A multi-scale mechanical behavior study of an electrical interconnection solution stretchable and removable for flexible electronic components for biomedical applications

Abstract

International audienceCurrently, the solutions for interconnecting electronic components with their active face facing on substrates are based on metallic soldering. The mechanical contacts are therefore rigid. In order to enhance the reliability of the bonding, underfill is usually used to redistribute the thermo-mechanical stress created by the Coefficient of Thermal Expansion (CTE) mismatch between the silicon chip and substrate. Underfill is done with epoxy resins containing silica fillers (SiO2). As a result, the removal of components is no longer possible. Moreover, this solution is not suitable for devices integrating ultra-thin silicon components (<100 μm) hybridized on flexible substrates that may be subject to deformation. This is the case, for example, of medical "patches" worn on the person and continuously solicited. Indeed, the rigid contact points are likely to break. To address this issue, we are developing an ultra-thin anisotropic conductive adhesive film integrated in a stretchable flex inspired by the adhesion of the gecko. This film can be placed between the electronic component and the substrate. Thanks to the microstructuration of its legs involving about 1 million setae, the gecko can develop a large contact surface and thus a large force of attraction by the multiplication of van der Waals interactions (1).In this work, this "dry adhesion" based on the principle of "contact splitting", was implemented in order to improve the adhesion of a flexible interconnection. For this purpose, the surface of a polydimethylsiloxan (PDMS) film was structured with micrometric mushroom-shaped patterns known as the most efficient form of contact (2,3). To this end, silicon molds with varying mushroom geometries were etched to shape the PDMS (with different flange and pillar diameters). This approach makes it possible to do reproducible achievement of micro structured films with few defects. Studies at the macro and micro-scale were conducted in order to better understand the PDMS-mushrooms mechanical behavior. Mechanical results (using shear tests, pull-off experiments and Nano indentation) were modeled by finite element numerical calculations. Modelling and mechanical experiments at imposed displacement make possible to deduce the stiffness for one PDMS-mushroom in order to predict the number of mushroom contributing to maintain the adhesion. In the other hand, macro-scale experiments have an interest in the mechanical understanding on the total PDMS-film micro patterned. Finally, the impact of adding localized electrical connections through the structured film on the adhesion properties was also evaluated in order to select the most suitable films to obtain a flexible interconnection for biomedical applications