The Good, Bad And Unknowns Of Flexible Devices

Flexible hybrid electronics are beginning to proliferate in consumer, medical, and industrial applications due to their comparatively low weight, thin profile, and the ability to literally bend the rules of design.

Open any smart phone today and you’re likely to find one or more of these flexible boards. Unlike standard printed circuit boards, FHE devices are printed using a combination of roll-to-roll processes, similar to how a newspaper is printed, with tiny chips bonded onto the same substrate. The result is generally inexpensive and can be highly customized, but as adoption increases, it also is becoming more sophisticated.

The key attributes of FHEs are weight, cost, and the ability to fit into any form factor, no matter how small or irregular. They can be applied to the skin as a patch, and they can be used in a variety of harsh environments, depending upon the substrate and the materials used to bond chips and inks to that substrate. The substrate itself can vary, too, involving everything from plastic to paper, foil, and even flexible glass — a material that supposedly dates back nearly 2,000 years.

In 2019, the FHE market was valued at $95 million, according to research and markets. The research firm estimated the market size would increase to $231 million by 2025. But market growth projections vary widely, and they have changed since the global pandemic began highlighting the value of remote diagnostics. In fact, IDTechEx estimates the need for “flexible, foldable, stretchable, conformal and lightweight” electronics will push the overall flexible electronics market as high as $8.3 billion by the end of this decade. How much of that will be FHE versus simple printed sensors remains to be seen, but the need to broadcast results wirelessly and securely points to more complex electronics than a simple RFID.

Fig. 1: Flexible hybrid electronics on a curved surface. Source: IDTechEx

Automotive is another target market, particularly as cars become increasingly electrified. Less weight equals longer range for electric vehicles, and wiring harnesses can weigh as much as 150 pounds, according to Doug Burcicki, senior director at Siemens Digital Industries Software. Replacing wiring harnesses with flexible wiring schemes that can be printed and quickly customized is an attractive option, providing reliability can be proven to be as good as existing systems.

“Right now, 5G is driving a lot of the demand,” said Yair Alcobi, president of KLA’s Orbotech PCB Division. “There’s a lot more connectivity required, and new board designs. What we’re seeing, though, is this is no longer assembly of different parts. It’s all going onto a flexible board. We’re expecting these will replace a lot of the wiring in a car, too.”

One of the first uses of FHE with 5G is an antenna array, which is essential for beamforming because 5G signals are more susceptible to interruption than previous wireless technologies. That’s one of simpler applications for FHE. There also are different materials for the chips mounted to those substrates, the wires that are printed on them, and the circuitry to move signals back and forth across these devices. For comparison, traditional PCBs use measurements like layers and discrete numbers of boards, while FHE vendors measure output in meters per day.

“A lot of this is roll-to-roll manufacturing,” said Alcobi. “Accuracy is plus or minus 10 microns, and lines/spaces are 10 to 15 microns.”

Compared with PCBs, which can include dozens of different layers and be used for the most advanced chips or packages, the use of FHEs has been much more limited. But they do have a significant role to play, and that hasn’t gone unnoticed. Money is being invested into this technology to push it more into the mainstream, including the introduction of CAD tools for FHEs, which allow designers to lay out circuits and chips on flexible substrates, as well as end-to-end flows that span from design to final inspection.

“FHE is at the onset of enabling a near-endless combination of applications and markets due to its ability to integrate with existing technology,” said Ryan Moss, director of new business development at Brewer Science. “Its distinct abilities of allowing for the customization of form factor, signal and robustness have made it more prevalent and adoptive with IIoT-type solutions.”

Challenges ahead
Just because these devices are not at the bleeding edge of design does not mean they are simple to manufacture, however. FHEs can be used anywhere, including harsh industrial environments. That requires a lot of materials science expertise around the substrate choice, the inks, and the materials used to bond small chips to the substrate, not all of which are readily available today.

“Ink and substrate choices carry with them immense considerations for each unique application and solution, ranging from power to survivability to resolution,” said Moss. “It requires choosing the most compatible and complementary materials for the solution, as well as the ability to adjust and modify key attributes in order to achieve optimal characteristics and performance.”

