Growth behavior of preferentially scalloped intermetallic compounds at extremely thin peripheral Sn/Cu interface

The growth behaviour of Cu₆Sn₅ intermetallic compound (IMC) at the solder height controlled post-spread Sn/Cu interface is investigated for different initial solder ball volumes, reflow temperatures and cooling rates. Because of the limited solder thickness at the periphery, the IMC retains a preferential scalloped morphology, even after cooling. For solder balls of initial diameters of 500, 1000 and 1700 μm, with the maximum solder height at the peripheral regime not exceeding 150 μm, reflowed at 250 °C and undergoing air cooling, it has been revealed that IMC characterized with larger layer thickness and grain diameter, correspond to the sample of smaller ball size. The increase in reflow temperature for a solder of initial size of 500 μm, is characterized by the increase in IMC thickness, developments of few but quite large faceted planes over the original scalloped morphology and non-uniformity in the grain diameter. In contrary the to air cooling (cooling rate ≥ 4.0 K/s), the IMCs obtained at the thin film zone, for furnace cooling (cooling rate = 0.037 K/s), are very larger, both in grain size and layer thickness. Moreover, the scalloped Cu₆Sn₅ surface in furnace cooled specimens, bear many but tiny facets. Finite element analysis is utilized to numerically estimate the diffusion of Cu into the geometrical volumes of solder. Ostwald ripening and film height are the important parameters defining the growth behavior of the compound in miniaturized solder joints.