AOI is the abbreviation of automated optical inspection, which refers to the equipment based on optical principle to detect common defects in welding production. AOI is a new type of testing technology, but it develops rapidly. Many manufacturers have introduced AOI test equipment. When automatic detection, the machine automatically scans PCB through the camera, collects images, compares the tested solder joints with the qualified parameters in the database, checks out the defects on the PCB after image processing, and displays / marks the defects on the PCB through the display or automatic mark for the maintenance personnel to repair.

The high-speed and high-precision vision processing technology is used to automatically detect various mounting errors and welding defects on PCB. PCB board range from fine pitch high-density board to low-density large-size board, and can provide online detection scheme to improve production efficiency and welding quality. By using AOI as a tool to reduce defects, we can find and eliminate errors in the early stage of assembly process to achieve good process control. Early detection of defects will avoid sending bad boards to the subsequent assembly stage, AOI will reduce repair costs and avoid scrapping irreparable boards.

Main features

1) High speed detection system

It has nothing to do with PCB mounting density

2) Fast and convenient programming system

Under the graphical interface

Automatic data detection using mounting data

Fast editing of test data by component database

3) Using rich special multi-functional detection algorithm and binary or gray level optical imaging processing technology to detect

4) According to the instantaneous change of the detected component position, the automatic correction of the detection window is carried out to achieve high precision detection

5) Check the detection electricity by marking directly on the PCB with ink or using graphic error on the operation display

The types of AOI errors include: After solder brushing and before mounting, bridging shifting solder insufficient; After reflow soldering, shifting, leakage, polarity, skew, foot bending, and wrong parts; After reflow or wave soldering, less tin / more tin, solder ball leakage polarity shift pin bending wrong parts; PCB industry bare board detection.

There are two main objectives for implementing AOI.

1) Final quality

Monitor the final state of products when they go off the production line. When the production problem is very clear, product mix is high, and quantity and speed are the key factors, this goal is preferred. AOI is usually placed at the end of the production line. In this location, the equipment can generate a wide range of process control information.

2) Process tracking

Use inspection equipment to monitor the production process. Typically, it includes detailed defect classification and component placement offset information. When product reliability is important, low mix mass production, and stable component supply, manufacturers give priority to this goal. This often requires that the inspection equipment be placed in several positions on the production line to monitor the specific production status online and provide necessary basis for the adjustment of production process.

Although AOI can be used in multiple locations on the production line, each location can detect special defects, AOI inspection equipment should be placed in a position where the most defects can be identified and corrected as soon as possible. There are three main inspection locations:

1) After solder paste printing

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

A. Insufficient solder on the pad.

B. There is too much solder on the pad.

C. The overlap between solder and pad is poor.

D. Solder bridge between pads.

In ICT, the probability of defects relative to these conditions is directly proportional to the severity of the situation. A small amount of tin rarely leads to defects, while severe cases, such as basic Wuxi, almost always cause defects in ICT. Insufficient solder may be one of the causes of missing components or open solder joints. However, deciding where to place AOI requires recognizing that component loss may be due to other causes that must be included in the inspection plan. Checking at this location most directly supports process tracking and characterization. The quantitative process control data at this stage include printing offset and solder quantity information, and qualitative information about printed solder is also generated.

2) Before reflow soldering

The inspection is completed after the components are placed in the solder paste on the board and before the PCB is sent to the reflow oven. This is a typical location to place the inspection machine, as most defects from paste printing and machine placement can be found here. The quantitative process control information generated at this location provides calibration information for high-speed film machines and close spaced element mounting equipment. This information can be used to modify component placement or indicate that the mounter needs to be calibrated. The inspection of this location meets the goal of process tracking.

3) After reflow soldering

Checking at the last step of the SMT process is the most popular choice for AOI, because this location can detect all assembly errors. Post reflow inspection provides a high degree of security because it identifies errors caused by paste printing, component placement, and reflow processes.