Process Parameters and Modeling Study of Thermosonic Flip Chip Bonding

Thermosonic flip chip (TSFC) bonding technology has become a developing area of chip packaging technology due to its advantages of low bonding temperature and bonding pressure. To explore the appropriate bonding parameters and to understand the relationship between each parameter and the stress/strain at the bonding interface, bonding tests were carried out, and then, simulated orthogonal tests were conducted using the finite element model (FEM). The bonding test results show that the initial position of the bumps does not affect the strength after bonding, but an unreasonable selection of bonding pressure and ultrasonic parameters can lead to different degrees of cracks at the bonding interface after bonding and destroy the bonding strength. Simulation results show that the order of influence of the three bonding parameters on von Mises stress/(shear stress) is: bonding pressure $>$ ultrasonic amplitude $>$ bonding temperature/(ultrasonic amplitude $>$ bonding temperature $>$ bonding pressure). Based on the simulation results, a multivariate nonlinear regression model between von Mises stress/shear stress and bonding parameters was constructed. Some recommendations for the selection of bonding parameters are given: 1) the bonding pressure was selected in the range of 9–11 MPa; 2) the bonding temperature was chosen to be less than 200 °C; and 3) the ultrasonic amplitude was $1.1 \mu \text{m}$ (power about 22 W). At the same time, the ultrasonic power can be increased by increasing the ultrasonic frequency to achieve better results in bonding. This study can provide some theoretical basis for the selection of bonding parameters for TSFC bonding.