A study of free air ball formation in palladium-coated copper and bare copper bonding wire

Due to the continuous rise in gold prices, copper (Cu) wire has become an alternative to gold (Au) wire in the field of LSI interconnection. The characteristics of Free Air Ball (FAB), which is used for the ball bonding, give a large influence on the bond reliability. Understanding the mechanism of the FAB formation is an important step to further the application of Cu wire, including the next generation high density packaging and in-automotive IC applications. In this study, the FAB formation process was investigated in detail employing the high speed camera as a means of visualization. The studies verified that the electronic flame-off (EFO) conditions have significant effects on plasma characteristics, initiation of wire tip melting, and rate of ball rising. Further study was performed on the formation process of the irregular FABs, such as off-centered FABs and pointed FABs, which have been a matter of concern in the ball bonding; the camera analyses showed that the ball-tilting occurs from the initial stage of the wire tip melting while the transition into the pointed ball takes place during the solidification process. The formation processes of the FABs elucidated from the experiments are useful for the consideration of the stable FAB formation.