Self-healing technology developed for circuit repair

Citation

Bogue, R. (2012), "Self-healing technology developed for circuit repair", Assembly Automation, Vol. 32 No. 2. https://doi.org/10.1108/aa.2012.03332baa.006

Self-healing technology developed for circuit repair

Article Type: Mini features From: Assembly Automation, Volume 32, Issue 2

A research group from the University of Illinois at Urbana-Champaign has demonstrated a self-healing technique which can be used to repair damaged electronic circuits. The emergence of ever-more complex, high-density, high-value ICs which frequently feature large numbers of thin, patterned conductive films (typically Cu or Al), separated by dielectric layers and interconnected through multiple conductive vias, has highlighted the need for effective repair mechanisms. Failure is frequently caused by a lack of conductivity arising from interconnect fractures, thin film cracking or the delamination of conductive pathways and although techniques such as manual repair or hardware redundancy have been proposed to address these issues; they have met with only limited success.

The new technique relies on the fracture-initiated release of a microencapsulated repair material; a concept which has already been applied to the structural repair of various composites, polymers and other materials by the Illinois team and other groups. Eutectic Ga-In alloy was selected as the healing agent because of its low melting point, ~16°C and its relatively high electrical conductivity (3.40×10⁴ S cm⁻¹). The alloy was encapsulated within ellipsoidal, polymeric urea-formaldehyde shells with a length of around 200 μm. The self-healing concept was tested on a multilayer electronic device which comprised a conductive circuit formed by patterning gold lines on a rigid glass substrate by a lift-off lithography technique and an epoxy dielectric layer which was deposited on top of the circuit. This had dimensions of 12.0×75.0×4.0 mm and acted as a resistor in an unbalanced, constant voltage Wheatstone bridge circuit during testing. Two configurations of the self-repair mechanism were studied: in one, larger diameter (~200 μm) microcapsules were embedded in the dielectric layer and in the other, smaller diameter (~10 μm) capsules were patterned directly onto the gold lines. The devices were bonded to a notched glass top layer and a ductile acrylic bottom layer and loaded in four-point bending to provide controlled circuit failures. At a critical bending load, a crack initiates at the notch root and propagates through the dielectric layer and conductive gold line, ending at the bonded acrylic interface. The embedded microcapsules are ruptured during crack propagation, releasing the liquid metal into the damaged circuit which restores the conductivity to >99 percent of its original value (Figure 1). The average healing time was 160 μs. It was found that the proportion of the samples where healing occurred rose as the volume fraction of the 200 μm diameter capsules increased; at the maximum fraction tested (0.16), 90 percent of the samples healed. However, all of the samples with the 10 μm diameter capsules patterned on the gold line were fully healed, even with a volume fraction as low as 0.007.

Figure 1 Schematic of the self-repair mechanism

This work, which was reported in the journal Advanced Materials (Blaiszik et al., 2011, “Autonomic restoration of electrical conductivity”, doi: 10.1002/adma.201102888), has demonstrated the autonomic restoration of electrical conductivity in a mechanically damaged circuit for the first time. The research group argues that self-healing circuits will lead to increased device longevity, fault-tolerance and reliability in adverse mechanical environments, allowing new applications in microelectronics, advanced batteries and other electrical systems. Beyond self-repairing devices, these concepts could be used to create novel microelectronics that generate new circuits along stress-activated pathways, yielding adaptive circuit architectures and improved circuit redundancy.

Robert Bogue