Buckling waves in aluminum on a polyimide sea: In situ analysis towards a reliable design strategy for stretchable electronics

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Buckling waves in aluminum on a polyimide sea: In situ analysis towards a reliable design strategy for stretchable electronics

‘Stretchable electronics’ refers to highly deformable devices in which compliant polymeric substrates support micron-size sensor units; these may provide spatially distributed measurements over complex surfaces. Stretchable sensors have opened new perspectives and applications in many fields, among which biomedicine is one of the most promising: instrumentation for tools which need to withstand significant bending or stretching during service has been demonstrated via balloon catheters and implantable patches, enabling a totally new generation of smart devices. Stretchability and bendability of such systems are achieved by selecting a compliant substrate for the sensing units and by granting sufficient deformability to the electrical interconnects. The results of this study provided an insight into the local mechanics involved in the onset of the delamination and buckling in S-shape deformable interconnects. In particular, a correlation was found between the interface failure phenomena and the geometrical parameters that define the design of deformable interconnects, allowing the identification of unexpected drawbacks related to specific S-shape design features

Micron-scale experimental-numerical characterization of metal-polymer interface delamination in stretchable electronics interconnects

Understanding the mechanical behavior and failure mechanisms of stretchable electronics is key in developing reliable and long-lasting devices. In this work a micron-scale stretchable system consisting of an aluminum serpentine patterned interconnect adhered to a polyimide substrate is studied. In-situ experiments are performed where the stretchable sample is elongated, while the surface topography is measured using a confocal microscope. From the resulting height profiles the microscopic three-dimensional deformations are extracted using an adaptive isogeometric digital height correlation algorithm. The displacement information is compared to realistic numerical simulations, in which the interface behavior is described by cohesive zone elements. It is concluded that despite fitting the traction separation law parameters, the model fails to correctly capture the distinct out-of-plane buckling (with magnitude of a few micron) of the interconnect. The model is updated with residual stresses resulting from processing and crystal plasticity induced behavior (decreased yield strength) in the aluminum layer, but both measures are not resulting in the experimentally observed deformations. Finally, mixed-mode cohesive zones are implemented, in which the properties are different in the shear and normal direction. After fitting the corresponding parameters to the experimental data, the model shows realistic in-plane and out-of-plane deformations. Also a predictive simulation for a different geometry results in the correct experimentally measured behavior. It is concluded that the aluminum-polyimide interface mode-angle dependency explains the observed microscopic failure mode of local delamination and buckle formation.</p

In-situ experimental characterization of interfacial toughness of aluminum thin films on polyimide substrates

Interfacial delamination embodies a major issue as concerns the mechanical reliability of metal-on-polymer thin films. The interfacial toughness of a metal/polymer material system is a relevant constraint when dealing with the design of electronic devices, which feature stacks of several dissimilar materials. This evidence provides the motivations for the experimental measurement of metal/polymer adhesion. In this study, the interfacial delamination of Aluminum thin films on Polyimide substrates has been investigated combining in-situ light microscopy with a miniaturized 90° peel test setup. Being peel forces in the mN range, dedicated clamps have been designed in order to avoid the use of conventional sliding fixtures meanwhile ensuring proper 90° peel geometry when testing samples on a uniaxial tensile stage. Peel tests revealed a very sharp delamination front, with no evidences of interface heterogeneities. In-situ imaging allowed to interpret the trend of the force/displacement curve. Meaningful data for the computation of the fracture energy (~90 J/m2) were clearly distinguished from data affected by undesired dissipation phenomena competing with delamination, such as plastic deformation of the Aluminum film

Mechanical reliability of microelectronics packaging: Small scale adhesion measurements and in-situ imaging

Delamination owed to misfit and thermal stresses is a relevant concern in the field of microelectronics packaging. This study is focused on the adhesion of a polymeric dielectric to inorganic passivation layers and metal caps. In-situ imaging of the delamination phenomena has been achieved by combining 90° peel testing and confocal laser scanning microscopy. This technique has shown promising to enable straightforward segmentation of the bent shape of the peeled layer. The latter has been used to estimate concurring plastic dissipation by means of elastoplastic beam theory and finite element analysis. The results indicate that the actual adhesion energy amounts to ∼30% of the external work associated to the measured peel force, which confirms the relevance of in-situ imaging in the field of microelectronics reliability

