Ultrasound's role in wire bonding

In almost all integrated circuit chips, the wires that connect the internal circuitry to the external packaging are attached by a process called wire bonding. In that technique, ultrasound is used in combination with heat and pressure to weld the tip of the wire, usually gold, to the surfaces to be connected. It's been known for 40 years that ultrasound can make metals easier to work, an effect called acoustic softening. But the process of working the metal can have its own impact on the metal's hardness. Thus it's been difficult until now to get a clear picture of what's going on, and wire bonding has remained a largely empirical process. By placing gold microballs under different levels of applied force and ultrasound and measuring their resulting deformation, a team of researchers from the University of Waterloo in Canada and Tsinghua University in Beijing has succeeded in separating the softening contributions of the ultrasound from the effects of the mechanical force. The researchers were also able to quantify the residual effects of ultrasound on gold, and they found residual softening that increased with greater ultrasound amplitude above a certain threshold. They attribute the residual effects to the net balance between ultrasound's dynamic annealing and its potential opposing effect on activating and multiplying dislocations. (I. Lum et al., J. Appl. Phys., in press.) — Richard J. Fitzgerald