An elastic-plastic microcontact model, that takes into account the directional nature of surface roughness, is proposed for elliptic contact spots between anisotropic rough surfaces. In addition, the plasticity index was modified to suit more general geometric contact shapes. This contact model, which expands the usefulness of the CEB model, is also utilized to determine the effect of effective radius ratio (γ) on microcontact behavior and to compare the results of this model and other models under different surface topographies. The results show that the elliptic contact model and circular contact model deviate considerably in regard to the separation (h), total real contact area (At), plastic area (Ap) and plasticity index (Ψ). The present model can be simplified to become other stochastic models.

Wire bonding is a popular joining technique in microelectronic interconnect. In this study, the effects of applied load, surface roughness, welding power and welding time on bonding strength were investigated using an ultrasonic bonding machine and a pull tester. In order to relate bonding strength to contact phenomena, the asperity model was used to compute real contact area and flash temperature between the wire and the pad. The experimental results show that a decrease in load or ultrasonic power produces a larger weldable range in which the combination of operation parameters allow the wire and pad to be welded. Regardless of roughness and applied loads, the bond strength increases to a maximum with increases in the welding time, and then decreases to fracture between wire and pad. The theoretical results and experimental observations indicate that bond strength curves can be divided into three periods. The contact temperature plays an important role in bonding strength in the initial period, and surface roughness is the dominant factor in the final period. The maximum bonding strength point occurs in the initial period for different loads and surface roughness values. Our results show that bond strength of ultrasonic wire bonding can be explained based on the input energy per real contact area.