下载掌阅APP，畅读海量书库
立即打开
Currently, wire bonding is the most popular first-level interconnection technology used between the die and package terminals [1] . It has been widely used as an interconnection technique in the electronic packaging industry since it was invented in the 1960s [2] . Approximately 9 trillion wires in electronic devices were bonded globally by this technique in 2008 [3] . Owing to the cost and flexibility, it will still play an important role for a long period [4] . Recently, gold has been extensively used in wire bonding because of its excellent performance [5] . Conversely, copper wire exhibits better tensile strength and lower cost compared with gold wire, increasing its use as a substitute for gold wire [6] . Nonetheless, it presents several drawbacks owing to its oxidation, high strength, and high hardness [7] . Therefore, the bonding mechanism of copper needs to be further evaluated.
Despite its long-term and excessive usage, the wire bonding mechanisms have not been completely evaluated [8] , and fundamental research is still required. From the chemical point of view, the wire bonding mechanism explains that the formation of solid-state metallic bonds between the surfaces of wire and substrate to achieve the required connecting strength [9] . Several methods have been used to form metal bonds that meet the bonding conditions, such as ultrasonic wire bonding. In this method, the ultrasonic energy is applied onto the contact interface to break the oxide films presented on the surface of the wire and substrate [10] , which gets metal atoms close enough to build metal bonds. In the past, experimental methods were used to analyze the changes of microweld [8] . However, these methods presented drawbacks like being destructive and not able to detect microscale changes [11] . Because of that, it is difficult to identify the changes in the reaction zone between the contact surfaces. Therefore, simulation is the only approach possible to evaluate the wire bonding. Although finite element (FE) simulations are widely used to predict mechanical behavior, the results depend on phenomenological models, which rely on experimental observations and practical inverse analysis of the bulk behavioral laws [12] . However, experimental data are difficult to obtain, and some macro mechanisms are no longer applicable from the micro perspective, hindering the use of the FE method to perform simulations with acceptable quality. Ding et al. [13] and Ding & Kim [14] established a 3D model and a 2D model to analyze the contact pressure and temperature rise on the contact surface, in which only the normal force was considered and the vibration was coarsely added in the analysis. The result showed that the largest contact pressure was located at the interface perimeter, which disagreed with the result reported by Winchell and Berg [15] . Consequently, a micro-scale approach is needed to deal with this problem.
Molecular dynamics (MD) simulation is more appropriate to investigate the local changes of microweld as the changes of microweld occur at an atomic level. Per-atom modeling, simple and accurate kinematic equations, complete and reliable potential function, and not high computational workload make MD suitable for studying the behavior of metal systems. Ding et al. [16] simulated the interfacial contact in Au-Au ultrasonic wire bonding and adhesion in pull test of the weld and obtained the estimated tensile strength. Long et al. investigated the evolution of surface morphology in the friction process based on the nano-friction model and discovered that the weld can be formed or broken instantly [8] . However, atomic stress analysis during the wire bonding process has not been evaluated yet. It is necessary to study the atomic stress during the contact process of Cu-Cu wire bonding, which is helpful to understand the mechanism of wire bonding and get better bonding parameters.
In this chapter, the contact model for the nanoindentation process between the wire and substrate is developed to simulate the contact process of the Cu wire and Cu substrate. The mechanism of microweld formation and breakage during Cu-Cu wire bonding is investigated by MD simulation. The evolutions of the indentation morphology and distributions of the atomic stress are investigated through the loading and unloading processes.