Pure Chemical Reduction of Tin Oxides to Metallic Tin by Atmospheric Plasma to Improve Interconnection Reflow of Pb-free Solders

Abstract

Tin alloys are widely used as solder for electronic interconnections. Tin solder surfaces tend to have tin oxides which need to be removed to improve the yield of interconnection reflow processes such as flip chip joining. Conventionally, a strong flux is employed to remove these oxides, however this process has the drawbacks of leaving flux residue which can cause underfill delamination or require a high-cost cleaning process. As solder bump volumes and bump-to-bump spacing decrease, these problems become more difficult to manage in manufacturing. We propose the use of Atmospheric Plasma to reduce these oxides from the bump surfaces to enable the use of very light fluxes, or no flux at all. This process has the advantages of plasma surface preparation without the cost and throughput penalty of vacuum plasma processes. Such a process can increase throughput and yield while reducing the cost. We describe an experiment in which tin foils were treated with a reducing-chemistry Atmospheric Plasma process and then analyzed with X-ray photoelectron spectroscopy (XPS) and Auger Electron Spectroscopy (AES). AES depth profile analyses indicate that the thickness of tin oxides was significantly reduced by the plasma. There was no evidence for any etching of underlying elemental tin. These results suggest that tin oxides are reduced to metallic tin without etching of the underlying tin metal. In another similar experiment using semiconductor chips with SnAg solders, XPS results suggest that the tin oxides were again reduced to metallic tin. In flip chip joining, the joining process with such Atmospheric Plasma-treated chips achieved high interconnect yield, even in the case of poor quality solder balls with excessive oxidation. It is our understanding that the pure chemical reduction of tin oxides with atmospheric plasma in ambient had not been previously reported.