Mechanisms of Ultrasonic Wire Bonding

Leitung:  Dr.-Ing. Jens Twiefel
Jahr:  2019

Ultrasonic (US) wire bonding is widely utilized in ICs, power devices and other applications since decades. A better understanding on the underlying bonding mechanisms, including friction, oxide removal, microweld formation and energy flows, can potentially enhance the bonging quality and reliability at a higher speed.

As US vibration starts, friction occurs at both the tool/wire interface and the wire/substrate interface. The relative displacements at these two interfaces were captured by a real-time observation system. In general, the relative displacement amplitude at the wire/substrate interface decreases and that at the tool/wire interface increases as process goes.

The friction at the wire/substrate interface causes the oxide removal and microweld formation. The oxide removal process takes four steps a cracking of the oxide scale, the detachment of discretized oxide flakes, the milling of flakes into particles and the transportation of these particles to the outside of the contact. Once the local oxides are removed, pure metal-metal contact is exposed and microwelds are formed. The total microweld area determines the final bonding quality.

he energy flows from the input power of the US transducer to the wire/substrate bonding interface were quantified. Large amount of energy was lost at the transducer. The majority of the output energy from the transducer was dissipated by the vibration induced friction at the two interfaces and was consumed by the vibration induced microweld formation. The energy for acoustoplastic effect was much smaller despite its importance to the continuous plastic deformation of the wire.