Modeling and Design of a Novel Cooling Device for Microelectronics using Piezoelectric Resonating Beams

Abstract

As thermal management in microelectronics becomes more and more important in insuring the reliable operation, a novel and effective cooling device by smart materials such as piezoelectric bimorph needs to be developed. Investigation of modeling and design of piezoelectric resonating structures was conducted. A dynamic performance prediction method was proposed to calculate tip deflections at resonances and investigate the effect of finite stiffness bonding layer in piezoelectric bimorph. Considering the product of resonance frequency and dynamic tip deflection as a performance merit, the effects of length and location of the actuators on passive piezoelectric structures as well as the boundary conditions were analyzed for generating acoustic streaming which may be used for cooling microelectronic components.

The cooling effects generated by vibrating non-slot and slotted piezoelectric bimorphs were experimentally investigated. A prototype, which is comprised of a piezoelectric bimorph actuator, an aluminum block with commercial cartridge heater served as heat source, four micrometer heads to adjust the gap size between bimorph and heat source, was constructed. Validated finite element analyses were employed to simulate the vibration characteristics including the natural frequencies and mode shapes of the proposed prototype. Setting the operation frequency at the fundamental resonance frequency, the cooling effects were measured by the temperature drops of the heat source above the vibrating bimorph. Electric field applied on the bimorph and the gap between heat source and actuator were adjusted to find out the best cooling result. Heat transfer coefficients between the heat source and vibrating bimorphs were calculated by ANSYS steady state thermal analysis and the lumped energy balance method. Air flow patterns around the bimorph actuator were visualized using particle tracking velocimetry (PTV) as well.

The experiments showed that there exists an optimal gap between the heat source and the vibrating bimorph which brings the maximum temperature drop and the cooling effect increases with the electric field strength. The enhancement of heat transfer between the heat source and the non-slot bimorph can be up to 210% with the acoustic streaming generated by the bimorph vibration. The presence of slots in the bimorphs may enhance the mixing of streams outside and inside the channel resulting in an amplified heat transfer performance. However, the number, location and size of slots may influence the vibration characteristics and the formation of swirling streaming in the channel between the heat source and the bimorph. Finally, the heat transfer coefficient of the prototyped cooling device in terms of mean Nusselt number was correlated as a function of streaming Reynolds number. This study may provide useful information on modeling the vibration characteristics of piezoelectric actuators and designing the miniature cooling device utilizing bimorph vibrations.