Localized heating and bond formation at solder/Cu interface induced by ultrasonic friction

The intense generation of frictional heat in fast rubbing process can locally melt contacting solids and join materials. This letter decribes a localized heating and soldering at solder/Cu interface induced by ultrasonic friction. Through probing the mean surface temperature of sliding solder bump, the transient nature of frictional heat generation can be deduced, which is associated with the interfacial thermo-mechanical interaction during ultrasonic slip. A thin molten solder film forms at the frictional interface and reacts with the Cu top surface metallization (TSM) pad, producing nano-scale Cu₆Sn₅ intermetallic compounds (IMCs), with some tiny Ag₃Sn particles attaching on them. The effects of ultrasonic sound on the interfacial reaction of IMCs were investigated, and the shear strengths of ultrasonic-soldered joints with different bonding times were also characterized.