A study has been conducted on the deformation mechanisms in metal substrates subject to aluminum ultrasonic wire bonding (UWB). Aluminum wires were bonded to copper, nickel, stainless steel, and aluminum bronze foil substrates and then removed in aqueous sodium hydroxide to permit thin sections of bonded areas to be examined in the transmission electron microscope (TEM). The results showed a variety of dislocation substructures formed during bonding, including dislocation cells, subgrains, and planar arrays. Aluminum and copper showed evidence of thermal effects on microstructural evolution during bonding, such as dislocation annihilation at cell walls in copper and complete recrystallization in aluminum. In the nickel and stainless steel substrates, which have higher recrystallization temperatures, thermal effects on microstructure were not observed. In addition, it was found that low stacking-fault energy (SFE) materials, such as aluminum bronze, were less likely to undergo cell formation, and only planar dislocation arrays formed. In general, it is clear that the process of UWB induces cyclic stresses in the substrates, which exceed the yield strength of the metals examined. In addition, there is some heat generated during the bonding process, which can influence the resultant deformation microstructure.

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 the Au-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.

Gold-aluminum ball bonds were thermally exposed at constant elevated temperatures, and the resultant phase transformations studied in detail. The asbonded microstructure of a Au-Al ball bond essentially consisted of a reaction zone (termed "alloyed zone" (AZ) in the as-bonded condition) between the Au bump and the bonded Al metallization. It is the growth of the reaction zone between the Au bump and the bonded Al metallization and also the nonbonded Al metallization during thermal exposure that gave rise to the various phase transformationsare the predominant phases that grew across the ball bond until the bonded Al metallization is available to take part in the interdiffusion reactions. After the complete consumption of the bonded Al metallization, the Au-Al phases reverse transformed resulting in the formation of the Au 4 Al phase in the entire reaction zone across the ball bond (RZ-A). The lateral interdiffusion reactions resulted in the nucleation and the growth of all of the Au-Al phases given by the phase diagram. Kidson's analysis and Tu et al.'s treatment were extended to a five-phase binary system to explain the phase transformations in thermally exposed Au-Al ball bonds. It is possible for all of the Au-Al phases to grow across a ball bond uninhibited as long as the bonded metallization is available. However, the supply limitation of the bonded metallization gives rise to reverse transformations where Al-rich phases transform to Au-rich phases and eventually result in the formation of the Au 4 Al phase in the entire RZ-A. If infinite time is allowed, Au 4 Al would dissolve; the extent of which is dependent on the solubility of Al in Au. No supply of Au lateral to the bond causes the reverse transformation of the Au 4 Al phase, giving rise to the lateral growth of the remaining Au-Al phases. If infinite time is allowed, the lateral phase transformations would result in the formation of a phase that is dependant on the relative proportion of Au and Al present in the nonbonded metallization (NBM) and Au 4 Al below the void line. Hence, the presence of a phase in a particular location of a ball bond is dependent on the time and temperature of thermal exposure.

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Thermal exposure of Au-Al ball bonds results in the transformation of Au-Al phases across and lateral to the ball bond that affects their service life. The phase transformations across and lateral to the AuAl ball bond were described by partial reactions at each of the interphase boundaries. Based on a model for the mass distribution of Au from the Au phase or the Au-rich phases towards the Al phase or the Al-rich phases, volume changes associated with the partial reactions were calculated. Except for the Au 4 Al and AuAl 2 phases, theoretical analysis showed that the growth of all the Au-Al phases across the ball bond results in volumetric shrinkage. The growth of the Au-Al phases across the bond is restricted due to the limited bonded Al metallization available to take part in the interdiffusion reactions. The supply limitation of the bonded metallization gives rise to reverse transformations that result in the formation of the Au 4 Al phase across the entire reaction zone across the ball bond. Except for the AuAl 2 phase, theoretical analysis showed that the reverse transformation of all the Au-Al phases across the ball bond is associated with minimal volume change. A "Criterion for Inward Lateral Growth" that governs the growth of Au-Al phases laterally into the ball bond was developed. Theoretical analysis showed that except for AuAl 2 , the lateral growth of all the Au-Al phases results in volumetric shrinkage. The lateral phase transformations are essentially reverse transformations due to no-supply of Au. Theoretical analysis showed that all the lateral reverse transformations also result in volumetric shrinkage.