Locating and checking of a BGA pin´s position using gray level

A machine vision system for SMT-mounting machine applications is usually 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 location. In this paper, a set of complete procedures is proposed to locate and check the BGA image. During the locating procedures, we first calculate a threshold of the frame using an iterative threshold algorithm. If an object is found under this threshold, then the pin´s area is calculated by the local threshold by using a momentum algorithm. After that, whether this object is a pin or not is decided by its neighbors´ relative positions, 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 momentum algorithm, then it 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´s problem and utilizes the symmetrical property of a circle to deal with the shape problem. Lastly, the CRC algorithm is used to check the correspondence between each pin and its pin type. This new method has a high accuracy and a low execution time. It can meet the crucial timing requirement of a high-speed SMT machine through experimental verification.