Abstract: The parameters of EFO(electronic flame off) process of boll bonds with fine diameter 20 μm copper wire during copper wire bonding were optimized.The results showed that EFO current and time had more affects on ball forming condition in different parameters process.As the EFO parameters increased,the diameter of FAB(free air ball) became larger.The flow of shield gas,whose impact on FAB size was not obvious,played an important role on preventing FAB oxidation and Cu FAB formation quality.Shield gas flow would reach a certain value in order to form a smooth surface without oxidation copper ball.The Cu FAB is composed by a few big columnar grains.There are two different regions in the same grain.The copper ball grain growth process is divided into two periods:the initial with dendrite growth,as well as the late approach to cellular crystal form.