Printed Circuit Boards (PCB) are widely used in electronic industry. As the PCB industry continues to adopt increasingly higher levels of integration and achieving higher and higher levels of component density, automatic PCB inspection during the manufacturing is critical and difficult. The manufacturing of PCB circuits uses the SMT (Surface Mount Technology). The SMT circuit assembly consists of three major processes. They are, in order, screen printing solder paste on the PCB, component placement and then solder re-flow in a convection oven. Correspondingly, there are three main tasks of vision inspection in PCB assembly: