W. A. Baeslack

Degradation and failure mechanisms in thermally exposed Au-Al ball bonds

During the manufacturing and the service life of Au-Al wire bonded electronic packages, the ball bonds experience elevated temperatures and hence accelerated interdiffusion reactions that promote the transformation of theAu-Al phases and the growth of creep cavities. In the current study, these service conditions were simulated by thermally exposing Au-Al ball bonds at 175 and 250 °C for up to 1000 h. The Au-Al phase transformations and the growth of cavities were characterized by scanning electron microscopy. The volume changes associated with the transformation of the intermetallic phases were theoretically calculated, and the effect of the phase transformations on the growth of cavities was studied. The as-bonded microstructure of a Au-Al ball bond typically consisted of an alloyed zone and a line of discontinuous voids (void line) between the Au bump and the bonded Al metallization. Thermal exposure resulted in the nucleation, growth, and the transformation of the Au-Al phases and the growth of cavities along the void line. Theoretical analysis showed that the phase transformations across and lateral to the ball bond result in significant volumetric shrinkage. The volumetric shrinkage results in tensile stresses and promotes the growth of creep cavities at the void line. Cavity growth is higher at the crack front due to stress concentration, which was initially at the edge of the void line. The crack propagation occurs laterally by the coalescence of sufficiently grown cavities at the void line resulting in the failure of the Au-Al ball bonds.

Resistance Diffusion Bonding of a Titanium Aluminide

Diffusion bonds and fusion welds have been produced between sheets of Ti-6wt%AI-4wt%V and Ti-14.8wt%Al-21.3wt%Nb titanium aluminide using the capacitor-discharge resistance spot welding process. Diffusion bonds in Ti-6wt%Al-4wt%V were characterized by beta grain growth across the bond line and the presence of occasional interface defects. Despite fracture through the bond region, optimum tensile-shear strengths for the diffusion bonds were comparable to those of fusion spot welds which failed by “nugget pullout”. High quality, defect-free diffusion bonds were also produced in a Ti-14.8wt%Al-21.3wt%Nb alpha-two titanium aluminide. The bond region was characterized by the growth of equiaxed beta grains across the bond line and an absence of interface defects. Fusion welds in the titanium aluminide exhibited a coarse, columnar beta grain structure. High cooling rates experienced in both the diffusion bonds and fusion welds in the titanium aluminide promoted the retention of beta phase down to room temperature throughout a large proportion of the bond and weld zones. Tensile-shear fracture of the titanium-aluminide welds initiated near the weld outer periphery and propagated principally through the unaffected base metal. The average fracture strength of the diffusion bonds produced at high axial force was markedly superior to that of bonds produced at low axial force and fusion welds. This difference was attributed to a more severe notch geometry at the outer periphery of the latter weld types.