As with all chips, processing generates heat. That creates a potential mismatch in coefficients of thermal expansion between the substrate and the chips, similar to the kinds of issues being dealt with in advanced packaging today. Because this market is still in the formative stages, not all of those issues have been addressed.

“At the moment, a lot of the flexible hybrid prototypes hybrid use relatively simple microprocessors, so the input/output ports are huge,” said Matthew Dyson, a technology analyst at IDTechEx. “If you go to more sophisticated chips with smaller and more input/output pads, there has to be some intermediate medium that will link together the outputs from the chip itself. You can print all of those to very high resolutions, but it’s very slow. This technology is still emerging. So if you want to do high-speed printing, your resolution is limited. People have tried doing combined approaches, where they’ll print to medium resolutions, and then laser ablate this printed ink to make the fingers that ultimately will link up to the to the to the integrated circuit. That whole area is a challenge, particularly with regard to durability. People can make a prototype and it will work, but would you put it in your car? Maybe not yet.”

Another issue involves the rigidity of the layer between the chip and the substrate. The chips themselves can be flexible, but the attachment of those materials to the substrate using various wire schemes is not. The elasticity of the middle layer causes a mismatch.

The mismatch problem grows with the level of complexity, which adds other issues to the mix. For example, building a more complex chip will require more sophisticated equipment that can test everything from structural integrity to flexibility.

“If the chip is very small, the bending radius becomes irrelevant,” said Dyson. “If you have an offset grade of 10% or 20% off that base, the absolute change is pretty small. But that also limits you to relatively simple chips. It’s going to be a very long time before you get to the kind of chips used in an iPhone mounted onto a fully flexible substrate. The chips that are currently mounted on flexible substrates tend to be packaged to some extent, and you’re not dealing with something that is fully flexible.”

Fig. 2: FHE vs. conventional and printed electronics. Source: IDTechEx

Markets
Still, a number of companies are exploring the flexible hybrid electronics space, and they are coming at it from different perspectives. So while some companies focus on silicon-on-polymer, others may use an approach of building up and then etching metal oxides.

There is room for both approaches, as well as others, because these devices could end up across a spectrum of applications. In a 2016 paper, researchers from UC Berkeley, Binghamton University, i3 and ExSys looked at printed sensors for medical uses. The conclusion was that despite the “mechanical mismatch” between flexible substrates and hard electronics, flexible devices had a higher signal-to-noise ratio than traditional sensors because of better contact with skin.

The mil/aero segment has a strong interest in FHE, as well. This is evident in the membership roster in NextFlex, one of eight “Manufacturing Innovation Institutes” set up under the U.S. Department of Defense. Two of the leading corporate members — Boeing and Lockheed Martin — are focused on avionics, where weight is a critical issue. The next lower membership tier focuses on materials. There also is a long list of academic and non-profit members.

And finally, there is widespread interest across a number of edge applications and markets, where FHEs can be used to speed time to market and accommodate last-minute engineering change orders. This is expected to propel the FHE market forward even faster, once there is more history about how well FHE devices function over time and under a variety of environmental stresses.

“The current vision of FHE and unique sensors appeals to the use of pre-calibrated, environmentally sealed components that are plug-and-play,” said Brewer Science’s Moss. “Components must be expedited to the point of need, without the requirement of additional service costs or fear of long-term reliability.”

Orbotech’s Alcobi agrees. “In the silicon and packaging world, they are looking for full-package solutions, not just assembly of different parts,” he said. “The cost of a PCB is rising. There is much more functionality required, and more complexity of the the PCBs. In the past, we could use large scan optics, but now it requires a multi-wavelength laser.”

Fig. 3: Markets and processes. Source: IDTechEx

Conclusion
The fusion of roll-to-roll printed circuits and traditional semiconductors is opening up a wide range of possibilities across a slew of end markets. How advanced these devices ultimately become will depend on some complex tradeoffs, both technological and economic, in individual end markets.

But the need for less weight and relatively simple, inexpensive devices is growing, particularly as the rest of the industry disaggregates from centralized processing units and a range of more complex, expensive and heavier PCBs and wiring schemes. And as they gain traction and push further into the mainstream, the chip industry will begin to understand where this technology will be most effective and how well it will perform over time.

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