Sn–Ag–Cu Soldering Reliability as Influenced by Process Atmosphere

To develop an optimal surface mount reflow soldering process with Sn-Ag-Cu, the influences of atmosphere and cooling speed on soldering reliability have been examined by using Sn plated chip components and of Pd plated small outline packages (SOPs) on a printed circuit board (PCB). Typical three Sn-Ag-Cu alloy pastes, i.e., Sn-3.0wt%Ag-0.5wt%Cu, Sn-3.8wt%Ag-0.75wt%Cu, and Sn-4.0wt%Ag-0.9wt%Cu, were used for reflow soldering in air or N₂ atmospheres. In the case of chip component joints, the solder compositions, cooling speed, and atmospheres during reflow treatment slightly affect the dendritic microstructure of the solder fillets. In contrast, these parameters rarely affect the solder wettability both on boards/components and shear strengths of the solder joints. In the case of the SOP joints, however, the atmospheres in reflow treatment and the fluxes strongly affect the appearances of solder fillet surfaces structure. Despite the types of solder fluxes, N₂ process atmosphere obviously improved wettability of the solders on the lead-frames of the SOP. Moreover, the scatter in shear strengths becomes smaller and the wetting of solders on the lead-frames becomes stabler in N₂ atmosphere than in air atmosphere.