Most electronic systems that power our digital life are inflexible and flat. Rigid electronic designs work for our computers and phones but not for our bodies. Humans are soft and curved. Electronic systems capable of bending, twisting, and stretching have great potential for applications in which conventional, stiff semiconductor microelectronics would not suffice. One promising area for conformable electronics is in biomedical applications such as wearable and implantable electronic systems. Design for Reliability of Multi-Layer Thin Film Stretchable Interconnects, to be presented at the Electronic Components and Technology Conference on May 28 – 31, 2013, discusses the use of stretchable interconnects (Figure 1) and the future shape of conformable electronics.

Figure 1. Multiple layers of stretchable interconnects crossing at the strain relief structure described below.

Previous stretchable interconnect designs are composed of only a single metal layer. Due to the nature of this single metal layer design, these stretchable electronic systems have large, sparse layouts. However, as the system gets more complex, single metal layer interconnects become impractical. Multi-layered stretchable interconnects allow for smaller, more intricate, and more practical systems and can be produced through conventional bottom-up micro-fabrication processes. When multiple meandering interconnects intersect, however, the junctions of the interconnects are subject to the specific orientation of each structure, complicating the design, risking the integrity of the small junction areas, and resulting in complex mechanics during stretching.

A circular structure to relieve strain at the junctions between multiple metal interconnect layers is proposed in the paper. In the proposed strain relief structure design (Figure 2), meandering horseshoe patterned metal interconnects contain circular rings that are stacked to allow the interconnects to overlap one another.  While the proposed strain relief structure is a universal design that can be applied to any number of systems with multiple interconnecting layers, structures composed of two metal layers are examined in this study.

Figure 2. Strain relief structure design with two layered intersecting interconnects.

A comprehensive investigation into the deformation behavior and failure mechanisms of the structures was carried out through both numerical and experimental analysis. Numerical analysis indicates that elongations of up to 20% cause no plastic strain in the structure, allowing it to operate indefinitely. Simulations show that the crests of the horseshoe interconnects are the regions with the greatest strain and that the structure will ultimately fail at one of these crests (Figure 3).

Figure 3. Right: Structure imaged during testing. Left: Simulated strain distribution.

Results of electromechanical testing show that the strain relief structure can stretch up to 285% of its initial length prior to failure. Initial results from fatigue life testing of the structures has demonstrated that they are able to withstand more than one million cycles of 100% elongation at a 200% per second strain rate. Fatigue life testing also verified the numerical simulations, indicating that the strain relief structure effectively dissipates strain from the interconnect junctions to the crests of the horseshoes. Both the simulation and experimental results show that this multi-layer strain relief structure for stretchable electronic systems is a durable and highly promising design with significant implications for the future of conformable electronics.