AOI inspection, or automatic optical inspection, is an important inspection tool and process quality control tool for ensuring product quality in the electronics manufacturing industry.

AOI takes a picture of the PCB using a single 2D camera or two 3D cameras, and then compares the board photo to a detailed schematic. If the board does not match the schematic to a certain extent, the mismatch will be marked as inspected by technicians, and AOI can detect fault problems in time.

However, AOI detection will not power the board and cannot detect all components 100% of the problems, so AOI will generally be used in combination with other test methods, and the commonly used test combinations are:

● AOI and flying probes

● AOI and Online Testing (ICT)

● AOI and functional testing

On the PCB production line, AOI inspection equipment is generally placed in these three places:

1) After solder paste printing. If the paste printing process meets the requirements, the number of defects found by ICT can be greatly reduced. Typical printing defects include the following:

A. Insufficient solder on the pad.

B. Too much solder on the pad.

C. Poor coincidence of solder pads.

D. Solder bridge between pads.

(2) Before reflow soldering. The inspection is done after the components are placed inside the solder paste on the board and before the PCB is fed into the reflow oven. This is a typical location for inspection machines, as most defects from paste printing and machine placement can be found here. The quantitative process control information generated at this location provides information on the calibration of high-speed chip machines and dense pitch component placement equipment. This information can be used to modify component placement or indicate that the placement machine needs to be calibrated. The inspection at this location meets the goal of process tracking.

(3) After reflow soldering. Inspection at the last step of the SMT process is the most popular choice for AOI because it is a location where all assembly errors can be found. Post-reflow inspection provides a high level of safety as it identifies errors caused by solder paste printing, component placement, and reflow processes.