Precipitation and coarsening of bismuth plates in Sn-Ag-Cu-Bi and Sn-Cu-Ni-Bi solder joints

In recent years there has been increased interest in Bi-containing solders to improve the reliability of electronics and drive down processing temperatures. When the Bi content is more than similar to 2wt% in Sn-Ag-Cu-Bi and Sn-Cu-Ni-Bi solders, (Bi) phase forms by a non-equilibrium eutectic reaction and also precipitates in the solid state. Here, we study the development of bismuth plates during room temperature storage in a range of Bi-containing solders. It is shown that Bi plates precipitate with one of two (Bi)-Sn orientation relationships (ORs). During coarsening the OR with the better lattice match grows at the expense of the other, leading to a single OR. The plate coarsening kinetics scale with the cube root of holding time and are relatively rapid, with plates exceeding 1 mu m in length within 1 day at room temperature. Prolonged room temperature storage of more than 1year resulted in Bi accumulation in 5-20 mu m (Bi) particles at Sn grain boundaries.