The reliability of wire bonding using Ag and Al

The combination of aluminum (Al) and silver (Ag) for wire bonding is long established: aluminum (Al) heavy wire bonding on silver (Ag) metallization, such as pastes consisting of Ag as main component or comprising Ag alloys, was first established in the 1970s and was widely discussed (particularly in terms of reliability under high temperature and relative humidity) until the 1990s. Recently, discussion of the Ag/Al material system has reemerged in the literature, albeit within the context of a new bonding approach - thermosonic (TS) ball/wedge (B/W) bonding with Ag or Ag alloy wires, instead of gold (Au) or copper (Cu) wires, on Al-metallized chips. Several forms of Ag wire are currently being evaluated, including pure Ag, Ag alloys and a preliminary version of a palladium(Pd) -coated Ag. The present paper bridges the gap in the literature between the two bonding approaches by reviewing reliability studies for both Ag-on-Al and Al-on-Ag bonding.