Abstract: As a pretreatment process, plasma cleaning is widely used in semiconductor industry such as wire bonding, copper-to-copper bonding and hybrid bonding. The effects of plasma cleaning on the surface state and mechanical properties of Cu/Al wire bonding joints were studied by microscopic characterization, statistical analysis and performance testing. The results showed that after plasma treatment, the contact angle of water droplets on the pad surface decreased from 96°to 12.6°, the average number of contaminated particles on each pad surface decreased from 18.7 ± 9.9 to 3.7 ± 1.6, and the average shear strength increased from 165.6 ± 5.9 to 179.3 ± 3.9mN. In addition, it is found that the shear strength and stability of the welding ball can be improved with the proper extension of the processing time. The bonding joints that failed after three times reflow and high temperature storage tests (without plasma treatment before the experiment) were analyzed. It was found that the crack originated at the interface of aluminum pad and copper ball, and it expanded along the CuAl₂ interface and led to bonding failure. The reason for the failure may be that the organic pollution on the surface of the pad promotes the formation of Kirkendall void, and then cracks are formed and the failure occurs under the influence of thermal mechanical stress. It is concluded that plasma cleaning can effectively improve the surface cleanliness of the pad and improve the bonding reliability.