Solder-assembled large MEMS flaps for fluid mixing

We describe surface-micromachined thermal actuator-based micro-electro-mechanical systems (MEMS) flaps with a length scale of 1,000 μm. These flaps were developed for the enhancement of fluid mixing in an experimental study of a planar air jet. The scales of the flow physics required the actuators to be much larger than the typical MEMS scales, and the experiment required an array of 10 flaps (1 cm in length) to be soldered onto a ceramic substrate with high precision. These unusual requirements made it difficult to achieve good assembly yields that could provide large deflections. To improve the yields and deflections, we modified the initial flap design by reducing the size of the solder pads, removing sharp corners, changing the number and the width of the actuator´s hot arms, and strengthening the support beams. In addition, we developed a controllable assembly process to reflow tin/lead solder on gold pads, and we used numerical simulations extensively in the design process to improve the performance. The experimental measurements corroborated these simulations: the deflection was increased from 6.4 to 11.1 μm under a 5 V applied voltage. Flow velocity measurements showed that these MEMS flaps amplified the natural instabilities of the planar jet.