Study of copper free air ball in thermosonic copper ball bonding

In order to meet the new requirements of higher performance and reliabilities for electronic devices, the copper wire ball bonding technology was developed to be an alternative method for the gold wire ball bonding technology because of its economic advantages, strong resistance to sweeping, superior electrical and mechanical performance. But copper wire ball bonding has not been widely implemented in mass production because of some difficulties. The major reason is that the copper is readily to be oxidized during FAB formation process. And the copper oxide would cause non-stick, crater and other reliability problems. Hence, to get standard FAB balls with consistent dimensions and unoxidized ball surfaces was crucial in copper wire ball bonding. In this paper, the impacts of different electronic flame off (EFO) parameters, such as EFO current, preset value of FAB size, gap length and forming gas flow rate and so on, on copper FAB sizes and surface appearance were investigated. It was found that the higher EFO current and preset value of FAB size was needed to form copper FAB balls comparing with gold FAB ball formation. Excessive gap length would cause more heat dissipation during EFO process and had an adverse effect on the copper FAB ball formation for 2 mil copper wire. Low flow rate forming gas would lead to FAB balls with top-ends and high flow rate would cause the tilted FAB ball. Excessive high flow rate would disturb the FAB formation environment. A medium flow rate was suitable for copper FAB ball formation; such as 0.8 l/min was preferred in this study.