As PCB factory china knows that，although AOI technology has been used in different forms in SMT assembly, they share the same working logic, that is, optical methods are used to capture the figure of inspected target and inspections, analysis and judgment can be made in certain way. The commonly-used inspection, analysis and judgment method in AOI system includes DRC (Design Rule Check) method and CAD (Computer-Aided Design) data comparison method.

•DRC Method

DRC method is used to inspect design pattern based on some given rules. For example, PCB circuit pattern is inspected in accordance with the rule that all lines should rely on solder joints as terminals, all lines share the same width and spacing should be equivalent or larger than a regulated value. DRC method is capable of guaranteeing the validity of the pattern from algorithm. In addition, DRC method features some advantages, including easy setup, high-speed processing, and small volume of programming and small data storage. Therefore, AOI system mostly depends on DRC method.

The obvious disadvantage of DRC lies in its weakness in determining boundary capability and boundary position should be determined by designing particular method.

•CAD Data Comparison Method

CAD data comparison method is used to compare the digital image patterns stored in AOI system with the practical image patterns that are captured to summarize inspection result. The accuracy of this method depends on inspection accuracy, definition and all inspection procedures and can be obtained high. However, disadvantages of this method include high volume of captured data and high requirement on data real-time processing.

As DRC method and CAD data comparison method are compared, the latter enjoys more merits.