Eliminating harmful intermetallic compound phase in silver wire bonding by alloying silver with indium

Recently, silver (Ag) alloys have been emerging as bonding wire materials for commercialized electronic products because of moderate hardness, high ductility, best thermal and electrical conductivities among metals and low growth rate of intermetallic compounds (IMCs). Many compositional designs such as Ag-Palladium (Pd), Ag-Gold (Au)-Pd have been demonstrated and relevant reliability issues on aluminum (Al) pad have been studied. Ag2Al and Ag3Al have been identified as the interfacial IMCs. However, the softness, facture toughness and corrosion resistance of Ag2Al are much better than those of Ag3Al. Therefore, Ag3Al and its interfaces between adjacent phases become weak part in terms of long term reliability. In this paper, an approach to eliminate Ag3Al phase is proposed by alloying indium (In) into Ag. Comprehensive studies are preformed after the inter-diffusion between Ag and Al. Focus ion beam (FIB) is utilized to create clean cross sections and sample preparation. Scanning electron microscopy (SEM) by using in-beam secondary electron (ISE) detector is carried out for cross-sectional examination and morphological evolution description. Furthermore, nanostructure, high spatial resolution compositional study and phase identification are conducted by transmission electron microscopy (TEM), energy dispersive spectroscopy (EDX) and selected area electron diffraction (SAED). The results show that the IMCs growth rate is high suppressed and Ag3Al layer has been replaced by an Ag-In-Al ternary phase. The crystal structure of the ternary phase is identified as hexagonal close packing (hcp), which is same as the structure of Ag2Al. As a result, the weak phase and interfaces due to the growth of Ag3Al are eliminated, which will definitely increase the reliability of joints. Also, alloying with indium will improve the mechanical property and tarnishing resistance of Ag, therefore, it should be promising in the future Ag wire bonding market.