Modeling for solder self-assembled MEMS

A new method of assembling MEMS is being developed that uses solder surface tension force to manipulate and assemble MEMS 3-D structures. Modeling is critical to design solder joints for precision assembly. An accurate model has been developed based on the principle of surface energy minimization. Using Surface Evolver software, this model considers three-dimensional MEMS configurations with different pad dimensions, geometries, and volumes of the solder joint. The software calculates solder shapes with local minimum surface energies and identifies the final shape with the global minimum energy. A two-plate popped-up MEMS structure was modeled and experimentally measured. The experiment confirmed the model could predict the final, equilibrium angle to within +/- 2 degrees. This accuracy level is actually limited by the experimental error bar of +/- 2 degrees, which was caused by the volume variation of the solder spheres used. The model's accuracy is expected to be much better. Nevertheless, the present model, with the verified accuracy, can help MEMS researchers design innovative 3-D MEMS assembled using solder.