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The electrical resistance of metal-polymer conductive inks increases as they undergo cyclic loading, posing a major challenge to their reliability as interconnect materials for flexible electronic devices. To characterize an ink’s fatigue performance, extensive electro-mechanical testing is usually performed. Phenomenological models that can accurately predict the resistance increase with cyclic loading can save time and be useful in flexible conductor design against fatigue failure. One such model was recently developed for only one composite ink type. The model is based on experiments monitoring resistance under monotonic stretch data and multiple experiments measuring the rate of increase of the resistance under different strain amplitudes and mean strains. The current work examines whether such resistance rate model could be generalized to apply for more types of composite inks. Two composite inks with different binder material, metal flake sizes and shapes, and substrate material were experimentally tested under monotonic and cyclic loading. It was found that the two new inks are also more sensitive to strain amplitude than mean strain. The resistance rate model accurately predicts early/catastrophic failure (<1000 cycles) in all inks and conservatively estimates high fatigue life for low strain amplitudes. A protocol detailing the procedures for applying the resistance model to new inks is outlined.

 

The electrical performance of stretchable electronic inks degrades as they undergo cyclic deformation during use, posing a major challenge to their reliability. The experimental characterization of ink fatigue behavior can be a time-consuming process, and models allowing accurate resistance evolution and life estimates are needed. Here, a model is proposed for determining the electrical resistance evolution during cyclic loading of a screen-printed composite conductive ink. The model relies on two input specimen-characteristic curves, assumes a constant rate of normalized resistance increase for a given strain amplitude, and incorporates the effects of both mean strain and strain amplitude. The model predicts the normalized resistance evolution of a cyclic test with reasonable accuracy. The mean strain effects are secondary compared to strain amplitude, except for large strain amplitudes (>10%) and mean strains (>30%). A trace width effect is found for the fatigue behavior of 1 mm vs 2 mm wide specimens. The input specimen-characteristic curves are trace-width dependent, and the model predicts a decrease inN_fby a factor of up to 2 for the narrower trace width, in agreement with the experimental results. Two different methods are investigated to generate the rate of normalized resistance increase curves: uninterrupted fatigue tests (requiring ∼6–7 cyclic tests), and a single interrupted cyclic test (requiring only one specimen tested at progressively higher strain amplitude values). The results suggest that the initial decrease in normalized resistance rate only occurs for specimens with no prior loading. The minimum-rate curve is therefore recommended for more accurate fatigue estimates.

 

Abstract Cyclic degradation in flexible electronic inks remains a key challenge while their deployment in life critical applications is ongoing. The origin of electrical degradation of a screen-printed stretchable conductive ink with silver flakes embedded in a polyurethane binder is investigated under uniaxial monotonic and cyclic stretching, using in-situ confocal microscopy and scanning electron microscopy experiments, for varying ink thickness (1, 2, and 3 layers, each layer around 8–10 μ m) and trace width (0.5, 1, and 2 mm). Cracks form under monotonic stretching, and the evolution of crack pattern (density, length and width) with applied strain is affected by ink thickness such that the 3-layer ink exhibits larger normalized resistance but slightly lower resistance than the 1-layer ink up to strains of 125%. For cyclic stretching, the crack density and length do not evolve with cycling. However, the cracks widen and deepen, leading to an increase in resistance with cycling. There exists a strong correlation between fatigue life, i.e. the number of cycles until a normalized resistance of 100 is reached, and the strain amplitude. The normalized resistance increase rate with respect to cycling is also found to scale with strain amplitude. The rate of change in resistance with cycling decreases with ink thickness and trace width. For practical applications, thicker ( ⩾ 25 μ m) and wider (⩾2 mm) inks should be used to lower resistance increases with repeated deformation. 

Abstract Flexible electronics often employ composite inks consisting of conductive flakes embedded in a polymer matrix to transmit electrical signal. Recently, localized necking was identified as a cause of a substantial increase in normalized resistance with applied strain thereby adversely impacting electrical performance. The current study explores two possible contributing factors for the formation of such localization—ink surface roughness and local variations in silver flake volume fraction. Uniaxial tension experiments of a DuPont 5025 type ink are used to inform a constitutive model implemented using finite element method on different substrates. Surface roughness was modeled by sinusoidal variation in ink height, whose amplitude and wavelength are informed by experimental laser profilometry scan data. Local flake fraction variations obtained from experimental measurements before applying any strain, were modeled as local variations in the elastic modulus according to an inverse rule of mixtures between the silver flake and acrylic binder material properties. The study identified that the ink height roughness is the most impactful contributor to the subsequent strain localization. The substrate elastic properties impact the number and magnitude of localization bands, with the stiffer substrate delocalizing strain and averting catastrophic crack formation seen with a more compliant substrate. The model incorporating surface roughness closely matches experimental measurements of local strain across different substrates. The study can inform designers of the adverse impact of ink surface roughness on localization and subsequent detrimental increase of the resistance.