Flexible electronic circuits: bend it, shape it, any way you want it

Electronic products always seem to start out ‘chunky’ and then slim down to a sleek, more sophisticated version. Think of the early designs for a digital watch, mobile phone, desktop computer and even a microwave oven. Compared to today’s versions of each, the early ‘bold and chunky’ look represented the ‘tech’ value of the product. As if there was so much electronics and technology crammed in that it had to be large and bulky.

As electronics manufacture progressed, the drive was towards miniaturisation, as digital watches became smart watches and mobile phones became smartphones (with no external battery pack required).

The next step is to have electronics that are so thin that they are flexible, ironically accommodating curves where there were none before in lighting design, displays, games and toys and in packaging labels.

Many companies are already developing and testing printed electronics for a new raft of thin, flexible products. Polyester has been used as a substrate for printed electronics for 20 or 30 years, says Justin Spitzer, manager, global business development for flex circuits and printed electronics solutions at Molex, an interconnect and moulding manufacturer. “It is a proven and reliable substrate, but others are being investigated for use in printed electronics, such as thermoplastic polyurethane (TPU) to create a more pliable substrate,” he says. TPU can be laminated on to fabric.

New substrates also mean that alternatives to traditional copper-based inks are used. These are mostly silver, carbon or a silver/carbon blend, says Spitzer. The most reliable and proven process is silver, which boasts better electrical properties even than copper, but the goal remains to reduce resistance while increasing power. The challenge, he points out, is that once the ink is printed on to the substrate, the resistance increases. This happens whether using standard copper on a PCB, or on a copper flex circuit, and regardless of whether the ink is silver or copper. Choosing lower-resistance inks can present problems for a complete system layout, however, as they often do not allow components to be attached very well.

Researchers are working on an ‘additive copper’ ink, where copper is added to a substrate layer without requiring additional processing to make the structure more robust. The use of additive copper is “down the road”, says Spitzer, as the drop in resistance is not yet achievable. Although there are lots of interesting copper models, they require many changes in processes, such as oven and vacuum deposition. Today, the cost is not warranted by the current low levels of demand.

The company is instead focusing on producing flexible circuits using silver ink on a clear, polyethylene terephthalate (PET) substrate. Although polyimide can accommodate finer traces than PET, it is more expensive. Molex Silver Flex Circuits are used in wearable devices, remote monitoring devices (telehealth products) and medical diagnostic equipment and sensors in industrial buildings. PET also allows small components to be attached using a surface mount, proprietary bonding process and a UV-cured encapsulant to protect both the solder joints and the component attachment points.   

Developers are currently working on solving the problem of adding more support for components. This can be done by encapsulating them. The process has to be balanced with the need to achieve dynamic flexibility – the goal is to produce a robust product that does not cause stress to the solder joints during movement.

Another limitation encountered by developers is the length of the printed electronics that can be achieved. The lower levels of resistance may affect the signal integrity. One solution is to increase the width of the trace to provide sufficient power. However, this can alter the design or dimensions of a product. For example, in medical applications, a strap linking sensors and monitors used to measure areas around the body is limited to around 75cm in length as the resistance levels have to be maintained.

Molex showed some prototype stretch fabric patches with sensors for monitoring in health and medical applications at Printed Electronics USA in November 2017. These are intended to be disposable, for one-time application. The stretch fabric substrate makes them comfortable for the wearer.

Another issue is the manufacturing processes of copper compared with silver, which requires less chemicals for processing and etching. This also extends to when the product comes to its end of life, making copper less environmentally friendly than silver when disposed of.

“We are working with customers to test other materials [for substrates]: glass, fabrics, non-woven materials and paper,” says Spitzer. “It is a new frontier,” he says. “Nobody quite knows what you can print on reliably.”

‘The balance is to create a robust product with dynamic flexing, but retaining its strength so as not to stress the silver joints’

One advance has been the company’s Copper Flex Circuit products and the use of polyamide material. In a form similar to a PCB, deposits of copper on the polyamide have low resistance. This model overcomes the length limitations, and can be used in long lengths for LED lighting. The flexible substrate enables it to be used in curved interior lighting in offices, homes and vehicles.

