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This study investigated the transient transport phenomenon during the pile up of molten lead-free solder via the inkjet printing method. With regard to the droplet impact velocity, the distance from nozzle to substrate can be controlled by using the pulse voltage and distance control apparatus. A high-speed digital camera was used to record the solder impact and examine the accuracy of the pile up. These impact conditions correspond to We = 2.1-15.1 and Oh = 5.4 × 10 −3 -3.8 × 10 −3 . The effects of impact velocity and relative distance between two types of molten droplets on the shape of the impact mode are examined. The results show that the optimal parameters of the distance from nozzle to substrate and the spreading factor in this experiment are 0.5 mm and 1.33. The diameter, volume and velocity of the inkjet solder droplet are around 37-65 μm, 25-144 picoliters, and 2.0-3.7 m s −1 , respectively. The vertical and inclined column structures of molten lead-free solder can be fabricated using piezoelectric ink-jet printing systems. The end-shapes of the 3D micro structure have been found to be dependent upon the distance from nozzle to substrate and the impact velocity of the molten lead-free solder droplet.

The novelty of this study is the laboratory formulation of silver ink adapted for the inkjet printing of conductive metallic features on flexible polyimide (PI) substrates with potential integrated circuit applications ranging from large-area electronics to low-end applications. A new silver precursor for printing conductive patterns with the empirical formula [Ag(dien)](tmhd), where tmhd ¼ 2,2,6,6tetramethyl-3,5-heptanedionato and dien ¼ diethylenetriamine, was synthesised using a simple and environmentally friendly method. The viscosity and surface tension of the organic solvent system were optimised through the addition of ethyl cellulose and hexylamine, yielding potential printing ink of high Ag wt%. Silver patterns on a flexible PI substrate were produced by thermal annealing of silver features prepared either by spin-coating or by directly drawing with a piezoelectric inkjet printer.Films were produced using a silver precursor (60 wt%) dissolved in hexylamine (39 wt%) and ethyl cellulose (1 wt%) with a viscosity of 9-11 mPa and annealed in air at 250 C. They displayed resistivity values in the range of 4.625-9.376 Â 10 À6 U cm. The composition of printing ink is [Ag(dien)](tmhd) : hexylamine : ethyl cellulose ¼ 45 : 54 : 1 by wt%. A resistivity of 7.44 Â 10 À6 U cm was found for a silver line with a width of 177 mm and a thickness of 106 nm. The silver patterns were characterised by scanning electron microscopy, FT-IR, X-ray photoelectron spectroscopy and X-ray diffraction. The resistivities of our silver patterns are lower than those previously prepared by other research groups using water-based silver salts as ink. We propose that the high Ag wt% achievable with an organic solvent-based system may explain this lower resistivity.

This study aims to determine the effects of appropriate experimental parameters on the thermophysical properties of molten micro droplets, Sn-3Ag-0.5Cu solder balls with an average droplet diameter of 50 μm were prepared. The inkjet printing parameters of the molten micro droplets, such as the dot spacing, stage velocity and sample temperature, were optimized in the 1D and 2D printing of metallic microstructures. The impact and mergence of molten micro droplets were observed with a high-speed digital camera. The line width of each sample was then calculated using a formula over a temperature range of 30 to 70 °C. The results showed that a metallic line with a width of 55 μm can be successfully printed with dot spacing (50 μm) and the stage velocity (50 mm∙s−1) at the substrate temperature of 30 °C. The experimental results revealed that the height (from 0.63 to 0.58) and solidification contact angle (from 72° to 56°) of the metallic micro droplets decreased as the temperature of the sample increased from 30 to 70 °C. High-speed digital camera (HSDC) observations showed that the quality of the 3D micro patterns improved significantly when the droplets were deposited at 70 °C.