Comparison of Bond Interface Reaction in Al-Ni and Al-Au Systems Formed by Ultrasonic Wedge Bonding

Considering two kinds of the most widely used wire bonding pads of the COB and BGA packaging styles, the characters of the bond interfaces in Al-Ni and Al-Au systems were compared after high temperature storage test through SEM observation. It was found that, according to the appearance, the Al-Au bond aged at 250degC in air for more than 40 days was expanded badly, and surface cracks were severe, but the Al-Ni bond changed little. At bond interface, AuAl₂ IMC was formed in Al-Au system, furthermore, many inner cracks initial the boundary of the wire and the IMC connected with the cracks in the wire, however, there was no such thick IMC formed in Al-Ni system. The causation was analyzed based on the metallurgical theory. Experimentally, the Al-Ni system was more reliable than the Al-Au system.