Production of tin and tin-based alloy spheres via multi-orifice uniform droplet spraying

Abstract

Small tin spheres, approximately 100 μm in diameter or less, can be used in high-performance electronics packaging applications, and therefore need to be produced to high levels of uniformity. The lead-free, tin-based alloy SN100C is of particular interest as it can be used as a solder in such applications. The goal of this research was to determine the feasibility of the commercial production of tin and tin alloy spheres for industrial applications by the uniform droplet spray (UDS) process with multi-orifice designs. The alloy was ejected from a pressurized crucible through 25-μm orifices drilled in stainless steel, as well as 30- to 100-μm orifices made of sapphire and ruby, into a chamber containing inert gas and allowed to solidify into spherical droplets. The resulting droplets were examined for uniformity and microstructural characteristics. The spray process allowed several parameters to be varied, including spray temperature, pressure, and oxygen content of the spraying atmosphere. Through experimentation, the optimum spray conditions for SN100C were found to include a melt temperature of 350 °C, a spray pressure of 414 kPa, and a spray chamber O2 content of ≤5 ppm. Multiple-orifice stable breakup of alloy streams into discrete, spherical droplets was demonstrated, thus proving that even very small orifices are capable of producing metal balls via the UDS process. It was determined that the optimum spray conditions favor the use of this spray process in industrial applications. Further work will be to continue the process on a larger scale.