Study of Ag-alloy wire in thermosonic wire bonding

Lower cost materials, such as copper (Cu) and palladium coated copper (PdCu) are the commonly chosen alternatives of gold (Au) wire in the package industry. However, the high hardness of Cu and PdCu wires brings concerns over the bonding quality and the long-term reliability of the packages. Silver (Ag) has drawn more attention in the package industry since it has similar properties like hardness, elongation and breaking load as Au, while having a comparable price to PdCu. Bondability of Ag-alloy wire, including performance of free air balls (FAB) and bonding capability on aluminum (Al) die pads, was first investigated. Inspection of intermetallic compound (IMC) and unmolded baking of the bonded packages with the Ag-alloy wire were also carried out for further understanding the reliability performance of the wire. Investigations of bonding capability comparison between pure Ag, Ag-alloy, and PdCu wires in processes with stand-off-stitch-bond (SSB) and peel sensitive dies were included in the study as well. Generally, Ag-alloy wire delivers good and stable bonding capability using N₂ as the cover gas. For applications with SSB and peel/lift sensitive bond pads which are normally difficult using PdCu wire, Ag-alloy wire also possesses good performance.