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Response of Long Sculpted Wire Bonds to Vibrational Excitation
Keywords: Wire Bonding, Vibration, Finite Element Analysis
The pitch of wire bond connections is decreasing to meet the need for higher interconnect densities, while at the same time, the ratio of wire length to diameter is increasing, which lowers the mechanical resonant frequency of the wire. In many applications in which MEMS sensors are coupled with ASIC front end electronics, the bonded wires can be subjected to a wide frequency spectrum of mechanical vibrations. One potential consequence is that the parasitic capacitances of the sensor could vary dynamically at a magnitude comparable to that of the sensor signal. In extreme cases, intermittent shorts or fatigue failures of the wire bonds could occur. A recent paper by Barber et. al, showed that wire bonds carrying alternating currents in a strong magnetic field could suffer fatigue failure. Their analysis and experiments focused on simple loop geometries. In many applications, more complex wire bond geometries are used to minimize loop height and obtain dense wiring in stacked chip configurations. These geometries give rise to many more vibration modes with unique resonant frequencies and displaced shapes. We have used simple analytical beam models in conjunction with finite element models to study various wire bond configurations subject to mechanical vibratory excitation. We focused on the effects of overall wire length and geometric shape on resonant modes, and fatigue life for a prescribed cyclic loading. The finite element models were also used to calculate the capacitance between adjacent wires subject to mechanical excitation at one or more of their resonant frequencies. We have summarized the results of our study in a set of dimensionless plots of wire displacement versus wire length scaled by amplitude of the mechanical excitation. These summary plots include results for wire loops with none, one and two bends.
Thomas F. Marinis, Principa member of Technical Staff
Charles Stark Draper Laboratory
Cambridge, MA

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