Formation of uniform metal traces using alternate droplet printing

Formation of uniform traces is fundamental, but challenging, in droplet-based 3D printing. Due to the scalloped surface topography of metal droplets and complex thermal deformation between them, sequentially printed traces tend to form low-quality shapes. Here, an alternate droplet printing method is proposed to print uniform metal traces. Instead of printing droplets successively, discrete droplet array with uniform intervals was first printed. Then, the gaps between droplets were fully filled up in a later printing iteration. Herein, the proper combination of the step interval and the thermal state in the droplet printing process were first investigated to ensure the success of the proposed printing method. Four typical shapes of segments were identified using multiple threedroplet deposition experiments. Moreover, metal trace printing experiments further revealed five distinct trace topographies. In addition, the relationship between the topographies of printed traces and the printing parameters (i.e., temperature of the substrate and printing step size) was studied to establish a trace-shape map. Finally, experiments show that metal traces with the dimensionless roughness (Ra/Dd) of 1.8% were successfully printed. The results demonstrate a significant reduction of the roughness comparing to the literature results printed in consecutive sequence.