Visualization and Measurements of Picoliter-Size Molten Droplet Impact Dynamics and Solidification on a Surface

Abstract

The basic problem of the impact and solidification of molten solder droplets on a flat substrate is of central importance to the novel micromanufacturing process of solder jetting, in which microscopic size solder droplets are dispensed for the attachment of microelectronic components. Under certain conditions, “frozen ripples” appear on the surface of solidified solder microbumps deposited using the solder jetting technology (Waldvogel et al. 1996). The mechanism for the formation of these “frozen ripples” was later explained and quantified in a theoretical study by Waldvogel and Poulikakos (1997) as a consequence of the dynamic competition between flow oscillations and rapid solidification. However, no analogous experimental results for the transient impact process have been reported to date to the best of our knowledge. Such a study is reported in this paper. Eutectic solder (63Sn37Pb) was melted to a preset superheat and used in a specially designed droplet generator to produce droplets with diameters in the range 50–100 μm. The size, temperature, and impacting speed of the molten droplets were maintained constant. The primary variable is the temperature of the substrate that was controlled in the range from 48 °C to 135 °C. The dynamics of molten solder microdroplet impact and solidification on the substrate was investigated using a flash microscopy technique. The duration of flash used in the study was 1 μs. The time for the completion of solidification from the moment of a solder droplet impact on the substrate varies between 150 μs and 350 μs. The dynamic interaction between the oscillation in the liquid region and the rapid advance of solidification front was visualized, quantified and presented in this paper. To the best of our knowledge, this study presents the first published experimental results on the transient fluid dynamics and solidification of molten microdroplets impacting on a substrate at the above mentioned time and length scales that are directly relevant to the novel solder jetting technology. Existing results on this problem pertain to time and length scales at least one order of magnitude higher (Jonas et al. 1997; Pasandideh-Fard et al. 1998; Zhao et al. 1996). The visualization results on the oscillatory motion and rapid solidification shed light on a host of interesting phenomena and also support the frozen ripple formation theory presented by Waldvogel and Poulikakos (1997).