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In this chapter, a MD nanoindentation model of weld in the wire bonding process was developed. Through the loading and unloading processes, the mechanism of the Cu wire and Cu substrate contact was investigated using the analysis of load force versus the indentation displacement curve, indentation morphology, and atomic stress distribution. The conclusions could be drawn as following.
(1) During the loading process, at the initial contact stage, the main force is an attractive force because of the wire and substrate adhesion. At the indentation displacement of 2.32 Å, the contact load is zero. Further indenting the wire to the substrate, the main force changes to repulsive force. During the unloading process, the repulsive force decreases significantly, changing to an attractive force. The loading and unloading curves do not coincide, and the unloading curve exhibits hysteresis.
(2) For the substrate, at the initial contact stage of the loading process, the maximum von Mises stress is under the tip of the wire. As increasing the indentation displacement, the maximum von Mises stress is increased and shift towards the edge of the contact area. The equivalent von Mises stress at the contact edges of the substrate is higher than at the center of contact. During the unloading process, the value of equivalent von Mises stress in the substrate is reduced first and then increased. And it is higher at the contact edges of the substrate than at the center of contact. There is the minimum von Mises stress at the indentation displacement of-18.4 Å, corresponding to the narrowest band.
(3) For the wire, stress concentration occurs around dislocations in the middle area of the Cu wire during the loading and unloading processes.
(4) Microweld can be formed and broken in an extremely short time. The attractive force between the two parts occurs and creates bonds at the initial stage of contact between the wire and substrate.