Modeling energy transfer to copper wire for bonding in an inert environment

Copper wire ball bonding has gained popularity in the last few years due to its economic advantage and superior electrical performance. However, because of copper´s reactive nature to oxygen, free air ball formation for copper bonding is more crucial than in the gold wire process. To create a free air ball (FAB), the wire bonder uses an electronic flame-off (EFO) unit. During operation, a high voltage is generated across the EFO gap, causing a high current (spark) to discharge and melts the tail of the copper wire to form a spherical ball. Unlike gold FAB, copper FAB must be formed in an inert gas environment as copper oxidizes readily, especially at elevated temperatures. If oxidization of copper FAB occurs during formation, the FAB does not form a spherical shape, which could result in poor first bond. Oxidization occurs if the flow rate is not sufficiently high to provide a complete inert gas envelope during the melting of the copper FAB. On the other hand, if the flow rate is too high, it could have a significant convection cooling effect, depending on the wire diameter, which could also result in a pointed FAB. The type of cover gas used during bonding also plays an important role in FAB size. When inert gas such as 100% purified nitrogen, is used, the effect is a total inert blanket and cooling effect. If standard forming gas (5%H₂/95%N₂) is used, it will deplete the oxygen to remove any oxide that is present at the copper wire surface. In addition, the hydrogen gas in the mixture also raises the heat energy to provide additional heat thermal energy for melting the copper wire. Therefore, both cover gas types, and their respective flow rates, affects the copper FAB size and characteristics. This paper describes a general method to model the copper FAB diameter at various conditions with different interactions of current setting, fire time, EFO wand feature, type of cover gas and corresponding flow rate. The model used is based on a parti- - ally enclosed glass tube gas delivery design. As a general method, it can be applied to different gas delivery designs