Interfacial reactions between a Pb-free solder and die backside metallizations

The interfacial reactions between Sn-3.0Ag-0.7Cu solder and backside metallizations on two semiconductor devices, field-effect transistors (FET) and diode, are studied. The metallizations on both devices were vacuum evaporated Ti/Ni/ Ag. The intermetallic compounds (IMC) formed near the diode/solder and FET/ solder joints during reflow, and the interdiffusion processes during solid state aging are characterized by the quantitative energy dispersive x-ray analysis and the x-ray mapping technique in a scanning electron microscope. Two different intermetallic compounds are found near the diode/solder interface. Both are in the form of particles, not a continuous layer, and are referred to as IMC-I and IMC-II. IMC-I corresponds to Ni3Sn4, with Cu atoms residing on the Ni sublattice. It is uncertain whether IMC-II is Cu6Sn5 or a Cu-Ni-Sn ternary phase. Near the as-reflowed FET/solder interface, both isolated scallops and a skeleton-like layer of Ni3Sn4 are observed. The primary microstructural dynamics during solid-state aging are the coarsening of IMCs and the reactions involving the Ni- and Ti-layer with Sn and Au. While the reaction with the Ni-layer yields only Ni3Sn4 intermetallic, the reaction involving the Ti-layer suggests the formation of Ti-Sn and Au-Sn-Ti intermetallics. The latter is due to the diffusion of Au from the substrate side to the die side. It is postulated that the kinetics of the Au-Sn-Ti layer is primarily governed by the diffusion of Au through the Ni3Sn4 layer by a grain boundary mechanism.