Origins of strain localization in a silver-based flexible ink under tensile load

Flexible electronics often employs 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 (FEM) 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 variation 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.

Research on the Working Mechanism of Melt Metallurgical Effect

This paper takes rectangular tinned silver flake as the research object to explore the working mechanism of melt metallurgical effect. Firstly, the paper designs and processes rectangular tinned silver flake through the electroplating technology, then conducts low-overload pre-arc experiment with the metallurgical samples. According to the experiment results, it is proposed that “Under low overload current, the formation of silver-tin alloy leads to the increase of current density of the metallic silver layer, which significantly advances the break time of the silver flake. At the same time, the silver-tin interpenetration speed is a linear function of the temperature.” Finally, the self-programming software is used to establish the transient electric heating field model of the metallurgical tinned silver flake. By comparing simulation and experiment, the results verify the accuracy of the hypothesis, and obtain the working mechanism of the melt metallurgical effect, which provides a reliable theoretical basis for the design and application of the metallurgical trigger.

Highly Stretchable Conductive Electrode Composed of Silver Flake and Ecoflex

Stretchable electronic devices commonly require interconnectors with high stretchability, conductivity, and durability. In this work, we demonstrated a highly stretchable electrode based on a composite of silver (Ag) flake filler and Ecoflex binder. The stretchable composite material was printed on polyurethane substrate. To improve the dispersibility and printability of the composite material, poly(dimethylsiloxane-ethylene oxide polymeric) was used. The effects of Ag flake content and Ecoflex binder materials on the electromechanical properties of the stretchable electrode were investigated via stretching and cyclic stretching tests. As the amount of Ag flake increased, the electrical conductivity of the electrode increased as well, but the stretchability decreased. Use of Ecoflex 00-10 material, with its softer and lower Young's modulus, greatly improved the stretchability of the electrode—allowing it to stretch to a strain of up to 120%. The stretchable electrode withstood repetitive cyclic stretching durability tests of 10,000 cycles. The addition of carbon black also had a great impact on its electromechanical properties. An electrode fabricated with 0.2 wt% carbon black showed the highest conductivity and excellent stretchability of more than 150%.

The Effect of Encapsulation Geometry on the Performance of Stretchable Interconnects

The stretchability of electronic devices is typically obtained by tailoring the stretchable interconnects that link the functional units together. The durability of the interconnects against environmental conditions, such as deformation and chemicals, is therefore important to take into account. Different approaches, including encapsulation, are commonly used to improve the endurance of stretchable interconnects. In this paper, the geometry of encapsulation layer is initially investigated using finite element analysis. Then, the stretchable interconnects with a narrow-to-wide layout are screen-printed using silver flake ink as a conductor on a thermoplastic polyurethane (TPU) substrate. Printed ultraviolet (UV)-curable screen-printed dielectric ink and heat-laminated TPU film are used for the encapsulation of the samples. The electromechanical tests reveal a noticeable improvement in performance of encapsulated samples compared to non-protected counterparts in the case of TPU encapsulation. The improvement is even greater with partial coverage of the encapsulation layer. A device with a modified encapsulation layer can survive for 10,000 repetitive cycles at 20% strain, while maintaining the electrical and mechanical performance.