Wire-bond void formation during high temperature aging

Voids formed in Au-Al intermetallic phases degrade the long-term reliability of gold wire bonds to aluminum pads. In this study, a series of microstructural studies were performed to evaluate void formation in wire bonds. Voids are classified as initial, annular or minute. Probe marks and Al pad contamination are the main causes of initial voids that block alloy diffusion and slow down intermetallic growth. Annular voids are caused by the ultrasonic squeeze effect of thermosonic wire bonding. These bonding gaps may become pathways for halide species that corrode and degrade wire bonds. Minute voids are formed during the Au₄Al phase. The two Au₄Al phase textures in these voids may be due to different Au₄Al phase formation reactions or be related to grain boundary effects on the surface layer of the Au ball.