Locating and checking of BGA pins’ position using gray level

Abstract

A machine vision system for SMT-mounting machine applications usually involves a two-stage algorithm. It first measures the centroid and rotation angle of the SMD, and then checks each pin’s area, position error, and grid coordinate. In this paper a set of complete procedures is proposed to locate and check the BGA image. During the locating procedures, one first calculates a threshold for the frame using an iterative threshold algorithm. If an object is found under this threshold, then the pin area is calculated by a local threshold. After that, whether this object is a pin or not is decided by its neighbouring pins’ relative positions, then the approximate rotation angle for finding the outer pins is calculated, and the centroid as well as the rotation angle of a BGA component is calculated by the rectangular least-squares algorithm. The checking procedure also measures each pin’s area using the moment algorithm, it then calculates the radius of the moving sum using each pin’s area, and finally measures the position error using a moving-sum algorithm and judges each pin’s type by gray level. The new method uses the gray level statistic information to solve the empty pad problem and utilizes the symmetrical property of a circle to deal with the shape problem. Lastly, the cyclic redundancy check (CRC) algorithm is used to check the correspondence between each pin and its pin type. This new method has a high accuracy and reduced execution time and meets the crucial time requirement of a high-speed SMT machine through experimental verification.