Bump morphology formation and transformation mechanism in solder droplet printing

Solder droplet printing is a direct and efficient approach for fabricating bump arrays in advanced packaging. However, emerging 3D packaging requirements for high bonding strength and low-temperature processing demand bump morphologies with large initial contact areas and precisely controlled micro-protrusions, challenging conventional surface tension-dominated solidification mechanisms. In this study, the impact and solidification of solder droplets on copper substrates were systematically investigated to reveal bump formation and transformation mechanisms. Three morphologies were examined: conventional spherical bumps and two novel non-spherical morphologies (flat-top and protruding-top). High-speed photography and numerical simulations were employed to capture droplet retraction behaviors and jet dynamics under varying thermodynamic conditions. A quantitative relationship was established between the solder bump morphology and the thermodynamic coupling ratio (λ =τ_d / τ_sol), where τ_d is the droplet dynamic time scale and τ_sol is the solidification time scale. An increase in λ was found to promote the formation of a central jet and satellite droplets, driving bump morphology transitions. Furthermore, it was shown that λ is governed by the Ste and Pe numbers, leading to the development of a Ste-Pe morphology map for predicting solder bump morphologies. By utilizing this map, uniform spherical, flat-top and protruding-top ball-grid arrays with high dimensional consistency (Δh/h <2%) were printed. This study advances the understanding of metal droplet solidification beyond the traditional surface-tension-dominated paradigm and serves as a predictive design tool, transforming the empirical, trial-and-error bump fabrication process into a deterministic method vital for advanced electronic packaging and additive manufacturing.