Investigating the Effects of Rapid Precipitation of Bi in Sn on the Shear Strength of BGA Sn-Bi Alloys

The potential of Sn-Bi alloys as low-temperature solders for electronics manufacturing has spurred significant research on their mechanical properties, both in the as-soldered condition and after aging. Previous studies have demonstrated that, because of the extreme temperature sensitivity of the solubility of Bi in Sn, the mechanical properties of Sn-Bi solder alloys are very sensitive to their thermal history. While the properties of the bulk solder alloy are a factor in its performance as a solder joint, the reliability in service is also affected by the joint geometry and the interaction of the solder alloy with the joint substrate. In the work reported in this paper the effect of thermal history on solder joints formed with representative Sn-Bi alloy solder balls was assessed by measuring the performance in a standard ball shear test of a solder ball reflowed to solder mask defined (SMD) copper pads with organic solderability preservative (OSP) or electroless nickel/immersion gold (ENIG) finishes. The solder ball/substrate combinations were tested within 10 min of reflow and after room-temperature storage for up to 10 days to determine the effect of aging on their response to the ball shear test. Our results show that the peak force and fracture mode of Sn-Bi solder joints is influenced by the Sn-Bi alloy composition, the substrate type, and the aging time. These observations provide new information on the capability of these alloys to deliver reliable service over a range of operating conditions.