Self-assembly Of MEMS Microcomponents And Laser-induced Actuation Behaviors Of Molten Droplets Based On Solder Ball Reflow

MEMS self-assembly, which utilizes molten solder surface tension forces to self-assemble MEMS microcomponents, can successfully fabricate sophisticated3-D microstructures with high aspect-ratio. It is significant for MEMS manufacturing. Moreover, actuation of microdroplet is a critical technique for microswithes and microfluidic devices, inspiring the developments of novel MEMS devices. This dissertation presented a new self-assembly method using laser reflowing, and a micromanipulation prototype was developed. Systematic studies were conducted on the factors affecting the equilibrium position and precision of self-assembly via the combined experimental and numerical approaches. Dynamic self-assembly process and solder-microcomponents interactions were also investigated. In addition, a pioneer research was conducted on actuating a microscale solder droplet on an open surface.Studies on the factors affecting the self-assembly precision show that variations of the equilibrium angle can be controlled within±2.5o; the increasing of pad size and aspect-ratio decrease the equilibrium angle; with smaller solder volume, the equilibrium angle is also smaller and the tendency restoring the microcomponents to equilibrium position is stronger; based on the assumption that the pads can be completely wetted, the variations of solder surface tension and contact angle have no effects on the equilibrium position; a parameter η representing the combination of pad size and solder volumes is proposed and the same η value almost achieves the same equilibrium angle. Energy investigations on the hingeless self-assembly structures demonstratethat the self-assembly system has the tendency decreasing the gap between the microcomponents (a visual hinge definition is proposed). This tendency decreases with the free microcomponent approaching the equilibrium position, and it vanishes at the equilibrium position. For hingeless structures, a wire limiter, which optimized the self-assembly precision to±0.5o, was developed. Based on the numerical method, dynamic wetting of a microscale solderdroplet was studied. It shows that the dynamic contact angle model is more appropriate for describing the initial fast wetting process, and has a poor accuracy in the vicinity of the equilibrium position. Moreover, the prediction shows that the relations between wetting radius and time can be perfectly described by Rw(t)~tn (n=0.32~0.45) in the initial fast wetting process. A further study indicates that the overall process of wetting can be described by the combined of two Rw(t)~tn relations, in which the change of n represents the difference of wetting mechanisms.Studies were conducted on the dynamic self-assembly process. The results show that the solder droplet tends to spreading on the free microcomponent firstly, and then spreads on the fixed microcomponent. The self-assembly rotation has the tendency increasing the dynamic contact angle of solder on the free microcomponent. This tendency can increase the wetting force, leading a fast and asymmetric spreading process. A torque analysis show that the net torque of self-assembly oscillates. The advancing of the contact line will increase the complexity of net torque. In the vicinity of the equilibrium position, the net torque oscillates between zero-value and almost presents same amplitudes at positive and negative sides. Energy analysis shows that the kinetic energies of the microcomponent and solder are small compared with the decease of the solder surface energy, which indicates the energy conversion efficiency is low and most of the energy needs to be dissipated.Analysis on the factors affecting the dynamic self-assembly process shows that the microcomponent rotates faster with the decrease of solder volume. For the slow spreading solder, self-assembly shows obvious “fast-slow-fast” three-stage, which is induced by asymmetric wetting. For the fast spreading solder, the wetting force enhancing effect induced by rotation is weakened, and the solder tends to spreading symmetrically. Investigation on the rotating rate shows that the smaller rate leads to a smaller solder surface energy. With a high rotating rate, the solder surface energy may increase during the self-assembly, and then decreases fast when the contact line advancing on the fixed microcomponent.In addition, microscale solder droplets were successfully actuated by laser offset-heating on an open surface. The actuations always allow the microdroplet moving to the center of laser beam (hot side), and then trap the droplet when the center of the laser beam and droplet coincides. This movement is opposite to the common thermally induced microdroplet actuations. Qualitative and quantitative analyses demonstrate that Marangoni flow and the vapor recoil forces of the soldering flux lead a movement towards the cold regions, weakening the driving tendency. Whereas, thermally induced wettability alternation is the main mechanism driving the droplet forward.