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Process-Property Relationships for Flexible Additive-Printed Electronics
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Authored By:Pradeep Lall, Nakul Kothari, Kartik Goyal Auburn University NSF-CAVE3 Electronics Research Center Department of Mechanical Engineering AL, USA Ben Leever US Air Force Research Labs Wright-Patterson AFB OH, USA Scott Miller NextFlex Manufacturing Institute CA, USA SummaryTraditionally, a combination of imaging and plating based subtractive processes have been used for fabrication of printed circuit assemblies to form the needed circuitry on rigid and flexible laminates. In addition to circuits, additive electronics is finding applications for fabrication of sensors for wearable applications and asset situational awareness. Aerosol-Jet printing has shown the capability for printing lines and spaces below 10 µm in width with a wide variety of materials, including nanoparticle inks, conductive polymers, insulators, adhesives, and even biological matter. The adoption of additive manufacturing for high-volume commercial fabrication requires an understanding of the print consistency, electrical and mechanical properties. In this study, the effect of process parameters on the resultant line-consistency, mechanical and electrical properties has been studied for single-layer and multi-layer substrates. Print process parameters studied include the sheath rate, mass flow rate, nozzle size, substrate temperature and chiller temperature. Properties include resistance and shear load to failure of the printed electrical line as a function of varying sintering time and varying sintering temperature. Printed samples have been exposed to different sintering times and temperatures. The resistance and shear load to failure of the printed lines has been measured. The underlying physics of the resultant trend was then investigated using elemental analysis and SEM. The effect of line-consistency drift over prolonged runtimes has been measured for up to 10-hours of runtime. Printing process efficiency has been gauged a function of process capability index (Cpk) and process capability ratio (Cp). Printed samples were studied offline using optical Profilometry to analyze the consistency within the line width, line height, line resistance and shear load to study the variance in the electrical and mechanical properties over time. ConclusionsIn this paper, a detailed study of the effect of aerosol-jet process parameters on the resistance, shear strength, and elastic modulus has been presented. Experimental measurements indicate that the resistivity improved with the increase in the number of passes. Increase in sintering time at a constant sintering temperature was found to increase resistivity of the printed line resulting from the increase in number of cracks in the printed lines. Multi-layer print process is developed and different parameters that adversely affect the print quality are studied. Parameters such as UV Curing for the dielectric, and sintering temperature for the metal ink, are found to be of great influence to the print quality of the micro-via. Increase in the sintering temperature at a constant sintering time was found to decrease the resistivity of the printed line – which based on SEM analysis can be attributed to the decrease porosity of the printed line. Mechanical properties including the elastic modulus and shear strength decreased with the increase in sintering temperature. Line-consistency measurements of line-width, line height, and resistance over a continuous 10-hour print run are reported. Process measurements indicate that viscosity of the ink has a pivotal role in maintaining print consistency. Line width, line height, and resistance measurements exhibit much lower variance with the use of bubbler, which enables control over ink-viscosity during the 10-hour print run. In addition, computation of the process capability index (Cpk) and process capability ratio (Cp) indicate that without bubbler, the resistance consistency is a 2.125-sigma process. Further, the use of bubbler allows for a wider process window where, resistance consistency is a 3.6-sigma process, height consistency is 3-to-3.25-sigma process, and width consistency is a 3-to-3.25-sigma process. The via-printing process exhibited a Cp of 1.1 and Cpk of 0.65. Initially Published in the SMTA Proceedings |
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