Mechanics of stretchable interconnects for stretchable electronics devices

Stretchable electronics is the newest field of research within the topic of the large-area electronics where stretchability is usually achieved by coupling micron-size electronic devices with stratchable polymer-based substrate and interconnects. Several approaches have been developed with the purpose to realize elastically stretchable interconnects. Among others, the use of metal films becomes straightforward: the conductive material is patterned onto an intrinsically elastic substrate. Two main limits represent critical aspects in the stretchable electronics interconnects: the occurrence of interface failure and the mechanical reliability of the metal line at the transitions between flexible and stretchable parts. Stretching-induced delamination has been studied considering 1 micron thin Al interconnects deposited on 10 micron PI substrate. A micro-tensile testing with simultaneous imaging of the samples has been carried out by means of a suitably developed micro-tensile equipment. Optical and ESEM observations have been obtained during stretching of the interconnects. The mechanical tests have been simulated through finite element modeling integrated with a cohesive approach able to explicitly account for the delamination phenomena. The sub-modeling technique has been used to consider more refined models focusing on the interface. The traction vector acting at the cohesive surface modeling the interface has been derived from the interfacial potential proposed by Xu and Needleman [X-P Xu and A. Needleman, J. Mech. Phys. Solids, 1994]. The cohesive mixed-mode formulation yielded delamination morphologies which were consistent with that observed in the experiments, thus establishing the importance of irreversible and coupled normal-tangential behavior. Moreover, in order to understand the effect of the flex-stretch transition on the mechanical reliability the cyclic behaviour of 17 micron thick copper interconnections with four different geometries embedded in 1 mm PDMS encapsulation have been studied with fatigue cyclic tests at 10% of elongation. Numerical models that simulate the mechanical behavior of the copper-based interconnects have been developed based on the submodeling technique. The fatigue life of all designs has been evaluated and mutually compared on the basis of the accumulation of plastic strain. Mechanical characterization of the constituent materials have been carried out, at the real scale, by using the nanoindentation technique and micro-tensile tests for the copper and tensile tests for the PDMS. A good correlation between the model and the experiments allowed to establish a fatigue life prediction by using the Coffin and Manson relationship between the accumulated plastic strain obtained through the models and the experimental meander life.