Finite element modeling and analysis of ultrasonic bonding process of thick aluminum wires for power electronic packaging

Abstract

Ultrasonic wedge bonding of aluminum (Al) wires is a widely applied interconnect technology for power electronic packaging. The joint quality of the wedge bonding is mainly affected by the process parameters and material properties. Inappropriate process parameters will lead to failure modes such as chip surface pit, metal layer peeling off, wire cracking, non-sticking to the pad, etc., which limits the long-term stability of power devices. In order to reach the desired reliability, the design of experiment (DoE) is generally deployed which is costly in terms of time and related materials. Therefore, simulation-assisted analysis is in demand to rapidly narrow down the process windows. In this paper, an ultrasonic bonding model involving thick Al wires (300 μm) was established based on the Finite Element Method (FEM), to optimize process parameters effectively with reduced time and cost. The model was designed in ANSYS utilizing the transient structural mechanics module with various stresses and ultrasonic power, to simulate the relative deformation of the bonded wires and the displacement against the substrate. The result was then verified by ultrasonic wedge bonding samples with 9 sets of process parameters. The stress distributions were simulated and analyzed with the failure modes of tensile strength tests, while the deformation of wires under various process parameters was measured and compared with shear strength tests. Further, the relationship between the failure modes of the joint and the deformation was then analyzed by Response Surface Method (RSM), and the regression equation of the wire deformation and related process parameters was established and fitted with the actual sample's data. Such analysis not only found the optimum range of the deformation of thick Al ultrasonic wire bonds but also quickly provided a range of optimized processes for Al thick wires applying ultrasonic wedge bonding techniques.