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Introduction
Wire bonding is a micro welding process of a thin metallic wire such as gold or copper to electrically connect the bond pad of a silicon device to the metal lead of a IC package using a thermosonic process that involves the combined application of ultrasonic energy, heat and mechanical compressive force at the bimetal interface of the weld ([1] Harman, 1997). Today wire bonding remains to be dominant technique accounting for 90 percent of all the chip to package interconnects. But in the electronic packaging industry there is a constant need to move towards thinner and smaller outline packages with more number of input/output (I/O) connections. Hence, there is a constant pressure to move towards smaller and thinner wires bonded at close pitch of less than 30 μ m. In a previous study, the reliability aspects of such thin wires were discussed in detail ([2] Murali et al. , 2006a).
Between the two ends of the bonded wire, the first bond, which involves bonding the gold wire to the Al bond pad of the Si device, is the crucial process step. The ball bonds are expected to possess a nice bell shape in order to have enough contact with the bond pad to grow the intermetallics as well as have the right thickness (∼7 μ m) to achieve good wire bond reliability. However, squash bonds of wire bonds are seen occasionally in some leadframe designs of surface mount package types such as quad flat no lead package (QFN). Literature review shows no detailed study has been made to investigate their cause. The presence of such defects on the wire bonds affects the wire bonds' performance during further reliability tests. The effect of neck fatigue can be quantified in terms of shear and pull strength. Hence, in the present study a detailed investigation was undertaken to understand the reasons of such damage. Since, the ultrasonic process involves application of a vibration at very high frequency (138 KHz), dynamic forces such as inertial and damping forces exist other than the spring forces. Hence, vibration characteristics of the bonding process was taken for detailed study for any clues as to what can induce such a squashed ball bond.
Materials and processes
QFNs are getting prominent as...