Super Fine Lead-Free Solder Powder for Fine Pitch Bump Applications

Abstract

We have successfully developed super fine lead-free and low alpha solder powder, which contains more than two elements by the method of wet chemical reduction. The size (D50) of super fine powder is around 2–3 micrometer to meet finer pitch assembly in the near future. This new method made it available to synthesize various compositions of solder powder like Sn-Ag, Sn-Cu, Sn-Ag-Cu, etc. Also, this method achieves very high yield compared to a gas atomization method. A solder paste for printing method composed of the fine solder powder has a superior printing ability because of the unique powder shape. The powder shows anisotropic shape, and it can make printed figure excellent after printing without bridge and coplanarity issues for finer pitch applications. With our super fine solder paste, we will be ready for <100um pitch of solder bumps which will come in a few years. Furthermore, the super fine powder is applied to the Cu pad pre-coat. The solder paste for pre-coat composed of the super fine powder shows an excellent coverage and solders flatness on the outer pad after reflow.