Self-healing Interconnects with Nearly Plastic Stretching of Repairs

Flexible electronic systems such as roll-up displays and wearable devices promise exciting possibilities that could change the way humans interact with the environment. However, they suffer from poor reliability of interconnects and devices. Interconnects on flex are prone to open-circuit failures due to mechanical stress, electrostatic discharge, and environmental degradation. Passive approaches such as the use of stretchable conductors and novel geometries improve their response to mechanical stress but cannot salvage the interconnect if a fault were to occur. Active approaches using self-healing techniques can repair a fault and have been demonstrated with use of methods that use relatively rare materials, change conventional interconnect-fabrication processes, address only faults due to mechanical stress, or do not permit stretching. In this work we discuss a self-healing technique that overcomes these limitations and demonstrate heals having metallic conductivity and nearly plastic stretchability. This is achieved by dispersion of conductive particles in an insulating fluid encapsulated over the interconnect. Healing is automatically triggered by the electric field appearing in the open gap of a failed interconnect, irrespective of the cause of failure. The field polarizes the conductive particles, causing them aggregate and chain up to bridge the gap and repair the fault. Using dispersions of copper microspheres in silicone oil, we show self-healing interconnects with the stretchable heal having conductivity of about $5\times {10}^5$ S/m and allowing strains from 12 to 60. Previously, stretchable interconnects used materials other than copper. Here we effectively show self-healing, stretchable copper. This work promises high-speed, self-healing, and stretchable interconnects on flex, thereby improving system reliability.

Figure 1

(a) The self-healing mechanism. (b) Demonstration of stretchable self-healing with the heal adaptively increasing in length to accommodate the stretching. The dispersion is contained in a well and over an interconnect with an artificially induced open fault. Initially, the fault is healed. On stretching of the substrate, the heal stretches while maintaining electrical connectivity. The events occurring inside the well are also shown. See also Videos 1 and 2 in Appendix pp2. GND, ground.

Figure 2

(a) Experimental setup for the study of the mechanics of self-healing. (b) Current versus time for several self-healing experiments conducted with dispersions at different temperatures. At higher temperatures, the viscosity of the fluid is low and the healing is quicker. The sudden jump in current is indicative of the sintering of the heal. (c) Photographs of the self-healing process (see also Video 3 in Appendix pp2). (d) SEM images of the sintered heal. (e) Transients of the heal resistance, $R_h(t)$, during healing for different external resistances, $R_a$. (f) The heal time and steady-state heal resistance are dependent on $R_a$ [56].

Figure 3

(a) Experimental setup for the study of the impact of stretching on the heal. (b) Photographs from experiments demonstrating the stretchability of the heal. At $t=0$, the open gap of length 40 $\mu \mathrm{m}$ is healed. The electrodes are then moved apart at a constant velocity of 5 $\mu \mathrm{m}/$s. (c) In response to stretching, the heal adaptively increases its length by involving more particles in the healing process. (d) Measurements of current versus time during the stretching [56]. (e) Maximum stretchable length, $\mathrm{\Delta }{l}_{max}$, as a function of the stretching velocity, $u$, applied voltage, $V_a$, and dispersion concentration [56]. (f) Prediction of $\mathrm{\Delta }{l}_{max}$ as a function of $u$, $V_a$, and dispersion concentration based on Eq. (1). (g) Buckling of the heal in response to compression and the typical dynamics of the current through the heal in response to stretching and subsequent compression. See also Videos 4–7 in Appendix pp2.

Figure 4

(a) Demonstration of stretching in two dimensions. The electrodes are moved axially and then tangentially. The initial direction of the heal across the open gap is shown before the onset of tangential movement. The final direction of the heal is shown, defining the moment the chain breaks and cannot stretch any further. The maximum angle subtended between the initial and the final direction is $\theta$. (b) Plot of the typical current transients during the two-dimensional movement [56]. (c) The subtended angle $\theta$ as a function of the velocity and the maximum displacement length as a function of the velocity [56]. See also Video 8 in Appendix pp2.

Figure 5

(a) Self-healing with printed silver interconnects on polyethylene naphthalate substrate. (b) Noise-measurement setup. The sample is placed in a copper-shield box. A battery-powered transimpedance amplifier is used to measure the noise current. (c) Noise measurement without direct current. (d) Noise measurement with direct current. (e) Measured noise current for several cases.