Modeling for Solder Self-Assembly of MEMS Microstructure Powered by Surface Tension

Due to the characteristics of miniaturization of size and high integration of functions and so on, MEMS is taken as high-tech frontier of 21^st century. The progress of surface micromachining and bulk micromachining promotes MEMS technology. But, the inherent defects of surface micromachining and bulk micromachining hinder the triumphant manufacture of complex three-dimensional. Surface tension powered MEMS self-assembly can deal with this "bottle neck" successfully. Self-assembly technology is based on the principle of surface energy minimization of molten material. During the process of minimizing the surface energy, surface tension may pull the horizontal hinge plate up to some angle and then achieve equilibrium state. In this paper, finite element method is employed, a new dynamic MEMS self-assembly model is developed by SURFACE EVOLVER. The model in this paper can dynamically simulate the angle change of microstructure during the process of evolvement. The main characteristics of this model are: (1) it is easier to obtain the final angle than static model for a certain volume of solder paste. (2) Under the condition of no change of parameters, such as solder paste volume, pad size and pad figure, the final angle may be more precise compared with the results of previous researches. It can provide guarantee to precisely control the final assembly angle of hinge plate. Designs of experiments are planned and the solder geometry shapes are estimated from this dynamic model, and the area, energy and forces are derived correspondingly. Further, the relationship between the solder parameters and the angle, energy and forces are conducted from the above results. From the experiments we can conclude that pad size and solder volume are critical parameters which influence the final assembly angle. The experimental results can be used to guide the design of the self-assembly for 3D MEMS microstructures