Flight trajectory analysis of CuSn-droplets generated by laser drop on demand jetting, using stereoscopic high-speed imaging.

Laser drop on demand jetting of Cu-base braze droplets was proven a suitable method for joining wires to electrode structures of electronic devices, particularly if the electrical contacts need to withstand high thermal loads. During joining, a braze preform of 600 µm diameter is placed inside a capillary, molten by a laser pulse and subsequently ejected from the capillary by inert gas overpressure similarly to conventional solder ball bumping processes. However, since the liquidus temperature of the used braze material of 990 °C is about 760 °C higher than of standard Sn-based solders used in electronics packaging, the system technology was modified significantly to enable jetting of CuSn alloys. In particular, the beam source emits a five times higher optical output power than standard machines designed for processing Sn-based solders. In addition, a modified capillary made from technical ceramic was machined, to withstand the significantly higher heating- and cooling rates during the process. In order to understand the influence of capillary geometry on droplet detachment, and flight trajectory, two capillary geometries were machined applying a picosecond laser ablation process. Subsequently, stereoscopic high speed videos of droplet detachment and flight phase were analyzed. Using this approach it is possible, to determine droplet flight trajectories, velocities and lateral positional deviations in dependency of relative inert gas overpressure inside the machining head, pulse power and capillary geometry. The findings indicate a significant influence of the capillary geometry and the applied overpressure on the droplet flight trajectory, whereas the role of the laser pulse power seems neglectable.