DfR Solutions Reliability Designed and Delivered

Flexible Electronics – Technology and Challenges

Posted by Ashok Alagappan on Dec 13, 2017 3:00:00 PM

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Electronic components have become more ubiquitous in the last few years. Thanks to technological advancement, they are finding their way into more product categories and industries than ever before. One such recent advancement is the Internet of Things (IoT), which is a network of interconnected systems that communicate using a network protocol. The difference between the current internet and IoT is the heterogeneity. Systems of different functionality, technology and applications will belong to the same communication environment. The mantra that every component and system manufacturer is adopting these days is to make things “smarter.” What started with smart phones is now evolving into the smart watch, smart home, smart city, smart grid, smart retail, smart farming and the list goes on.

An example of smart health care is this Smart Cap that fits on a prescription bottle to help you stick to your regimen by sending reminder messages at the right time. It even takes care of order refills and doctor coordination.  For anyone who has forgotten to take a prescription, this could be a life-saver.

In this blog post, I’ll discuss the technology and challenges behind the electronic components that make an object smart and flexible.

Technology behind Flexible Electronics

Silicon was the heart of semiconductor technology driving the growth until now. Every major Integrated Circuit (IC) component manufactured was using silicon substrates. While silicon technology has its advantages, and will not run out of steam in the near future, it has its challenges when it comes to IoT applications. Silicon technology is not flexible, it is high cost and it takes a long time to manufacture. Because of those factors with silicon, market trends seem to favor form factor flexibility as seen in the massive growth of wearable electronics with products such as biomedical sensors on clothing and shoes, or pain relief patches becoming ever more popular. Form factor is an aspect of design which defines the size, shape and physical specifications of components. The need for form factor flexibility is not met by silicon, which means that a fundamental shift from the current silicon based technology is required to enable the growth. 

Flexible electronics are essentially traces and circuits printed on paper-based flexible substrates combined in some cases with traditional silicon die mounted on them.  In order to make flexible electronics practical, 2D materials are being used.

2D Materials:

Graphene is the common material used in devices to interconnect material. Graphene is a form of carbon with a monolayer of hexagonal lattice with one atom at each vertex. Some of the properties of graphene that makes it beneficial are:

  • very strong material
  • highly flexible
  • unique optical properties
  • high mobility
  • conducts heat and electricity

The properties that make it unique are flexibility, strength and optical transmittance. Graphene has an Elastic Modulus of 1TPa compared to ITO with 116 GPa and an ultimate Strength of 130 GPa. Graphene has high device operating frequencies to enable IoT applications.

The flexible materials are used as atomic sheets, which are then used to manufacture transistors and discrete components. Downsides of graphene include joule heating and poor On/Off ratio.

Other Transitional Metal dichalcogenide (TMD) materials such as MoS2 (Molybedenum diulphide), Phosphorene made out of black phosphorous, and Indium Tin Oxide(ITO), which has been shown to provide higher mobility and On/Off ratio are also being used.

Devices:

Active devices such as CMOS transistors in traditional silicon based technology are replaced by tunnel diodes and tunnel field effect transistors (TFTs).

TFT is 2D semiconductor based device that makes use of the quantum tunneling phenomenon. TFT’s achieve a much higher Ion/Ioff ratio for a given gate voltage compared to planar devices. Low Ioff currents makes it a suitable for low power IoT applications.

Tunnel diode integration with traditional FET’s offer significant advantages in terms of circuit speed, lower power, and reduced circuit complexity.  Tunnel diodes are printed with ultra-thin TiO2 interfacial layers on ITO substrates.

What are the Challenges of Flexible Electronics

Stretchable devices and power sources: Technology needs to advance to implement more discrete components such as inductors, capacitors and power systems in 2D materials. In order for the flexible system to be self-sufficient, more discrete components including power sources and inductors need to be made stretchable as well. There is a lot of research ongoing in this field.

Interconnect reliability: Bending stresses on flexible electronics are high due to the nature of its applications. As the strain in the film increases over time, the crack density and crack propagation increase due to mechanical deformation. Crack propagation is a wear out phenomenon that could result in lower fatigue life and eventually cause functional failure of the system. A few things to consider to improve reliability are: select material with right fatigue limit (graphene is shown to have higher limit than ITO) for the application, design appropriate trace thickness (smaller is better for a given stress amplitude) and other design rule optimizations must be made.

Manufacturability: Flexible electronics circuits are printed using different print technologies. The ability to print complex patterns with high throughput at low temperatures and high reliability is a manufacturing challenge. For more information on printing techniques and the reliability issues, refer to this DfR Solutions’ article.

Conclusion

According to a Market Research report, the global flexible electronics market is expected to reach $87.21 billion by 2024. The factors contributing to growth are ruggedness, light weight, low-cost production, unique electrical and optical properties.

The future of IoT will be driven primarily by flexible substrates with active components built on top using 2D materials that have been traditionally silicon based technologies. Flexible technology still needs to evolve to improve circuit functionality, density, reliability and manufacturability. But, flexible technology is here to stay and will continue to grow with advancement in technology.

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Topics: Wearable Electronics, IoT