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The reliability of nanocomposite conductive inks under cyclic loading is the key to designing robust flexible electronics. Although resistance increases with cycling and models exist, the exact degradation mechanism is not well understood and is critical for developing inks. This study links cracking behavior to changes in electrical resistance by performing in situ cyclic stretch experiments in scanning electron microscopy (SEM) with synchronized resistance measurements. Two screen-printed conductive inks, PE874 and 5025, on thermoplastic polyurethane (TPU) and polyimide (PI) substrates, respectively, were tested using the in situ technique. The obtained SEM images were analyzed with digital image correlation (DIC) to map the strain across cycles. The strain maps show that fatigue damage mainly occurred within the cracks formed during the initial monotonic stretch. There was no delamination at the ink–substrate interface or crack extension along the surface with cycling. Instead, fatigue damage resulted from a combination of crack widening and local shearing within the existing cracks. Crack depth varied based on the ink and substrate properties. The cracks in the 5025 ink on the PI substrate were only partially through the ink thickness, while fully through-thickness cracks were more prevalent in the PE874 ink on the TPU substrate. The 5025 ink showed a faster resistance increase with cycling than the PE874 ink because fatigue damage affected more bridging ink material for partial through-thickness cracks. Higher strain amplitudes caused greater crack widening and shearing and therefore faster resistance increase per cycle.
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Understanding resistance increase in composite inks under monotonic and cyclic stretching
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.
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- Award ID(s):
- 2026936
- PAR ID:
- 10442703
- Date Published:
- Journal Name:
- Flexible and Printed Electronics
- Volume:
- 7
- Issue:
- 4
- ISSN:
- 2058-8585
- Page Range / eLocation ID:
- 045010
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract 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 in N f by 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.more » « less
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Nanocomposite conductive inks consisting of metal flakes embedded in a polymer binder are employed as interconnect materials in flexible hybrid electronics (FHE) devices. Crack formation has been hypothesized to play a key role in the ink’s electrical degradation (increase of resistance), but the two have not been directly correlated. To address this gap, two classes of inks are studied using uniaxial stretch experiments, with in situ SEM imaging, and synchronous electrical resistance measurement. In plane strain maps obtained from digital image correlation (DIC) analysis of wide-field SEM images (~300 μm in width) identify crack patterns at various applied strains. From these strain maps, crack length measurements are obtained. A finite-element based numerical model adapted for non-uniform crack lengths is used to predict normalized resistance increase from the crack lengths. The model predictions are compared against the experimentally measured resistance changes. There is a remarkable correlation between the predicted and experimentally measured normalized resistance values. The linear crack density was also used to help understand the relation between the effective crack length and resistance increase. Closeup images of the ink top surface and the ink cross-section during the in situ SEM stretch experiment are used to explain differences between predicted and measured normalized resistance.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1088/2058-8585/aca98a
