Booming demand for high speed and high accuracy 3D inspection, especially in the area of manufacturing of electronic devices and components, causes fast progress in research related to this area and significant improvement in these systems capabilities, specification and features. Accuracy can be considered as one of the most important features for 3D machine vision inspection systems, probably along with speed and robustness.
Phase shift based systems are believed to be among the most accurate 3D measurement systems with resolution at sub-micron level, though to achieve such a high resolution and accuracy considerable efforts should be made and various physical effects should be taken into consideration.
3D accuracy problems comprise of 2D accuracy problems, such as camera radial and perspective distortions, subpixel measurement problems, etc. combined with special issues related specifically to the phase-shift measurement profilometry. Among them there are distortions of the projected light pattern, including projector radial, perspective distortions and non-sinusoidality of the projected pattern (harmonic distortion) and dynamic range problems caused by camera's signal to noise ratio and by constraints of limited digitized signal bit width. To eliminate or minimize negative influence of the above mentioned factors, a number of measures should be carried out when making 3D measurement head design followed by compensation and calibration procedures. Experimental results on various accuracy problems based on real 3D measurement system developed by authors as well as simulation results are presented in this paper.
Recently 3D inspection tasks become more and more important, especially in electronic manufacturing industry segment in such areas as solder paste inspection, wafer bump inspection, ball grid array (BGA) and leadless packages inspection, pre- and post-reflow surface mount technology (SMT) board inspection and others, number of these tasks is rapidly growing. The main trend in these applications is that sizes of objects that are being inspected are decreasing down to tens of microns thus increasing requirements in measurement accuracy for inspection systems including increasing of both range and lateral resolutions. All these factors form strong demand for 3D measurement methods that could combine high resolution and accuracy with high-speed scan and measurement capabilities. That is why phase shift profilometry methods, based on projection of structured moire-like light pattern on object's surface and measurement of the resultant phase shift, are becoming more and more popular due to their efficiency, precision and robustness. To ensure these method's precision it is necessary to make the projected pattern perfectly sinusoidal and also having predefined grid pitch (period). These parameters are subjects for various distortions due to several factors. Among these factors there are variations of projector's working distance due to changes of objects position, grating distortions, non-sinusoidality of grating's transmission profile, grating pitch variations, etc. Compensation methods for neutralizing the influence of the above mentioned factors are presented in this paper along the experiment results based on 3D measurement head developed by authors for solder paste inspection utilization as well as simulation results.