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The separation of golden ball and die pad due to the poor bonding strength of 1st ball solder joint is the main failure mode in thermo-sonic ball wire bonding. In this paper, nonlinear elastic and elasto-plastic quasi-static analysis of FAB (free air ball) in ultrasonic vibration process is carried out by finite element method with ANSYS. We study the evolution way of compressive stress distributions of FAB and sliding area in the contact interface under elastic and elasto-plastic deformation. According to the results of the analysis and the mechanism of micro-slip based on elastic theoretical assumption, we propose a new bonding process control that can improve the bonding strength of 1st ball solder joint effectively.