The motor assembly transforms the electrical energy into mechanical energy through the ultrasonic motor made up of piezoelectric rings. Here the piezoelectric effect of the motor assembly is neglected to understand the dynamic characteristics of the compact geometry, it was shown that the motor of the motor can be characterized by few radial vibration modes [13
]. The connection of horn holder and clamper was modeled as completely connected and the clamper was fixed through four screws holes, as shown in Figure 5
. The whole model is shown in Figure 6
. The model for horn holder fixed directly via eight points (Cases 2 and 4 in Table 2
) is shown in Figure 7
With horn holder diameter 13.0 mm, from the experimental results for transducer horn completely held by clamp and fixed by four screws, it can be seen the resonant frequencies of Mode 1 and Mode 2 in the concerned low frequency range are 1132 Hz and 1332 Hz respectively. The results are nearly the same with capillary clamped vertically and horizontally. For the case of horn completely clamped via horn holder and fixed via four screws, the comparison between the FEM simulation and experiments is in Table 3
It can be seen clearly that they agree very well. FEM is very accurate. Through the mode shape by FEM simulation, other information can also be clearly seen, for example, the corresponding stress distribution of each mode. Figure 8
shows the stress distribution of mode at 1,140 Hz, obviously it is a bending mode, the lower modes are useless and harmful for the system itself, only longitudinal modes can be used for electronic packaging.
Based on the same modeling techniques, the transducer constraints were changed with horn fixed only by eight points directly to analyze the changes of transducer characteristics. For the inertia and frequency calculation, the transducer is constrained in all directions at the eight clamping locations. It can be seen that both two corresponding modes are improved, i.e. 1,444 Hz and 1,454 Hz, respectively. It means the constraint is much stiffer; this is not good for longitudinal mode displacement transform at high frequency. For the Case 3: the barrel horn holder diameter was decreased to 11.0 mm, the transducer is fixed by four screws through the clamper, the first two resonance modes also decreased a little bit. The results still agree very well with the experimental results. After loosening the screw tightening torque to 0.3 N·m, the resonant frequencies also decreased. For the horn diameter decrease to 11.0 mm, the two lower resonant modes had been increased to 1,540 Hz and 1,652 Hz. Through the results comparison of Table 4
, it was found that the clamping state has obvious effects on the dynamic characteristics, the eight points fixed condition is not suitable for mounting the transducer on a wire bonder assembly, the mounting of transducer to machine system through the clamper's four screws is better, but the horn holder diameter has little affect on the vibration characteristics, so this horn holder clamper allows the converter to be attached on the wire bonder not only in axial (longitudinal) nodes but also in radial nodes of the ultrasonic field excited in the horn. Based on this, the useful longitudinal modes of transducer with 13.0 mm horn holder mounted via clamp by four screws were studied numerically in detail. FEM results show that there are a lot of natural frequencies and vibration modes of transducer system within 150 kHz. The vibration mode shapes can be categorized as axial mode, flexural modes, torsion modes, and coupling modes. From the mode shapes, it can be judged that the useful longitudinal modes are 95 kHz and 138 Hz. When driven by electrical signals with appropriate frequency, those vibration modes would be excited for wire bonding usage. The predicted useful longitudinal modes are 95 kHz and 138 kHz, the nearby resonant modes are far from these two modes. The predicted two useful mode shapes and their corresponding displacements along the transducer body are also analyzed in detail, as shown in Figures 10
, respectively. The longitudinal displacement of the transducer tip is 18 μm for the mode at 95 kHz, the displacement for mode 138 kHz can be up to 15 μm, and the displacement can be adjusted through the different driving power for different bonding usages. The impedance of transducer at high frequency range 80∼150 kHz was also measured via the Agilent 4294 A, as shown in Figure 12
. It can be seen the main mode frequency remains very clean around 95.8 kHz and 137.5 kHz due to design feature in the assembly, as shown in Figures 13(a, b)
. It can be seen that the FEM predicted results are also quite convincing after updated at low frequency range by modal testing results, the higher useful longitudinal modes are agree very well.