A study on intermetallic compound formation in Ag–Al system and evaluation of its mechanical properties by micro-indentation

Abstract

In electronic packaging in recent years, silver (Ag) alloy wires have been widely adopted in wire-bond processes. The bond pads on the device chips are mostly aluminum (Al). Thus, the bonding interface is mainly Ag–Al. In recent Ag alloy wire-bond publications, it is unclear what intermetallic phases form at the interface. In this research, experiments were designed to understand the Ag–Al intermetallic compound (IMC) formation in the Ag–Al system and evaluate its mechanical properties. First, the Ag–Al alloys with compositions from 19 to 43 at.% Al were evaluated to identify the phase equilibrium and crystal structure of the Ag–Al intermetallic phases. Microstructures and phase compositions of the designed Ag–Al alloys are presented. To further study the intermetallic compound formation at the Ag/Al interface, the interfacial reaction of the Ag/Al joints at 200 °C was investigated. The µ-Ag3Al and δ-Ag2Al IMC were identified to form at the Ag/Al interface and stabilize after long-time annealing at 200 °C. At last, deformation and fracture behaviors of the bulk µ-Ag3Al and δ-Ag2Al were analyzed by the micro-indentation. The measured results reveal that µ-Ag3Al exhibits significantly higher hardness and lower fracture toughness as compared to δ-Ag2Al. Indentation crack propagation in µ-Ag3Al demonstrates the fracture characteristics of brittle materials. In the case of δ-Ag2Al, the presence of slip bands exhibits the ductility of δ-Ag2Al to endure plastic deformation prior to fracture. The effect of the mechanical properties of the µ-Ag3Al and δ-Ag2Al IMC on the Ag–Al joint reliability is discussed. New information obtained in this research is important for future study of the joint reliability and failure mechanism of Ag–Al wire bonds.