AOI stands for Automated Optical Inspection systems, mainly used for PCB manufacturing, bare board inspection, board assembly, board rework and assembled board inspection. AOI is a contactless method of inspection of PCBs and is a highly efficient for improvement in process quality. In AOI, an autonomous camera scans the board continuously. A generic CCD camera, IR camera, or a microscope is mounted with zoom capabilities. The AOI is used in all stages of PCB assembling from paste printing, components placement, reflow soldering and post soldering components inspection.
A PCB is inspected during and after manufacturing. For example, after manufacturing the ET (Electrical Test) test is performed that ensures that there is no short circuiting between two different nets or tracks. Actually, it is an error of UV exposure or etching process where a small hair-like copper remains unexposed to the UV light and cannot be removed during the etching process.
Post manufacturing PCB inspection with AOI includes:
1.Line or track width violations: The track should be within certain range of width specified by the designer. Over-etching makes thin tracks and removes extra copper. Under-etching makes wider and short circuited tracks because unnecessary copper is not etched or removed.
2.Excessive copper: Copper not etched from large areas of copper over PCBs and small dots of copper remains intact.
3.Missing Pad: Paste of insufficient etch-protective layer from keeps the pads or vias uncovered and removes it completely during the etching process.
4.Clearance Violation: Clearance violation is most of the times by the designer, however, insufficient etching makes small clearance than required.
5.Sort circuit: Short circuit appears in the PCBs like a very small and thin trace connecting two tracks / pads and/ or vias. This type of short circuit is not visible by the naked eye especially in the inner layers of the multilayer PCBs. For this purpose high resolution microscope or X-ray scope is used.
6.Copper Cuts: copper cuts are generally found in the PCBs which mainly occurs due to the scratches over the PCB sheet. This type of fault is also very thin hair like structure and cannot be resolved with naked eye.
7.Scratched or Hole Breakage: In the electrolysis process the holes are not formed properly and have broken from inside the hole or form the annular ring or pads are ripped-off.
8.Silkscreen placed over pads: The components information like designators, assembly marks, PCB names and notes and identification marks are placed over the solderable areas like pads. It makes a poor soldering joint between pads and pins of the components.
9.Solder Bridging: AOI inspections are also used for post assembly and soldering inspection of the PCBs for solder bridging or short circuiting due to extra solders over two nearby pins.
Fig. 1:- Assembled PCB components placement and soldering inspection by AO machines