Trace width effects on electrical performance of screen-printed silver inks on elastomeric substrates under uniaxial stretch

Stretchable electronics can be realized using different manufacturing methods and hybrids thereof. An example of the latter is the combination of stretchable circuit boards with screen-printing, which will be discussed in this work. The hybrid stretchable electronics structures are based on photolithographically structured and rigid copper islands and screen-printed silver ink interconnections. This enables the assembly of components with a high number of contacts onto the copper islands and deformable silver ink lines between islands. The transition area between islands and lines is critical due to local stress concentration. The effect and potential mitigations were studied by measuring the electrical resistance of test interconnections under mechanical loading. The first set of samples was elongated up to 30 % in tensile tests. The second set of samples was elongated 10 %, 20 %, and 30 % in cyclic tests up to 10.000 cycles. After the tests, extensive failure analysis, e.g., scanning electron microscope, and finite element analysis were conducted.In tensile tests at maximum load, the interconnections either snap apart or their resistance increases by 640 % in the transition area. Adding protective structures around the transition area, the resistance increase can be reduced to 12 %. Stress concentration in the transition area can be controlled with the layout of the structures, as shown in the cyclic tests. Depending on a layout, the structures protect interconnections in the transition area (resistance < 4 Ω at 10 % and 20 % throughout 10.000 cycles, and up to 5000 cycles at 30 % elongation), or with particular designs, cause fatal damage of the circuitry and fail early. The identified failure mechanism is typically fatigue damage caused by the repeated bending of the protective structure. The observed resistance increase at the interface was closely related to the crack propagation phase in the protective structures.

Section: Electrical Measurement Resultsmentioning

confidence: 91%

Mechanical properties of structured copper and printed silver hybrid stretchable electronic systems

Salo

Werft

Adams

et al. 2023

“…The R/R 0 of the 3-layer 0.5 mm-wide specimens increased much more quickly with ε than the 2 mmwide specimens (see figure 2(b)). Therefore, the trace width effect found by Cahn et al [48] in the 1-layer ink can be extended to multi-layer inks. In other words, the width should be 2 mm or more to minimize both normalized resistance and resistance (since wider traces have lower R 0 ) with strain.…”

Section: Monotonic Behaviormentioning

confidence: 78%

“…Numerous works [44,45] have shown that serpentine shaped interconnects could greatly improve electrical performance under tensile strain compared to straight trace lines and can tolerate cyclic loading, due to the maximum local strain being more than an order of magnitude lower than the macroscale applied strains [44,46]. To characterize the electrical resistance evolution of a conductive ink with both monotonic and cyclic uniaxial elongation for a wide range of strain, specimens with straight trace lines were studied in a number of prior works by Cahn et al [34,47,48] due to the relatively uniform deformation and ease of achieving high strains experimentally. The conductive ink's electrical resistance evolution with monotonic or cyclic strain, obtained from tests using the straight trace line specimens, can potentially be used to map and predict the resistance evolution in more complex configurations such as serpentines.…”

Section: Introductionmentioning

confidence: 99%

Understanding resistance increase in composite inks under monotonic and cyclic stretching

Li¹,

Αντωνίου²,

Pierron³

2022

Self Cite

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 (SEM) experiments, for varying ink thickness (1, 2, and 3 layers, each layer around 8-10 µm) 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 exhibit 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. However, the cracks widen and deepen, leading to an increase in resistance with cycling. There exists a strong correlation between fatigue life as defined by 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 µm) and wider (≥2 mm) inks should be used to lower resistance increases with repeated deformation.

“…The trace width effect on the electrical behavior of the PE 874 ink has been investigated previously, mainly for monotonic loading [29]. The size effect was related to the crack size being commensurate with the trace widths, the same crack pattern having a more detrimental effect on resistance for smaller trace widths.…”

Section: Effect Of Trace Width (1 Vs 2 Mm) On Fatigue Properties and ...mentioning

confidence: 96%

“…As the PE 874 conductive ink is stretched, strain localization occurs in the ink by surface cracking starting from very low strains (∼1%) [28,29]. In both the monotonic and cyclic cases, the evolving crack pattern is linked to a resistance increase.…”

Section: Introductionmentioning

confidence: 99%

Modeling resistance increase in a composite ink under cyclic loading

Chung

Αντωνίου

et al. 2023

Self Cite

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 Nf 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 specimen with no prior loading. The minimum-rate curve is therefore recommended for more accurate fatigue estimates.