Effects of lead design on ultrasonic bond quality of wire bonds

Wire bonding is an important method of interconnection in microelectronics. The micro welding of the contact pad to the gold wire is achieved by a thermosonic wire bonding process. Ultrasonic energy is known to soften metallic materials and hence when used in the wire bond process to decrease the flow stress similar to thermal energy. But experimental results shows for some lead frame designs neck fatigue, squashed bonds and poor wire pull strength are observed for some particular leads. Detailed resonance studies shows that those leads have closeby resonance compared to the bonding frequency which causes enhanced vibration resulting in smeared and squashed ball as well as localized fatigue at both the neck and heel of the wirebond. Hence precautions during lead frame design are necessary to avoid closeby resonance motion that are in the in-plane direction along the capillary vibration direction. This can be determined apriori using finite element method