Effect of wire diameter on the thermosonic bond reliability

Thermosonic bonding process is a viable method to make reliable interconnections between die bond pads and leads using thin gold and copper wires. This paper investigates interface morphology and metallurgical behavior of the bond formed between wire and bond pad metallization for different design and process conditions such as varying wire size and thermal aging periods. Under thermal aging, the fine pitch gold wire ball bonds (0.6 mil and 0.8 mil diameter wires) shows formation of voids apart from intermetallic compound growth. While, with 1-mil and 2-mil diameter gold wire bonds the void growth is less significant and reveal fine voids. Studies also showed void formation is absent in the case of thicker 3 mil wire bonds. Similar tests on copper ball bonds shows good diffusional bonding without any intermetallic phase formation (or with considerable slow growth) as well as any voids on the microscopic scale and thus exhibits to be a better design alternative for elevated temperature conditions.