Fracture Behaviors of Simulated Sn-Ag-Cu Solder Microbumps: Effects of Joint Scale and Aging

Solder microbumps are widely applied in 3D packages using fine-pitch Cu-pillar or Through Silicon Via (TSV) technologies. Due to the small scale of these joints, the volumetric proportion of intermetallic compounds (IMCs) formed in these joints is typically very high. This renders microbump-joints much more brittle compared to traditional solder joints (flip-chip or BGA). In particular, the reliability of microbumps during a drop, which corresponds to a mixed-mode high strain rate fracture test, is of substantial concern because of the brittleness of these joints. This study reports on the fracture mechanics and mechanisms of simulated microbumps, which have similar thicknesses and IMC contents as actual microbumps, but are laterally scaled up to constitute valid fracture mechanics samples. Compact mixed mode (CMM) specimens with adhesive solder joints (Sn-3.0%Ag-0.5%Cu) between massive Cu substrates were utilized to measure the fracture properties. The fracture behavior was characterized as a function of joint thickness and proportion of IMC, the latter being controlled by adjusting the dwell time and aging time. It was found that the fracture toughness G_C decreased monotonically with joint thickness (h_Joint) due to increased triaxial constraint imposed by the substrates. With aging, the proportion of IMC thickness relative to the joint thickness (2h_IMC/h_Joint) increased, as did h_Joint. This resulted in lower G_C values. The associated mechanisms of fracture that led to these effects are discussed.