Interfacial delamination and delamination mechanism maps for 3D printed flexible electrical interconnects

Abstract

Flexible electronic systems integrate heterogeneous materials such as ceramics, metals, and elastomers, which results in interfaces prone to delamination under stress. In this research, we show that delamination can be initiated in a polyimide-based flexible interconnect system directly printed on a polydimethylsiloxane substrate where the metallic interconnect acts as the crack initiation site. This problem is experimentally and analytically evaluated to identify the controlling parameters and propose pathways to prevent delamination. The driving force for delamination is shown to be the vapor pressure of the absorbed moisture in the polymer. Based on the dimensions of the cracks in our system (20–60 μm) and the thickness of the delaminated polymer films (2–6 μm), nonlinear von-Kármán plate theory is utilized to capture both membrane stretching and thin plate bending behavior in the polymer films. The model yields ‘delamination mechanism maps’ that relate the energy release rate to the geometric dimensions of the flexible interconnects/circuits. For thin polymer films, a ‘vapor starved’ regime is shown where insufficient moisture reduces the driving force for delamination. For thicker films, a higher resistance to fracture is observed due to an increased rigidity of the polymer layer which behaves as a ‘plate’ rather than a ‘membrane’. Under these conditions, however, the higher retained moisture in the thicker films sustains the driving force for fracture capable of reaching the critical energy release rate. The mechanism maps also reveal the width of the metallic conductor (i.e., the initial crack size) as an important factor controlling fracture. For example, it is shown that the energy release rate for fracture is reduced from 20.5 to 4.6 J/m2 when the conductor width is reduced from 50 μm to 30 μm for a polymer film thickness of 6 μm. These predictions are shown to be in reasonable agreement with our experimental observations. A finite element model is also developed and used to further validate the analytical model. The work presented in this paper provides highly important and practical design guidelines for improved reliability of flexible electronic systems.