The effect of the echo-time of a bipolar pulse waveform on molten metallic droplet formation by squeeze mode piezoelectric inkjet printing

•The ink-jet printing process from pressure generation to droplet formation is simulated in this study.•The value of techo in a bipolar waveform determines the combination of the pressures induced at tfall and tfinalrise.•The pressure shock reduction provides a stable breaking process of the liquid thread.•Droplet shapes can be determined by adjusting the value of techo.

The effect of the echo-time of a bipolar pulse waveform on pressure propagation in a tube is investigated. Micro droplets of molten lead-free solder, Sn–3.0 wt%Ag–0.5 wt%Cu, are ejected at 230 °C using a piezoelectric ink-jet printing process. The numerical model is used to calculate for the detailed information concerning the formation of droplet after the reliability of the numerical model has been verified by finding the simulated modes of droplet formation consistent with the experimental observations. The echo-time of a bipolar waveform plays an important role in determining when the pressure variation induced at the finalrise-time influences the pressure variation induced at the fall-time. A single droplet can be obtained by applying a negative pressure variation to reduce the pressure shock in the negative pressure period which influences the contraction and break-up of the liquid thread in the droplet formation.