As lines on Flexible PCB manufacturer’s factory (Printed Circuit Board) ready to be used for SMT (Surface Mount Technology) assembly become finer, components miniaturized and SMDs (Surface Mount Devices) high-density assembled, visual inspection is far, far away from being sufficient to be compatible with SMT assembly quality inspection demands. As a result, AOI (Automated Optical Inspection) becomes increasingly commonly used in SMT assembly as a key method to inspect the performance of soldering joints formed in SMT assembly procedure. In addition, AOI has been applied in wider application fields and its inspection technology becomes gradually advanced and optimal. Right now, AOI system is moving towards intelligentization as an inspection mode used in SMT assembly.

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 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 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.

AOI technology can be used to implement inspections on PCB bare boards, solder paste printing, components and solder joints.

Bare Flexible PCB manufacturer’s inspection and solder joint inspection usually depend on independent AOI instrument and the inspections are not made in real time. Solder paste inspection and component inspection usually depend on the AOI instrument matching solder paste printer and chip mounter and their inspections are made in real time. For example, currently advanced solder paste printer is capable of carrying out real-time inspections on printing thickness and printing edge collapse through the matched AOI system.

AOI items for bare PCBs cover disconnection, wiretapping, scratches, pinhole, line spacing and tracing, edge roughness and mass-area defects. AOI items for solder joints cover lead arrangement and bending, missing components, misplacement, components placement direction, component number, solder joint quality etc. As AOI system finds out unqualified components, signals will be usually sent to operators who will be able to replace them with qualified components and stop errors from making disasters in volume.