Mechanical analysis of encapsulated metal interconnects under transversal load (original) (raw)

In this paper, the effect of encapsulation on deformation behavior and failure mechanisms of stretchable interconnects is presented. Extensive numerical modeling is conducted for mechanical analysis of which the results are correlated with in-situ experimental observations. The numerical results reveal that by adding an encapsulation layer of various thickness (from 0.0 to 0.5 mm) on top of the stretchable interconnect, the out-of-plane deformation and in-plane geometrical opening are reduced. Consequently, not only the plastic strain in the metal increases but also the in-plane shear stress at the interconnect/substrate interface. In-situ electromechanical experiments combined with scanning electron micrographs and optical images confirm the numerical analysis. More specifically, it is found that two failure mechanisms are involved during the stretching process: interfacial delamination in a S-shape alongside the metal conductor and metal breakdown at the crests of the metal conductor. The encapsulated stretchable interconnect shows both failure mechanisms at a lower percentage elongation than the non-encapsulated stretchable interconnect. Even so, the onset point of interfacial delamination for the encapsulated stretchable interconnect occurs only at an impressive number of 63% elongation and metal rupture only at 120%. The in-plane shear strain contour, obtained by numerical simulation, agrees well with the delamination failure location observed in the experiment.

In this work, the design of flexible and stretchable interconnections is presented. These interconnections are done by embedding sinuous electroplated metallic wires in a stretchable substrate material. A silicone material was chosen as substrate because of its low stiffness and high elongation before break. Common metal conductors used in the electronic industry have very limited elastic ranges; therefore a metallization design is crucial to allow stretchability of the conductors going up to 100%. Different configurations were simulated and compared among them and based on these results, a horseshoe like shape was suggested. This design allows a large deformation with the minimum stress concentration. Moreover, the damage in the metal is significantly reduced by applying narrow metallization schemes. In this way, each conductor track has been split in four parallel lines of 15 mum and 15 mum space in order to improve the mechanical performance without limiting the electrical characteristics. Compared with the single copper or gold trace, the calculated stress was reduced up to 10 times.

The performance of the flexibility and stretchability of flexible electronics depends on the mechanical structure design, for which a great progress has been made in past years. The use of prestrain in the substrate, causing the compression of the transferred interconnects, can provide high elastic stretchability. Recently, the nonbuckling interconnects have been designed, where thick bar replaces thin ribbon layout to yield scissor-like in-plane deformation instead of in- or out-of-plane buckling modes. The nonbuckling interconnect design achieves significantly enhanced stretchability. However, combined use of prestrain and nonbuckling interconnects has not been explored. This paper aims to study the mechanical behavior of nonbuckling interconnects bonded to the prestrained substrate analytically and numerically. It is found that larger prestrain, longer straight segment, and smaller arc radius yield smaller strain in the interconnects. On the other hand, larger prestrain can also ...