Printing conductive traces of sinterable metallic nanoparticle (NP) based inks is one of the key components of 3D additively manufactured (printed) electronic circuits. In this study, we employ 3D aerosol jet printing to print conductive traces of silver NP based ink and provide insights into the risk of cohesive longitudinal cracking inside the printed and sintered traces. This was investigated for a wide range of carrier and sheath gas flow rates, which implies a wide range of particle deposition rates and velocities. These cracks appear for both single and double pass traces that are several microns in height and hundreds of microns in width. We examine different sources of stress that could be the potential driver for the cracking and infer that the most likely source is the classical capillary-pressure-driven crack formation mechanism for the evaporating NP-based thin films. Most importantly, this hypothesis raises the possibility of crack formation at several locations of the NP-based conductive traces printed using an extremely popular recent technique for printed electronics, AJP.

This paper focuses on the influence of carrier gas flow rate (CGFR) and sheath gas flow rate (SGFR) on the quality of conductive traces printed with nanoparticle inks using aerosol jet printing (AJP). This investigation was motivated by previous results of two AJP specimens that were printed at different gas flow rates and yielded significantly different thermal cycling durability lifetimes. A parametric sensitivity study was executed by printing and examining serpentine trace structures at 15 different combinations of CGFRs and SGFRs. The analysis included quantifying the trace's macroscale geometry, electrical properties, and micromorphological features. Interesting macroscale results include an increase in effective conductivity with increasing CGFR. At the microscale, image processing of high magnification scanning electron microscope (SEM) images of the printed traces revealed that agglomerations of silver clusters on the surface of traces became coarser at higher CGFR and also that agglomerates in the bulk were finer than those on the surface. Crystalline silver deposits were observed at all flow rates. In addition, cross sectioning of the printed traces showed higher incidences of buried cohesive cracking at higher gas flow rates. These cohesive cracks reduce the robustness of the traces but may not always be visible from the surface. The degree of cohesive cracking was seen to be broadly correlated with the coarseness of the surface agglomerates, thus suggesting that the coarseness of surface agglomerates may provide a visible surrogate measure of the print quality. The results of this study suggest that print quality may degrade as gas flow rates increase.

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