Design of a multi-screen deflectometer for shape measurement of solder joints on a PCB

Solder joint shape measurement is one of the most challenging technique which is essential for automation of PCB inspection. It is difficult because the surface reflection is specular. Among several kinds of specular shape measurement techniques, deflectometry is suitable to measure the 3D profile of a specular free-form surface because of its high accuracy and short inspection time. In that a pattern displayed on a screen is reflected to be imaged on a image sensor. The surface shape is reconstructed from the distortion of the pattern. The major limitation of deflectometry is its limited measurable slope angle caused by the physical size of the screen. In this paper, we propose a multi-screen deflectometer to increase the measurable slope angle for solder joint measurement. And proper calibration method and shape reconstruction algorithm are developed. This idea is verified by experimental results, in which phase information is successfully extracted from the area which are not normally measurable due to its high curvature surface.