In situ studies of the effect of ultrasound during deformation on residual hardness of a metal

A method is developed using an ultrasonic wire bonder to systematically study in situ the effect of superimposed ultrasound during deformation on the residual hardness of a metal wire. The method is applied during ultrasonic ball bonding of Au wire. It is found that 69 mW applied ultrasonic power during deformation of the Au balls leads to lower residual hardness than similarly deformed Au balls without applied ultrasound; this may be beneficial in reducing defects in applications sensitive to bonding stresses. It is confirmed that, although samples deformed with applied ultrasound experience larger strain rates than samples deformed without, strain rate hardening (if it does exist) does not change the finding that ultrasound applied during deformation of Au leads to lower residual hardness.