What is AOI

What is AOI testing technology

AOI is a new type of testing technology which has been rising rapidly in recent years. At present, many manufacturers have launched 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.

1. Implementation objectives: the implementation of AOI has the following two main types of objectives:

(1) End 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.

2. Placement position

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 the paste is printed. 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 no tin at all, 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 at present, 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.