Influence of intermetallic phases on reliability in thermosonic Au-Al wire bonding

Since roughly 2002, reliability problems occur often at the ball bonds after wire bonding or reliability testing procedures. The failure mainly appears as ball lift-off or reduced pull force values. Applying Focused Ion Beam (FIB) techniques as well as Scanning and Transmission Electron Microscopy (SEM, TEM) alternative failure modes in addition to Kirkendall voids were found that could be divided into five morphological categories. These failures include the missing of any Au-Al intermetallic compound (IMC) formation at the pads after wire bonding, no uniform horizontal/vertical IMC formation after wire bonding, micro cracks between the interface IMC and Au ball during reliability stressing, micro cracks between the interface of the two IMCs during temperature aging and finally corrosion of the interface IMC/Au ball during temperature aging or humidity testing. Although there is strong indication that part of these failure modes are related to front end process steps, the practical limitations resulting from separation between semiconductor manufacturing and assembly line require a back end solution. It is shown that the use of an improved high security (HS) or high reliability wire (HR) contributes significantly to reliability improvement. The wires contain special dopands which reduce the IMC and defect growth. The influence of these wires on the reliability of bonds will be explained and shown in selected examples.