(Invited) Self Healing Interconnects for Reliable Flexible Electronics
Authors: Sambandan Sanjiv , Kumar Amit , Yaswant Vaddi , Parab Virendra 
Published: February 2020

The recent decade has seen a surge in research and development on flexible and wearable electronic systems. These systems consist of thin film transistor integrated circuits or off-shelf intergrated circuit chips fabricated and packaged onto a mechanically flexible substrate. Mechanically flexible electronics offers a host of functional advantages resulting in technologies such as foldable displays, x-ray imagers conformable to the human body, spherical image sensors offering wide angle view etc. However these advantages are accompanied by a number of reliability problems of which one major problem is the reliability of interconnects. Interconnects on flexible and wearable devices are prone to significant structural and electrical stress due to mechanical flexing, electro static discharge etc. and are subject to open circuit failure [1].

Interconnects have been conventionally made more robust to open circuit failure by several passive approaches such as the use of geometry (eg. meandered interconnects stretch more) or novel materials (eg. conductive elastomers) [2]. However, despite these precautions, open faults are expected to occur and there isn’t much that can be done once they do occur.

Active techniques on the other hand offer online repair of the interconnect fault [3]-[7]. We propose an active technique for self healing open faults in interconnects by the use of a dispersion of electrically conductive particles in an insulating fluid [4]-[7]. This dispersion is contained and isolated over each interconnect. Upon the occurrence of an open fault in a current carrying interconnect, the field appearing across the open gap polarizes the conductive particles in the dispersion. The polarized particles experience dipole-dipole attractive forces and eventually chain up to create a bridge across the gap thereby healing the fault (seen in (a) in the figure). This mechanism is truly self healing as it turns on and shuts off by itself without external interference. The occurrence of the fault creates the electric field that triggers the mechanism. After the repair of the fault, the electric field across the open gap disappears thereby stopping the mechanism.

The heal effectiveness is determined by the dispersion concentration, geometry of particles used, viscosity and permittivity of the fluid and the electric field across the open gap. We discuss the methods of implementation with circuits on flex and rigid substrates (seen in (b) in the figure) and the impact of circuit parameters on the heal time and heal impedance (current carrying capacity) in real world implementation. The limitations and structural stability of the heal are also discussed.

In this talk we not only discuss the mechanism of self healing, but also some remarkable properties shown by the heal. Firstly, the heal shows active re-routability i.e. it can be re-oriented from one set of electrodes to another thereby permitting a relay like operation [6] (seen in (c) in the figure). Secondly, the heal shows a neuromorphic behaviour where it ‘remembers’ a connection between routes, ‘forgets’ the connection if programmed, and ‘re-learns’ the connection faster once re-engaged [6]. Finally, the heal shows high stretchability that makes it very suited to electronics on elastomers.

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Figure 1