An investigation of capillary vibration during wire bonding process

As the bond pitch size decreases, understanding the behaviour of the capillary tube and monitoring bond quality becomes increasingly important. This paper uses finite element analysis (FEA) and laser Doppler interferometer to study the vibration of the capillary tube during the wire bonding process. Using a laser Doppler interferometer, the vibrations were measured along the x, y and z axis under two different conditions, when the capillary was vibrating against a hard surface (low friction) and during wire bonding operations (higher friction). The head of the capillary was shown to describe a circular path when the friction was low and an elliptical path when there was more frictional resistance at the capillary tip. The forces acting on the capillary tube were studied and it was shown that the reason for the vibration in the y-axis was the horizontal reaction due to the frictional resistance when the capillary was not aligned with the x-axis. The path described by the tip of the capillary is discussed. The significance of these results is discussed and using the forces acting on the capillary in the deformed shape, the difference between the circular and elliptical paths is explained. FE analysis is then used to further understand and demonstrate the behaviour of the capillary. The conclusions of the paper show that when the capillary vibrates in a stable condition, the deformed shape does not change, but revolves around the central vertical z-axis.