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. This work 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 compression caused cure-induced shrinkage of the adhesive and degradation is by stress relaxation of the adhesive material during temperature and moisture cycling throughout the life cycle. However, temperature cycling conducted here raises questions about the validity of this hypothesis since no such progressive degradation is found under temperature cycling. Instead, overstress failures were found at cold temperatures. The alternate interconnection mechanism between the Au bumps, suggested by this study, is metallurgical bonding by cold welding or mechanical interlocking. Experimental and modeling results showing evidence of this cold welding phenomenon are presented here. First, the surface roughness of unmated and mated Au bumps are characterized on flip-chip dies, since the amount of surface flattening provides insights into both mechanical interlocking as well as propensity for cold-welding. A corresponding elastic-plastic, large-deformation finite element modeling with nonlinear contact surfaces is used to further understand and quantify this surface-flattening phenomenon. Next, pull tests for flip-chip to flip-chip assemblies were designed to measure the bond strength and observe the temperature & time dependence of the strength at the interface. The results clearly showed strong evidence that the bonding mechanism may be either diffusionassisted metallurgical bonding or increase in contact area over time due to new bond formation. This work is somewhat novel, as prior examples of low-temperature cold welding are mostly for very thin gold films.

This content is only available via PDF.
You do not currently have access to this content.