The fabrication of printed electronic devices via molten metal droplet jetting has enormous potential in flexible electronic device applications due to the extremely high electrical conductivity and excellent substrate adhesion of printed features. However, large pinholes (which could be detrimental to the feature performance) have been experimentally observed when molten metal droplets of aluminum 4043 alloy are deposited and solidified on a polyimide (PI) substrate. In this study, we have shown that subjecting the polymer substrate to elevated temperature during droplet deposition considerably reduces the number and size of pinholes. The formation mechanism behind the large pinholes is interpreted as the release of the adsorbed/absorbed moisture from the polymer substrate into the solidifying droplet due to the rapid rise in temperature of the substrate upon droplet impact. Through numerical modelling, we have shown that the temperature of the polyimide substrate underneath the deposited droplet exceeds the boiling point of water while the metal droplet is still in liquid state, showing the possibility of water vapor escaping from the substrate and causing pinholes in the solidifying metal.
Magnetohydrodynamic Jet Printing (MJP) is a novel additive manufacturing technique that offers tremendous promise for the fabrication of highly conductive electronic circuits with excellent adhesion on flexible substrates. MJP is an on-demand droplet jetting process in which the fluid is molten metal rather than a conductive nanoparticle ink. The work reported here examines the influence of drop spacing and jetting frequency on line morphology and electrical resistivity. Furthermore, the equivalent wire gauge of printed lines is estimated as a function of the drop volume and drop spacing. Under optimized jetting conditions, electrical resistivity as low as 3.2 ΜΩ cm (equivalent to bulk resistivity) has been achieved in aluminum 4043 alloy printed onto flexible polyimide. Little or no substrate cleaning is needed prior to printing, and post processing steps such as drying and curing are eliminated with this technique. The process uses metal wire as the feedstock material, making it significantly less expensive than conventional nanoparticle ink printing techniques.
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