Uncertainty in Manufacture and Assembly of Multiple-Joint Solder Self-Assembled Microelectromechanical Systems (MEMS)

Surface-micromachined microelectromechanical systems (MEMS) are two dimensional in their “as fabricated” form. Surface-micromachined MEMS can be assembled after fabrication to realize systems with a more three-dimensional form and function. One reliable method of assembly, suited to commercial mass production, is to use the surface tension of microsized droplets of molten solder to assemble the microsized structures, otherwise known as “solder selfassembly.” In other works, single-joint solder self-assembled structures have been demonstrated without emphasis on the uncertainty involved in the assembly position. In this work, a reliable process for manufacturing multiple-joint solder self-assembled MEMS was developed, and the impact of process and component tolerances on assembly precision was investigated using statistical and worst-case tolerance analysis techniques. It was determined that assembly precision was affected, from greatest to least impact, by the following assembly variables: solder volume, scavenging and overwetting, residual stress in bilayer structures, temperature, structure dimension, solder pad warpage, hinge play, and residual stress in single-layer structures, respectively. Guidelines on increasing assembly precision are discussed.