Mechanism Study of Ultrasonic-Vibration-Assisted Underfill Process for Flip-Chip Encapsulation

Owing to the poor filling ability of the traditional underfill process, a novel underfill, called ultrasonic-vibration-assisted underfill, was proposed. Ultrasonic vibration was introduced into the underfill to improve the process. The driving mechanism of the ultrasonic-vibration-assisted underfill process was studied in this paper. A corresponding physical model, based on a close coupling algorithm, was established to characterize the ultrasonic-vibration-assisted underfill process. In this model, the motion of the fluid and solid structures was solved separately and coupled by the data transfer at the interface within each time-step. Ultrasonic vibration was exerted on the structure as a displacement boundary condition. The model was verified by comparing the calculated results to the experimental measurement. Using this model, the kinetic behavior of the solid structure and the fluid under the ultrasonic vibration was analyzed to study the driving mechanism of ultrasonic vibration in the underfill process. According to this paper, the driving mechanism was revealed as follows. First, in the chip structure, the input ultrasonic vibration was converted into the vibration in the flow direction by the elastic deformation of the structure. Second, the chip vibration could induce the encapsulant flow in the channel because of the viscosity effect of the fluid. Moreover, the encapsulant was also driven owing to pressure variation caused by surface-normal motion of solder bumps. Finally, owing to the blocking effect at the entrance caused by the accumulated encapsulant, the fluid was driven to produce a net directional flow to the exit.