Membrane-based mechanical characterization of screenprinted ink films

The evolution of flexible sensing devices towards smaller and more sensitive structures would benefit from the ability to understand the mechanical properties of printable inks, simulate their behaviors in multiphysics frameworks and enable the precise design of mechanical sensors, such as piezoresistive pressure sensors. The key mechanical properties of screenprinted inks to be determined for such application are the Young’s modulus, Poisson coefficient and residual stresses. Several techniques are well known for thin film characterization, such as microtraction and nanoindentation. However, none is directly adaptable to ink films. The two main reasons are that (i) these methods require the material to be deposited on a substrate that has much lower Young’s modulus and residual stress to guarantee the decorrelation of the substrate and film behaviors and (ii) most of these techniques require the prior knowledge of the Poisson coefficient of the material. This work proposes a microfabricated characterization platform for screenprinted inks on silicon substrate. It consists in a set of well-controlled ultrathin flexible polyimide membranes of several sizes, both square and circular shaped, from 500 to 2000 μm side or diameter, onto which the inks to be tested can be screenprinted. The ink-coated membrane responses are measured using white light interferometry or laser Doppler vibrometry, to quantify (1) their deflection under pressure, known as the bulge test, and (2) their dynamic resonance modes under vibration. The combination of the two tests is intended to decorrelate the main mechanical parameters of inks using structural equations. The objective is to be able to decorrelate Young’s modulus, Poisson coefficient and residual stresses with a relative variation to polyimide smaller than 20%.