At the Printed Electronics USA event, chemicals and materials company DuPont was exhibiting sportswear made from its Intexar stretchable electronic inks and flexible substrates. The stretchable inks and films for smart clothing were released in July last year. On the company’s stand was a sports top fitted with a sensor and an accelerometer to measure the step and movement of the wearer and the heart and breathing rates. A microelectromechanical (MEMS) sensor in the transmitter wireless relays data to a healthcare monitoring device. The same technology was used in a sports bra, where the circuitry was used to bridge sensors that monitor heart and respiratory signs.

The stretchable fabric can be used in garments or small patches to make pre- or post-operative triage more convenient and comfortable for the patient by wirelessly transmitting medical data to monitor the patient’s condition.

The Intexar ‘smart’ material is based on a film and ink system, explains Michael Burrows, global venture leader, photovoltaic and advanced materials, at DuPont. When it was found that the available films were not robust enough, the DuPont laboratory created a new film. Launched last summer, this version is printed on TPU, which is applied to fabric, using standard clothing manufacturing processes, then coated with an adhesive layer. Burrows says it improves performance in resistance compared with the company’s earlier stretchable fabric.

Importantly, the resulting garment can be washed over 100 times without affecting its stretching and circuitry performance. This is one of the main obstacles to ‘smart’ clothes, as they are required to withstand the rigours of detergents and wash cycles without disconnecting components or dislodging solder joints.

Another theme at the show was In-Mold Electronics (IME). This process eliminates the circuit board for the printed circuitry and discrete components to be integrated into injection-moulded plastics.

According to Dr Khasha Ghaffarzadeh, research director at market research and event company IDTechEx, IME is on the threshold of wide adoption, with the first products ready to go to market soon. Manufacturers are moving beyond conductive inks and using transparent conductive films as well as sensors for products that are lighter, occupy less space in the final system and are less expensive to assemble. IME products also produce less waste than the manufacturing of a PCB or a flexible printed circuit.

Pieces are created by taking an item of formed plastic, pressing the circuitry into the mould and then over-moulding the plastic over it. Both DuPont and Molex were showing IME prototype products at the show. Spitzer explains that the moulded interface can be backlit for use in switches and vehicle dashboards.

Spitzer cautions that incorporating the circuitry requires a lower temperature for the moulding, limiting the materials that can be used. Thinking further ahead, he says that 3D printing will also play a part. Rather than layers being deposited on to the flat surface and moulded into a shape, presently used in prototyping IME units, he envisages a form being created using 3D printing and then components being attached to it. This will require an investment in equipment, as metallisation will require processes such as laser direct structuring (LDS).

One of the reasons for the expected growth in IME, according to Ghaffarzadeh, is that large-contract manufacturers are becoming involved in processing and designing. As a result, he says, “the industry is learning how to optimise the many trade-offs involved in going from a pilot design to one streamlined for mass manufacture. The cost of production is still higher than the incumbent solution, therefore IME products must create additional value such as better aesthetics or thinner designs.”

In vehicles and aerospace applications, as well as in consumer electronics and appliances, IME can eliminate buttons and wires on dashboards and displays to save weight. It also simplifies production by reducing both the number of parts required and the manufacturing steps. Assembly time is reduced, as the formed part can be as a complete unit, which also increases reliability, through fewer moving parts. Another cost saving is that existing in-mould processes are used, so there are no retooling costs. As well as reducing the width or thickness of the product, IME further opens up possibilities for design innovation, as capacitive LED switches can be placed anywhere on the final form.

The challenge facing the industry today is how to balance the high melting point temperatures required with the lower temperatures needed in plastics moulding. Spitzer points out that PET cannot handle the high temperatures, so manufacturers may have to use lower-temperature plastics instead. “This is not ideal for plastics manufacturers,” says Spitzer. “Today, we have printed flexible electronics, using a printed circuit and the equivalent of four layers of PCB with components attached.

“Depending on what you are trying to achieve, there is ‘dynamic flexing’ and ‘flex to install’. Flexing to install can be less rigid, as the components are supported once the unit is installed,” he explains.

Dynamic flexing requires more secure attachment of the components and a more robust design to withstand vibration. “More support can be added by encapsulating the components for dynamic flexing applications – we cannot rely on silver joints to hold the components in place. The balance is to create a robust product with dynamic flexing, but retaining its strength so as not to stress the silver joints,” he concludes.

There are several ‘smart’ labels, which can track the location of goods or monitor the environment in which they are kept. A Cambridge-based company, PragmatIC, has taken the concept of a smart label one step further. It produces flexible electronics that are thinner than a human hair and which can be embedded in everyday items for security or marketing purposes.

