Performance of Fine and Ultra-Fine Lead-Free Powders for Solder Paste Applications

The continuing demand for smaller and lighter electronic products has driven the use of miniature components. The assembly of these miniature components requires fine solder joints; and, finer solder joints require advanced solder pastes with finer particle sizes. In general, there are four key characteristics for any powder: purity, particle size distribution, surface oxide, and morphology. However, a complete understanding of the effect of key solder powder characteristics on the paste properties is still not achieved. This understanding becomes much more important when new solder pastes with finer powder size (Type 5, 6, 7, and 8) are developed for advanced applications in semiconductor packaging. In this work, SAC305 (Sn-3Ag-0.5Cu) powders is produced with a proprietary atomizing technology, particularly effective in producing solder powder ranging from 1 to 25 μm. Powder characteristics considered are particle size distribution and powder oxidation. The surface oxide layer is characterized using Auger Electron Spectroscopy and Transmission Electron Microscopy and characterization of the surface oxide layer is presented for powders with various particle size distributions. The relation between surface oxide thickness and reflow performance is described. Finally, a powder treatment will be shown to be required to improve the robustness of solder paste in certain conditions. The influence of the powder treatment on the reflow performance of solder paste will be discussed. The results and knowledge obtained by the systematic study presented in this paper can be applied to design new and advanced solder pastes with fine powders.