Study of Pd mixing during Pd-Cu wire ball formation and impact on wire bond quality

The skyrocketing price of gold has driven the rapid adoption of copper wire in semiconductor packaging. Cu wire is being used for increasingly advanced applications requiring copper wire bond to finer bond pad geometries and structures. Palladium coated Cu wire (Pd-Cu wire) has emerged as the preferred material to realize the cost benefits of Cu wire while achieving the same yield and reliability levels as Au wire. However, during the ball bond formation, the Pd-Cu wire is melted causing mixing of the Pd and Cu. This paper presents the results of a fundamental study of Pd-Cu wire mixing during the ball formation process. An Electronic Flame Off (EFO) process is used to form the ball. Pd mixing in the ball due to the EFO condition is studied. Visual appearance and quality of the ball will also be evaluated. Several bonding wires suppliers are analyzed. Resultant hardness of formed ball is measured. The impact on the stability of the bonding process to the aluminum bond pad is studied, including analysis of the bond interface and Al splash during bonding. Through this study fundamental insight into the variability of Pd mixing and appearance in formed ball related to wire supplier and EFO parameter is gained.