Delamination phenomena in aluminum/polyimide deformable interconnects: In-situ micro-tensile testing

The deformation and failure mechanisms of metal/polymer electrical interconnects with S-shaped planar meanders are investigated. Samples consist of 1 μm thick aluminum conductive coating evaporated on a 10 μm thick polyimide substrate. Uniaxial tensile tests up to 40% stretch with in-situ optical and scanning electron microscopy (SEM) were performed to assess the effects of different meander geometries on the local mechanics. As a consequence of the large strain experienced by the underlying polymeric substrate, two different delamination modes were observed at the metal/polymer interface, namely, (a) shear-based and (b) buckling-based delamination. Mechanisms (a) and (b) are activated depending on the specific meander geometry: interestingly, a crucial role was played by the length of rectilinear arms, which was shown to influence the extent of transverse contraction experienced by the interconnect. Upon increasing stretch, in-situ SEM observations revealed detrimental effects related to the interfacial failure, as metal fracture localizes in the delaminated areas. Experimental results suggest that, in addition to the need of surface treatments aimed at improving the metal/polymer interface adhesion, it is also crucial to conceive optimal geometrical designs to achieve mechanical reliability of stretchable interconnect

Modeling and Experimental Studies of Coating Delamination of Biodegradable Magnesium Alloy Cardiovascular Stents

Biodegradable magnesium alloy stents exhibit deficient corrosion period for clinic applications, making the protective polymer coating more crucial than drug-eluting stents with the permanent metal scaffold. We implemented a cohesive method based on a finite element analysis method to predict the integrity of adhesive between coating and stent during the crimping and deployment. For the first time, the three-dimensional quantitative modeling reveals the process of polymer coating delamination and stress concentration. The fracture and microcracks of coatings were consistent with the simulation result, confirmed by the scanning electron microscopy observation. Moreover, we analyzed four possible factors, i.e., stent design, strut material, coating polymer, and thickness of the coating, affecting the stent-coating damage and the distribution of the stress in coatings. Mg-Nd-Zn-Zr alloy with lower yield strength performed a more uniform strain distribution and more favorable adhesion of the coating than the commercial magnesium alloy AZ31. Shape optimization of stent design improves the strain and stress distribution of coating remarkably, avoiding coating delamination. Additionally, PLGA coating with lower elastic modulus and yield strength tends to follow the deformation of the stent better and to adhere on the surface more tightly, compared to PLLA polymer. A reduction in coating thickness and an increase in the strength of stent-coating interface improve the resistance to delamination. Our framework based on cohesive method provides an in-depth understanding of stent-coating damage and shows the way of computational analyses could be implemented in the design of coated biodegradable magnesium stents

Adaptive isogeometric digital height correlation: application to stretchable electronics

A novel adaptive isogeometric digital height correlation (DHC) technique has been developed in which the set of shape functions, needed for discretization of the ill-posed DHC problem, is autonomously optimized for each specific set of profilometric height images, without a priori knowledge of the kinematics of the experiment. To this end, an adaptive refinement scheme is implemented, which refines the shape functions in a hierarchical manner. This technique ensures local refinement, only in the areas where needed, which is beneficial for the noise robustness of the DHC problem. The main advantage of the method is that it can be applied in experiments where the deformation mechanisms are unknown in advance, thereby complicating the choice of suitable shape functions. The method is applied to a virtual experiment in order to provide a proof of concept. A second virtual experiment is executed with stretchable electronics interconnects, which entail localized buckles upon deformation with complex kinematics. In both cases, accurate results were obtained, demonstrating the beneficial aspects of the proposed method. Moreover, the technique performance on profilometric images of a real experiment with stretchable interconnects was demonstrated.\u3cbr/\u3