Ultrasonic welding is one of the most popular techniques for joining thermoplastics and plays an important role in MEMS applications such as fabrication and packaging of MEMS devices. In this paper, an attempt was made to further understand the heating mechanism during ultrasonic welding. Firstly, the equation governing heat generation was derived assuming adiabatic heating. A thermal equivalent circuit model was also developed to describe the heat transfer process from the joint interface into the surroundings, and the governing equation of temperature distribution in the welding sample was deduced. Finite element method was then engaged to solve these equations to reveal the transient heating behaviour. Lastly, temperatures of the joint interface and the point adjacent to the joint were measured. The temperatures of the point adjacent to the joint calculated from finite element model are matched well with the experimental results. Based on the correlation, the temperature distributions of welding samples can be derived from the finite element model. Since the new developed model can be used to obtain the dynamic temperature distributions of welding samples during ultrasonic welding, the model provides an effective way for detailed understanding of the thermal behaviours and monitoring of the ultrasonic welding process.

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