Creep deformation of Sn-3.5Ag-xCu and Sn-3.5Ag-xBi solder joints

Abstract

Creep properties of lead-free Sn-3.5Ag-based alloys with varying amounts of Cu or Bi were studied by single lap-shear test. Solder balls with five different compositions of Cu (0 wt.%, 0.75 wt.%, 1.5 wt.%) and Bi (2.5 wt.%, 7.5 wt.%) were reflowed on Cu. The Cu-containing alloy had a lower creep rate than the Bi-containing alloy. The Sn-3.5Ag alloy showed the lowest creep rate on Cu, implying that the Cu element already dissolved in the Sn-3.5Ag alloy during reflow. The Cu-containing alloy was strengthened by dispersed small precipitates of Cu6Sn5. As the Cu content increased up to 1.5 wt.%, the Cu6Sn5 coarsened and platelike Ag3Sn intermetallics were found, which deteriorated the creep resistance.