Effects of ultrasonic capillary dynamics and pad material on the mechanics of thermosonic ball bonding

A frequency response model of thermosonic ball bonding is reported studying the effects of the ball/pad materials on the mechanical stresses of ultrasound under the bonding pad. The model includes chip, wire, and capillary tool, is matched to experimental results, and calculates the underpad stress fields. Assuming perfect elasticity, the stress values increase when replacing Au wire with Cu wire and decrease when using Ni as a pad material. For the standard Au-Al process, the maximum principal stress magnitude (MPS) under the pad is 490 MPa and compressive. With the harder Cu wire instead of the Au wire, less ultrasound amplitude is required while MPS increases with the same bond quality (interfacial shear) maintained. When Ni is used to replace the softer Al pad material, MPS is reduced. A Cu wire combined with a Ni pad results less than 2% higher MPS than with the Au-Al combination.