Cold welding: A new factor governing the robustness of adhesively bonded flip-chip interconnects

In the conversion to Pb-free electronics, there has been increasing interest in conductive adhesive interconnects, as they combine Pb-free materials with the added benefit of low temperature, processing (Lau et al., 2002). This paper explores the degradation mechanisms and kinetics in adhesively bonded Au-bumped flip chip interconnects. Earlier researchers have suggested that electrical contact is by mechanical interfacial pressure and that stress relaxation of the adhesive material is a major degradation mechanism, since temperature cycling and moisture cycling throughout the life cycle can progressively lower the contact pressure between the contacting parts, resulting in progressive cyclic increase in contact resistance. However, temperature cycling conducted by this group raised questions about the validity of this hypothesis since no such progressive degradation was found under temperature cycling, but overstress failures were found instead at cold temperatures (Haase et al., 2008). The alternate bonding mechanism suggested by this study was metallurgical bonding between the Au bumps by cold-welding In the present paper, experimental results and modeling results showing evidence of this cold welding phenomenon are presented. These results are somewhat novel, as prior examples of low-temperature cold welding are mostly for very thin gold films (Ferguson et al., 1991). The surface roughness of unmated and mated Au bumps is characterized on flip-chip dies, since the amount of surface flattening provides insights into both mechanical interlocking as well as propensity for cold-welding (Taylor et al., 1991). Using techniques developed in related studies (Farley et al., 2008), elastic-plastic, large- deformation finite element modeling with nonlinear contact surfaces is used to further understand and quantify this surface-flattening phenomenon. The consequences that these bonding mechanisms have on the robustness of the adhesive interconnect and how this affects the failure modeling approach are discussed.