Adding Bi and/or Sb to Sn-Ag-Cu solders is of interest for producing higher reliability solder joints. However, a microstructure-level understanding of these solders is lacking. This thesis investigates the effects of Bi and/or Sb additions on the microstructures and thermal cycling performance of Sn-Ag-Cu solder joints in two ball grid array (BGA) packages.
After -55/125°C thermal cycling, the crack paths and β-Sn recrystallization of Sn-Ag-Cu-Bi joints are correlated with their performance. The result suggests that, while increasing Bi content enhances solder strength and can improve thermal fatigue resistance, a balance between solder strength and plastic deformation is necessary to prevent brittle fracture along the substrate/intermetallic layer interface under high strain.
Time-lapsing imaging provides insights into the precipitation and coarsening of (Bi) phase on the surface of Sn-2.25Ag-0.5Cu-6Bi joints. The accumulation of (Bi) on the surface significantly exceeds the equilibrium volume fraction for this alloy. Tracking individual (Bi) particles reveals that coalescence causes jumps in (Bi) particle size and the formation of anomalously large particles, highlighting the role of coalescence ripening within the coarsening mechanism of (Bi) on the surface.
The crystallographic aspects of (Bi) precipitates are explored using electron backscatter diffraction (EBSD). The variants of the ORs between (Bi) precipitates and the β-Sn matrix were analysed, revealing the interplay between interfacial coherency, morphology, and stability of (Bi) precipitates. Re-examining (Bi) surface precipitation from a crystallographic perspective shows that this phenomenon may be driven by the volumetric misfit between (Bi) and β-Sn and influenced by the anisotropic diffusivity of Bi in β-Sn.
Combined Bi and Sb additions enhance the thermal cycling performance of Sn-Ag-Cu solders; however, premature failure occurs in certain solder/package combinations with Sn-3.4Ag-0.7Cu-3.2Bi-3Sb and Sn-3.8Ag-1.2Cu-1.5Bi-3.5Sb. In these solders, SbSn particles coarsen substantially during room-temperature storage and thermal cycling, potentially degrading the mechanical properties of solder joints.