In the manufacturing process of flip-chips, ultrasonic bonding has been widely used. In the past, however, the ultrasonic horn was designed to move horizontally while the bonding force was applied vertically, the motion control of the horn thus cannot be controlled accurately. In addition, non-coplanar effects were unavoidably occurred and the quality of bonding became unacceptable. In this paper, the non-coplanar phenomenon between the horn and the substrate was attempted to improve by designing the horn with the following features: 1. The resonant mode of the horn is purely longitudinal; 2. The contact surface between the horn and the flip-chip in the substrate remain plane; The finite element method (FEM) was used to design the ultrasonic horn with the above features. In the future, the amplitude-fluctuation electronic speckle pattern interferometry (AF-ESPI) will be used to verify the FEM results.