Shape Measurement of Solder Bumps by Shape-from-Focus Using Varifocal Mirror

Abstract

We measured solder bumps on an LSI package board presented for inspection based on the shape from focus. We used a copper-alloy mirror deformed by a piezoelectric actuator as a varifocal mirror to build a motionless yet fast focusing mechanism. The varifocal mirror was at the image focal point of the image-taking lens so that lateral magnification was constant during focusing and orthographic projection was established. A focused plane was shifted along the optical axis with a precision of 1.4μm in a depth range of 1.5mm 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 spaced 500μm on the board was measured. The 10mm×10mm board was segmented into partly overlapping 3×4 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. Error in measuring bump heights was less than 12μm.