The Process Optimization of EFO for Copper Wire Bonding

This paper investigated the process optimization of electronic flame-off (EFO) which was the melting of a wire tip by an electrical spark to form a free air ball (FAB) for 1.8mil copper wire bonding. The results showed that sparking current and sparking time rather than tail length were the key factors to influence the form of FAB. The FAB was skew and coarse with very small diameter when sparking current was 110mA. However, the FAB would be burned-out when sparking current was 180mA. And the appropriate sparking current was proved to be 130mA~150mA. When sparking time was too short (0.8ms~1.0ms), the FAB would appear to be pointed ball or golf ball. However, when sparking time was too long (3ms), the diameter of FAB was much bigger which led to offset ball and golf-bond. The suitable sparking time was proved to be 1.2ms~2ms.