Calibration tool for a CCD-camera-based vision system

A special calibration tool has been developed for a CCD camera based vision system in an automatic assembly machine. The machine is used to attach orifice plates onto a silicon wafer in a production process. The center locations of the positioning circular holes on the plate must be controlled accurately to coincide with those on the wafer die before they are attached together by UV curing. Although CCD camera based vision systems are widely used for accurate positioning and dimensional measurements in precision engineering, electronics and semiconductor industry, their calibrations are normally done by artefacts with plane patterns. These artefacts are therefore restricted to only two dimensional measurements. The calibration tool we developed was to check the positioning accuracy of circular objects in a two-layered structure. It can also be used to determine parallax errors, non-linearity and spatial non- uniformity errors as well as repeatability of the vision system with an uncertainty at sub-micrometer level. The design, calibration and performance of the tool are described in detail in this paper.