The effect of Pb contamination on the solidification behaviors and mechanical properties of backward compatible solder joints

The Pb contamination on the solidification behaviors and shear properties of Backward Compatible solder joints were investigated. The solidification behavior of solder and joint examined by using a DSC instrument, and it was found that the addition of Pb in Sn3.0Ag0.5Cu would change the primary phase during solidification and change the microstructure of backward compatible solder and joint, meantime, it is easy form a low melting temperature Sn-Ag-Pb ternary eutectic alloy, and the pasty rang between two endothermic(or exothermic) peak decreases with the increasing of Pb content in solder matrix. The discrepancy in solidification behavior results in different microstructures significantly. The average sizes of β-Sn and IMC particles(Ag₃Sn and Cu₆Sn₅) in solder matrix are larger than those in Sn3.0Ag0.5Cu solder relatively. However, the shear force of backward compatible mixed joints is higher than SAC/Cu joint independent of Pb content when Pb content in solder matrix is less than 30 wt.% by using ball shear test, it seems that the low melting temperature Sn-Ag-Pb has little influence on the shear properties of mixed joints with a single interface in as-solidified condition. The addition of Pb in Sn3.0Ag0.5Cu solder apparently facilitated the slip of β-Sn and stress release under shear stress to some extent.