Layers of polymer, metal or metal oxide materials make up the thin, flexible integrated circuits (ICs). Unlike a conventional silicon IC, no packaging is required, as the polymer substrate does not need it, says Richard Price, chief technology officer, PragmatIC. The resulting FlexICs are “around 10µm”, confirms Price, making them thin and flexible and barely perceptible when embedded in labels or paper or card-based products.

The company operates a standalone facility, FlexLogIC, to produce the flexible ICs, using an enclosed, fully-automated production line. In addition to using the existing infrastructure, the thin and flexible substrate makes them compatible with print label conversion technologies.

The ICs can be embedded in various examples of product packaging, from make-up to food packaging. Using radio-frequency identification (RFID) or near-field communication (NFC) the ICs communicate via a smartphone to engage in ‘push marketing’, to advise the holder of a special promotion associated with the product, for example.

The concept can also be used in toys and games, using ICs to communicate with a dedicated reader or a smartphone or tablet for interaction, information or instructions. PragmatIC is part of the Printed Intelligent NFC Game Cards and Packaging or PING project, which won the Innovation Product Award at the European Forum for Electronics Components and Systems 2017.  

Other uses will be in anti-counterfeit and brand protection to verify a product is genuine. “The holograms currently used in security protections are not very secure,” explains Price. “Adding an electronic certification at a low cost will allow brands to protect against counterfeiting.”

At the moment, such ICs are used in conjunction with a hologram as multiple security features to protect high-worth goods. The thin and flexible nature of FlexICs means that they could be used in a variety of products that at the moment will be obscured or weighed down by attaching a large tag or label. The value of the product and the brand dictates the investment, observes Price, but as production costs are reduced, the additional security can be accessible to a wider range of high-volume goods.

The problem with connected devices is that they demand an ‘always on’ state and require battery power. In small, thin, flexible products, a battery is cumbersome at best.

Lithium-ion coin cell batteries help in many of the wearable devices, but still add weight to the finished product.

It is possible to include low-power wireless circuitry in the printed electronics and attach components for RFID, Bluetooth Low-Energy and NFC applications for tracking and locating productions using these wireless protocols.

Wireless charging is limited by transmission and receive range. Alta Devices uses flexible electronics processes to produce flexible solar cells that can be applied to health and fitness trackers and integrated into garments.

Although silicon-based photovoltaic cells are increasingly common on buildings, they are bulky and rigid, making them hard to integrate. This can exclude their use in some mobile applications. The alternative, thin-film solar technology cells, are less bulky but relatively inefficient, says the company.

Rich Kapusta, Alta Devices’ chief marketing officer and head of sales, explains why the company’s flexible solar cells, based on its AnyLight technology, are different. “We use a gallium arsenide (GaAs) substrate to grow our solar film. The film is removed and reused over and over. This allows us to control our manufacturing costs, while still being able to produce the world’s most efficient solar cells.”

The solar cells use energy harvesting to extend battery life. They are lightweight, with cells less than 100µm thick. A fully assembled solar module weighs 170g per square metre. A typical IoT (Internet of Things) office sensor with one solar cell might harvest up to 6mWh per day, half its total requirement, and so increase the time between battery replacements. In low-power sensor applications, it can eliminate the need for a battery. As well as being used on the backs of smartphones, for example, the solar cell modules can be added to the roof of a car or the curved wings of an aeroplane or unmanned aerial vehicle (UAV) to avoid stops for recharging. “On a typical fixed-wing UAV, we can extend flight times from around two to three hours to over 11 hours,” says Kapusta.

The company has announced a project with German car maker Audi to integrate its solar cells into the glass roofs of electric vehicles, to increase journey range. Initially, the project will focus on the vehicle’s glass roof, although the solar cells are flexible enough to be used across the entire roof surface. They could supply power for functions in the vehicle, such as air conditioning. Further ahead, solar energy could directly charge the traction battery of the electric vehicles.

“We continue to improve both efficiency and reduce weight,” says Kapusta. “In 2018, we expect to set new world records for efficiency and introduce even more products to the market.”

The industry seems optimistic that although there are currently technical limitations to flexible electronics, these can be overcome. The scope of how and where flexible electronics will be used may be limited only by the engineer’s imagination and design flair.