<title>CCD photoresponse calibration and contrast adjustment for reliable material discrimination in the inspection of electronic packages</title>

Abstract

Inspection of complex electronic packages requires discrimination between the various materials used in such packages. Variations in the appearance of these materials and in the equipment's illumination complicates the segmentation process. In addition, some materials have similar reflectance and absorption characteristics. As a result, the segmentation process is. sensitive to small variations in the illumination settings, photoresponse nonuniformity, and contrast fluctuations. In this paper, we present two techniques that reduce these variations: 1) a new method to calibrate and correct the photoresponse characteristics of optical inspection systems, and 2) a method to automatically correct for contrast variations between the inspected packages. This results in a more repetitive appearance of the used packaging materials, which in turn results in improved segmentation performance.The photoresponse correction procedure, models the output of each photosite as a linear function of input illumination and the parameters of the model are measured. The response is corrected Using image processing hardware. Experimental results show that the non-uniformity is corrected to within 1 % of the AID dynamic range which agrees with the error analysis. The contrast adjustment method adjusts the image contrast based on histogram features and is adjusted using vendor and custom developed hardware. The relationship between the two techniques is also discussed.