In order to avoid the toxicity of lead and satisfy consumers desiring "green" electronic products, considerable R&D efforts have been focused on the composition modification and optimization of some lead-free solder candidates since the early 1990s. For successful commercialization of several promising lead-free solders, the production of fine solder balls with tighter dimensional tolerance at lower cost is greatly desired. However, it is very difficult or uneconomical to make high quality powders industrially for lead-free solders by conventional atomization methods. To produce acceptable lead-free solder balls efficiently and industrially, a novel powder-making process, Hybrid Atomization that combines free fall gas atomization and centrifugal atomization effectively, was invented and developed recently by us. This new technique can produce very fine, spherical tin alloy powders with mean diameters of about ten micrometers, very narrow size distributions, few satellites and low production costs. Taking Sn-9mass%Zn alloy as an example, process experiments were carried out and the optimal processing conditions obtained. Results show that the influences of processing parameters and optimum conditions are very different from those in conventional atomization processes. Compared with conventional gas atomization, hybrid atomization results in more uniform particle microstructures. A new powder-making concept and a special atomization mechanism for this process were presented in this study. The cooling and solidification of droplets, and the formation of uniform particle microstructure in hybrid atomization were also discussed.
