Modelling of free air ball for copper wire bonding

Copper wire ball bonding has gained considerable attention due to its economic advantage, strong resistance to sweeping and superior electrical performance. In order to have a good first bond, consistent free air-ball formation for copper bonding are even more crucial than they are in the gold wire process. To create a free air ball (FAB), the wire bonder uses an electronic flame-off (EFO) unit, where high voltage is connected. During operation, the EFO gap is breached by a high current, creating a high voltage spark, which melt the tail of the copper wire in a glow discharge to form a spherical ball. Unlike gold wire, copper wire oxidizes readily. Hence, during the formation of the FAB, the copper wire must be enclosed in an inert gas environment in order to prevent oxidisation of the FAB. In this study, an empirical methodology was developed to model a consistent FAB for copper wire diameters ranging from 0.8mil to 2.0mil. Cherry pit bonds were created to study the FAB. Numerous tests were run on an automatic ball bonder in which current and time were varied and the resulting ball size measured. These data points were then used as inputs for the empirical model. This methodology uses the EFO current and time as measurable energy inputs and the FAB size as the measurable energy output. It is simple to use and has the advantage of avoiding complex computation of phenomena analysis, which involves the phase change during FAB melting and formation. It is also able to predict copper wire FAB to provide consistent FAB size.