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This paper reports on surface micromachined hinged structure assembly using thermokinetic forces in the molecular flow regime. Ultrasonic vibration energy is used to reduce the static friction making the thermokinetic force comparatively significant. The thermokinetic force, resulting from the more energetic gas molecules emanating from the heated substrate, increases with pressure and substrate temperature in the molecular flow regime. The transition from viscous to molecular regime occurs as the molecular mean-free-path approximately equals the flap length, making the pressure threshold for thermokinetic flap actuation size dependent. In addition to the experimental results, one-dimensional (1-D) and two-dimensional (2-D) force models are presented. Examples of assembled structures are shown and assembly jig suitable for automated MEMS batch assembly is demonstrated.