ABSTRACT This paper describes a method of measuring the shape of solder bumps arrayed on an LSI package board presented for inspection based on the shape-from-focus technique. We used a copper-alloy mirror deformed by a piezoelectric actuator as a varifocal mirror to build a simple yet fast focusing-mechanism. The varifocal mirror was situated at the focal point of the image-taking lens in image space so that lateral magnification was constant during focusing and orthographic projection was perfectly established. A focused plane could be shifted along the optical axis with a precision of 1.4 µm in a depth range of 1.5 mm by driving the varifocal mirror. A magnification of 1.97 was maintained during focusing. Evaluating the curvature of field and removing its effect from the depth data reduced errors. The shape of 208 solder bumps 260-µm high arrayed at a pitch of 500 µm on the board was measured. The entire 10 mm x 10 mm board was segmented into 3 x 4 partly overlapping sections. We captured 101 images in each section with a high-resolution camera at different focal points at 15-µm intervals. The shape of almost the entire upper-hemisphere of a solder bump could be measured. Errors in measuring the bump heights were less than 12 µm. Keywords: Visual inspection, Shape from focus, LSI package, Solder bump, Varifocal mirror
['Akira Ishii', 'Jun Mitsudo']
Shape measurement of solder bumps by shape-from-focus using varifocal mirror