AXI (Automated X-Ray Inspection), automatic X-ray inspection, a kind of optical inspection system.

AXI is a new type of testing technology that has only emerged in recent years. When the assembled circuit board (PCBA) enters the inside of the machine along the guide rails, there is an X-Ray emission tube above the circuit board. The X-ray emitted by the PCBA passes through the circuit board and is placed on the detector below (usually a camera). ) Accepted, because the solder joints contain a large amount of lead that can absorb X-rays, compared with the X-rays that pass through other materials such as glass fiber, copper, silicon, etc., the X-rays irradiated on the solder joints are largely absorbed and appear black The dots produce good images, making the analysis of solder joints quite intuitive, so simple image analysis algorithms can automatically and reliably inspect solder joint defects.

 

 

AXI technology has developed from the previous 2D inspection method to the 3D inspection method. The former is a transmission X-ray inspection method, which can produce a clear image of the component solder joints on a single panel, but for the widely used double-sided mounting circuit boards, the effect will be very poor, which will cause the visual images of the solder joints on both sides to overlap. It is extremely difficult to distinguish. The 3D inspection method uses layered technology, that is, the beam is focused on any layer and the corresponding image is projected on a high-speed rotating receiving surface. The high-speed rotating of the receiving surface makes the image at the focal point very clear, while the images on other layers It is eliminated, so the 3D inspection method can independently image the solder joints on both sides of the circuit board.

In addition to inspecting double-sided mounting circuit boards, 3DX-Ray technology can also perform multi-layer image "slicing" inspections on invisible solder joints such as BGA, that is, thorough inspection of the top, middle and bottom of the BGA solder joints . At the same time, this method can also be used to measure through-hole (PTH) solder joints to check whether the solder in the through-hole is sufficient, thereby greatly improving the quality of solder joint connection.

ICT testing is the most commonly used testing method in the production process. It has the advantages of strong failure capability and faster testing speed. This technology is very convenient and fast for manufacturers with large batches and finalized products. However, for users with a small batch and a variety of products, the needle bed needs to be replaced frequently, so it is not suitable. At the same time, as circuit boards are becoming more and more complex, traditional circuit contact testing has been greatly restricted. It is difficult to diagnose defects through ICT testing and functional testing. With the increasing density of most complex circuit boards, traditional testing methods can only increase the number of test contacts of the online tester.

However, with the increase in the number of contacts, the cost of test programming and needle bed fixtures has also risen exponentially. It usually takes several weeks to develop test programs and fixtures, and it may take more than a month for more complex circuit boards. In addition, increasing the number of ICT contacts will result in an increase in the number of ICT test errors and retests. AOI technology does not have the above problems. It does not require a needle bed. Driven by the computer program, the camera automatically scans the PCB in different areas, collects images, and compares the tested solder joints with the qualified parameters in the database. After image processing, check out Defects on the PCB. Very short test program development time and flexibility are the biggest advantages of AOI. In addition to detecting defects that cannot be detected by visual inspection, AOI can also collect the work quality and types of defects in each process in the production process, and feed them back for analysis and management by process control personnel. However, the AOI system also has shortcomings, such as the inability to detect circuit errors, and it is also powerless to detect invisible solder joints. And after our investigation, we found that AOI testing technology will have some problems in the actual application process:

1) AOI has high requirements for test conditions. For example, when the PCB is warped, it may cause test failures due to changes in focus. If the test conditions are relaxed, the test purpose will not be achieved.

2) AOI judges whether the component is incorrectly attached by recognizing the shape or text of the component. If the component type changes frequently (such as components provided by different companies), it is necessary to frequently change the component library parameters, otherwise it will lead to misjudgment.

AXI technology is a relatively mature test technology, and its coverage of process defects is very high, usually over 97%. The process defects generally account for 80%-90% of the total defects, and invisible solder joints can be inspected, but the AXI technology cannot test the defects and faults in the electrical performance of the circuit. Nevertheless, the application prospects of AXI technology in the electronic communication industry are promising. For example, Shanghai Bell and Qingdao Langxun have adopted this new technology.

From the perspective of application, testing strategies that use a combination of two or more technologies are becoming a development trend. Because each technology compensates for the shortcomings of the other technology: From the perspective of combining AXI technology and ICT technology for testing, on the one hand, X-rays are mainly focused on the quality of solder joints. It can confirm whether the component exists, but it cannot confirm whether the component is correct, the direction and the value are correct. On the other hand, ICT can determine the direction and value of the component but cannot determine whether the solder joints are acceptable, especially components with solder joints at the bottom of the package, such as BGA, CSP, etc. It needs to be pointed out that with the development of AXI technology, AXI system and ICT system can "talk to each other". This technology called "AwareTest" can eliminate the repeated testing between the two. By reducing the redundant test coverage of ICT/AXI, the number of ICT contacts can be greatly reduced. This simplified ICT test requires only 30% of the original number of test contacts to maintain high test coverage, while reducing the number of ICT test contacts can shorten ICT test time, speed up ICT programming, and reduce ICT fixture and programming costs. In the past two to three years, the use of combined test technology, especially the combination of AXI/ICT to test complex circuit boards, has seen an amazing increase, and the growth rate is still accelerating, because more industry-leading manufacturers are aware of this. The advantages of this technology and put it into use.