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Fatigue Cycling of Dispensed Electrical Interconnections on Flexible Substrate; Materials Evaluation and Mechanical Behavior Understanding
Keywords: Flexible electronics, Dispensed Interconnections, Silver Pastes
Flexible electronics are circuits fabricated on thin, compliant substrates such as polyimide (PI), polyethylene terephthalate (PET), thermoplastic polyurethane (TPU) and paper. The advantage of flexible electronics over conventional electronics is that they can be twisted, bent, folded and stretched without significant effect on their functionality [1,2]. Conventional electronics components, such as batteries and integrated circuits, complex integrated components that cannot be easily made thin and flexible, and flexible elements, such as interconnects, resistors, capacitors and transistors, can be integrated on a flexible substrate, resulting in flexible hybrid electronics (FHE). Flexible electronics can be fabricated either by conventional microfabrication techniques (e.g. lithography) or printing techniques (e.g. inkjet printing). However, fabrication of flexible electronics using conventional techniques has limitations represented by the cost associated with it and the complex multistep processes involved. Furthermore, many flexible substrates are not compatible with microfabrication processes, such as etching and developing, used in conventional fabrication. The advantages of printing technologies over conventional techniques include high throughput, low cost, rapid prototyping, low temperature process and wide range of materials that can be used [3 – 6]. Dispenser printing is an additive manufacturing process that directly deposits dots or lines of material based on a programmed toolpath. This technique is capable of printing complex structures using a wide range of functionalized inks, like those used in screen printing while removing the need to invest in a screen. In presented work, electrical interconnects were directly printed on a flexible substrate (2mil PI) by a dispenser. Two silver pastes, creative material 125-13 and Asahi SW 1400, were utilized to dispense fine conductive lines, for the first time, on the flexible substrate. The printing quality of both materials was investigated and the dispenser parameters, which included pressure, dispensing speed, vacuum, needle size and stand-off distance, were optimized for both pastes to dispense traces with a width of 170µm and a thickness of 7-10µm. The dispensed pastes were cured at 150 °C for 20 minutes in a convection oven. The mechanical tensile test was conducted using single column Instron 3344. The mechanical durability of the dispensed paste samples was investigated by measuring the change in the electrical resistance of the test samples during tensile tests at different strains, strain percentage of 1.50%, 2.00% and 2.5%. The test samples were subject to these strains for different number of cycles. In addition, the microstructure of both dispensed pastes was compared before and after tensile test. Considering 20% change in the initial resistance as failure criteria, the life time of the dispensed interconnects versus the applied strain was modeled using Coffin-Manson relation. Evaluation the damage in the printed interconnections caused by mechanical tension is not easy because of presence of the flexible substrate that support the trace and reduce the effect of the applied load significantly. However, monitoring the change in the resistance of the interconnections while tension is a good way to measure the damage evaluated on the printed traces. Based on the change in the trace’s resistance, we concluded that the dispensed Asahi SW 1400 silver paste was less robust than the dispensed creative material 125-13 paste. The finer microstructure and smaller particle size of the creative material 125-13 silver paste enhanced its durability when subject to tensile strain. For example, the resistance of the test sample after 100 cycles at 2.0% strain was increased by 12% for the creative material 125-13 paste and 16% for the Asahi SW 1400 paste. Small increase in the initial resistance of the interconnections is very significant when it comes to use these interconnections in devices such as human vital signs monitors. Both materials showed good printing quality but creative material 125-13 paste presented better and more uniform printing quality.
Mohammed Alhendi, PhD Student
Binghamton University
Binghamton, NY

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