in the appropriate place within the circuit sheet optimizes the electronics and allows them to work as well as those on a solid wafer.

Fig.1 CONSTRUCTION: Flexible circuits are most commonly manufactured using one of two base materials, either polyimide or polyester. Polyimide is favoured where soldering of the assembly is required, while polyester is generally used in low-cost applications. Polyimide is also the material of choice for manufacturing nearly all CSPs (cyclic segmented prefix) and flex BGAs (ball and grid array). However, polyester has been successfully used in the creation of Smart Cards, which are arguably a large format chip package and due some recognition.

TYPES OF FLEX CIRCUITS: Single-sided flex circuits Single-sided flexible circuits are the most common type in production today. They are also the construction most often employed

and best suited to dynamic flexing applications. Their construction consists of a single patterned conductor layer on a flexible dielectric film termination features on these circuits are accessible only from one side. Single-sided flex circuits can be fabricated with or without such protective coatings as cover layers or cover coats. While many different metal foils can be used as the conductor, copper foil is the most common. Back-sided flex circuits Back-sided flex circuits (also known as double-access flexible circuits), contain only a single conductor layer, but are processed to allow access to the conductors from both sides. This construction is often employed in IC packaging. Tape automated bonding (TAB), for example, relies almost exclusively on this construction (Fig.2). Similarly, most flex-based CSPs employ this construction.

Fig.2 Double-sided flexible circuits Double-sided flex circuits contain two conductor layers and can be produced with or without plated-through holes, depending on design requirements. Two-metal layer flex circuits will likely see increased use in chip packaging, as operating frequencies continue to rise and the need for controlled impedance constructions increases.

Multilayer flexible circuits Flex circuits that have three or more conductor layers are referred to as multilayer flex. The layers of the circuit are interconnected with plated-through holes(Fig.2-11). Newer methods of constructing multilayer flex circuits may limit the need for high aspect ratio plating. Newer multi-chip packages (MCPs), with their higher interconnection density, may employ such structures in the future.

Rigid-flexible circuits Rigid-flex circuits are a hybrid construction, consisting of rigid and flexible substrates laminated together into a single package and electrically interconnected by means of plated-through holes. Such flexible circuit types have also enjoyed tremendous popularity among military product designers, but, in recent years, this type of construction has made gains in the commercial world, as well. Rigid-flex boards are normally multilayer designs, but double-sided (two-metal layer) constructions are possible, as well, and, in fact, have been selected for certain microelectronic chip-packaging applications, most notably in the construction of hearing aids.

BENEFITS: There are numerous motivational reasons for using flex circuit technology. As an interconnection methodology they are unmatched in terms of their versatility. Following is an examination of some situations where flex circuits can solve packaging problems Size and weight reduction Flex circuits are among the thinnest dielectric substrates available for electronic interconnection. In extreme cases, it is possible to produce flexible circuits less than 0.002" total thickness, including the cover layer Flex circuits can also help reduce the weight of an electronic package significantly (up to 75% weight reduction or more is possible) These attributes have not been lost on the IC packaging community, which packages a significant and growing percentage of CSP devices using flex circuits.

Reduced Assembly Time and Costs Because flex circuits can seamlessly integrate form, fit and function, flexible circuits can provide an excellent means of reducing assembly time of a product. Other benefits are derived from the ability to reduce the number of assembly operations required, and from the user's ability to construct and test the circuit completely, prior to committing the circuit to assembly. Increased system reliability Reliability engineers note that when an electronic package of any type fails, it is typically at a point of interconnection. When employed to greatest advantage, flexible circuits are an excellent means of reducing the number of levels of interconnection required in an electronic package. Improved controlled Impedance Signal Transmission Design and Manufacture Materials used for flexible circuits are very uniform in both thickness and electrical properties. This feature facilitates the production of circuits needed for high-speed packaging applications. Improved heat dissipation Capability Flat conductors have a much greater surface-tovolume ratio than round wire. This extra surface area facilitates the dissipation of heat in conductors. In addition, the short thermal path in flex circuit constructions further improves heat dissipation. Three-dimensional packaging capability Much has been written in recent years about the advantages of injection moulded boards as a means of achieving a truly threedimensional interconnection structure. This advantage appears poised for exploitation in IC packaging.

THE NEXT GENERATION:

The newer flex circuit IC packaging constructions are showcasing the technology's ability to meet the demand for increased circuit density. These new packages often occupy a small fraction of the volume of more traditional approaches. The result is a continuation of the electronics industry's familiar rubric of smaller, faster and lighter. In fact, some TAB was made on thin glass epoxy substrates, adding testimony to the lack of true concern about flexibility(Fig.4), beyond the material’s ability to be wound and unwound from large film reels.

Fig.4 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. In addition to cameras, a common application of flex circuits is in computer keyboard manufacturing; most keyboards made today use flex circuits for the switch matrix. Flexible circuit offers value added services that will help with bringing the design to reality with following advantages:

 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 rockets and satellites.

 Electrical connections where board thickness or space constraints are driving factors.

 Flexible circuitry could help efforts to create clothing containing wearable gadgets.

 Human implantable and wearable circuits(Fig.6).

Fig.6 CONCLUSION: The innovations in the electronic devices will surely be able to reach much higher and unimaginable limits with the use of these flex circuits. They enable the young researchers and developers to make all the impossible lighter, slimmer but still efficient range of electronics possible in a short period of time. These flexible circuits has also opened a wide gateway in Bio-medical engineers as it paves the way for more efficient artificial circuits that could be made wearable or implanted in humans to make human life even more better. On the whole, these flex circuits would enlighten the future range of technological advancements in the packaging circuitry.