Characterization and Performance of Ultrafine Lead-Free powders

Abstract

With the miniaturization of components in advanced packaging, interconnect requires fine solder joints to be formed. To be able to form small solder joints, solder paste with ultrafine solder particles is required. For ultrafine powder there are important powder characteristics such as particle size distributions (PSD), surface oxide and aspect ratio which need to be considered.A proprietary process technology is used to produce ultrafine SAC305 (Sn-3Ag-0.5Cu) powder with particle size ranging from $2 \mu \mathrm{m}$ to $25 \mu \mathrm{m}$ with tight particle size distributions and high sphericity. Focus will be on new ultrafine powders $(28 \mu \mathrm{m})$ and characterization will be presented for PSD, surface oxide content and aspect ratio. Surface oxide content will be characterized using inert gas fusion infrared technology while particle size distribution is characterized using laser diffraction method.The ultrafine powder was made into solder paste (water soluble) for feasibility studies. Impact of PSD and surface oxide on paste characteristics like solder balling, solder bridging and cold slump will be discussed. Finally, results for printability on ultrafine pitch and solder volume after reflow will be discussed.