AOI automatic optical inspection for PCBA

SMT electronic patch and plug-in processing manufacturers must ensure the quality of their products. In order to ensure the quality of products, it is particularly important to monitor the quality of semi-finished or finished products in each production link during the product manufacturing process. With the printing of surface mount technology (SMT) The development trend of refined circuit board circuit patterns, miniaturization of SMD components, and high-density assembly and rapid assembly of SMT components. The use of visual inspection or manual optical inspection is no longer suitable. Automatic optical inspection (AOI) technology is used as quality inspection Technical means has become the general trend.

The general flow of the aoi automatic optical inspection equipment is the same, mostly through the pattern recognition method. That is, the standard digital image stored in the AOI system is compared with the actual detected image to obtain the detection result, which is equivalent to automatic and intelligent manual visual inspection.

For example: When inspecting a solder joint, a standard digital image is established based on a good solder joint, and compared with the measured image. Whether the inspection result passes or fails depends on the standard image, resolution and inspection procedures used. Various algorithms are used in pattern recognition, such as finding the ratio of black to white, color, composition, averaging, summing, difference finding plane, finding corners and so on.

AOI’s light irradiates two types of equipment: white light and colored light. White light uses 256 levels of grayscale, and color uses red, green, and blue light. The light irradiates the surface of the solder/components, and then the light reflects into the lens. Generate a three-dimensional display of two-dimensional images to reflect the height and color difference of solder joints/components. People see and recognize objects by judging by the amount of light reflected back. The amount of reflection is mostly bright, and the amount of reflection is less, which is dark. AOI has the same principle as human judgment.

AOI has single-lens and multi-lens in terms of the number of lenses. This is only a choice for the realization of the technical solution. It is difficult to say that that method is necessarily good, because a single lens can be well detected through different angles of multiple light sources. image. Especially for the rough surface of lead-free soldering, solder joints of different shapes will be produced, bubbles are easy to form, and parts are prone to be upended at one end. The new AOI inspection equipment has also been updated with adaptive hardware and algorithms. .

The current PCB AOI detection algorithms are roughly divided into three categories:

  1. Detection algorithm based on design rules
  2. Detection algorithm based on image processing
  3. Combination of the above two

The logic of AOI reporting shortcomings is generally divided into two types: design specification (DRC) and motherboard (CAM reference) comparison.

Design specification
AOI judges based on the parameters you set, and reports that violate the parameters are disadvantages.

Such shortcomings are mainly minimum line width, minimum spacing, pinholes and copper slag.

Motherboard comparison
AOI compares the motherboard data learned with the CAM data with the actual scanned images, and reports the shortcomings that are not within the tolerance range.
Such shortcomings are mainly open circuit, short circuit, pad, and pinhole.
The design specification and motherboard comparison are organically combined, so that only the shortcomings you want to find are reported, and the number of false points is minimized.

AOI automatic optical detectors can generally find most of the defects in the soldering of SMT patches and DIP plug-ins, and there are a small number of missed inspections, but the main problem that affects its reliability is the false inspection. Dust, contamination and poor reflectivity of some materials during PCB processing may cause false alarms. Therefore, at present, manual verification must be performed after defects are detected by AOI. The initial link is also the source of most defects. About 60%-70% of the defects appear in the printing stage. If the defects are eliminated in the initial link of the production line, losses and costs can be minimized. Therefore, many SMT production lines are equipped with AOI testing for the printing process.

There are many types of printing defects, which can be roughly divided into insufficient solder paste on the pad and too much solder paste; solder paste scratching in the middle of a large pad, solder paste sharpening at the edge of a small pad; printing offset, bridging, and Contamination, etc., the reasons for these defects include poor rheology of solder paste, improper template thickness and hole wall processing, unreasonable printer parameter settings, low precision, improper selection of squeegee material and accuracy, poor PCB processing, etc., through AOI It can effectively monitor the quality of solder paste printing and analyze the number and types of defects to improve the printing process.
Since the reflow soldering process is immediately followed by the placement process, the inspection after the placement is sometimes referred to as the reflow soldering front end inspection. From the point of view of quality assurance, the reflow soldering furnace cannot be detected due to problems that occur in the reflow soldering furnace. It seems meaningless. In the reflow oven, the solder has a self-correcting displacement after melting, so the mounting displacement and solder printing status cannot be detected on the substrate after soldering, but in fact the front-end inspection of the reflow soldering is the focus of quality assurance. Information that cannot be detected after reflow soldering, such as the component mounting status of each part before soldering, can be seen at a glance. At this time, there are no irregularities on the substrate, which is most suitable for image processing, and the pass rate is very high, and the misjudgment caused by excessively harsh detection is greatly reduced.

After AOI detects a problem, an alarm will be issued, and the operator will visually confirm the board. The accidental problem report of missing parts can be corrected by repairing tweezers. In this process, when the operator visually inspects the same problem point repeatedly, he will ask the person in charge of each production equipment to reconfirm whether the machine setting is Reasonable, the feedback of this information is very helpful to the improvement of production quality, and can achieve a leap in production quality in a short time.

