Shape and height measurement of solder bumps using focus measure difference

摘要

This paper describes a technique, based on the shape-from-focus method, to measure the shapes and heights of an array of solder bumps through visually inspecting an IC chip-size package (CSP). We devised a focus measure difference method for detecting the horizontal section of a bump quickly and precisely. This was where we selected bump image pixels that had the same focus measure at two specified focal points along the height axis. The selected pixels corresponded to the contour points of the section at the mid-height between the two focal points. We applied it to measuring the height of an array of 208 bumps that were 260 μm high arranged at intervals of 500 μm. By measuring the area of the horizontal section at a specified height and using it to fit a sphere in the bump, the height of the bump top was efficiently estimated to match the sphere top. This sphere fitting achieved a height measurement accuracy of 8 μm (2σ) at a resolution of 3.4 μm pixels on the bump using a 2/3-inch high resolution CCD camera with 1300 * 1030 pixels. The accuracy was sufficient to inspect the coplanarity of an array of solder bumps.