Oxidation and Au-Sn reaction of laser reflowed micro-solder joints protected by N2 or exposed to air atmosphere

Purpose-The purpose of this paper is to investigate oxidation and the Au-Sn reaction of laser reflowed (LR) micro-solder joints when different protective atmospheres were applied. Design/methodology/approach-A N2 atmosphere at room temperature, 608C, 1008C and 1308C, or an air atmosphere at room temperature were utilized in this study. The solder balls were composed of Sn-2.0Ag-0.75Cu-3.0Bi, and 120mm in diameter. The surface finish of one pad of the joints was 4.0mm Au/0.1mm NiFe/0.01mm Ta, another pad was made of Cu plated with 3.0mm Au. The laser reflow process time was controlled to within 10 ms. Auger Electron Spectroscopy (AES) was used to identify the oxidation condition of LR solder joints with or without protection from a N2 atmosphere at room temperature. The appearance and cross-sections of the joints protected by a N2 atmosphere at different temperatures were evaluated using SEM analysis. Findings-Oxidation of LR solder joints from an air atmosphere was extremely severe, and the surfaces of solder were rough as compared with joints protected by a N2 atmosphere. Au-rich phases and needle-like AuSn4 intermetallic compounds (IMCs) formed at the interfaces of the solder and the pads. As the temperature of the N2 atmosphere was increased above 1008C, almost all of the Au-rich phases disappeared. More needle-like AuSn4 IMCs formed at the interfaces, as compared with that in joints protected by a N2 atmosphere at room temperature and 608C. In addition, the orientation of the IMCs had clearly changed. Originality/value-The results may provide a guide for controlling oxidation and the Au-Sn reaction of micro-solder joints during the LR process, and improving the properties of joints between solder and pads with Au surface finishes, by regulating the protective atmosphere.