There are two types of problems and found problems, the inspection classification and prevention problems after printing and placement, and the inspection after reflow is classified as development problems. In the back-end inspection of reflow, the inspection system can check the missing, offset and Skew conditions, as well as all polarity defects, must also be tested for the correctness of solder joints, insufficient solder paste, solder short circuits, and lifted feet. The back-end inspection of reflow soldering is currently the most popular choice for AOI. This position All assembly errors can be found, providing a high degree of safety. Figure 7 shows the inspection image of a certain type of AOI on the PCB after reflow soldering. It uses 3 different lighting modes, focusing on solder joints, parts and laser printing text images. Collection. Figure 8 shows different types of defects identified by AOI after reflow soldering.

Analysis on the conclusion of reasonable arrangement of AOI automatic optical detector:
AOI can play a role in testing all aspects of SMT production, but AOI is currently very expensive. For the majority of small and medium-sized electronics manufacturers, it is not appropriate to configure AOI for each link, so it is a production line When there is only one AOI, which link should be placed, this is very worthy of discussion.

(1) Leading thought
There are two main ideas for inspection: defect prevention or defect discovery. The appropriate method should be defect prevention. In such a method, the AOI machine should be placed after the solder paste printer of the SMT production line, or after the component placement. The main idea is that when defects are found, the AOI machine should be placed after the reflow oven, which is the last step in the manufacturing process to ensure product quality.

(2) Implementation goals

The main goal of applying AOI is final quality and process tracking.

The final quality attention is mainly focused on the final state of product production. When the production problem is very clear, the product mix is ​​high, quantity and speed are the key factors, this goal is preferred. The equipment can generate a wide range of process control information. Use AOI Equipment to monitor the production process, typical content includes detailed defect classification and component placement offset information. When the product reliability is high, the mixing degree is low, the mass manufacturing and the component supply are stable, this goal is preferred to monitor the specific production status online, and provide the necessary basis for the adjustment of the production process.

(3) Implementation strategy
The position prevented by the AOI equipment can achieve or hinder the inspection target, and different positions can produce correspondingly different process control information. AOI placement is determined by the following factors:

1) Special production problems. If there are special problems in the production line, the inspection equipment can be added or moved to this position to detect defects and find repetitive defects as soon as possible.

2) Implementation goals. For AOI equipment, there is no one best place to deal with all the defects of the production line. If the goal of applying AOI is to improve the overall final quality, placing the AOI equipment in the front of the process may not have much value behind. In order to avoid adding value to the defective products, in the early stages of the process, the cost of repairing the defective product is much lower than the repair cost before and after delivery. However, many defects appear in the later stages of production, which means that no matter how many defects are found in the front, a comprehensive visual inspection is still required before shipment.

(4) Placement
The key to implementing AOI is to configure the inspection equipment to a location that can identify and correct the most defects as early as possible. Although AOI can be used in multiple locations on the production line, three inspection locations are the main ones:
1) After printing, 60%-70% of solder joint defects in SMT are caused during printing. If the solder paste printing process meets the requirements, the number of defects in the later period can be effectively reduced.

2) Before reflow soldering, this is a typical placement location, because most defects from solder paste and machine placement can be found. Program-controlled information can be generated at this location, and placement machines and fine-pitch component placement equipment approvals are provided. The information is used to modify component placement or indicate that the placement machine needs to be calibrated to meet the goal of process tracking.

3) After reflow soldering, this is the most popular choice for AOI, because all assembly errors can be found at this location, preventing defective products from flowing into the hands of customers. Post-reflow soldering inspection can provide a high degree of security. It can identify printing, solder paste, and errors caused by component placement and reflow processes support the final quality goal.

The development of AOI automatic optical inspection technology in the direction of intelligence is an inevitable requirement brought by the development of SMT. Under the development characteristics of SMT’s miniaturization, high density, rapid assembly, and diversified varieties, the amount of inspection information is large and complex. In terms of real-time detection and feedback, it is still in terms of the correctness of analysis and diagnosis. It is almost impossible to rely on manual analysis and diagnosis of quality information obtained by AOI. Intelligent AOI technology that replaces manual automatic analysis and diagnosis has become an inevitable development. Figure 11 shows a schematic diagram of an intelligent AOI system that uses solder joint shape recognition and expert system analysis [9]. It is based on solder joint shape theory and the method is similar to automatic visual inspection, that is, using optical systems and image processing measures. Online measurement of the shape of the formed solder joints, the computer compares the actual shape of the obtained solder joints with the reasonable shape of the analysis and evaluation expert system inventory, quickly identifies the faulty solder joints that exceed the allowable shape range, and uses intelligent technology to determine the fault type Automatic analysis and evaluation of failure causes, formation of real-time control information for optimization and adjustment of process parameters, real-time feedback control of solder joint quality, and recording and statistical processing of analysis and evaluation information. Research in this area is in progress at global.