The interfacial reactions between Sn-3.0Ag-0.7Cu solder and backside metall
izations on two semiconductor devices, field-effect transistors (FET) and d
iode, are studied. The metallizations on both devices were vacuum evaporate
d Ti/Ni/ Ag. The intermetallic compounds (IMC) formed near the diode/solder
and FET/ solder joints during reflow, and the interdiffusion processes dur
ing solid state aging are characterized by the quantitative energy dispersi
ve x-ray analysis and the x-ray mapping technique in a scanning electron mi
croscope. Two different intermetallic compounds are found near the diode/so
lder 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 C
u6Sn5 or a Cu-Ni-Sn ternary phase. Near the as-reflowed FET/solder interfac
e, both isolated scallops and a skeleton-like layer of Ni3Sn4 are observed.
The primary microstructural dynamics during solid-state aging are the coar
sening 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 su
bstrate 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.